Natural-inspired tooth replacement: design and strategies
Abstract
A perda de dentes é uma condição que afeta pessoas de várias faixas etárias. Pode ser provocada por diferentes condições como a higiene oral deficitária, factores associados a condições ambientais e/ou hábitos de vida, mesmo doenças como a doença periodontal, bruxismo ou outras sistémicas, de que constitui exemplo a diabetes mellitus. Hoje em dia substituir um dente por um implante é um processo relativamente comum. Contudo, as atuais estratégias aplicadas apresentam inúmeras limitações, nomeadamente a falha do implante devido à sua fraca estabilidade, e consequente falta de adesão óssea funcional, definida como osseointegração. Os implantes análogos de raiz personalizados surgiram, assim, como uma alternativa aos convencionais, com o objetivo de proporcionar ao paciente uma solução que preserve mais tecidos duros e moles, com um tempo de cicatrização reduzido. Neste sentido, o objetivo deste projeto de doutoramento é o desenvolvimento de um implante dentário que se adeque às características de cada pessoa e assim melhorar o desempenho do implante melhorando o processo de osseointegração e minimizando os riscos inerentes a tal cirurgia, bem como os custos relacionados com os implantes dentários. A estratégia desenvolvida inclui a aquisição da geometria real do dente ou alvéolo através de Tomografia Computorizada de Feixe Cónico (CBCT), modelação 3D do implante e fabrico por Controlo Numérico Computadorizado (CNC). Para assegurar a estabilidade mecânica e biológica do implante, técnicas como a incorporação de microcanais na superfície e outras características de superfície foram exploradas e avaliadas. Para analisar e avaliar a estratégia desenvolvida foram feitos testes como a validação in silico, hidrofilicidade da superfície e adesão bacteriana. Além disso, para validar o processo global de aquisição de imagem e manufatura do implante, foi realizado um estudo numa mandíbula de um cão cadáver. Espera-se que o desenvolvimento deste produto conduza ao início de uma nova era de implantes dentários, uma vez que permitirá a produção de implantes dentários semelhantes ao dente natural com um melhor desempenho biológico e mecânico, com o objetivo principal de melhorar a qualidade de vida do paciente.
Full text
Universidade do Minho Escola de Engenharia Telma Sofia Alves Dantas Natural-inspired tooth replacement: design and strategies abril de 2022 UMinho | 2022 Telma Sofia Alves Dantas Natural-inspired tooth replacement: design and strategies
Telma Sofia Alves Dantas Natural-inspired tooth replacement: design and strategies Tese de Doutoramento Programa Doutoral em Líderes para Indústrias Tecnológicas Trabalho realizado sob a orientação de Professor Doutor Filipe Samuel Correia Pereira da Silva Professora Doutora Paula Cristina dos Santos Vaz abril de 2022
ii DIREITOS DE AUTOR E CONDIÇÕES DE UTILIZAÇÃO DO TRABALHO POR TERCEIROS Este é um trabalho académico que pode ser utilizado por terceiros desde que respeitadas as regras e boas práticas internacionalmente aceites, no que concerne aos direitos de autor e direitos conexos. Assim, o presente trabalho pode ser utilizado nos termos previstos na licença abaixo indicada. Caso o utilizador necessite de permissão para poder fazer um uso do trabalho em condições não previstas no licenciamento indicado, deverá contactar o autor, através do RepositóriUM da Universidade do Minho. Licença concedida aos utilizadores deste trabalho Atribuição-NãoComercial-SemDerivações CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/ Universidade do Minho, 06 /04 /2022 Assinatura: ________________________________________________ Assinado por: TELMA SOFIA ALVES DANTAS Num. de Identificação: 12923468 Data: 2022.04.06 14:13:09+01'00'
iii ACKNOWLEDGMENTS First of all, I would like to thank my supervisor, Professor Filipe Samuel Silva, for all the support, encouragement, and guidance. Thank you for all the shared knowledge, teachings, enthusiasm, and dedication throughout these years. Thank you for all the motivational words and for this opportunity. I will, certainly, be eternally grateful. To my co-supervisor, Professor Paula Cristina Vaz, for all the support and insights on the dentistry field. Thank you for being available to help me and to clarify my doubts. I would like to express my sincere gratitude to all members of the Microfabrication and Integrated Systems Laboratory for their shared knowledge, team spirit, and willingness to help. Thank you for being such an outstanding and supportive team! Thank you for the friendship and the memories we created and that I will, certainly, take with me for life! A special thanks to Sara for being the big sister I needed in the most difficult times. To all people from other laboratories and institutions, especially Jorge, that in one way or another helped me during these years. To all my friends, for always being present and for supporting me in such a moment of my life. Thank you for putting up with my concerns, for cheering me up, and for always being with me. To Mafalda and Fábia for the constant support, encouragement, and companionship. To my sister, Soraia, and my now-fiance João, for the unconditional support, encouragement, friendship, and patience! Thank you for always cheering me up and for being the best friends I could ever ask for!! Thank you for being such inspirations. To my parents for their unconditional love and support. Thank you for giving me the best education I could have had, and for never letting me down. Without you, nothing of this would have been possible! Finally, I would like to acknowledge Fundação para a Ciência e a Tecnologia for my PhD grant PD/BD/140202/2018.
iv Statement of Integrity I hereby declare having conducted this academic work with integrity. I confirm that I have not used plagiarism or any form of undue use of information or falsification of results along the process leading to its elaboration. I further declare that I have fully acknowledged the Code of Ethical Conduct of the University of Minho. University of Minho, 06 /04 /2022 ____________________________________________ Telma Sofia Alves Dantas
Natural-inspired tooth replacement: design and strategies v RESUMO Substituição dentária de inspiração natural: design e estratégias A perda de dentes é uma condição que afeta pessoas de várias faixas etárias. Pode ser provocada por diferentes condições como a higiene oral deficitária, factores associados a condições ambientais e/ou hábitos de vida, mesmo doenças como a doença periodontal, bruxismo ou outras sistémicas, de que constitui exemplo a diabetes mellitus. Hoje em dia substituir um dente por um implante é um processo relativamente comum. Contudo, as atuais estratégias aplicadas apresentam inúmeras limitações, nomeadamente a falha do implante devido à sua fraca estabilidade, e consequente falta de adesão óssea funcional, definida como osseointegração. Os implantes análogos de raiz personalizados surgiram, assim, como uma alternativa aos convencionais, com o objetivo de proporcionar ao paciente uma solução que preserve mais tecidos duros e moles, com um tempo de cicatrização reduzido. Neste sentido, o objetivo deste projeto de doutoramento é o desenvolvimento de um implante dentário que se adeque às características de cada pessoa e assim melhorar o desempenho do implante melhorando o processo de osseointegração e minimizando os riscos inerentes a tal cirurgia, bem como os custos relacionados com os implantes dentários. A estratégia desenvolvida inclui a aquisição da geometria real do dente ou alvéolo através de Tomografia Computorizada de Feixe Cónico (CBCT), modelação 3D do implante e fabrico por Controlo Numérico Computadorizado (CNC). Para assegurar a estabilidade mecânica e biológica do implante, técnicas como a incorporação de microcanais na superfície e outras características de superfície foram exploradas e avaliadas. Para analisar e avaliar a estratégia desenvolvida foram feitos testes como a validação in silico , hidrofilicidade da superfície e adesão bacteriana. Além disso, para validar o processo global de aquisição de imagem e manufatura do implante, foi realizado um estudo numa mandíbula de um cão cadáver. Espera-se que o desenvolvimento deste produto conduza ao início de uma nova era de implantes dentários, uma vez que permitirá a produção de implantes dentários semelhantes ao dente natural com um melhor desempenho biológico e mecânico, com o objetivo principal de melhorar a qualidade de vida do paciente. Palavras-Chave: Implantes dentários análogos de raiz, Customização, CAD/CAM, CNC, Zircónia.
Natural-inspired tooth replacement: design and strategies vi ABSTRACT Natural-inspired tooth replacement: design and strategies Tooth loss is a problem that affects both old and young people. It may be caused by several conditions, such as poor oral hygiene, lifestyle choices or even diseases like periodontal disease, tooth grinding, or diabetes. Nowadays, replacing a missing tooth by an implant is a very common process. However, many limitations regarding the actual strategies can be enumerated, namely implant failure due to weak mechanical and biological stabilities, and consequent lack of functional bone adherence, defined as osseointegration. Customized root-analogue implants have, therefore, emerged as an alternative to conventional ones, aiming to provide the patient with a solution that preserves more hard and soft tissues, with a reduced healing time. In this sense, the aim of this PhD project is the development of a dental implant that fits each person’s characteristics and thus improve the implant performance by improving the osseointegration process and minimizing the risks inherent to such surgery as well as the costs related to dental implants. The developed strategy encompasses the CBCT scan of the real tooth or socket geometry, implant 3D modelling and CNC manufacturing techniques. To ensure both mechanical and biological stabilities, techniques such as the incorporation of surface microchannels and other surface features were explored and evaluated . In silico validation, as well as bacterial adhesion to different surfaces, were also assessed. To validate the overall image acquisition process and implant manufacturing, a study was performed in a cadaver dog mandible. The development of this product is expected to lead to the beginning of a new era of dental implants since it will allow producing natural tooth-like dental implants with enhanced biological and mechanical performance, with the main goal of improving the patient’s quality of life. Keywords: Root-analogue dental implants; Customization, CAD/CAM, CNC, Zirconia.
Natural-inspired tooth replacement: design and strategies vii CONTENTS Acknowledgments ..................................................................................................................... iii Resumo ..................................................................................................................................... v Abstract .................................................................................................................................... vi Contents.................................................................................................................................. vii List of Symbols, annotations and abbreviations ........................................................................ xii Figures .................................................................................................................................... xv Tables ..................................................................................................................................... xix Chapter 1 Introduction .............................................................................................................. 1 1.1. Motivation ................................................................................................................. 2 1.1.1. Leaders for Technical Industries Doctoral Program ................................................. 3 1.2. Objectives ................................................................................................................. 3 1.3. Structure of the Thesis .............................................................................................. 4 Chapter 2 State of the Art ......................................................................................................... 7 2.1. Jaw Characterization ................................................................................................. 8 2.1.1. Jaw Anatomy ......................................................................................................... 8 2.1.2. Teeth Anatomical Structure .................................................................................. 10 2.1.3. Bone Classification .............................................................................................. 12 2.1.4. Bone Geometry Evaluation After Tooth Extraction ................................................. 14 2.2. Dental Implants ....................................................................................................... 16 2.2.1. Materials Used in Dental Implants ........................................................................ 17 2.2.2. Dental Implants Types and Geometries ................................................................ 21 2.2.2.1. Root-analogue Dental Implants ......................................................................... 23 2.2.3. Dental Implants Market........................................................................................ 24 2.2.4. Dental Implants Failures ...................................................................................... 26 2.3. Manufacturing Techniques....................................................................................... 28 2.3.1. CAD/CAM Technology ..................................................................................... 29 2.3.1.1. Digital Radiography ...................................................................................... 30 2.3.1.2. Clinical Applications of CAD/CAM ................................................................ 32 2.4. Dental Implants Surface Modifications ..................................................................... 35 2.4.1. Surfaces for Improved Mechanical and Biological Stabilities .............................. 36 2.4.1.1. Machining ................................................................................................ 36 2.4.1.2. Sandblasting ............................................................................................ 36
Natural-inspired tooth replacement: design and strategies xiv STL Stereolithography TBZ Titanium Blasted with Zirconia TBZA Titanium Blasted with Zirconia then Acid etched TCP Tricalcium Phosphate TI Threaded Implant Ti Titanium Ti6Al4V Titanium-6Aluminum-4Vanadium TiO2 Titanium dioxide TZP Tetragonal Zirconia Polycrystal US United States WCA Water Contact Angles WHO World Health Organization Y2O3 Yttria YSZ Yttria Stabilized Zirconia ZrO2 Zirconia β-TCP Beta-Tricalcium Phosphate
Natural-inspired tooth replacement: design and strategies xv FIGURES Chapter 2: Figure 2.1 The facial skeleton: (a) external anatomy of the right side of the skull; (b) internal anatomy of the left side of the skull. Adapted from [1]. ...................................................... 8 Figure 2. 2 Mandible, right lateral view. Adapted from [1]. ......................................................... 9 Figure 2. 3 Maxilla, right lateral view. Adapted from [1]. ............................................................ 9 Figure 2. 4 Median section of a canine tooth and its alveolus. Adapted from [5]. ...................... 11 Figure 2. 5 Different tooth types and respective roots. Adapted from [6]. ................................. 12 Figure 2. 6 Jawbone Quality Index (BQI) representation. Adapted from [9]. .............................. 12 Figure 2. 7 Mandibular Cortical Index (MCI) representation [10]. ............................................. 13 Figure 2. 8 Influence of bone density on implant success rate [9]. ........................................... 14 Figure 2. 9 Schematic drawing representing the location where the histometric measurements were performed [14]. ..................................................................................................... 15 Figure 2. 10 Overview of the extraction site after: (a) 1 week; (b), 2 weeks; (c) 4 weeks, and, (d) 8 weeks. B, Buccal; L, Lingual; PM, Provisional Matrix; C, Blood Clot; WB, Woven Bone; BM, Bone Marrow [14]. ......................................................................................................... 16 Figure 2. 11 The early dental implants made of stone, shells, bones and other materials [16]. . 16 Figure 2. 12 Implant components. Adapted from [18]. ............................................................ 17 Figure 2. 13 Different implant designs: (a) subperiosteal; (b) endosseous; and (c) transosteal. Adapted from [55]. ......................................................................................................... 22 Figure 2. 14 Global dental implants market size. Adapted from [68]. ....................................... 25 Figure 2. 15 Global dental implants market share. Adapted from [68]. .................................... 26 Figure 2. 16 CAD/CAM process workflow. Adapted from [94]. ................................................. 30 Figure 2. 17 CBCT apparatus: (a) equipment; (b) CBCT data acquisition software; (c) CBCT images and 3D reconstruction. ....................................................................................... 32 Figure 2. 18 CAD/CAM applied in dentistry: (a) ceramic crown; (b) software layout [105]. ....... 33 Figure 2. 19 Surgical guide: (a) surgical guide design and surgery simulation; (b) clinical procedure with the manufactured surgical guide [106]. .................................................. 33 Figure 2. 20 Dental implants manufactured by CAD/CAM: (a) titanium root-analogue implant [109]; (b) zirconia root-analogue implant [112]. .............................................................. 34 Figure 2. 21 Schematic representation of the implant stability [23]. ......................................... 35 Figure 2. 22 Schematic representation of the sandblasting process [123]. ............................... 37 Figure 2. 23 Schematic representation of the anodization process [134]. ................................ 38 Figure 2. 24 Schematic representation of the plasma-spraying process [119]. ......................... 40 Chapter 3: Figure 3. 1 Search strategy flowchart, adapted from [8]........................................................... 58 Figure 3. 2 Schematic representation of an endosseous implant, a subperiosteal implant and an improved solution. .......................................................................................................... 71 Chapter 4: Figure 4. 1 Search strategy flowchart, adapted from [16]. ....................................................... 86
Natural-inspired tooth replacement: design and strategies xvi Figure 4. 2 Schematic representation of the materials, design and manufacturing techniques, and surface treatments of the selected and analysed studies........................................... 90 Chapter 5: Figure 5. 1 Schematic representation of the studied models: (a) STI - Semi-Threaded Implant; (b) TI - Threaded Implant; (c) RAI - Root-Analogue Implant; (d) RAIF - Root-Analogue Implant with Flaps; and (e) NT - Natural Tooth. .......................................................................... 111 Figure 5. 2 Boundary conditions: (a) Applied loads; (b) Fixed constraint. ................................ 113 Figure 5. 3 Analysed cortical bone volume (partition) for the different models: (a) STI; (b) TI; (c) RAI; (d) RAIF; and (e) NT. ............................................................................................. 113 Figure 5. 4 Von Mises stress distribution in the cortical bone for the studied models: (a) STI; (b) TI; (c) RAI; (d) RAIF; and (e) NT. ................................................................................... 115 Figure 5. 5 Von Mises stress distribution in the circular partition, as a function of the volume, for the different models. .................................................................................................... 116 Figure 5. 6 Hydrostatic pressure in the circular partition, as a function of the volume, for the studied models. ............................................................................................................ 118 Figure 5. 7 Percentage of the circular partition volume in the four µ-strain zones: Disuse atrophy (< 50 µ-strain); Steady state (100 - 2000 µ-strain); Physiologic overload (2000 - 4000 µstrain); and Pathologic overload (> 4000 µ-strain). ........................................................ 119 Figure 5. 8 Implant/Tooth total displacement for the studied models: (a) STI; (b) TI; (c) RAI; (d) RAIF; and (e) NT........................................................................................................... 120 Chapter 6: Figure 6. 1 Specimens design and final dimensions: (a) P1; (b) P2; (c) P3. ............................ 133 Figure 6. 2 SEM micrographs of the surface topography of the three specimens with the respective micro-channels: (a), (b) P1 specimen; (c), (d) P2 specimen; (e), (f) P3 specimen. .................................................................................................................................... 136 Figure 6. 3 Representative figures of the profilometry analysis software outputs: (a) C2 microchannel depth and width; (b) C2 micro-channel topography. ......................................... 137 Figure 6. 4 Roughness (mean ± SD) of the specimens’ top surface (P1 TS, P2 TS and P3 TS) and in the bottom of the micro-channels. ............................................................................. 138 Figure 6. 5 Representative images of a water droplet and measured contact angle: (a) immediately after reaching the surface; (b) 10 s later. ................................................... 139 Figure 6. 6 Water and FBS contact angles (mean ± SD) of P1 (a), P2 (b) and P3 (c) specimens, for 0 s and 10 s. .......................................................................................................... 140 Figure 6. 7 Fluid height variation with time, in the P1 micro-channels, during the capillarity test: (a) pigment; (b) FBS. .................................................................................................... 141 Figure 6. 8 Fluid height variation with time (mean •} SD), in the P2 micro-channels, during the capillarity test: (a) pigment; (b) FBS. ............................................................................. 142 Figure 6. 9 Fluid height variation with time (mean ± SD), in the P3 micro-channels, during the capillarity test, for both the pigment and FBS fluids. ...................................................... 143 Figure 6. 10 Representative images of the fluid rise in the capillarity tests: (a) P1; (b) P2; and (c) P3. .............................................................................................................................. 143
Natural-inspired tooth replacement: design and strategies xvii Chapter 7: Figure 7. 1 Schematic representation of the produced specimens: (a) Tititanium with SLA; (b) ASzirconia as-sintered; (c) SbEzirconia with sandblasting and acid etching; (d) MCzirconia with micro-channels on the surface; (e) MCEzirconia with micro-channels and acid etching. ........................................................................................................................ 158 Figure 7. 2 Schematic representation of the developed surfaces and tested bacteria. ............. 160 Figure 7. 3 SEM micrographs of the surface topography of the produced specimens: (a) Ti; (b) AS; (c) SbE; (d) MC; (e) MCE. ....................................................................................... 162 Figure 7. 4 Contact angles (mean ± SD) of tested materials, for 0 s and 10 s. ....................... 164 Figure 7. 5 Representative images of the fluid rise in the capillarity tests: (a) Ti; (b) AS; (c) SbE; (d) MC; (e) MCE. .......................................................................................................... 165 Figure 7. 6 Bacterial adhesion (CFU/mL) on the tested surfaces: (a) Escherichia coli ; (b) Staphylococcus aureus ; (c) Pseudomonas aeruginosa . The error bars represent the standard deviation. Different letters between distinct columns denote significant differences using one-way ANOVA with Tukey’s multiple comparison test (p < 0.05). ....................... 166 Figure 7. 7 Bacterial adhesion (CFU/mL) in co-culture: (a) Staphylococcus aureus ; (b) Pseudomonas aeruginosa. The error bars represent the standard deviation and (*) indicates statistically significant differences (p < 0.05). ................................................................ 168 Figure 7. 8 Representative micrographs of the adhered bacteria: (a) and (b) E. coli on Ti and MC, respectively; (c) and (d) S. aureus on Ti and MC, respectively; (e) and (f) P. aeruginosa on Ti and MC, respectively. ................................................................................................... 170 Figure 7. 9 Representative micrographs of the adhered bacteria in co-culture: (a) Ti; (b) MC. . 171 Chapter 8: Figure 8.1 Flowchart representing the performed procedure. ................................................. 187 Figure 8.2 3D reconstructions of the Cone-Beam Computed Tomographies: (a) dog skull with soft tissuesCBCT1; (b) dog maxilla after soft tissues removalCBCT2; and (c) dog mandible after soft tissues removalCBCT2. ................................................................................ 190 Figure 8.3 Selected teeth: (a) selected teeth from the mandible; and (b) selected tooth from the maxilla. ........................................................................................................................ 191 Figure 8.4 M1 tooth segmentation in RadiAnt : (a) application of a threshold based on the image grey levels; (b) selection of the region of interest; and (c) M1 model resulting from the software. ...................................................................................................................... 191 Figure 8.5 M1 treatment in Materialise Magics : (a) tooth before artefacts removal; and (b) tooth after artefacts removal and surface smoothening. ......................................................... 192 Figure 8.6 Designing In1 based on the socket geometry: (a) region of interest segmentation; (b) 3D mesh creation in AutoDesk Meshmixer ; and (c) final model. .................................... 192 Figure 8.7 Superimposition of the SLT files resulting from the 3D scan (green) and CBCT scan (orange): (a) M1; (b) M2; and (c) In2. ........................................................................... 193 Figure 8.8 Deviations of the superimposed surfaces (measurement in millimetres): (a) M1; (b) M2; and (c) In2. ........................................................................................................... 194 Figure 8.9 Original vs. manufactured teeth: (a) M1; (b) M2; (c) In1; and (d) In2. .................... 197 Figure 8.10 X-ray scans after teeth placement: (a) In1; (b) In2 and M1; and (c) M2. .............. 198
Natural-inspired tooth replacement: design and strategies xviii Figure 8.11 Distances between the manufactured tooth and the bone, measured in RadiAnt: (a) In1; (b) In2; (c) M1; and (d) M2. ................................................................................... 199 Figure 8.12 Distances between the original tooth and bone, measured in RadiAnt. ................. 200 Chapter 9: Figure 9.1 Implant prototype. ................................................................................................ 210
Natural-inspired tooth replacement: design and strategies xix TABLES Chapter 2: Table 2. 1 Width of the bone tissue at the buccal and lingual walls of the extraction sites. Adapted from [14]. ...................................................................................................................... 15 Chapter 3: Table 3. 1 Summary of the selected articles. ........................................................................... 58 Table 3. 2 Comparing subperiosteal and endosseous implants. ............................................... 67 Chapter 4: Table 4. 1 Articles included in the review................................................................................. 86 Table 4. 2 Summary of the selected articles: implant material, design and manufacturing techniques and implant surface treatments. .................................................................... 88 Table 4. 3 Summary of the clinical trials/case reports parameters and results. ........................ 91 Chapter 5: Table 5. 1 Material properties in FEA. ................................................................................... 112 Table 5. 2 Average element quality, and number of nodes and tetrahedrons of the mesh of the different models. .......................................................................................................... 112 Table 5. 3 Average μ-strain values for the five models. .......................................................... 118 Chapter 6: Table 6. 1 Guide for cutting Zirconia with DWX – 50 [37]. ..................................................... 131 Table 6. 2 P1 micro-channels dimensions and respective designation. ................................... 132 Table 6. 3 Micro-channels depths and widths acquired by profilometry: CAD model vs. real dimensions. ................................................................................................................. 137 Chapter 7: Table 7. 1 Mean (SD) of surface roughness of the produced specimens. Roughness of MC and MCE specimens was measured on the top surface. ...................................................... 162 Chapter 8: Table 8.1 Milling parameters. ............................................................................................... 189 Table 8.2 Deviations between the two compared surfaces - 3D scan of the original tooth and developed tooth. ........................................................................................................... 195 Table 8.3 Comparison of surface area change between the 3D scan of the original ................ 195 Table 8.4 Comparison of surface area change between the 3D digital surface models and the fabricated teeth. ........................................................................................................... 196
Natural-inspired tooth replacement: design and strategies 1 CHAPTER 1 INTRODUCTION The first chapter of this PhD thesis aims to present the purpose of this project and includes the motivation and the objectives that led to the development of such work as well as the structure of the thesis.
Natural-inspired tooth replacement: design and strategies 2 1.1. Motivation When I finished my master’s degree in Biomedical Engineering, I have started working on an innovative project entitled Hybrid Additive Manufacturing for Bio Inspired Components, which aims to develop new strategies to create and produce advanced materials to implement in the medical field. This project has increased my motivation to develop new products to keep up with the constant advances in medicine and apply all the acquired knowledge in a more industrial and commercial approach. Nowadays, tooth loss is a very common problem and, as consequence, dental implants are each time more and more usual. In fact, it is estimated that 10% of people will need a dental implant in their lifetime. Despite all the advances in the comprehension of dental implants designs, materials and techniques, there is still a lot of work to do regarding the better use of biomaterials, implant design, surface modification and functionalization of surfaces to improve the osseointegration process and the overall implant performance. Traditional implants have many limitations and, once each patient has a different oral condition, implants may not fit adequately in everyone’s characteristics. Titanium and its alloys have been extensively used in dental implants due to their high corrosion resistance, low weight, biocompatibility, and mechanical properties. However, some drawbacks are associated with these metallic materials. Their dark colour may become an aesthetical issue if gingival recession occurs. Additionally, they tend to release ions when in contact with fluids and their electrical conductivity may compromise the implant performance. In this context, ceramic materials such as Zirconia have also been investigated for such applications. Zirconia is a tooth-like colour ceramic with high wear and thermal resistance, biocompatibility, and low bacterial affinity. Despite the advantages of Zirconia, the implant osseointegration process also needs to be optimized. The constant developments regarding dental implant materials and geometries are also notorious. However, few investigations have been conducted regarding customizing a dental implant according to each patient's oral condition. Developing a strategy that customizes the implant according to each patient's geometrical and biological characteristics will not only preserve more hard and soft tissue but also improve the healing time and reduce implant failure rates. Having this in mind, the main outcome of this work is the development of a customized dental implant that pretends to mimic the original tooth geometry and function, using different
Natural-inspired tooth replacement: design and strategies 3 techniques to ensure that it fits perfectly in the patient’s oral cavity. To achieve this, different technologies and approaches were used: image acquisition techniques, 3D modelling, Computer Numerical Control (CNC) machining, in silico and in vitro validation, and validation in dog cadaver mandibles. The main motivation of this work is the belief that this new approach will considerably improve the patient’s quality of life and well-being, through a less invasive and more comfortable procedure. 1.1.1. Leaders for Technical Industries Doctoral Program This PhD work aims to focus not only on the development of a new product but also on its integration in the market world, considering other competitor solutions. The Leaders for Technical Industries (LTI) doctoral program, within the Engineering Design and Advanced Manufacturing (EDAM) area, - MIT Portugal Programis designed to promote a close relationship between the industrial environment and academia. This PhD program aims to generate new scientific knowledge and engineering solutions that will compete in different industrial sectors. This is the main reason why this PhD program was chosen for the development of the present research work. This relationship between academia and industry empowered by the LTI doctoral program contributed for a continuous awareness of the market evolution and needs, and, consequently, contributed to the development of an optimized product to fulfil the market needs. Additionally, this MIT graduate program provided me tools to work in a team, improved my management skills, as well as promoted my leadership spirit. 1.2. Objectives As previously mentioned, the main goal of this PhD work is to develop a strategy to create a customized dental implant that fits each person’s characteristics, through a less invasive surgical procedure, improving the patient’s quality of life and satisfaction. This approach avoids bone drilling aiming to reduce the healing time, minimize the risks inherent to such surgery, as well as reduce the costs related to dental implants. To achieve such purpose, it is necessary to fulfil the following main objectives: • To develop a minimally invasive customized dental implant, targeting the improvement of the patient’s overall quality of life.
Natural-inspired tooth replacement: design and strategies 4 • To validate the implant development strategy. Some specific objectives were therefore established to ensure that these two general objectives are accomplished: • To validate the implant geometry in silico . • To implement strategies to promote an effective primary stability, inducing a decrease in the surgery healing time. • To develop a functionalized implant surface in order to promote a fast osseointegration and avoid bacterial adhesion. • To get accurate CBCT images to properly obtain the tooth socket geometry. • To produce the zirconia implant by CNC. • To test the developed strategy in dog cadaver mandibles. 1.3. Structure of the Thesis This PhD thesis is organized as a compilation of research papers that are already published, accepted or submitted in international ISI journals. Apart from the research papers, this thesis is also composed by three other chapters: chapter 1, chapter 2 and chapter 10. Chapter 1 corresponds to the Introduction. In this chapter it is possible to find the motivation and objectives of this research work, as well as the structure of the thesis. Chapter 2, entitled “State of the Art” presents a literature review on the most relevant concepts related to the developed work. The main purpose of such chapter is to contextualize readers from different backgrounds to the topics that will be discussed in the subsequent chapters. In this sense, the second chapter addresses topics such as the human jaw anatomy, dental implants materials, types, market, failures and manufacturing techniques, as well as the most common dental implants surface modifications. As previously mentioned, from chapter 3 to 9 it is possible to find research papers that correspond to the work developed in the scope of this PhD project. Chapter 3 – “Subperiosteal Dental Implants: Past or Future? A Critical Review on Clinical
Natural-inspired tooth replacement: design and strategies 11 Figure 2. 4 Median section of a canine tooth and its alveolus. Adapted from [5]. Adults normally have 16 teeth in the mandible and 16 in the maxilla. From the midline to the rear of each jaw, there are two incisors, a canine, two premolars, and up to three molars. The incisors are chisel-like cutting teeth used to bite off a piece of food. The canines are more pointed and act to puncture and shred it. They serve as weapons in many mammals but became reduced in the course of human evolution until they now project barely above the other teeth. The premolars and molars have relatively broad surfaces adapted for crushing and grinding [5]. The root of a tooth is characterized by its vulnerability to injury and infection due to the absence of protective enamel. The number of roots and root canals that a tooth has is an important issue when it comes to performing certain treatments. It usually depends on the type of the tooth (Figure 2. 5): the upper incisors, upper canines, and the lower incisors, canines, and premolars are characterized by one root; the upper premolars and the lower molars are characterized by two roots; the upper molars are generally characterized by the existence of three roots [5].
Natural-inspired tooth replacement: design and strategies 12 Figure 2. 5 Different tooth types and respective roots. Adapted from [6]. Understanding the anatomy of human teeth is one of the basic functions of dental education, once it is one of the prerequisites for achieving success in any clinical intervention [7]. In this sense, several imaging techniques that allow the three-dimensional analysis of the human teeth structure have already been developed and will later be further explored. 2.1.3. Bone Classification Quantitative and qualitative measurements of mandibular bone from radiographs have been assessed in order to evaluate ´bone quality´. Some of these techniques require specialized facilities and need high standards radiographies. In general practice, two simple assessments have been used: Bone Quality Index (BQI), and Mandibular Cortical Index (MCI) [8]. From those, BQI describes ‘bone quality’ according to four different types, regarding the cortical thickness and trabecular pattern (Figure 2. 6): Type I: homogeneous cortical bone. Type II: thick cortical bone with marrow cavity. Type III: thin cortical bone with a dense trabecular bone of good strength. Type IV: very thin cortical bone with a low-density trabecular bone of poor strength. Figure 2. 6 Jawbone Quality Index (BQI) representation. Adapted from [9].
Natural-inspired tooth replacement: design and strategies 13 MCI is classified by the appearance of the lower border of the mandibular cortex distally from the mental foramen, according to panoramic radiographs. This index is divided into three types (Figure 2. 7) [8,10]: C1: endosteal margin of the cortex was even and sharp on both sides. C2: the endosteal margin showed semilunar defects (lacunar resorption) or seemed to form endosteal cortical residues on one or both sides. C3: the cortical layer formed heavy endosteal cortical residues and was clearly porous. Figure 2. 7 Mandibular Cortical Index (MCI) representation [10]. Bone Mineral Density (BMD) is extremely related to BQI and refers to the density of minerals, such as calcium, in the bones, and is extremely related to the strength of the bones [9]. It is used for clinical issues as an indirect indicator of osteoporosis and fracture risk. Over the last few years, many investigators have documented the influence of bone density on dental implants’ success rate. As seen in Figure 2. 8, higher success rates are found in better bone quality (pink) when compared to poor bone quality (blue) [9].
Natural-inspired tooth replacement: design and strategies 14 Figure 2. 8 Influence of bone density on implant success rate [9]. 2.1.4. Bone Geometry Evaluation After Tooth Extraction Following a tooth extraction, the geometry of the bone is expected to change since the recovery of the extraction socket starts immediately. From the first week after removal, osteoclasts start to accumulate in the alveolar bone part and bone remodeling lasts for several more weeks [11]. However, if not stimulated, the bone will start to reabsorb. The rate of bone resorption is greatest in the first year, especially in the first 3 months. The bone loss depends on the patient’s age as well as noticeable differences between the maxilla and the mandible. Bone loss postextraction is also influenced by systemic factors such as smoking [12]. One study performed by Elsubeihi, E., et al [13] found that after 14 days of the extraction the BMD is expected to increase about 25%, and between 56 and 112 days after extraction differences were not so significant. They also concluded that healing was associated with a reduction in cross-sectional mandible area of 32.89%, 21% in height, and 12.84% reduction in the mandible width [13]. Lindhe, et al., aimed to evaluate the dimensional changes in the alveolar ridge after premolars tooth extraction, as well as the respective bone remodeling, through an experimental study in dogs [14]. In Figure 2. 9 it is shown a schematic representation of the locations where the measurements were performed.
Natural-inspired tooth replacement: design and strategies 15 Figure 2. 9 Schematic drawing representing the location where the histometric measurements were performed [14]. Results revealed that the most important alterations occurred during the first 8 weeks after tooth extraction. In Table 2. 1 it is possible to observe the results regarding the width of the bone tissue at the buccal and lingual walls of the extraction sites, during the first 8 weeks. Table 2. 1 Width of the bone tissue at the buccal and lingual walls of the extraction sites. Adapted from [14]. 1 week 2 weeks 4 weeks 8 weeks buccal lingual buccal lingual buccal lingual buccal lingual Level A 0.6 (0.1) 1.4 (0.2) 0.6 (0.1) 1.3 (0.3) 0.7 (0.2) 1.3 (0.1) 0.5 (0.1) 1.2 (0.1) Level B 1.3 (0.1) 2.0 (0.3) 1.1 (0.3) 1.9 (0.4) 1.1 (0.2) 1.6 (0.1) 1.1 (0.1) 1.7 (0.3) Level C 2.0 (0.0) 2.8 (0.8) 1.9 (0.4) 2.7 (0.7) 2.0 (0.4) 2.9 (0.3) 1.6 (0.1) 2.7 (0.5) In the first week of healing, osteoclasts were observed in all areas of the socket, as well as a periodontal ligament characterized by fibroblasts, collagen fibers, and inflammatory cells (Figure 2. 10.a). It is possible to denote large amounts of provisional matrix and a blood clot in the center of the socket. After 2 weeks (Figure 2. 10.b) it is possible to observe large amounts of woven bone (immature bone) in the lateral and apical portions of the socket.
Natural-inspired tooth replacement: design and strategies 16 Figure 2. 10 Overview of the extraction site after: (a) 1 week; (b), 2 weeks; (c) 4 weeks, and, (d) 8 weeks. B, Buccal; L, Lingual; PM, Provisional Matrix; C, Blood Clot; WB, Woven Bone; BM, Bone Marrow [14]. Four weeks after the extraction (Figure 2. 10.c), it is possible to observe a structure characterized by newly formed woven bone. The bundle bone of the crestal region of the buccal wall was resorbed and partially replaced by woven bone. Additionally, the marginal portion of the old buccal wall (arrow) is ‘‘apical’’ to its lingual counterpart. After 8 weeks (Figure 2. 10.d), the inner side of the socket was full of bone marrow, and trabecular mineralized tissue. The overall results showed that the biggest dimensional differences were observed in the first two weeks [14]. 2.2. Dental Implants Replacing a missing tooth is a very old process. Stone, shells, bones, gold, or even animal teeth were used by our ancestors to replace a missing tooth (Figure 2. 11). Once this practice was proved to be the cause of infectious diseases or eventually death, evolution had been carried out in order to overcome such issues [15]. Figure 2. 11 The early dental implants made of stone, shells, bones and other materials [16].
Natural-inspired tooth replacement: design and strategies 17 The biggest evolution in the history of dental implants goes back to the 60’s when Branemark studied the osseointegration of titanium (Ti) implants [17]. Nowadays, despite all the efforts to implement other materials (such as zirconia – ZrO2) in the implants industry, Ti and its alloys are still the most used materials in dental implants, with success rates over 90% [17]. The commercially available implants most used in our days are typically composed of three components, as seen in Figure 2. 12: the implant itself, the abutment, and a crown. Figure 2. 12 Implant components. Adapted from [18]. The implant is usually a threaded screw that is surgically inserted in the jawbone and aims to replace the natural roots. The abutment is a structure responsible for holding the crown and is screwed into the implant. The crown is the structure that is visible and pretends to mimic the natural tooth. 2.2.1. Materials Used in Dental Implants The variety of materials used nowadays in dental applications is huge. Metallic materials, such as stainless steel, chromium-cobalt (Co-Cr), or titanium (Ti) and its alloys, ceramics like zirconia (ZrO2), or even polymers such as poly-ether-ether-ketone (PEEK) have been of great interest for biomedical applications, namely for dental implants [19,20]. When selecting a material for a dental implant, it is crucial to take into consideration its mechanical properties (fatigue and fracture strengths), biocompatibility, and more recently, its aesthetic properties. Titanium Titanium and its alloys have been applied in a wide range of biomedical applications, such as hard tissue replacements (artificial bone, joints, bone plates, screws, and dental implants) and
Natural-inspired tooth replacement: design and strategies 18 cardiovascular applications (vascular stents and artificial heart valves) [21]. Their excellent mechanical properties, high biocompatibility, and high corrosion resistance make them the metals of choice for biomedical applications, namely for dental applications [20,22]. Ti6Al4V is one of the most required Ti alloys due to its unique properties – high strength, good fracture toughness, low density, low thermal expansion, and good corrosion resistance [23]. This corrosion resistance of Ti6Al4V is related to the formation of a stable and dense oxide layer of Ti (TiO2) on its surface, responsible for protecting the material and promoting chemical stability in the organism [24,25]. This oxide layer (typically with 2-5 nm of thickness) is spontaneously formed when the surface is exposed to oxygen or environments containing oxygen [26]. Despite all the advantages of Ti6Al4V, its young modulus Ti6Al4V (85-115 GPa) is considerably higher than the one of the cortical bone (8-18 GPa) and consequently loading occlusion can cause stress shielding in the peri-implant area, leading to bone loss and, eventually, implant failure [19,27,28]. On the other hand, Ti6Al4V is characterized by a poor wear resistance that is intensified when in contact with body fluids, resulting in the release of metallic ions to the surrounding tissues [27]. In the specific case of the dental implants, the contact between the abutment and the implant body is characterized by sliding wear and abrasive wear due to the hard particles that come from food intake or tooth brushing [24,25]. The metallic ions that are released from the Ti6Al4V alloy have already been proved to be toxic for the human body and are associated with inflammatory tissue reactions, such as allergic reactions and hypersensitivity [29]. Additionally, the growing concern with aesthetical issues has led to a decrease in the use of titanium as a dental material. Its grey color gets visible and more pronounced when the soft tissues are not healthy, and the dark color appears through the thin mucosa [30]. In this sense, the urge for solutions that overcome these negative aspects associated with titanium implants has led to an increased interest in the development of non-metallic implants. PEEK By overcoming some limitations of the titanium, PEEK has been applied in dental implantology since the 90’s [21]. PEEK, a high-performance polymer, is characterized by a semicrystalline, linear, polycyclic, aromatic, and thermoplastic structure [31]. Its young modulus (3-4 GPa), very similar to the cortical bone, is what makes this material so popular in the dentistry field [32]. Moreover, the addition of carbon fibres to the PEEK has been used to increase its elastic modulus to high levels, as demand for specific applications. Due to its good mechanical properties and high biocompatibility this polymer has been applied in different dentistry components: the
Natural-inspired tooth replacement: design and strategies 19 dental implant itself, abutments, prosthesis, and also for the manufacturing of screws to bond the implant and abutment [33]. Despite the advantages of this polymer, it has been proved to be a bioinert material and, therefore, does not have osteoconductive properties. Many researchers have compared PEEK implants with titanium and results revealed that PEEK promotes less osteoblast differentiation than Ti [21]. In this sense, efforts have been carried out aiming to increase PEEK’s bioactivity, namely by increasing the surface roughness and/or hydrophilicity [34,35]. Zirconia The drive towards non-metallic implants to satisfy the increasing aesthetic demands and the metal-free request has also empowered zirconia as a promising dental material. Due to its high mechanical strength, excellent wear resistance, and biocompatibility, zirconia is one of the most used ceramics in the biomedical field [36]. ZrO2 is a white crystalline zirconium oxide and a polycrystalline ceramic with no glassy phase [37]. This ceramic is usually found in three different crystalline forms: monoclinic, tetragonal, and cubic. At room temperature, pure zirconia assumes a monoclinic configuration, and this structure is stable up to 1170 °C. In turn, the tetragonal zirconia phase is stable from 1170 °C to 2370 °C and cubic zirconia is formed at temperatures above 2370 °C up to the melting point - 2680 °C [38]. The transition between the different crystalline forms of zirconia is due to a force on its surface and results in a volumetric change in the crystal where the stress was applied. The cracking energy resultant from the stress creates a tetragonal-monoclinic transition [39]. The previously mentioned characteristics of ZrO2 (high mechanical toughness and strength) are a consequence of the stabilization of the tetragonal phase of zirconia trough the addition of metal oxides such as yttria, ceria, calcia, or magnesia [37]. Although its sintering is much more difficult, yttria (Y2O3) stabilized zirconia, also known as tetragonal zirconia polycrystal (Y-TZP) is the kind of zirconia that is most considered for medical use due to its good mechanical properties, excellent chemical stability, and adequate level of oxygen-ion conductivity in both oxidizing and reducing environments [39]. Zirconia’s first proposal for medical, namely orthopedic, applications goes back to the 60’s. By then, a zirconia hip head was placed in a monkey femur, as an alternative to titanium and alumina components and results revealed no adverse reactions [39]. As far as the dentistry field is concerned, zirconia has been considered for dental applications since the early 90’s. In fact, the first research work on zirconia for such purposes was published in 1975, by Cranin et al. [40]. Authors aimed to assess and evaluate the addition of a ceramic coating over a Vitallium implant to increase the implant's biologic acceptability in the oral
Natural-inspired tooth replacement: design and strategies 20 environment, in beagles. Zirconia implants have been addressing many limitations of conventional implants. The advantages of zirconia for dental applications are numerous. Its white color, very similar to the natural teeth, improves the aesthetical outcomes of such dental restorations; its excellent flexural strength and fracture toughness allow the implant to withstand the physiological loads from the masticatory muscles [21]; additionally, zirconia excellent osseointegration capabilities, low plaque affinity and good adhesion to bonding substrates are expected to decrease the risk for peri-implant diseases [30,41]. In one study, performed by Wenz et. al [42], a systematic review of the literature was conducted aiming to assess the clinical success of yttria-stabilized zirconia implants and whether its osseointegration is comparable to that of titanium. Osseointegration was evaluated at 4 weeks to 24 months after implant placement and results revealed that zirconia implants osseointegration was comparable (or even better) to that of titanium implants, and the mean bone-implant contact percentage was above 60% in almost all experimental groups. Roehling et. al [43], also evaluated whether zirconia implants demonstrate differences in hard and soft tissue integration compared to titanium implants in preclinical studies. Again, results showed similar qualitative soft tissue integration for zirconia and titanium implants. In another work, Sivaraman et. al [44], aimed to understand if zirconia is a viable alternative to titanium for an oral implant. The main outcomes of such work are that zirconia implants are a promising substitute to titanium standard implants, with a superior soft-tissue response, biocompatibility, and aesthetics, and with comparable osseointegration capability. Despite these advantages of zirconia, its physical and mechanical properties tend to degrade as a result of a low-temperature ageing process (LTD) [45]. This process is characterized by a slow surface transformation of the tetragonal phase into the monoclinic structure of zirconia, associated with an increase in the volume of the ceramic that stresses the particles and results in subcritical crack growth (SCG), offering a way for water to penetrate inside the material. The stage of the crack growth depends on various microstructure patterns, such as porosity, residual stresses, and particle size [46]. If, in one hand, a certain degree of tetragonal-monoclinic transformation can improve the mechanical properties of Y-TZP, on the other hand, a narrow range exists between improvement and deterioration of mechanical properties [45]. In this sense, some authors have been investigating the effect of LTD on the mechanical properties of zirconia, in the context of dental applications. Att et. al [47], simulated , in vitro , the effect of ageing on zirconia used for oral rehabilitation and found that despite reducing the mechanical properties of the ceramic, that decrease occurs within clinically acceptable values. In addition, Alghazzawi et. al [48]
Natural-inspired tooth replacement: design and strategies 27 of functional load whereas late failures occur after applying occlusal load [71]. Early implant failure corresponds to a failure in establishing the osseointegration of the implant, which means it is solely a biologic complication. On the other hand, a late implant failure corresponds to the failure of either the implant osseointegration or function, resulting from both biological or mechanical complications [71,72]. The biologic complications that lead to early implant failure may be a consequence of factors such as overheating, contamination, and trauma during the surgical intervention, poor bone quality or quantity, and poor primary stability. These biologic mechanisms usually induce the resorption of soft and hard tissues around the implant [73]. Late implant failure, in turn, may be induced as a result of inadequate implant loading design and consequent implant fracture, occlusal trauma, overloading, stress shielding, allergic reactions, bacterial infections, or even poor oral hygiene [71,74]. There are some studies in the literature reporting the factors that influence the failure rates of dental implants. Moy P. et al. [75] conducted a study, between January 1982 and January 2003 to analyse the failure rates of dental implants and determine associated risk factors. Data regarding age, gender, implant location, medical history, and bone quality and volume were recorded and correlations between these factors were used to establish relative risk ratios. Results revealed that increasing age is strongly related to the risk of implant failure. Patients in the 60-79 age group had a significantly higher risk of implant failure when compared to patients less than 40 years. Smoking, diabetes, and postmenopausal estrogenic therapy were proved to have a strong influence on the implant failure rate. On the other hand, gender, hypertension, coronary artery disease, pulmonary disease, steroid therapy, and chemotherapy were not associated with a significant increase in implant failure. The overall results of this investigation reported implant failure rates of 8.16% in the maxilla and 4.93% in the mandible. Raikar S. et al. [76] conducted a study to assess several factors affecting the implants’ survival rates. Results revealed that age, length, and diameter of the implant, bone quality and location of the implant are detrimental to the survival of the implants. Furthermore, the authors concluded that implants above 11.5 mm length, with a diameter <3.75 mm, placed in the mandibular posterior region, in type III bone showed the highest failure rates. Sun H. et al. [77] assessed the factors that influence the failure rates of short dental implants and concluded that most failures can be attributed to poor bone quality and machined surfaces. Parihar A. et al. [78] conducted a study to assess the failure rate of dental implants in medically compromised patients (diabetes, osteoporosis, hypothyroidism, organ transplant, and cardiovascular diseases). Results showed that among medically compromised patients, higher
Natural-inspired tooth replacement: design and strategies 28 failure rates were found in patients with diabetes. Do T. et al. [71] also summarized the influence of different risk factors on late implant failure. Results indicate that the factors that induce late implant failure may be classified into three different groups: the patient history (radiation therapy, bruxism, periodontitis); clinical parameters such as the bone type and location of the implant; and factors resulting from the doctor’s choices, such as low initial stability. The aforementioned factors, individually or a combination of them, will, eventually, lead to peri-implantitis, an inflammatory process that affects the soft and hard tissues surrounding dental implants [79]. Peri-implantitis is characterized by the creation of peri-implant pocket formation, with subsequent loss of bone around the implant and ultimately its failure [80]. Factors such as diabetes, occlusal overload, genetic factors, smoking habits, periodontal disease, and poor plaque control are reported in the literature for having a huge contribution to the development of periimplantitis [81,82]. A special concern is given to the formation of plaque in the peri-implant area. This accumulation and progression of inflammatory cells in subepithelial tissue has an adverse effect on peri-implant tissue health leading to considerable supporting bone loss [80,83]. In general, those conditions that were found to be correlated with an increased risk of failure should be considered during implantation planning in order to minimize dental implants failure rates. 2.3. Manufacturing Techniques As previously mentioned, titanium dental implants are still the gold standard solution for replacing a missing tooth and the variety of techniques to produce this type of implant is huge. Titanium dental implants are often produced by rods of commercially pure titanium (cpTi) or its alloy Ti6Al4V, by a manufacturing process that involves a first milling stage and the posterior application of surface treatments [15,84]. Injection molding has also been applied for the production of such medical devices. This technique is composed of five main steps: selection of the powder and binder, production of feedstock, injection molding, debinding, and sintering [85]. Additive manufacturing (AM) technologies have gained an increased interest in the production of dental implants via 3D printing, mainly due to their simplicity and effectiveness [84,86]. Among the most promising AM technologies for metallic dental implant fabrication is direct metal laser sintering (DMLS). This laser-based technology allows the production of complex structures, layerby-layer, using powdered metals, radiant heaters, and a computer-controlled laser [87]. Briefly,
Natural-inspired tooth replacement: design and strategies 29 objects are fabricated on a moveable platform by applying incremental layers of the desired material. For each layer, the machine lays down a film of powder, which will subsequently be subjected to a high-power laser beam, to fuse the metal powders according to a computer-aided design (CAD) file, resulting in a thin metal layer. This process is repeated, layer by layer, until the desired 3D object is fabricated [87]. With DMLS it is possible to control the porosity of each layer, the pore size, shape and distribution, by changing processing parameters such as the thickness of the layer, the laser power and peak power, the laser spot diameter, the scan speed, and scanning strategy, among others [87]. This AM technique allows the reduction of the manufacturing time and waste, by avoiding post-processing steps [86]. Additionally, recent studies have been reporting DMLS as a promising technique for the production of custom-made root-analogue implants (RAI), by making use of cone-beam computed tomography (CBCT) to fabricate perfect copies of the radicular units to be replaced [88]. M. Figliuzzi et al., fabricated and inserted into a fresh extraction socket a custom-made DMLS titanium root-analogue implant. After 1 year of follow-up, the implant was functional and aesthetically satisfactory, with no bone resorption or soft tissue recessions [88]. The constant developments in the technological industry have led to the introduction of new technologies in the healthcare sector, including the frameworks of modern dentistry [89]. Making use of technology to produce dental applications plays an important role not only in terms of practice in the dental office but also in dental technical laboratories. In this sense, over the last years, dentistry is evolving towards digitalization, in order to simplify the clinical procedures and reduce the treatment and healing times, by adopting new technologies such as Computer-Aided Design/ Computer-Aided Manufacturing (CAD/CAM) [90]. 2.3.1. CAD/CAM Technology The CAD/CAM technology encompasses three main steps/components: an image acquisition equipment (usually a digitalisation scanner or a Computed Tomography (CT) equipment) that transforms geometry into digital data to be processed by the computer; the software that processes the data and produces a digital 3D model for the fabrication of the product; and a production technology that transforms the data into the final product [91]. After image acquisition and 3D modelling, the CAD model is either (1) produced by a 3D printer or (2) manufactured by a milling machine. In this second approach, the CAD model is converted into milling strips for the CAM-processing, and loaded into the milling device, that can have different number of axis (3, 4 or 5 axis) [91]. A computer numerical control (CNC) machining system, uses
Natural-inspired tooth replacement: design and strategies 30 power-driven tools to mechanically cut a block of the desired material into the final geometry, according to the computer instructions [92]. In this subtractive manufacturing technique, the milling machine will perform the commands for removing unwanted material from the block, resulting in a product with the final desired geometry. The application of CAD/CAM in the dentistry field goes back to the 1970’s when Dr. Duret fabricated crowns with the functional shape of the occlusal surface, through a system that encompassed an optical impression of the abutment tooth, the designing of the crown and its milling using a numerically controlled milling machine [93]. Some years later, Dr. Moermann (CEREC® system), introduced the concept of this new technology in a dental office at the chairside of patients, by measuring the cavity with an intra-oral camera with the posterior design and carving of an inlay made from a ceramic block [93]. At the beginning of the 1980s, when titanium has emerged as a promising material for dental applications, and since the precision casting of this metallic material was still difficult, Dr. Andersson attempted to fabricate titanium copings by spark erosion. He introduced the CAD/CAM technology as a specialized procedure as part of a total processing system that later evolved to a processing center networked with satellite digitizers targeting the fabrication of frameworks composed only of ceramic materials. These digitalized production systems are nowadays being employed by dental companies worldwide [93]. Producing implants or other dentistry components by this technique has been proved to reduce the production time of the implants and to produce very complex geometries that would be difficult to create with conventional techniques [91,92]. In Figure 2. 16 it is possible to observe a representative image of the CAD/CAM process workflow. Figure 2. 16 CAD/CAM process workflow. Adapted from [94]. 2.3.1.1. Digital Radiography The current developments in the dentistry field lead to the need for more precise and accurate diagnostic tools, especially imaging methods. These imaging techniques have been used
Natural-inspired tooth replacement: design and strategies 31 to evaluate the bone quality and quantity in a diagnostic stage, to evaluate the bone and teeth geometry, and also to create 3D replicas/models of the patient bone/teeth anatomy [95]. Digital imaging has, therefore, been used to diagnose, treat, place, and restore dental implants. It enables the evaluation of the implant sites and provides diagnostic information that other techniques cannot provide [96]. With this technology, analogue photos are converted into digital images to be displayed in a computer that assigns a number to every single shade between white and black, based on the voltage present in the analogue form. A binary image is then created to be used by the technicians or dentists where 0 corresponds to black and 255 refers to white [97]. These digital images can be characterized as intraoral or extraoral depending on whether an X-ray sensor is placed internally or externally in the mouth. The CT technique, which allows to differentiate and quantify hard and soft tissues, has been developed over time also for dental applications, moving from the medical CT to the CBCT [98]. The conventional medical CT is characterized by high levels of radiation exposure and high costs which makes CBCT the most suitable tool for dental practice. CBCT creates images through an X-ray source and detector placed in a moving platform that rotates about a pivot that, in turn, circles the area to be imaged, usually, the patient’s head (Figure 2. 17) [99]. This technique generates cone-shaped beams, and the images are obtained in one single rotation, resulting in considerably low levels of radiation. It creates a 3D picture of both hard and soft tissues in a small examination time, reduces the image unsharpness and distortion (related to patient translation and internal movements), and increases the X-ray tube efficiency [96].
Natural-inspired tooth replacement: design and strategies 32 Figure 2. 17 CBCT apparatus: (a) equipment; (b) CBCT data acquisition software; (c) CBCT images and 3D reconstruction. The accuracy of such technique has been evaluated by Al-Ekrish et al., that found mean absolute errors of 0.49 mm for the overall data [100]. In another study, conducted by Suomalainen, A. et al. results revealed errors in the range of 2.3%-4.7% for CBCT scans [101]. The images acquired by CBCT are of three types: axial images with a computer-generated curve of the alveolar process; alveolar cross-sectional images; and panoramic-like images. The continuity of the cortical bone plates, residual bone in the mandible and maxilla, the relative location of vital structures, and the contour of soft tissues covering the osseous structures are possible to identify with CBCT scans. Also, it allows the measurement of the distance between the alveolar crest and the mandibular canal avoiding impingement of the inferior alveolar nerve [102]. The main advantage of making use of this technology is that it helps to relate the radiographic images with information of a precise anatomic location or potential implant site. 2.3.1.2. Clinical Applications of CAD/CAM As previously mentioned, CAD/CAM has been applied in the dentistry field aiming to facilitate and reduce the rehabilitation treatment time, but also to overcome the limitations of conventional techniques, namely the difficulty in achieving the total fit of the implants and frameworks [103].
Natural-inspired tooth replacement: design and strategies 33 This digital design and manufacturing technique is, nowadays, applied in a large number of clinical applications. It has been used to produce fixed prosthetic inlays, onlays, crowns, and veneers, to manufacture structures for implant bridges, to custom abutments and bars, among many others [89,104]. CAD/CAM allows the production of biocompatible structures with outstanding precision and aesthetic characteristics in a short period of time. In Figure 2. 18 it is possible to observe a ceramic crown manufactured by this technique and an example of a designing software layout (Exocad GmbH) [105]. Figure 2. 18 CAD/CAM applied in dentistry: (a) ceramic crown; (b) software layout [105]. Another application of CAD/CAM in this medical field, is the design and production of computerized prototyped surgical guides (Figure 2. 19) to implant placement based on real images of the bone anatomy [104]. In this approach, a virtual surgery simulating the implant placement is performed, by manipulating the CT scan images on a specific software, thus enabling to find the best implant position and bone anchorage [106,107]. Guided surgeries have been reported in the literature as a successful therapy for the rehabilitation of edentulous, partial or single tooth restorations, characterized by optimal aesthetic and functional results [108]. Figure 2. 19 Surgical guide: (a) surgical guide design and surgery simulation; (b) clinical procedure with the manufactured surgical guide [106].
Natural-inspired tooth replacement: design and strategies 34 Apart from the previous dental applications of CAD/CAM, this technology has also been applied for the production of totally customized implants. Chen, J., et al. created a CAD model of a simplified maxillary bone block with 1 mm layer of cortical bone, based on CT scans of the patient. Two types of titanium customized dental implants were designed: a root-analogue implant and a root-analogue threaded implant and a selective laser melting technique was used to print titanium implants [57]. Some other authors have also been developing titanium dental implants from CT scans data, 3D reconstruction of the root, and posterior processing by DMLS [88,109,110]. Pirker, W. et al [65,111,112] has been manufacturing zirconia dental implants based on the laser scanning of the original tooth and posterior implant milling by CNC. In Figure 2. 20 it is possible to observe a titanium and a zirconia dental implant, both manufactured by the CAD/CAM technology. Figure 2. 20 Dental implants manufactured by CAD/CAM: (a) titanium root-analogue implant [109]; (b) zirconia rootanalogue implant [112]. It is unquestionable that CAD/CAM systems have revolutionized dentistry and its market. There are numerous studies reporting the successful clinical outcomes of such technique, mainly due to the improved and more accurate implant (or implant parts) fit, when compared to conventional methods [113–116]. It allows the production of both implant parts, such as crowns or abutments, or entire customized implants, making use of different materials, according to each specific clinical situation. In addition, it has become a useful tool for the simulation and planning of implant placement, making surgeries less invasive and more efficient [104]. Despite all the advantages of CAD/CAM technology, it involves several steps that should be carried out with
Natural-inspired tooth replacement: design and strategies 35 caution, targeting an optimal clinical performance and balance between all the biological and mechanical factors [104]. 2.4. Dental Implants Surface Modifications The term “osseointegration” was first introduced and developed by Dr. Par-Ingvar Branemark in 1985, to define the direct structural and functional connection between a living bone and the surface of a load-carrying artificial implant, typically made of titanium [117]. This process requires an initial interlocking between the alveolar bone and the implant - primary stabilityand later, biological fixation through continuous bone remodelling toward the implant - secondary stability). Osseointegration is a very complex process and there are many factors that influence the formation and maintenance of bone at the implant surface [117]. The implant stability is, therefore, composed of two components: the primary stability, also denominated mechanical stability, and the secondary or biological stability [118]. Primary stability is characterized by the mechanical anchorage of the implant immediately after the implant insertion. There are key factors that influence an implant primary stability: surface roughness, the precision of implant dimensions, thread pitch and respective drills, bacterial adhesion, and many others [23]. On the other hand, secondary stability is related to bone ingrowth and subsequent biologic healing. This biological stability will ensure the prosthesis fixation over time, as well as the osseointegration process, improving the implant longevity [23]. The combination of these two stabilities results in the implant total stability (Figure 2. 21). Figure 2. 21 Schematic representation of the implant stability [23].
Natural-inspired tooth replacement: design and strategies 36 Over the last years, aiming to enhance dental implants success, different surface treatments have been developed to improve both primary and secondary stabilities. 2.4.1. Surfaces for Improved Mechanical and Biological Stabilities Surface composition, roughness, and hydrophilicity have been proved to have a huge impact on the implant surface interactions with the surrounding tissues and consequent stability [21]. To induce modifications in these parameters, some techniques have been developed and applied, at the macro, micro, and even nano scales [119]. 2.4.1.1. Machining Machined surfaces are surfaces that after being manufactured are not subjected to any treatment. After being manufactured, implants with machined surfaces are simply cleaned, decontaminated, and sterilized [15]. At the microscopic level, these surfaces are characterized by the presence of marks and grooves resultant from the machining process [120]. The main disadvantage of these surfaces is that the osteogenic cells tend to attach and deposit bone along the surface grooves delaying the surgery healing-time [16,120]. Computer-Aided Manufacturing systems have been used to produce machined ceramic implants and implant parts. On the other hand, the machining of titanium has been proved to be inefficient and a time-consuming process [16]. 2.4.1.2. Sandblasting Sandblasting is one of the most common surface modifications applied in dental applications. This technique is characterized by an increased implant surface roughness resulting from the projection, through a nozzle, of hard ceramic particles, usually alumina (Al2O3), hydroxyapatite (Ca5(PO4)3(OH)) or titanium dioxide (TiO2), at high velocity towards the implant surface (Figure 2. 22) [121,122].
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Natural-inspired tooth replacement: design and strategies 59 Obwegeser, 1959 [9] Experiences with subperiosteal implants 35 23 complete and 2 partial mandibular subperiosteal implants 8 complete and 2 partial subperiosteal maxillary implants Weber, 1968 [10] Complete bilateral subperiosteal implants for partially edentulous mandibles 1 Complete bilateral subperiosteal implant. Kratochvil and Boyne, 1972 [11] Combined use of subperiosteal implant and bonemarrow graft in deficient edentulous mandibles: A preliminary report 1 Combination of subperiosteal implant and bone-marrow graft Boyne, 1974 [12] Restoration of deficient edentulous ridges by bone grafting and the use of subperiosteal metal implants 12 Combination of subperiosteal implants and bone-marrow graft Bodine, 1974 [13] Evaluation of 27 mandibular subperiosteal implant dentures after 15 to 22 years 27 Complete mandibular subperiosteal implants Bloomquist, 1982 [14] Long-term Results of Subperiosteal Implants Combined with Cancellous Bone Grafts 19 Combination of subperiosteal implants and cancellous bone graft Hess et al., 1982 [15] Two cases of incompatibility to carbon-coated subperiosteal implants 2 Complete mandibular subperiosteal implants Kreutz and Carr, 1986 [16] Bilateral oronasal fistulas secondary to an infected maxillary subperiosteal implant 1 Complete maxillary subperiosteal implant Kay et al., 1987 [17] Hydroxyapatite-coated subperiosteal dental implants: Design rationale and clinical experience 339 66 complete maxillary; 165 complete mandibular; 11 unilateral mandibular; 97 unilateral maxillary Truitt et al., 1988a [18] Use of computer tomography in subperiosteal implant therapy 41 3 submerged full subperiosteal implants; 33 exposed full subperiosteal implants; 3 circumferential subperiosteal implants; 2 unilateral subperiosteal implants. Truitt et al., 1988b [19] Morphologic replication of the mandible using computerized tomography for the fabrication of a subperiosteal implant 10 Mandibular subperiosteal implant Cranin et al., 1988 [20] Reconstruction of the Edentulous Mandible with a Lower Border Graft and Subperiosteal implant 15 Combination of subperiosteal implant and lower border bone graft Falomo, 1988 [21] A retrospective survey of patients treated with subperiosteal and endosseous implants 16 2 maxillary subperiosteal implants; 14 mandibular subperiosteal implants Bailey et al., 1988 [22] The mandibular subperiosteal implant denture: A fourteen-year study 74 Mandibular subperiosteal implants Fischer, 1993 [23] CAD/CAM subperiosteal implants in Australia. Case report 1 Mandibular subperiosteal implant Yanase et al., 1994 [24] The mandibular subperiosteal implant denture: A prospective survival study 81 Mandibular subperiosteal implants Bodine et al., 1996 [25] Forty years of experience with subperiosteal implant dentures in 41 edentulous patients 41 Mandibular subperiosteal implants
Natural-inspired tooth replacement: design and strategies 60 Perry, 1998 [26] Reconstruction of advanced mandibular resorption with both subperiosteal and root-form implants 3 Circumferential mandibular subperiosteal implants Mansueto, 1999 [27] Replacement of a mandibular subperiosteal implant 1 Mandibular subperiosteal implant Fish and Misch, 2000 [28] Mandibular bone growth induced by a Hydroxyapatitecoated subperiosteal implant: a case report 1 Mandibular subperiosteal implant Sirbu, 2003 [29] Subperiosteal implant technology: Report from Rumania 2 Mandibular and maxillary subperiosteal implants Minichetti, 2003 [30] Analysis of ha-coated subperiosteal implants 22 6 full maxillary 1 full mandibular 6 unilateral maxillary 9 unilateral mandibular Moore and Hansen, 2004 [31] A descriptive 18-year retrospective review of subperiosteal implants for patients with severely atrophied edentulous mandibles 40 Mandibular subperiosteal implants Lozada et al., 2004 [32] Immediate functional load of mandibular implant overdentures: A surgical and prosthodontic rationale of 2 implant modalities 5 Mandibular subperiosteal implants Kusek, 2009 [3] The use of laser technology (er;cr:ysgg) and stereolithography to aid in the placement of a subperiosteal implant: Case study 1 Mandibular subperiosteal implant Loperfido et al., 2014 [33] Severe mandibular atrophy treated with a subperiosteal implant and simultaneous graft with rhBMP-2 and mineralized allograft: a case report. 1 Mandibular subperiosteal implant Nazarian, 2014 [34] Placement of a modified subperiosteal implant: A clinical solution to help those with no bone 1 Maxillary subperiosteal implant Mapkar and Syed, 2015 [35] Revisiting the maxillary subperiosteal implant prosthesis: A case study 1 Maxillary subperiosteal implant Peev and Sabeva, 2016 [36] Subperiosteal Implants in Treatment of Total and Partial Edentulism - A Long Term Follow Up 93 Partial and total subperiosteal implants Gellrich et al., 2017 [5] A customised digitally engineered solution for fixed dental rehabilitation in severe bone deficiency: A new innovative line extension in implant dentistry 3 2 mandibular subperiosteal implants 1 maxillary subperiosteal implant Nguyen et al., 2018 [37] A subperiosteal maxillary implant causing severe osteolysis 1 Maxillary subperiosteal implant Cerea and Dolcini, 2018 [38] Custom-Made Direct Metal Laser Sintering Titanium Subperiosteal Implants: A Retrospective Clinical Study on 70 Patients 70 Maxillary and mandibular subperiosteal implants Mangano et al., 2020 [4] Custom-made 3D printed subperiosteal titanium implants for the prosthetic restoration of the atrophic posterior mandible of elderly patients: a case series 10 Mandibular subperiosteal implants After a careful analysis of the thirty-three articles, authors were able to extract a lot of information to be analysed and compared. In this sense, and as previously mentioned, results of this study were divided into five main groups: the materials and surface coatings; the implant
Natural-inspired tooth replacement: design and strategies 61 design and manufacturing techniques; the methods of implantation; complementary strategies; and the main clinical outcomes. 3.3.2. Materials and Surface Coatings The materials applied for the fabrication of subperiosteal implants have changed throughout the years. The subperiosteal implants first described in literature were made of different biomaterials. Chromium-cobalt-molybdenum alloys (particularly Vitallium) and tantalum are the most reported ones [5,9]. In fact, the first subperiosteal implant, placed in 1948 by Gershkoff and Goldberg [39], was made of Vitallium. Their electric inertness, mechanical strength, hardness, insolubility in body fluids, resistance to corrosion and also their biocompatible nature made these alloys appropriate materials for such applications [17]. Later, titanium and its alloys have also been indicated as good candidates for subperiosteal implants [33]. A controversy regarding the side effects of the metallic materials in the body environment, namely the ions release to the surrounding tissues has led to the development of alternative solutions. In a first attempt, it was thought that the subperiosteal implants would be better tolerated if coated with carbon due to its supposed outstanding biocompatibility [40]. This thin coat over the metallic substrates was thought to minimize the formation of a connective tissue capsule around the implant frame. However, this approach was not much developed since the purported biocompatibility of the carbon-tissue interface was not effectively confirmed. A study performed by Hess [15] reported two cases of incompatibility to carbon-coated subperiosteal implants. Authors believe that the implants failure may be related to the fragmentation of the carbon coating on the implant surface and demonstrated the histopathologic effects that this type of implant material can induce in human tissues that would eventually lead to the implant failure. Later, in the 1980s, another strategy was implemented and evaluated in this regard - the creation of hydroxyapatite (HAP) coatings over the metallic structures [17,34]. By then it was already known that this ceramic material has a high similarity in composition to the bone mineral, is bioactive and osteoconductive and has a high ability to form a strong interface with bone. The incorporation of HAP over metallic substrates had already been found to combine the excellent mechanical properties of the metallic materials with the biocompatibility and bone bonding characteristics of the ceramic. When comparing coated to uncoated implants, during some experiments in dogs, results revealed that the coated metallic implant induced a stronger implantbone adherence [17]. Since then, several studies have reported clinical results on the use of HAP-
Natural-inspired tooth replacement: design and strategies 62 coated subperiosteal implants. Golec [41] performed a study on the performance of 241 HAPcoated mandibular subperiosteal implants and, in a follow-up period of 7 years, a survival rate of 98% was achieved [31,41]. In another study [24], survival rates of 79% were achieved in a followup period of 10 years, and 60% after 15 years. A more recent study [30] evaluated the performance of HAP-coated subperiosteal implants over 10 years and a success rate of 91% was achieved. However, 36% of those implants needed a corrective intervention. Despite these rates not being so high as desirable, the use of HAP coatings over metallic substrates is still recommended since it has been proved to improve the interactions between the implant and bone, to decrease strut dehiscence and to improve the soft tissue environment [34]. 3.3.3. Design and Manufacturing Techniques The design of the subperiosteal implants must allow the transfer of load from the denture to the post, and from the post to the strut structure, with neglectable stress concentrations and static or cyclic fatigue mechanisms [17]. The technique of designing this type of implants was first described by Boyne and Kratochvi [11]. This technique used to start with a direct impression of the bone where the implant would lay, leading to the need of a two-stage surgery, as will later be explained, in the Methods of implantation section. In this method, a previously prepared plastic tray used to be fitted over the exposed bone and over the remaining anterior teeth, followed by some corrections to ensure a good fit between the implant and bone [10]. Then, the tray was filled with elastic materials, namely rubber adhesive. After approximately 10 minutes, the impressions were removed, and the occlusal registration was made [10,25]. It is important to mention that some more recent studies performed this step without the need of using the prefabricated trays, by simply making bone impressions with materials such as polysulfides, silicones, and polyethers [42]. During this procedure, the patient was occluded onto a prepared baseplate and occlusion rim, which was seated on the exposed mandible. The base of the occlusion rim was lined with soft wax to ensure that it would seat directly onto the bone. This record was later used to determine the interocclusal distance and to serve as a guide to the height of the implant posts [10]. Following the impression step, a bone model was made in dental stone and mounted at the proper vertical dimension for the implant fabrication [30]. This method was characterized for inducing a significant postoperative discomfort to the patients due to the excessive bone exposure [14]. In this sense, aiming to reduce the patient discomfort and avoid a two-stage surgery, in 1985 Truitt [43] developed a non-invasive technique for the design of mandibular subperiosteal
Natural-inspired tooth replacement: design and strategies 63 implants, based on the computerized tomography (CT) scanning technique. The main advantage of this method is that the clinician is able to obtain the bone model by making use of a CT and a computer-generated model. The CT is performed prior to any surgical intervention, and only one surgery is needed to insert the implant, making the overall process much less invasive [30]. Since the introduction of this new technique, some studies can be found in literature with quite satisfactory results [4,5,18,19,23,34,38]. Apart from its non-invasive nature, this technique also avoids the potential toxic effects of foreign bodies of the impression material and does not require the use of anesthesia [19]. After CT acquisition, stereolithography, has been used to fabricate very precise anatomical models of the patients’ jaws anatomy. This highly accurate technology uses the data from the 3D computer model to, layer by layer, fabricate a 3D model of the patient anatomy. The model is then delivered to a dental laboratory to ultimately fabricate the cast framework [3,33]. Some surface treatments such as polishing, sandblasting, acid etching, HAP coating and sterilization are also reported. Direct metal laser sintering (DMLS) is an additive manufacturing technique characterized by its ability to produce custom-made grids and implants, perfectly adaptable to specific anatomical requirements. In this sense, in the past few years, DMLS has emerged as a potential manufacturing technique for the production of subperiosteal implants. One study performed by Cerea [38] evaluated the clinical performance of 70 custom-made DMLS titanium subperiosteal implants and a satisfactory survival rate of 95.8% was reported in a two-year follow up. Briefly, in this technique, the implant was fabricated on a moveable platform by applying layers of grade 5 titanium micro-powders. For each layer, the machine lays down a thin film of the metallic powder with a specific thickness. The laser melts selected areas and the platform then moves down by the pre-stablished layer thickness, a fresh film of metal powder is poured and the next layer is melted via exposure to the laser source [38]. This process is repeated, layer by layer, until de implant is complete. In this specific case, the implant was then polished by electroerosion and finally sterilized. Even more recently, Mangano [4] evaluated the clinical outcomes of ten subperiosteal implants fabricated by DMLS. Despite the fit of two of the implants not being satisfactory, at the one-year follow-up no implants were lost, leading to a 100% survival rate. This approach of fabricating subperiosteal implants by DMLS is a novel technique that still requires further clinical evidence to corroborate these positive preliminary clinical outcomes. Clinical studies on a larger number of patients and longer follow-up periods are needed.
Natural-inspired tooth replacement: design and strategies 64 3.3.4. Methods of Implantation As previously mentioned, the first generation of subperiosteal implants used to encompass two surgeries for complete implant insertion. The first surgery was related to the bone impression for the implant design. This surgery was characterized by excessive bone exposure. According to Sirbu [29] this procedure used to start with the sterilization of the oral cavity followed by the administration of anesthetic blocks. A crestal incision was made in the periosteum, around the entire arch. To facilitate reflection of the mucoperiosteal flaps, a vertical anterior relieving incision was also required. The vital bearing areas were thus exposed after flaps reflection with a sharp periosteal elevator. If bone irregularities or protrusions were observed at this time, corrections were made with bone files or rongeur forceps. Cross tongue dorsum ligatures were used to promote an adequate flaps retraction and the impression tray (mentioned in the designing techniques) was then placed or made over the bone [29,44]. At this time, the bone was exposed and prepared for the impression method described in section 3.3.3. An impression was considered adequate if it included the external oblique ridge area, the mental nerve region and the mental symphysis area of the buccal side; and the mylohyoid ridges, the genial tubercles of the lingual side [45]. As far as the second surgery is concerned (that corresponds to the unique surgery in the cases where the bone geometry is acquired by CT), it corresponds to the implant placement procedure. As previously mentioned, with the introduction of the CT for the fabrication of subperiosteal implants, in 1985, this became the only needed surgery for the placement of subperiosteal implants. There is no consensus in the literature regarding the time to wait between the two surgeries. Obwegeser [9] mentioned that, while some previous authors waited three to six weeks between the two surgeries, one to three weeks is the most acceptable time to wait between bone impression and implant placement, in order to ensure a better implant fit. This author refers that waiting three to five weeks for the implant insertion would result in variable degrees of misfit between the implant base and bone and waiting more than five weeks may lead to such a poor fit that implant placement would no longer be recommended [9]. However, a wide range of periods of time can be found in literature as regards the most appropriate time for the second surgical intervention: Kratochvil [11] performed the second surgery ten weeks after the first; Cranin [20] placed the implant 12h after the bone impression procedure; Mansueto [27] performed the second surgery four weeks after the first one; Sirbu [29] waited twenty-eight to forty-five days to place the implant; Mapkar [35] performed the implant insertion eight weeks after bone impression acquisition, claiming it was enough time for the tissues to re-establish blood supply and avoid the
Natural-inspired tooth replacement: design and strategies 65 risk of incision line opening; Leake [40] scheduled the second surgery for two or three weeks after the first one; and Truitt [43] inserted the implant twenty days after the first surgical intervention. This second surgery is more rapid and causes less swelling and discomfort to the patient when compared to the first surgery [42]. In this surgical intervention, the patient was again sedated under local anesthesia, after oral cavity sterilization, and the wounds were then reopened, using the same incision lines [29]. After exposure of the bone, the sterilized implant was inserted under the mucoperiosteal flaps and seated on the bone. The position and fit of the implant were adjusted and screws were used to fix the implant [4,10,11,34,35,38]. There are, however, some authors that reported that there was no need to wire or screw the implant to the bone [9,20]. It was believed that with adequate bone preparation and enough implant framework extension, the implant would be stable enough during the initial postoperative period and after ten days it would already be fixed by connective tissue. The procedure was finally completed by a careful suturing to obtain a tensionfree, first-intention closure [4,29,38]. 3.3.5. Complementary Strategies Subperiosteal implants used to be placed in mandibles with extreme bone resorption. In this sense, some strategies used to be applied in order to rebuild the bone and, at the same time, give structural support for the implant base. These strategies were also used to reduce the fit mismatch between the implant and bone when placing the implant [11]. The main advantage of such procedure was the ability to manage and reduce the inaccuracies related to the implant design technique, namely the bone impressions or the CT. Autologous marrow and cancellous bone grafts used to be very common in the first subperiosteal implants [11,12,14,18,19]. The bone grafts were commonly harvested from the iliac crest, and patients had to be under general anesthesia [14]. In more recent studies, HAP particles were applied to induce bone augmentation and fill the void between the bone and the implant [17,27,30,34]. Additionally, a combination of autogenous bone and hydroxyapatite for augmenting the bone inferior border has also been reported [20]. In another clinical report, performed by Kusek [3], the implant struts were grafted with the patient’s blood and beta-tricalcium phosphate (β-TCP) and then covered with a membrane. The development of grafts of bone morphogenic proteins (BMPs) has also been reported by Loperfido [33]. BMPs are multifunctional proteins with a wide range of biologic activities. BMP-2, specifically, had already been reported of being capable of driving multipotent cells into an
Natural-inspired tooth replacement: design and strategies 66 osteoblastic phenotype culture. In this sense, these authors treated a severely atrophic mandible with a HAP-coated titanium subperiosteal implant together with a BMP-2/mineralized allograft. Results revealed that no clinical complications were observed and grafting the mandible with mineralized allograft and BMP-2 may lead to considerable bone formation. However, not many studies are available on the literature regarding this matter and therefore further studies are required to confirm the benefits of bone morphogenic proteins when placing a subperiosteal implant [33]. As mentioned in section 3.3.2, strategies to improve the biocompatibility of subperiosteal implants have also been reported. In a first attempt, coatings of carbon were used [15,40] and their clinical outcomes assessed. This strategy did not lead to promising results and therefore did not prevail. These coatings were later replaced by HAP coatings and better outcomes are reported [17,28,32,34]. The constant developments in the field of biomaterials enabled doctors and clinicians to enhance the implant’s biocompatibility and rebuild atrophic bones before implant insertion. However, quite invasive surgeries are associated with these procedures, as, for instance, the harvesting of autologous bone from the patient’s hip. Further research should be performed in order to create alternative and effective solutions for dental rehabilitation. 3.3.6. Main Clinical Outcomes One of the main reasons why dental implants fail is the lack of functional bone adherence. Osseointegration is a very complex process and there are many factors that influence the formation and maintenance of bone at the implant surface [46]. The implant material and surface properties, the congruence between the implant and bone, the surgical procedures, among others, have a huge impact on this biologic response. In fact, the lack of direct contact between the implant and bone is expected to lead to fibrous integration, rather than osseointegration [47]. In this sense, the clinical reports evaluated in the scope of this review present varying clinical outcomes. Complications such as pain, swelling, and inflammation have been reported. Removing a subperiosteal implant is characterized by an extremely complicated prosthodontic treatment and, as can be seen in Table 3. 1, at least one-third of the analysed articles reported implant removal during the follow-up period [13,14,38,15,17,21,24,25,27,36,37]. Despite most of the articles not reporting the need for implant removal, complications were commonly observed. Post-operative infection, strut dehiscence, bone resorption, and fibrous encapsulation with
Natural-inspired tooth replacement: design and strategies 67 subsequent implant movement were some of the reported complications. Only a few articles reported satisfactory results [11,12,35,18–20,26,28,31–33]. These satisfactory results mean that, during the follow-up periods, no major complications were observed, and the implants were still in place, which does not necessarily indicate that the implants had been successful. 3.4. Discussion Subperiosteal implants were used for several years, however, some drawbacks are associated with this kind of dental rehabilitation. The positioning of the implant was a very complex process, and high complication rates have been reported, as described in the previous sections of this work [4]. In this sense, subperiosteal implants have been replaced by endosseous dental implants, first introduced by Branemark. Endosseous implants overcome some limitations of the subperiosteal implants and have been proved to be a reliable and successful solution for dental restoration [48]. Despite being a worldwide practice, standard endosseous dental implants only provide limited options for implant diameter, length, and thread parameters [49]. Additionally, the lack of congruency between the implant and socket is expected to lead to plaque formation and accumulation, bone loss (and consequent implant exposure leading to aesthetical issues), poor implant stability and, ultimately, its failure [50]. In this sense, in this section of the present critical review, the authors aimed to compare and discuss some aspects (positive and negative) related to both subperiosteal and endosseous implants. Moreover, a new optimal/improved solution for dental rehabilitation is proposed based on the limitations and advantages of the current solutions. In Table 3. 2 it is possible to observe a summary of the main differences/similarities between subperiosteal and endosseous implants, as well as the parameters that an optimal solution should encompass. Table 3. 2 Comparing subperiosteal and endosseous implants.
Natural-inspired tooth replacement: design and strategies 68 3.4.1. Number of Surgeries and Clinical Procedure When placing a dental implant, the number of surgeries that are necessary to complete the process of implantation plays an important role. The patient will always prefer a treatment that is the less invasive possible and, therefore, one surgery will be preferable when comparing to two surgeries. As before mentioned, the first generation of subperiosteal implants used to encompass two surgical procedures. With the constant technological developments, new strategies were implemented, and it became possible to perform this kind of subperiosteal dental restoration in one surgery only. To place an endosseous dental implant, one surgical intervention is usually enough. Despite being possible to place an endosseous implant in only one surgical intervention, very aggressive techniques are associated with this type of dental restoration. Before placing the implant, the dentist needs to prepare and drill the bone, making it an extremely invasive procedure that causes a lot of discomfort to the patient. Additionally, the trauma caused by this type of surgical intervention, together with other factors such as occlusal overload and presence of micro-gaps may induce bone loss around the implant [51]. In its turn, the loss of bone in the peri-implant zone is expected to lead to the implant’s exposure and inherent aesthetical issues and, ultimately, induce the implant’s mobility and consequent failure [52]. To place a subperiosteal implant there is no need to drill the bone but, given the dimensions and geometry of the implant, big incisions have to be performed in the gingiva and high quantity of bone needs to be exposed for the settling of the implant. Furthermore, the strategies used to reduce the fit mismatch between the implant and bone make this procedure even more invasive and literature reports high levels of discomfort to the patient.
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Natural-inspired tooth replacement: design and strategies 81 CHAPTER 4 Customized root-analogue implants: a meta-analysis review on outcomes from clinical trials and case reports Published in Materials 2021, 14, 2296 doi:10.3390/ma14092296 T.A. Dantas1,2*, S. Madeira1, M. Gasik3, P. Vaz4, F.S. Silva1 1 CMEMS (Center for Micro Electro Mechanical Systems), University of Minho, 4800-058 Guimarães, Portugal; 2 MIT Portugal Program—School of Engineering, University of Minho, 4800-058 Guimarães, Portugal 3 School of Chemical Engineering, Aalto University Foundation, 02150 Espoo, Finland; michael.gasi[email protected] 4 Fixed Prosthodontics, Genetics—Faculty of Dental Medicine, University of Porto, 4200-135 Porto, Portugal; [email protected] * Correspondence: [email protected]m
Natural-inspired tooth replacement: design and strategies 82 Abstract (1) It is estimated that 10% of the world’s population will need a dental implant in their lifetime. Despite all the advances in the comprehension of dental implant designs, materials and techniques, traditional implants still have many limitations. Customized root-analogue implants are, therefore, gaining increased interest in dental rehabilitation and are expected to not only preserve more hard and soft tissues but also avoid a second surgery and improve patient overall satisfaction. In this sense, the aim of this review was to collect and analyse the clinical trials and case reports on customized root-analogue implants available in the literature; (2) This review was carried out according to the PRISMA Statement. An electronic database search was performed using five databases: PubMed, Google Scholar, Medline, Science Direct, and Scopus. The following keywords were used for gathering data: custom-made, dental implants, root-analogue, anatomical, customized and toothlike; (3) 15 articles meeting the inclusion criteria—articles reporting clinical trials, case reports or animal studies and articles with root-analogue implants and articles with totally customized implant geometries—were selected for the qualitative synthesis. The design and manufacturing techniques, implant material and surface treatments were assessed and discussed; (4) The performance of some root-analogue implants with specific features (i.e., macro-retentions) was successful, with no signs of infection, periodontitis nor bleeding during the follow-up periods. Keywords: root-analogue implants; custom-made; CAD/CAM technology; zirconia; titanium; clinical trial 4.1. Introduction Dental implants are an attractive option for replacing missing teeth, providing many advantages, reliability and comfort for improving quality of life [1]. There is a variety of different implants systems on the market [2,3] and some companies are already offering an implant selection system for their customers. However, the approaches aiming at implementation of completely customized dental implants are still uncommon [4]. Osseointegration has been defined as a direct and functional connection between bone and an artificial implant [5]. Traditional implants have a cylindrical or tapered geometry with threads along the screw length and over the placed abutment, followed by the crown (for a single tooth). Due to the geometry and design, they may only provide limited options for available implant
Natural-inspired tooth replacement: design and strategies 83 length, diameter, and thread parameters, and, therefore, cannot completely meet the personalized requirements of every patient [4,6]. This lack of proper congruency between the implant and the socket bone can eventually lead to implant failure due to stability loss and osseointegration [7,8]. To overcome this problem, novel approaches are being evaluated to manufacture customized root implants which are explicitly tailored to each patient’s condition. This is expected to reduce the bone and soft-tissue trauma and promote a better primary stability, being thus a promising alternative for dental rehabilitation [6,7]. Additionally, the placement of such root-analogue implants (RAI) is a minimally invasive procedure, since they do not usually require bone drilling, sinus lifting, bone augmentation or other traumatic procedures [9]. Root-analogue implants were first described by Hodosh et al. back in 1969 [10]. A polymethacrylate implant was developed and clinically tested at that time, but outcomes were not satisfactory. Some years later, Lundgren D. et al. [11] reintroduced the topic by developing a titanium RAI and testing it in beagle dogs. Results revealed that the use of titanium instead of a polymeric material led to a success rate of 88% (28 of the 32 implants were successfully osseointegrated). Titanium remains still as “the gold standard” metallic material for dental implants. However, an increased concern with aesthetical issues has led to an increased interest in ceramic materials, namely zirconia, for such applications [12]. Apart from its toothlike colour, its high corrosion resistance, biocompatibility and high wear resistance make zirconia a promising material for dental implants [13]. This is being already deployed in dental practice with zirconia abutments [14,15]. Despite the recent developments in the design and implementation of totally customized root-analogue implants, reliable data on the long-term use of RAI in humans are still scarce. The present review aims on collection and analysis of the few clinical trials and case reports available in the literature on customized root-analogue implants. The objective is to compare their clinical performance together with additional parameters of these implants (materials used, surface treatments, design and manufacturing techniques). Also, aspects related to the surgical procedure, such as extraction of the teeth, the time between tooth extraction and implant placement, the time between placement and final reconstruction, and main biological outcomes were accessed.
Natural-inspired tooth replacement: design and strategies 84 4.2. Methods 4.2.1. Search Strategy This review was carried out according to the PRISMA Statement (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) [16]. An electronic database search was performed using five databases: PubMed, Google Scholar, Medline, Science Direct, and Scopus. To conduct the search, Boolean operators such as “AND” and “OR” were used to correlate the keywords. The following keywords were explored and inserted with the field tag (Title/Abstract/Keywords): “custom-made”, “dental implants”, “root-analogue”, “anatomical”, “customized” and “tooth-like”. In addition, the references of review articles, as well as articles published in the International Journal of Oral and Maxillofacial Implants, International Journal of Oral and Maxillofacial Surgery, International Journal of Prosthodontics, Journal of Periodontology, Journal of Oral Implantology, Journal of prosthetic dentistry, and International Journal of Prosthodontics, were manually searched to include all the relevant articles available in the literature. 4.2.2. Study Selection A screening process was conducted over the titles and abstracts retrieved by the databases search, to select the articles for full-text reading. From this screening process and after duplicates removal, 64 articles were selected for full-text reading. In order to assess their eligibility to be included in this review, the following inclusion criteria were applied: (1) articles written in English, (2) articles dated from 1990 to 2020, (3) articles reporting clinical trials, case reports or animal studies, (4) articles with root-analogue implants, (5) articles with totally customized implant geometries. Articles not meeting these inclusion criteria were excluded from the review (articles comprising just a review were also excluded). Articles that met the inclusion criteria were entirely read and analysed considering the aim of this review. 4.2.3. Data Collection and Extraction The information extracted from each article was divided into two main groups: the implant characterization and the aspects related to the clinical trial/case report. As far as the implant is concerned, a brief summary of the materials, design and manufacturing techniques, as well as surface modifications, were analysed. For the second, aspects related to the clinical procedure were also evaluated (the number of subjects, the number of implants per subject, the treated tooth,
Natural-inspired tooth replacement: design and strategies 91 five subjects [30]. The treated tooth varied from article to article. After a careful analysis of the clinical procedures carried out in these studies, it was possible to conclude that almost half of the strategies (in 7 articles) perform the implant placement immediately after tooth extraction, in the same surgical procedure [17,23–25,27,29,30]. In the pre-clinical trial performed in beagle dogs [11], four teeth were extracted on the same day of the implant placement, whereas the other twenty-eight teeth had been extracted two weeks before. In the remaining articles, implants were placed between 1–8 days after the tooth extraction. All authors mention that the implants were placed into the socket under finger pressure and subsequent gentle tapping with a hammer and a mallet. Additionally, primary stability was commonly checked by palpation and percussion. As far as the final restoration is concerned, three to four months is the most reported time to wait between the placement of the implant and the crown, being reported in ten articles [19–25,27,28,30]. The follow-up periods reported in these studies range from six months to three years, being one year the most common period of time [23–25,27,30]. In the results section of the analysed articles, it is possible to find which were the main clinical and/or biological complications that occurred during the follow-up period, as well as the overall performance of the implant and whether success was achieved or not. Table 4. 3 Summary of the clinical trials/case reports parameters and results. Author/Year Type of Study Subjects * Implant Per Patient Tooth Time between Extraction and Implantation Implant Placement Details Time between Placement and Reconstructi on FollowUp Technical and/or Biological Complications; Overall Performance (Lundgren, et al., 1992) [11] Preclinical trial (beagle dogs) 4 (2–3years-old) 8 Variable Dog 1–3 (2 weeks) Dog 4 (0 day) Intra-alveolar soft tissue was removed, and the bone walls were not scraped. The implants were immediately placed. The mucoperiosteal flaps were repositioned and sutured 2 months 3 years 2 implants were lost in the first week. 30 implants were retained in the socket (of these, 2 were denuded because of late mucosal perforation but remained clinically stable). The remaining 28 implants fulfilled the clinical criteria for osseointegration
Natural-inspired tooth replacement: design and strategies 92 (Kohal et al., 1997) [17] Preclinical trial (Macac a fascicul aris) 3 4 Upper central and lateral incisors 0 day The implants were tapped into their respective socket The mucoperiosteal flaps were repositioned and sutured - 6 months Buccal bone fracture at the time of implantation. Some of the implants could not be inserted to the intended depth. Four implant exposures (6 days, 8 days, 9 days and 2.5 months). None of the implants was surrounded by soft connective tissue. None of the implants were lost and all were clinically stable at the end of the experiment (Heydecke et al., 1999) [18] Case report (human ) 1 (45, gender not specified) 1 Max. Sn. lateral incisor 1 day The implant was placed into the socket using the attached insertion bar under finger pressure and subsequent tapping with a hammer and a mallet. Primary stability was checked with the handles of 2 dental mirrors. The insertion bar was removed and a custom-made healing cap was placed 6 months No data Bony resorption and buccal soft tissue recession The final result was compromised by metal shining through the thin gingiva (Pirker and Kocher 2008) [19] 1 (63, gender not specified) 1 1st max. Dx premola r 4 days The implant was placed into the socket under finger pressure and subsequent gentle tapping with a hammer and a mallet. Primary stability was achieved as checked by palpation and percussion 4 months 2 years Stable implant. No changes on the peri-implant marginal bone level. No bleeding No signs of periodontitis nor bone resorption (Pirker and Kocher 2009a) [20] Clinical trial (human ) 18 Group A (4 F, 2 M, 27–60years-old) Group B (4 F, 8 M, 27–65years-old) 1 Variable Group A 1–4 days Group B 1–8 days Group A No restoration Group B 3–13 months 2 years Primary implant stability was achieved in all patients and no complications, such as swelling, inflammation, bleeding and pain (Group A) 5 implants were lost within 26– 128 days (Group A). Implants were lost suddenly without prior pain or infection (Group A). Implant lost after 624 days. However, the lack of osseointegration was already observed on day 18 (Group B) All 11 remaining implants healed uneventfully with no complications (Group B). Soft tissue retraction ranged from 0–1.5 mm within the first year and remained stable thereafter. Many implants (58%) had no observed soft tissue retraction and maintained an aesthetic gingival architecture (Group B). There was no wound infection, no signs of periodontitis, and no implant mobility/dislocation (Group B)
Natural-inspired tooth replacement: design and strategies 93 (Pirker and Kocher 2009b) [21] Case report 1 (27 M) 1 Dx. lateral maxillar y incisor 7 days 3 months 15 months Stable implant, unchanged periimplant marginal bone level. No bleeding. Excellent aesthetic result. No signs of periodontitis nor bone or soft tissue recession (Pirker et al., 2011) [22] Case report 1 (50 F) 1 1st mand. Sn. molar 4 months 2.5 years Stable implant, unchanged periimplant marginal bone level. Complete apical peri-implant ossification with no signs of peri-implantitis (Mangano et al., 2012) [23] Case report 1 (55 M) 1 1st max. Dx. premola r 0 day 3 months 1 year Primary stability was achieved, due to the perfect correspondence between the implant and the post-extraction socket The implant was still in function after a one-year follow-up The implant was stable, with no signs of infection, unchanged peri-implant marginal bone level and no peri-implant radiolucency The radiographic profile of the implant–crown complex was very similar to that of a natural tooth No prosthetic complications. The prosthetic restoration showed optimal functional and aesthetic integration (Figliuzzi and Mangano 2012) [24] Case report 1 (50 F) 1 2nd max. Dx premola r 0 day The implant was placed into the socket under finger pressure and subsequent gentle tapping with a hammer and a mallet. Primary stability was achieved as checked by palpation and percussion 3 months 1 year Implant in function after one year. The implant was stable with no signs of infection. Good conditions of the peri-implant tissues. Unchanged peri-implant marginal bone level and no periimplant radiolucency. No prosthetic complications (Mangano et al., 2014) [25] Clinical trial 15 (8 M, 7 F, 39–55years-old) 1 Premol ars (8 max; 7 mand) 0 day No implants were lost, leading to a survival rate of 100%. All implants were stable with no signs of infection. Unchanged peri-implant marginal bone level and no peri-implant radiolucency. The radiographic profile of the implant–crown complex was very similar to that of natural teeth. No prosthetic complications (Pirker and Kocher 2015) [26] Case report 1 (41 F) 1 Maxillar y 2nd Sn. molar 6 day 7 months 3 years The implant completely filled the extraction socket, ensuring perfect osseointegration. Unchanged peri-implant marginal bone levels. No signs of periodontitis, bone resorption nor bleeding. Excellent aesthetic result
Natural-inspired tooth replacement: design and strategies 94 (Figliuzzi et al., 2016) [27] Case report 1 (45 M) 1 Lateral Dx. max. incisor 0 days The implant was gently inserted in the socket using a little percussion hammer. Primary stability was achieved, as a consequence of the congruence between the implant and the socket. Then, sutures were positioned 3 months 1 year After one year, the implant was still in function. No biological complications were reported. The peri-implant tissues were mature and stable. Little or no periimplant bone loss, and no soft tissue recession (Patankar et al., 2016) [28] Case report 1 (22 F) 1 Dx. mand. 1st premola r 3 days The implant was placed into the socket under finger pressure and subsequent gentle tapping with a hammer and a mallet. Primary stability was achieved as checked by palpation and percussion 4 months 18 months Stable implant. Unchanged periimplant marginal bone level and complete apical peri-implant ossification. No signs of periimplantitis and no bleeding (Pour et al., 2017) [29] Case report 1 (35 F) 1 Max. Sn. central incisor 0 day The implant was inserted and seated with cautious tapping into the socket and buccal augmentation was achieved with BioOss for stabilizing the tissue architecture. The relief cut was sewn up with three single button sutures 6 months 16 months Satisfactory aesthetics and stability of the surrounding tissues. Stability of the bone and implant functionality observed (Moin et al., 2018) [30] Clinical trial 5 1 Premol ars 0 day The implant was placed into the socket under finger pressure and subsequent gentle tapping with a hammer and a mallet. Primary stability was achieved as checked by palpation and percussion 3 months 1 year In one patient, the implant showed mobility and symptoms of peri-implant infection after 4 weeks. The implant was removed at the 12 months evaluation, all remaining implants were successful. Two patients showed an absence of buccal bone around the implant. Healthy mucosal appearance in all remaining implants * Remarks: 00A refers to age (years) and gender of patients (e.g., 45F is a 45 year old female).
Natural-inspired tooth replacement: design and strategies 95 After evaluating the performance of the implants of the case reports [18,19,21–24,26–29], it is possible to observe that outcomes of the different studies are however rather similar (Table 4. 3). In fact, none of the implants was lost after the follow-up period. All the authors, except Heydecke et al. reported good implant stability, no bleeding, no signs of periodontitis nor bone recession. In study [18], bony resorption and soft tissue recession led to a slight discoloration of the marginal peri-implant mucosa. Excellent aesthetic results were reported in the other case reports, mainly in the ceramic-based implants. In general, all authors mentioned a quite satisfactory implant performance during the follow-up period of the case reports. This is an interesting observation especially for the peri-implantitis appearance. It is well-known that the acid etching or electrochemical treatment of titanium aimed to generate an anatase layer is very beneficial for the prevention of biofilm formation [33,34]. However, this is not straight possible for zirconia and there have been concerns that full zirconia implants might be not so resistant to biofilm formation. This issue was recently demonstrated to be possible to solve with a thin coating of zirconia with TiO2forming formulations [35], but no such treatment was used in the reported cases. It looks that this matter might need an additional investigation. For the less satisfactory clinical trials the outcomes were mainly limited due to the higher number of tested implants (Table 4. 3). In the pre-clinical trial performed in beagle dogs [11], 2 of the 32 implants were lost in the first week. Other two implants were not clinically stable after one year. The remaining 28 implants fulfilled the clinical criteria for osseointegration. In the other preclinical trial [17], performed in monkeys, some complications were also observed. Despite 4 of the 12 implants being exposed after a certain period of time, none of the implants was lost. However, the follow-up period of this study was too short (6 months), which may be influencing these results. In the clinical trial performed by [20] two different implants were evaluated: one designed by laser scanning of the original tooth root (Group A) and other designed also by laser scanning and with the incorporation of macro-retentions in its surface (Group B). The results of the two tested groups are very different. In Group A, five (out of six) implants were lost. The incorporation of the macroretentions (Group B) led to more satisfactory results since only one implant was lost (out of twelve). The other eleven implants presented no signs of infection nor mobility. F. Mangano et al. [25] carried out a clinical trial in fifteen patients. After a one-year follow-up, none of the implants was lost and all of them were stable with no signs of infection. In the last analysed clinical trial [30] one (out of five) implant was lost after four weeks, due to implant mobility. The other four implants were considered successful after a one-year follow-up.
Natural-inspired tooth replacement: design and strategies 96 4.4. Discussion This review has evaluated the clinical performance of customized root-analogue implants (RAI). The most differentiating aspects found in the selected literature were the type of clinical study (case reports, clinical trials or pre-clinical trials), the implant material(s) and the designing techniques. Two materials-titanium alloys) and zirconia-were the only materials found in RAI used in these studies. The excellent mechanical properties of titanium, its biocompatibility, high corrosion resistance and low weight, are well known for this material as a solution for dental implants [36]. However, its colour together with the possible long-term corrosion and release of ions to the body environment has led to an increase in the interest in zirconia as an alternative to this metallic material [37]. Zirconia is characterized by its biocompatibility, sufficiently low bacterial affinity (yet higher than treated titanium), high mechanical flexural and compressive strengths, excellent wear resistance, and adjustable white colour, being a promising solution to overcome the aesthetic issues caused by metallic dental implants [38]. The implants’ surface finishing, namely their roughness has been proved to have a huge influence on the implant osseointegration. Some published studies indicate that rough surfaces promote a faster osseointegration comparing to smooth ones [39–41], and many techniques have been applied for the creation of the desired implant roughness. The most common found in literature, and also in the clinical studies of this review are sandblasting and acid etching [42,43]. Sandblasting followed by an acid etching treatment might be considered as the gold standard surface modification in the dental implants market world [44]. On the other hand, it has been shown [34,45,46] that using simple roughness value as a single parameter is a significant oversimplification, as other factors together with porosity, hydrophilicity, nanoand macrotopology are important for implant success (at least, for metallic titanium materials). One of the biggest differences found in the evaluated literature was the implant designing technique. Some authors opted to scan the original tooth by laser, others designed the implant before tooth extraction, using the patient radiographic (CT) images, followed by 3D image manipulation, and posterior implant milling. This approach, known as computer-aided design/computer-aided manufacturing (CAD/CAM) technology, has become increasingly popular in the dentistry field over the past years [47]. In fact, many dental offices worldwide have been
Natural-inspired tooth replacement: design and strategies 97 trying to implement modern IT solutions in their daily practice in order to reduce costs, work more efficiently, and increase patient satisfaction [48]. Modern CAD/CAM solutions seem to be the future of the dentistry field, namely for the customization of dental implants. The success of an implant is known to be directly dependent on its osseointegration process. This process requires an initial interlocking between the alveolar bone and the implant (primary stability) and later, biological fixation through continuous bone remodelling toward the implant (secondary stability) [49]. There are key factors that influence implant stability: surface roughness (as previously mentioned), the congruity between the implant and bone, the period of time between tooth extraction and implant placement, bacterial adhesion, among others [50]. The majority of the studies that are being analysed reported ideal primary stability, where a perfect correspondence between the implant and the post-extraction socket was observed. However, in one study [17] some implants could not be inserted to the intended depth leading to implant exposures. Nevertheless, none of those implants was lost. For the latency time (between tooth extraction and implant placement), a healing period of 6–9 months was previously recommended (a late implant placement). Later, insertion of implants after 2–3 months was suggested (a delayed implant placement), and more recently, immediate implantation has been clinically tested too [7]. Despite new interest in immediate implant insertion, the literature reports it encompassing two main problems: (a) maintaining the implant primary stability and (b) preventing soft tissue ingrowth during the healing period [7]. Additionally, it is might to higher infection risks, flap dehiscence over the extraction site, and incongruity between the socket wall and the implant [3,14,33]. The articles evaluated in the scope of this review performed the implant placement immediately after tooth extraction or a few days later. However, and contrarily to what was reported in the literature, it seems that this strategy did not trigger any specific side effects nor biological complications. In fact, primary stability was achieved in most of the tested implants. Another aspect that is worth discussing is the incorporation of macro-retentions on the implants’ surface. Some authors mention the incorporation of these protrusions on the implant surface aiming to promote an improved attachment to the bone and consequently improved mechanical stability. Pirker et al. [20] have even compared the performance of RAI with and without macro-retentions, and results were clearly conclusive: RAI without the macro-retentions (n = 5) were suddenly lost, without prior pain or infection, in the first 128 days. On the other hand, among the RAI with macro-retentions (n = 12), only one was lost. These results were later corroborated in
Natural-inspired tooth replacement: design and strategies 98 one study performed by Moin D. et al. [51] that analysed, by means of FEA (Finite Element Analysis), the influence of 5 custom root-analogue implant designs on the stress distribution of peri-implant bone. The results of this study revealed that the addition of macro-retentions to an RAI standard design would have a positive effect on the stress distribution, reduce the concentration of bone stress, and provide a better primary stability. Despite being a promising alternative, the authors believe that the macro-retentions alone may not be enough to ensure implant mechanical stability. Since the RAI geometry is characterized by its conical shape, there is a risk that the implants may tend to be expelled and hence other strategies to avoid this risk should be further explored and developed. It is also notable, that FEA analysis conventionally used in dentistry usually suffers from an oversimplification of tissue properties, which are not well known (especially for soft tissues), and where anisotropy is seldom considered [52,53]. The advantage of the linear elastic models is of course in the provision of simple and direct prediction of the tissues properties for the sake of the computational efficiency but the usefulness of such data is very questionable (e.g., “elastic modulus of mucosa” ranging from 0.1 to 680 MPa [54]). FEA outcomes should be considered as a complement to the clinical studies, aiming to better understand the influence of some variables on the implants’ clinical performance. Together with the aforementioned mechanical features, there are also some mechanobiological aspects expected to improve the implants’ osseointegration. Some reported techniques are being developed and evaluated aiming to promote the infiltration and supply of nutrients and fluids around dental implants, consequently inducing vascularization at the implant’s surface [55]. However, in this review not much information regarding these biological stimuli has been found: only one article referred that small perforations were created in the palatal tissue of the socket to stimulate bleeding [29]. Other techniques, such as the creation of hydrophilic surfaces or the incorporation of micro-channels on the implant’s surface would also have a positive impact on the implant’s vascularization [56], and eventually needed to be further explored to reveal their potential clinical benefits. Despite the satisfactory clinical results observed in the selected articles, none of these solutions is widely available on the market—the manufacturing companies and dentists tend to prefer standard products with lower associated costs; most of the dental clinics do not have the necessary equipment (CBCT) for the design of such customized solution; despite its weaknesses, conventional dental implants have been reported with success rates of 90–95% for 10 years follow
Natural-inspired tooth replacement: design and strategies 99 up periods [57] based on current definitions of success, which are questionable in the opinion of the authors of this paper.. Authors believe that these factors may be hindering the worldwide practice of such dental treatment and studies should proceed, focusing on the implementation of RAI in the global dental market. The main findings of this review show that: • Titanium and zirconia are the selected materials for the manufacturing of RAI. • CAD/CAM technology followed by surface treatments such as sandblasting has been the preferred manufacturing technique for such applications. • The clinical outcomes of the analysed studies suggest that further investigations should be performed aiming to evaluate whether RAI may be considered a promising solution for the replacement of missing teeth or not. The limitations of this study rely, mainly, on the reduced number of articles meeting the inclusion criteria. 4.5. Conclusions This review includes 15 identified clinical studies and cases with RAI. These studies have a high heterogeneity of follow-up periods, sites, techniques and thus it is difficult to compare results and draw definitive conclusions about validated RAI outcomes. Nevertheless, some general considerations and trends in RAI application can be made: • Clinically tested RAI are made of titanium and/or zirconia; no other materials were reported for the analysed period. • CAD/CAM is an effective technology to design and manufacture RAI and it is being implemented in the dental practice. • Sandblasting and/or acid etching on the implant surface seems to be effective in promoting the implant osseointegration. • The addition of macro-retentions on the implant surface induces a positive effect on the stress distribution in the bone surrounding the implant. However, this strategy alone may not be enough to promote the implant mechanical stability, due to its conical geometry.
Natural-inspired tooth replacement: design and strategies 100 • Immediate implant placement may be considered successful in RAI, unless there are no clinical contraindications. • The performance of some RAI with specific features on its surface, namely the incorporation of macro-retentions, was proved to be successful, with no signs of infection, periodontitis nor bleeding during the follow-up periods. Given the results of the evaluated clinical studies, customized root-analogue implants (cRAI) may be the future of dental rehabilitation. However, and as expected, the literature on the clinical performance of these implants is still scarce, and further well-designed clinical studies, namely long-term randomized controlled trials, are required to corroborate these findings. Author Contributions: Conceptualization, T.D. and F.S.; methodology, T.D. and F.S.; validation, M.G., P.V. and F.S.; formal analysis, T.D.; investigation, T.D.; writing—original draft preparation, T.D.; writing—review and editing S.M., M.G., P.V. and F.S.; visualization, M.G. and F.S.; supervision, P.V. and F.S.; project administration, F.S.; funding acquisition, T.D. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by FCT-Portugal through the grant PD/BD/140202/2018, and the projects POCI-01-0145-FEDER-030498-FunImp and UID/EEA/04436/2019. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments The authors would like to acknowledge the following projects.
Natural-inspired tooth replacement: design and strategies 107 CHAPTER 5 In silico evaluation of the stress fields on the cortical bone surrounding dental implants: comparing root-analogue and screwed implants Published in Journal of the Mechanical Behavior of Biomedical Materials 2020, 104: 103667 doi:10.1016/j.jmbbm.2020.103667 T.A.Dantasa,b⁎, J.P. Carneiro Netoa, J.L. Alvesa, Paula C.S. Vazc, F.S. Silvaa aCenter for MicroElectroMechanical Systems (CMEMS-UMinho), University of Minho, Campus de Azurém, Guimarães – Portugal bMIT Portugal Program, School of Engineering, University of Minho, Guimarães, Portugal cFixed Prosthodontics, GeneticsFaculty of Dental Medicine, University of Porto, Portugal
Natural-inspired tooth replacement: design and strategies 108 Abstract Tooth loss is a problem that affects both old and young people. It may be caused by several conditions, such as poor oral hygiene, lifestyle choices or even diseases like periodontal disease, tooth grinding or diabetes. Nowadays, replacing a missing tooth by an implant is a very common process. However, many limitations regarding the actual strategies can be enumerated. Conventional screwed implants tend to induce high levels of stress in the peri-implant bone area, leading to bone loss, bacterial bio-film formation, and subsequent implant failure. In this sense, root-analogue dental implants are becoming promising solutions for immediate implantation due to their minimally invasive nature, improved bone stress distribution and because they do not require bone drilling, sinus lift, bone augmentation nor other traumatic procedures. The aim of this study was to analyse and compare, by means of FEA, the stress fields of peri-implant bone around root-analogue and screwed conventional zirconia implants. For that purpose, one root-analogue implant, one root-analogue implant with flaps, two conventional implants (with different threads) and a replica of a natural tooth were modelled. COMSOL was used to perform the analysis and implants were subjected to two simultaneous loads: 100 N axially and 100 N oblique (45°). Results: revealed that root-analogue implants, namely with flaps, should be considered as promising alternatives for dental implant solutions since they promote a better stress distribution in the cortical bone when compared with conventional implants. Keywords: Peri-implant bone loss; Root-analogue implants; Finite Element Analysis; Minimally invasive approach; Zirconia; Immediate load 5.1. Introduction Nowadays, there are several implant options for edentulous people. However, and despite being a widespread treatment, the success of a dental implant depends on many factors and may be compromised as a result of postplacement complications. The loss of the supporting bone, and consequent loss of stability, is, probably, the main reason why dental implants fail (John et al., 2016). Surgical trauma, occlusal overload, presence of micro gaps, peri-implantitis, allergic reactions, foreign body rejection, the material and design of the implant or even poor oral hygiene may lead to this bone loss (Levin, 2008; Macedo et al., 2017). The presence and maintenance of surrounding bone is what dictates whether an implant succeeds or not (Dantas et al., 2019). In this sense, one of the biggest challenges in implantology nowadays is the process of bone
Natural-inspired tooth replacement: design and strategies 109 resorption around the implant immediately after insertion and during its use. There are many theories regarding the relation between bone adaptation and strain rate and magnitude. The Frost mechanostat theory (Frost, 1987; Mahnama et al., 2013) classifies bone behaviour on the basis of the mechanical strain. This theory describes four micro-strain zones, that correspond to certain strain intervals, and their relationship with the bone mechanical adaptation (Vidyasagar and Apse, 2003). The extreme zones-disuse atrophy and pathologic overload - are expected to lead to bone loss. Disuse atrophy occurs for strain magnitudes below 50–200 μ-strain, whereas pathologic overload tends to occur when the magnitude strain values overcome 4000 μ-strain. Between these two zones, the frost mechanostat theory describes two other μ-strain zones: the steady state zone (100–2000 μ-strain) that is thought to enable a favourable bone reaction and the physiologic overload zone (2000–4000 μ-strain) that is expected to lead to an increased bone mass (Vidyasagar and Apse, 2003). In the past, it was believed that before implant insertion, healing periods of 6–9 months were required (late implant placement). A few years later, 2–3 months was proposed as the most appropriate time to wait between tooth removal and implantation (Regish et al., 2011). Lately, immediate implant insertion after tooth extraction has emerged as a great alternative, due to a reduction in the treatment time and surgical interventions, consequently leading to a reduction of costs and a quality of life improvement (Pirker and Kocher, 2008; Regish et al., 2011). Additionally, immediate implant insertion is expected to avoid alveolar bone resorption and softtissue regression as a consequence of early load (Pirker and Kocher, 2008). Standard implants have a cylindrical or tapered geometry with threads along their length and their main limitation is that they only provide limited options for implant length, diameter and thread parameters. Consequently, they cannot completely meet the requirements of every patient (Chen et al., 2014, 2012; Didier et al., 2017). On the other hand, the incongruence between the socket wall and the implant shape leads to the need of using barrier membranes or bone augmentation to avoid the formation of connective tissue between the implant and socket (He et al., 2016; Rodrigues et al., 2017). In this sense, customizing immediate load dental implants tailored to each patient condition would, not only preserve more hard and soft tissues, but also reduce rehabilitation time and avoid a second surgical intervention (Chen et al., 2014; He et al., 2016). Customized root-analogue implants were first introduced by Hodosh et al. (1969). This novel approach is becoming a promising solution for immediate single stage replacement of teeth and is acquiring a special interest due to its minimally invasive nature, and also because it does not require bone drilling, sinus lift, bone augmentation nor other traumatic procedures (Pirker and Kocher, 2011).
Natural-inspired tooth replacement: design and strategies 110 Additionally, it respects the natural anatomy of the socket, preserving it and is expected to promote an excellent primary stability as well as osseointegration (Rodrigues et al., 2017). Root-analogue titanium implants were developed by Lundgren (Prithviraj et al., 2011) and proved to be effective and to promote osseointegration in the first-month follow up period. However, studies indicate that after 9 months of the insertion, failure rates of approximately 48% were observed, which is not acceptable for clinical purposes (Kohal et al., 2002). Pirker et al. (Pirker and Kocher, 2011, 2008), developed a root analogue implant completely made of zirconia, due to its good mechanical properties, biocompatibility and aesthetical advantages (Inokoshi et al., 2014; Teng et al., 2001). Other features were implemented in this implant: micro and macro retentions, reduced diameter of the implant next to the cortical bone and a single-stage implantation process. Authors found that this new strategy leads to better results when comparing to root-analogue titanium implants with a uniform surface (Pirker and Kocher, 2008). Finite Element Analysis (FEA) has been applied as an effective method to investigate the biomechanical behaviour of dental implants, namely the bone stress/strain around implants (dos Santos Marsico et al., 2017; van Staden et al., 2006). Studies regarding the biomechanical response of standard dental implants are abundant, however, when it comes to root-analogue dental implants studies are scarce. Moin et al. (2016), performed a FEA analysis on five different root-analogue implants and evaluated the stress distribution of peri-implant bone. The five press-fit designs with a standard identical abutment, had some different macroretentions on their surface and were compared to a control implant (without any features). Results revealed that the addition of targeted press-fit geometry to the standard surface results in a positive response on the stress distribution and lower bone stresses, providing a better primary stability. Chen et al. (Chen et al., 2017), evaluated the stress distribution and micro-motion (for both immediate and delayed loading protocols) of two custom-made dental implant designs: one with the geometry of the natural root and another with a root-analogue design accompanied by a rotational symmetric body with a threaded characteristic. These approaches were compared to standard cylindrical dental implants. Results of this study suggest that the use of immediate rootanalogue implants with a threaded characteristic, lead to smaller micro-motion in the bone-implant interface and a favourable stress distribution was observed. Given what was mentioned before, the main goal of this research was to evaluate the stress fields in the peri-implant bone area of four different implant designs: a conventional Semi-Threaded Implant (STI), a conventional Threaded Implant (TI) a Root-Analogue Implant (RAI) and a Root-Analogue Implant with Flaps (RAIF). Additionally, a replication of a Natural Tooth (NT), surrounding bone and periodontal ligament (PDL)
Natural-inspired tooth replacement: design and strategies 111 geometry were modelled, to act as reference. FEA was performed in all models and zirconia was used as the implants’ base material. 5.2. Materials and Methods 5.2.1. 3D Modelling All five models addressed in the scope of this research work were designed in a CAD software (SolidWorks, 2018, Dassault Systems SolidWorks Corp., Concord, MA, USA). For simplification, all implants were modelled as a single body structure, with a simplified geometry (perfectly rounded), and placed in a bone model composed by trabecular bone and 2 mm of cortical bone. As seen in Figure 5. 1(a), in the STI model the thread is at the subcrestal level, whereas in the TI (Figure 5. 1(b)) the thread extends to the cortical bone. On the other hand, in the RAI, RAIF and NT models (Figure 5. 1(c), (d) and (e), respectively), a thin layer (1 mm) of cortical bone was designed (lamina dura), mimicking the tooth socket (Huynh-Ba et al., 2010), once, in these three strategies, this thin layer is expected to be preserved. The flaps observed in the RAIF model were designed to promote a better mechanical stability, as well as to improve the stress distribution along the implant. Additionally, in the NT geometry (Figure 5. 1(e)) it is possible to observe the periodontal ligament between the cortical bone and tooth. Figure 5. 1 Schematic representation of the studied models: (a) STI - Semi-Threaded Implant; (b) TI - Threaded Implant; (c) RAI - Root-Analogue Implant; (d) RAIF - Root-Analogue Implant with Flaps; and (e) NT - Natural Tooth. 5.2.2. Finite Element Modelling The five models described above were then imported into COMSOL Multiphysics 5.3, COMSOL Inc. Sweden, to generate the FE models. The mechanical properties of the materials were provided to the software based on previous studies and literature (Table 5. 1). All materials were
Natural-inspired tooth replacement: design and strategies 112 considered isotropic and homogeneous. Additionally, the PDL was considered as a non-linear elastic material. Table 5. 1 Material properties in FEA. Poisson’s ratio Young’s modulus (GPa) Cortical bone 0.3 (van Staden et al., 2006) 13.7 (van Staden et al., 2006) Trabecular bone 0.3 (Zhang et al., 2016) 0.7 (Dorogoy et al., 2017) Zirconia 0.3 (Fuh et al., 2013) 210 (Fuh et al., 2013) Dentin 0.31 (van Staden et al., 2006) 18.6 (van Staden et al., 2006) PDL 0.45 (Yan et al., 2012) - To simplify the simulation, the interfaces between zirconia and trabecular or cortical bone, cortical bone and trabecular bone, dentin and PDL, and between the PDL and cortical bone were treated as perfectly bonded connections. The number of nodes and tetrahedrons of each FE mesh can be found in Table 5. 2. Average element qualities are also described, according to the skewness parameter (in which values closer to 1 represent higher element quality). Table 5. 2 Average element quality, and number of nodes and tetrahedrons of the mesh of the different models. Tetrahedral elements Nodes Average element quality STI 1368886 229928 0.6619 TI 1479941 248406 0.6616 RAI 141095 24309 0.6562 RAIF 180003 31169 0.6541 NT 339593 61003 0.6586 Once the problem was defined, two external loads of 100 N were applied, at both axial and oblique directions. Oblique loading was performed from the buccal side at 45° relatively to the implant/tooth axis (Figure 5. 2(a)). A fixed constraint was implemented in the surfaces of the cutting section and the bottom of both the trabecular and cortical bone. The fixed constraint was applied to all the degrees of freedom of the nodes belonging to the highlighted surfaces (Figure 5. 2(b)). Mesh convergence was achieved, and its analysis was performed in function of the implant/tooth displacement.
Natural-inspired tooth replacement: design and strategies 113 Figure 5. 2 Boundary conditions: (a) Applied loads; (b) Fixed constraint. 5.2.3. Results Analysis The Von Mises equivalent stresses, the hydrostatic pressures and the octahedral shear strain values in the cortical bone caused by the loading were analysed. To better evaluate the response of the peri-implant bone, authors limited the results analysis to a circular section (hereafter referred as partition) of cortical bone around the implant/tooth with 8.6 mm of diameter and 5 mm height, as seen in Figure 5. 3. Figure 5. 3 Analysed cortical bone volume (partition) for the different models: (a) STI; (b) TI; (c) RAI; (d) RAIF; and (e) NT. Since this region is considered the most relevant in terms of loss of the implant stability due to bone resorption, it is crucial to understand and predict whether it is being correctly loaded or not. To determine the hydrostatic pressures in the partition, equation (1) was used:
Natural-inspired tooth replacement: design and strategies 114 𝑝 = − 𝑚= − ( 𝑥𝑥+ 𝑦𝑦+ 𝑧𝑧 3), (Eq.1) where, xx, yy, zz are the normal stress tensors. In order to compare results with literature (Frost mechanostat theory), equation (2) was used to determine the octahedral shear strain values. Implant and tooth displacements were also evaluated and compared. ϒ𝑜𝑐𝑡 = 2 3 √(𝜀1−𝜀2)2+(𝜀2−𝜀3)2+(𝜀3−𝜀1)2 , (Eq.2) where, ε1, ε2 and ε3 are the principal strains. 5.3. Results and Discussion 5.3.1. Stress Distribution and Hydrostatic Pressure in the Peri-implant Cortical Bone Figure 5. 4 displays the numerically determined stress fields in terms of the equivalent von Mises stresses at the cortical bone, for the five models. As can be seen, the higher values are observed in the peri-implant bone for all models, except for the NT. In this specific geometry, it is possible to observe that the stresses are better distributed, due to the colour gradient along the tooth socket. Also, the maximum value observed in the NT model (18 MPa) is quite lower than the ones observed for the other four geometries. The STI and TI models were the ones with the highest maximum stresses, around 70 MPa and 60 MPa, for the semi-threaded and threaded implant, respectively. Additionally, the incorporation of flaps in the root-analogue implant induced a decrease in the von Mises stresses in the peri-implant bone, transferring them to the bone adjacent to the flaps. Since this graphical analysis might be misleading and since these maximum values may only affect a neglectable volume of bone and might not be representative of the significant peri-implant stress states, results from volume-based analysis of peri-implant bone on von Mises stress and hydrostatic pressure are shown posteriorly. In this analysis, authors determined which quantity of the model volume was subjected to a certain von Mises or hydrostatic pressure value, thus, neglecting residual volume results.
Natural-inspired tooth replacement: design and strategies 115 Figure 5. 4 Von Mises stress distribution in the cortical bone for the studied models: (a) STI; (b) TI; (c) RAI; (d) RAIF; and (e) NT. In Figure 5. 5 it is possible to observe the von Mises stress distribution (on pre-defined von Mises intervals) in the studied circular partition, as a function of the volume fraction. Analysing the two threaded implants, despite having a similar behaviour, results showed that placing an implant at the crest level (semi-threaded implant) increases the maximum stresses in the peri-implant bone. In fact, some similar studies have already suggested that placing an implant in the trabecular bone would negatively affect the stress distribution along the bone (León et al., 2014; Rismanchian et al., 2013). On the other hand, placing a threaded implant in the cortical bone has already been proved to have some advantages regarding the implant stability (De Castro et al., 2014). Comparing the threaded implants with the root-analogue ones, it is possible to observe that the last ones present a better stress distribution once, for the same von Mises stresses, higher volume fractions are observed. Additionally, the threaded implants tend to induce some hot spots, evidenced by the high values of von Mises for very small volume fractions. The addition of flaps in the root-analogue model reduced the maximum von Mises stresses values and, as can graphically be seen, it was the geometry that presented the most similar behaviour to the natural tooth. The natural tooth model, in turn, was the one that presented the best stress distribution, as should be expected. In a natural tooth, the PDL absorbs the loads placed on the tooth during mastication and distributes them to the surrounding bone (Chu et al., 2013), leading to a more uniform stress distribution. A further refinement in the analysis reveals that the volume percentages with von Mises stresses higher than 10 MPa were quite small, for all models: 7.2% for the STI model, 7.5% for the TI, 2.9% for the root-analogue implant, 0.2% for the RAIF and the natural tooth presented 0.7% of
Natural-inspired tooth replacement: design and strategies 116 the studied volume with stresses higher than 10 MPa. The maximum values of the von Mises stresses are not reported in this analysis once, as previously stated, they are associated with vestigial volume fractions and might result from the geometrical process of the model construction and highly local effect of the FE mesh, without any physical meaning. Figure 5. 5 Von Mises stress distribution in the circular partition, as a function of the volume, for the different models. The hydrostatic pressure in the partition volume was also analysed in the scope of this study, once, with this evaluation, it is possible to understand which volume fractions of the partition were subjected to tensile or compressive mean stresses, through the state of tension at each gauss point. It is possible to observe, in Figure 5. 6, that the hydrostatic pressure values in the threaded implants reached higher compressive values, associated with quite small volume fractions, which indicates that these models tend to induce hydrostatic pressure hot spots and consequent stress
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Natural-inspired tooth replacement: design and strategies 127 CHAPTER 6 Design and optimization of zirconia functional surfaces for dental implants applications Published in Ceramics International 2020, 46: 16328–16336 doi:10.1016/j.ceramint.2020.03.190 T.A.Dantasa,b*, Paulo Pintoa, Paula C.S. Vazc, F.S. Silvaa aCenter for MicroElectroMechanical Systems (CMEMS-UMinho), University of Minho, Campus de Azurém, Guimarães – Portugal bMIT Portugal Program, School of Engineering, University of Minho, Guimarães, Portugal cFixed Prosthodontics, GeneticsFaculty of Dental Medicine, University of Porto, Portugal
Natural-inspired tooth replacement: design and strategies 128 Abstract Zirconia is becoming a promising solution for biomedical applications, namely for dental implants, due to its biocompatibility, and mechanical and aesthetical properties. Despite the constant developments in the dentistry field, strategies to promote an effective vascularization at the implant's surface and consequently improved osseointegration are still not enough. In this sense, with the aim of promoting the vascularization at the implant's surface, zirconia surfaces with micro-channels were designed and evaluated regarding their hydrophilicity and capillarity. A CAD/CAM system was used to design and produce the specimens and different techniques were used to characterize the surfaces. The obtained average surface roughnesses are in accordance with the literature for similar materials. Results revealed that the produced materials present high levels of hydrophilicity, whether in contact with water or FBS - Fetal Bovine Serum. Additionally, micro-channels with 200 µm of width and 100 µm of depth were the ones that presented higher capillarity, thus being promising solutions for the promotion of implants vascularization, and consequently improved osseointegration. Keywords: Zirconia dental implants; Implant vascularization; Surface hydrophilicity; Capillarity; CAD/CAM technology 6.1. Introduction Zirconia, namely Yttria stabilized zirconia (YSZ), a ceramic material, has been widely used in biomedical applications, namely in the dentistry field [1]. Its biocompatibility, low bacterial affinity [2,3], good chemical and dimensional stability, high mechanical strength, excellent wear resistance, and its white color are the main features that make zirconia a promising solution for such applications [4,5]. Despite its advantages, the physical and mechanical properties of zirconia tend to degrade as a result of a low-temperature aging process [6]. This process is characterized by a slow transformation of the tetragonal zirconia phase into the monoclinic, associated with an increase in the volume of the ceramic that stresses the particles and results in subcritical crack growth, offering a way for water to penetrate inside the material [6,7]. This phenomenon is strongly affected by the material grain size [8]. Above critical grain sizes, Y-TZP is less stable and, consequently, more susceptible to tetragonal-monoclinic phase transformation. On the other hand, for grain sizes below 0.2 mm, the transformation does not occur, leading to a reduced fracture toughness [8]. In this sense, the sintering conditions have a huge impact on the stability and
Natural-inspired tooth replacement: design and strategies 129 mechanical properties of zirconia, through the control of the grain size. In order to avoid the stressinduced transformation phase, sintering temperatures between 1350 and 1550 °C have been applied in the dentistry field [9]. Additionally, some authors believe that to avoid the tetragonalmonoclinic phase transformation, zirconia should no be grinded nor sandblasted. Regardless of all the efforts that have been carried out in the last few years, long-term dental implants survival and osseointegration is still an issue. The term “osseointegration” was first introduced and developed by Dr. Par-Ingvar Branemark in 1985, to define the direct structural and functional connection between a living bone and the surface of a loadcarrying artificial implant [10,11]. This process requires an initial interlocking between the alveolar bone and the implant (primary stability) and later, the biological fixation through continuous bone remodeling towards the implant (secondary stability). Osseointegration is a very complex process and there are many factors influencing the formation and maintenance of bone at the implant surface [10]. Additionally, some authors believe that the success of a dental implant is highly dependent on the interaction between the implant and intraoral tissues [12]. Surface properties such as roughness have a huge impact on the implant's osseointegration once they will influence the adhesion and proliferation of the osteoblasts on the implant's surface. Bone regeneration involves a sequence of cellular and extracellular biologic events in the interface boneimplant, regulated by growth and differentiation factors released by blood cells [13]. In this sense, one of the biggest challenges in implantology nowadays is controlling the process of bone resorption around the implant immediately after insertion and during its function (after 12 months) [12,14]. Some investigations have been carried out to improve the zirconia implant-bone interface interactions, and promote effective osseointegration [6,15]. Techniques such as sandblasting [16], coatings of zirconia, spray drying of a bioactive material [17] and chemical etching [18] have been applied in this field to promote an increased implant surface roughness and microporosity, and, consequently, induce the implant osseointegration. All these approaches are based on the mechanical interlocking between the host bone and the implant but do not take into consideration biological mechanisms such as vascularization. Osteoconduction, the process by which bone grows on a surface is crucial to achieving the implant osseointegration and consequent long-term stability [19]. On the other hand, to achieve osteoconduction, bone growth factors together with a proper blood supply are mandatory. This blood supply, responsible for providing nutrition is achieved through angiogenesis, which
Natural-inspired tooth replacement: design and strategies 130 corresponds to the formation of new blood vessels from the pre-existing vascular network [20–22]. As far as the bone regeneration is concerned, angiogenesis plays a crucial role in bone remodeling by providing the functional connection between the implanted material and the surrounding host tissues [20], by assisting and accelerating the regenerative process. To achieve so, it is suggested to decorticate the surrounding bone to assist the connection between blood vessels in the bone and the implant surface [23,24]. Some studies have been conducted to evaluate the role of dental implant surface characteristics, such as topography, hydrophilicity, surface charge, and other treatments, on the angiogenesis process surrounding the bone after dental implant insertion. Mohammad-Ali Saghiri et al. [25], concluded that the surface topography and hydrophilicity are the main features that an implant should encompass to promote angiogenesis and osteogenesis, once they induce the production of many pro-angiogenic growth factors. Schwarz et al. [26] reported that the hydrophilicity of a dental implant surface is more efficient for soft and hard tissue integration than the surface microtopography. In another study [27], it was concluded that hydrophilic dental implant surfaces tend to promote faster growth of human umbilical vein endothelial cells and osteoblast-like cells when compared with smooth hydrophobic implant surfaces. Also, the growth and function of undifferentiated endothelial progenitor cells that produce pro-angiogenic factors, were proved to be promoted by hydrophilic implant surfaces [28]. Lang N. et al. [29], also evaluated and compared the osseointegration rate and degree of hydrophilic and hydrophobic implant surfaces during the early phases of healing in a human model. Results showed that the degree of osseointegration after four weeks was higher for the hydrophilic when comparing with the hydrophobic surface. On the other hand, capillarity, a spontaneous movement of a fluid resulting from the cohesive forces between the fluid and the surrounding surface, is also required to achieve an efficient vascularization [30]. An implant surface with capillary properties will allow fluids, namely blood, to flow against gravity and spread, promoting, consequently, the cell infiltration and the proper supply of nutrients [31]. In this sense, strategies such as the production of scaffolds with interconnected pores or micro-channels have been investigated for biomedical applications to find an optimal design to facilitate these cell and nutrient flows. Results have shown that scaffolds or implant surfaces with capillary action may be a promising solution to improve bone regeneration through the flow of fluids into narrow spaces, such as pores or micro-channels [31–33].
Natural-inspired tooth replacement: design and strategies 131 Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) has revolutionized the dental implants world and its impact in society has been increasing lately [34]. This technique encompasses three big components: the digitalization scanner that transforms geometry into digital data to be processed by the computer; the software that processes the data and produces a data set for the fabrication of the product; and production technology that transforms the data into the final product [35]. After image acquisition and 3D modeling, the CAD model is converted into milling strips for the CAM-processing and loaded into the milling device, that can have different numbers of axis (3, 4 or 5 axes) [34,35]. This manufacturing process can be performed by a computer numerical control (CNC) machining system, that uses power-driven tools to mechanically cut the material into the desired geometry [36]. This subtractive manufacturing technique has been proved to reduce the production time of the implants and to produce very complex geometries that would be difficult to create with conventional techniques [35,36]. Targeting dental implants applications, this research work aimed to design and assess the hydrophilicity and capillarity of zirconia substrates with micro-channels on the surface, machined by a computer numerical control system. To find an optimal solution, micro-channels with different widths and depths were evaluated. 6.2. Materials and Methods 6.2.1. Specimen Design and Manufacturing Regarding specimen manufacturing, as previously stated, a CAD/CAM system was used. To perform this manufacturing process, the 3D CAD models were firstly drawn according to the desired geometries (SolidWorks Corporation, Dassault Systèmes S.A, USA). That models were posteriorly implemented in the CAM software that, in turn, created a code to be understood by the machine firmware (DWX – 50 by Roland, Serbia). This process was performed in a zirconia green compact disc (High Strength Zirconia – 3Y-TZP, with a scaling factor of 1.249) from Dental Direkt, Germany, and had to follow some specifications, according to the manufacturer (Table 6. 1). Table 6. 1 Guide for cutting Zirconia with DWX – 50 [37]. Tool Process X & Y direction cutting Z direction cutting Spindle speed (rpm) Path Interval (mm) Cut-in amount (mm) Finish margin (mm)
Natural-inspired tooth replacement: design and strategies 132 After the green machining process, specimens were sintered in a furnace (Zirkonofen 700 Ultra-Vakuum from Zirkonzahn - Zirkonofen) at 1500 °C, with a heating/cooling rate of 8 °C/min and a holding time of 2 h. This equipment allows the sintering of zirconia substrates in a hermetically sealed chamber, thus guaranteeing no contamination. In a first attempt, twelve different micro-channels with different dimensions were designed in a zirconia surface (Table 6. 2), as referred to as P1. Table 6. 2 P1 micro-channels dimensions and respective designation. Milling tool diameter (µm) Micro-channel designation Micro-channel depth (µm) 100 100 C1 200 C2 300 C3 500 C4 80 100 C5 200 C6 300 C7 500 C8 50 100 C9 200 C10 300 C11 500 C12 After wettability and capillarity analysis, the channels with the most promising results (C2 and C3) were designed in another zirconia surface (hereafter referred to P2). In this second specimen, micro-channels were designed in both horizontal and vertical orientations. Finally, given the results obtained in P2, a third specimen was designed. This third specimen - P3has a cylindrical shape, that intends to mimic a dental implant geometry. To sum up, three zirconia specimens were produced. The dimensions of the micro-channels on their surfaces resulted from an iterative process that evolved according to the wettability and capillarity results. The first speed (mm/min) speed (mm/min) R1 ball Rough cutting 2100 1200 25000 1 0.8 0.15 R0.5 ball Finish cutting 900 900 25000 0.1 0.1 -