Development of a medical device compatible with MRI/CT to measure ankle joint laxity

Tomás Francisco Freitas Correia Development of a medical device compatible with MRI/CT to measure ankle joint laxity October 2022 2 Tomás Francisco Freitas Correia Development of a medical device compatible with MRI/CT to measure ankle joint laxity Msc thesis in Mechanical Engineering Work done on the orientation of Óscar Samuel Novais de Carvalho Ana Isabel Neto Cardoso Leal i 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 CC BY-NC https://creativecommons.org/licenses/by-nc/4.0/ ii AGRADECIMENTOS Esta dissertação apenas foi possível pelo apoio de várias pessoas e entidades que gostaria de dar uma palavra de apreço. Em primeiro lugar, gostaria de agradecer ao Professor Filipe Samuel pela oportunidade de desenvolver a minha dissertação de mestrado no Center for Microelectromechanical Systems (CMEMs), na Universidade do Minho em parceria com a Clínica do Dragão. Ao Professor Doutor Óscar Carvalho, pelo apoio, motivação e confiança que depositou em mim para a realização deste projeto. À Professora Doutora Ana Leal, pela dedicação e pela partilha de conhecimento. Sem isso este trabalho não poderia ter sido levado a cabo. Ao Doutor Renato Andrade, pela disponibilidade e pelo acolhimento e apoio que me facultou sempre que foi necessário ir à Cliníca do Dragão. Aos meus amigos, por todos os momentos de ajuda, partilha e convívio, foi um prazer. Por fim à minha família, pelo apoio e pelo amor que sempre me deram. ¡Sí se puede! iii DECLARAÇÃO DE INTEGRIDADE Declaro ter atuado com integridade na elaboração do presente trabalho académico e confirmo que não recorri à prática de plágio nem a qualquer forma de utilização indevida ou falsificação de informações ou resultados em nenhuma das etapas conducente à sua elaboração. Mais declaro que conheço e que respeitei o Código de Conduta Ética da Universidade do Minho. STATEMENT OF INTEGRITY I declare that I have acted with integrity in the elaboration of this academic work and confirm that I have not resorted to the practice of plagiarism or any form of misuse or falsification of information or results in any of the stages leading to its preparation. I further declare that I know and have respected the Code of Ethical Conduct of the University of Minho. Universidade do Minho, 24 de outubro de 2022 iv ABSTRACT The ankle joint in the human body experiences the most significant number of injuries across the whole musculoskeletal system, both in daily life and sports. The actual effect of the ankle's ongoing functional demand, particularly while engaging in sports that need the body to be raised off the ground regularly, is that this structure is exposed to challenging circumstances contrary to the optimal settings for balance and stability. The analysis and diagnosis stages of these injuries are carried out during two distinct phases: a manual examination by an orthopaedist to assess the functional capacity using simulated movement and palpation of the area of peripheral pain and an imaging examination. Since the two tests are run at different times, it is challenging to combine the data. Furthermore, it is impossible to obtain the precision and repeatability needed for a procedure of this sort when using such diagnostic approaches. That being said, Clínica do Dragão wanted to overcome these problems and needed a medical device to do the manual examination in an MRI environment. The manual exam needs to be made by a machine, and the creation of such a device is the goal of this work. Firstly, the objectives that this device should complain with were created to define the final attributes of the equipment. Several meetings with the work group were conducted to determine the requirements that the product should respect and that would constitute the technical specifications in order to fulfil and define the project's primary aims. The device was improved step by step for the conceptual design until a final concept could comply with all the demands of the design requirements. After developing a viable concept, the preliminary parts started to be designed until the initial design was finally fulfilled. After, the device was validated, beginning with the mechanical simulation of the different components and sub-systems until the cinematic validation of the final device. The result is a medical device capable of linking a physical exam to a precise imaging test, such as an MRI. So, the laxity of the ankle joint may be quantified, which is beneficial for both patients and medical experts. Keywords Ankle Joint, Diagnosis, Medical Device, MRI, Laxity v RESUMO A articulação do tornozelo é a articulação do corpo humano que sofre o maior número de lesões em todo o sistema musculoesquelético. A constante solicitação funcional do tornozelo, particularmente durante a prática de desporto, é que esta estrutura é exposta a circunstâncias perigosas que podem comprometer o equilíbrio e estabilidade desta articulação. As etapas de análise e diagnóstico destas lesões são realizadas em duas fases distintas: um exame físico feito por um ortopedista para avaliar a capacidade funcional por meio de movimentos simulados e palpação da área de dor periférica, e um exame de imagem, onde a condição anatômica desta articulação é avaliada pela análise da posição relativa das diversas partes. Como os dois testes são executados em momentos diferentes, é difícil correlacionar os dados. Além disso, é impossível obter a precisão e a repetibilidade necessárias para um procedimento desse tipo ao usar tais abordagens diagnósticas. Posto isto, a Clínica do Dragão pretendia ultrapassar estes problemas e por isso precisava de um dispositivo médico que permitisse fazer o exame manual dentro de RM. Este exame manual necessita de ser executado por um dispositivo, e a criação de tal dispositivo é o objetivo deste trabalho. Primeiramente, foram criados os objetivos que este dispositivo deve possuir, com o intuito de definir os atributos finais do equipamento. Foram realizadas várias reuniões com o grupo de trabalho para determinar os requisitos que o dispositivo deveria respeitar e que constituiriam as especificações técnicas do projeto. Em termos de projeto conceitual, o dispositivo foi aprimorado passo a passo até que um conceito final fosse capaz de preencher todas as especificações do projeto. Depois de desenvolver um conceito viável, as peças preliminares começaram a ser modeladas até que, finalmente, se desenvolveu o dispositivo preliminar. Após isso, foi feita a validação do mesmo, iniciando com a simulação mecânica dos diferentes componentes e subsistemas até a validação cinemática do dispositivo final. O resultado é um dispositivo médico capaz de conciliar o exame físico a um exame de imagem preciso, como a RM. Assim, a laxidez da articulação do tornozelo pode ser quantificada, o que é benéfico tanto para pacientes quanto para especialistas médicos. Palavras-chave ARTICULAÇÃO DO TORNOZELO, DIAGNÓSTICO, DISPOSITIVO MÉDICO, RM, LAXIDEZ vi TABLE OF CONTENTS Agradecimentos ................................................................................................................................... ii Abstract.............................................................................................................................................. iv Resumo............................................................................................................................................... v Table of Contents ............................................................................................................................... vi List of Figures ....................................................................................................................................viii List of Tables ..................................................................................................................................... xiv List of Symbols .................................................................................................................................. xv 1. Introduction ................................................................................................................................ 1 1.1. Motivation ........................................................................................................................... 1 1.2. Objectives ........................................................................................................................... 1 1.3. Structure of the Dissertation ................................................................................................ 2 1.4. Contributions to this work .................................................................................................... 2 2. State of the art ............................................................................................................................ 3 2.1. Anatomy of the ankle joint ................................................................................................... 3 2.2. Ankle Kinematics................................................................................................................. 5 2.3. Pathologies and methods of injury evaluation ....................................................................... 8 2.4. Summary and Discussion .................................................................................................. 12 3. Porto Ankle Testing Device Design ............................................................................................. 13 3.1. Problem Description .......................................................................................................... 13 3.2. Objectives definition .......................................................................................................... 13 3.3. Design Requirements ........................................................................................................ 15 3.4. Technical Specifications Definition ..................................................................................... 17 3.5. Evolution of Concepts ........................................................................................................ 19 3.6. Preliminary Design ............................................................................................................ 28 xiii Figure 1373-way valve final design. .................................................................................... 87 Figure 138Flow regulator, section view. .............................................................................. 87 Figure 139Exploded view of the flow regulator. ................................................................... 88 Figure 140Slot added to measure the eversion angular movement. ..................................... 89 Figure 141Slot added to measure the abduction angular movement. .................................. 89 Figure 142Slot added to measure the anterior translation linear movement......................... 90 Figure 143PATD final design. ............................................................................................. 90 Figure 144Schematic view of the wheel and rack forces. .................................................... 93 xiv LIST OF TABLES Table 1Design requirements. ............................................................................................... 15 Table 2Technical specifications ........................................................................................... 18 Table 3List of components of the 2.0 version of the PATD .................................................... 20 Table 4List of components added to version 2.2 of the PATD. .............................................. 24 Table 5PATD version 3.0 component list. ............................................................................ 24 Table 6Component list of the 4.0 version of the PATD. ......................................................... 26 Table 7Different materials available with their main properties ............................................. 36 Table 8List of components of the flow regulator. .................................................................. 88 xv LIST OF SYMBOLS Initials, abbreviations and acronyms CAD Computer-Assisted Design CAE Computer-Aided Engineering FEA Finite Element Analysis MRI Magnetic Resonance Imaging CT Computer Tomography PATD Porto Ankle Testing Device PKTD Porto Knee Testing Device Units N Newton Nm Newton meter Nmm Newton millimetre m Meter mm Millimetre MPa Mega Pascal Development of a medical device compatible with MRI/CT to measure ankle joint laxity 1 1. INTRODUCTION 1.1. MOTIVATION Testing for ankle ligament laxity has always been a somewhat archaic and subjective process where each health professional could have a different opinion from another professional colleague when both examined the same patient. This is one of the reasons for patients with injuries in very complex areas, such as the ankle [1], often consult different experts in order to obtain a more objective assessment. Despite all the advances that medicine has been making over the years due to the introduction of cutting-edge technologies, such as magnetic resonance, and improving treatment as a whole, there are still no deterministic methods that allow rigorously ascertaining the need for medical intervention, [1], [2]. Therefore, the objective of this dissertation is the creation/development of a device [3], [4] that tests the laxity ligaments during an MRI (Magnetic Resonance Imaging). This allows the medical team to assess laxity and determine the need for treatment and the type (operation, physiotherapy, etc.). 1.2. OBJECTIVES This dissertation aims to develop solutions to create/ develop a medical device to measure ankle joint laxity quantitively. It is intended to carry out the device's design, modelling and virtual validation. The objectives can be defined as follows: ➢ Objective 1A literature review aimed at apprehending the morphology of the ankle and knowing the types of injuries, as well as the way of testing the ligaments of this joint and understanding how to determine whether the patient needs treatment. ➢ Objective 2Modelling the improvements to be implemented and changing existing geometries. ➢ Objective 3Mechanical simulation of the various movements of the device in order to verify the feasibility of the conjunction of the multiple degrees of freedom and the strength of the components. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 2 1.3. STRUCTURE OF THE DISSERTATION This thesis aims to develop/ improve a new medical device to support the diagnosis of ankle ligament instability. This work is a very complex process because it is required to know the basics of ankle anatomy, kinematics and pathologies as diagnosis methods and all the design processes. For these reasons, this work was divided into six chapters: Chapter 1 covers an overview of the thesis and presents the objectives, structure and contributions of this work. Chapter 2 explains the concept of laxity, along with a brief description of this joint's ankle anatomy and kinematics. Finally, the pathologies and diagnosis methods are presented before showing the existing devices that are now being used. Chapter 3 covers the design phase, from the definition of the objectives to the various concepts developed and, finally, the preliminary design. In Chapter 4, the device is verified from the mechanical and kinematic points of view. Also, some geometries are changed to provide more mechanical resistance to some critical areas. Chapter 5 briefly describes what the actuating system needs to be as well as the valve and flow regulator design. The layout of the pneumatic system is also presented. In Chapter 6, the final device is shown with some systems that allow measuring the displacements of the foot. Chapter 7 summarizes the conclusions of this dissertation while also giving future work perspectives. Finally, the Gear Design is explained in the Appendix, and the technical sheet of the various materials possible to use is shown. 1.4. CONTRIBUTIONS TO THIS WORK The first two chapters of the current thesis offer a survey of the literature and gather crucial background information. Introduction to the anatomy and function of the ankle, the laxity concept and the movements of the ankle articulation are described. The clinical methods used to diagnose ankle instability are reviewed, and the medical devices for diagnosis are explored. The project's techniques are described and applied to all phases, from the generation of ideas to the choice of the final concept, to help design and develop the new medical equipment. This work contributes to creating a medical device capable of measuring ankle joint laxity quantitively. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 3 2. STATE OF THE ART This chapter will present a brief description of the anatomy of the ankle joint and expose the concept of laxity as well as the various movements that this joint allows. Then, the different injuries that can arise will be shown, as well as the various methods of evaluating the pathologies. The differences between deterministic and non-deterministic methods will also be highlighted. First, it is necessary to define what laxity is because this work aims to measure the laxity of the ligaments of the ankle joint. Now laxity is the relaxation of the ligaments that become loose during an accident that causes an injury. However, if the articulation is very lax, it will be necessary to undergo treatment. If this parameter is within acceptable limits, the patient will not need any intervention[1]. It is, therefore, necessary to define this factor in measurable quantities (SI units). However, measuring displacements and rotations of ligaments and/or bones with the naked eye is challenging. This is why this device has to act in the context of MRI or CT (Computerized Tomography). Thus, it is possible to carry out all the measurements required by the medical panel in order to ensure that the decision to intervene or not to intervene is correct [2]. Currently, this type of deterministic evaluation does not exist, and the assessment that is performed may have a different conclusion from doctor to doctor, which makes the definition of the severity of the injury highly subjective. The objective of this work is to help in this decision-making through the use of deterministic methods [3]. 2.1. ANATOMY OF THE ANKLE JOINT Anatomy is a science that studies the body's structures and how they work and interact with each other. In this section, it will be analysed the area in question in this work, the ankle. First, it is essential to define the ankle and its location in the human body. The ankle corresponds to the joint existing in the connection between the foot and the leg, formed by three bones: talus, fibula and tibia [4] as shown in Figure 1. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 4 Figure 1Ankle location. (Adapted from [5], [6]) The talus has a cubic shape and is supported by the calcaneus, the heel bone that bears much of the impact during human locomotion [4], [7]. In contrast, the cuneiform, navicular and cuboid provide more flexibility to the foot. The tibia is a bone with a considerable length that supports much of the body's weight. It consists of a body and two ends. In the extremities, it connects to two joints, the knee and the ankle [4], [7]. Finally, the fibula is similar to the tibia, a body with two ends, whose main function is muscle fixation [4], [5] as seen in Figure 2. Figure 2Nomenclature of the ankle bones. (Adapted from [8]) Development of a medical device compatible with MRI/CT to measure ankle joint laxity 5 Ligaments connect these bones and the rest of the foot and leg. In turn, these are activated through the muscles, thus allowing the movement of the other body parts [4], [7]. Ligaments are fibrous structures of collagen and elastic fibres. These elastic fibres allow some degree of deformation. However, they can be damaged if it is excessive. Thus, the ligaments unite the joints, stabilizing the bone sets and only allowing specific movements. The ankle joint is made up of 4 ligaments [7], as shown in Figure 3: • The deltoid ligament is located in the medial zone of the ankle joint. • The anterior and posterior talofibular ligaments connect the talus to the fibula. • The calcaneus-fibular ligament, which connects the calcaneus to the fibula. Figure 3Location of the different ankle ligaments. (Adapted from [9]) 2.2. ANKLE KINEMATICS So far, the anatomy of the ankle has been briefly discussed. The following section will show what types of degrees of freedom the ankle structure allows. These will be the movements that the device will have to perform in order to test the ligaments of the foot without causing or worsening the patient's injury. First of all, it is necessary to address the convention of the principal planes of ankle movement [10]–[12]. The sagittal plane is located vertically from the front of the foot to the back, dividing the ankle into left and right zones. The frontal plane is located vertically from left to right, dividing the ankle into front and back. Finally, the transverse plane is positioned horizontally, dividing the ankle into an upper and lower zone, as schematically shown in Figure 4. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 6 Figure 4Ankle movement planes. (Adapted from [13]) In terms of axes, see Figure 5. Figure 5Ankle joint axis. (Adapted from [14]) Now that the different agreed planes and axes have been addressed, the various degrees of freedom of the ankle will be shown [15], [16]. The ankle allows six movements that are performed in a single plane along an axis. Thus, the intersection of the various axes, the origin, is located in the centre of the ankle. This centre of rotation must be respected so that the movements performed are passive. The allowed motions are: • Abduction and Adduction. • Inversion and Eversion. • Dorsal and Plantar Flexion. The abduction and adduction movements are carried out along the longitudinal axis, thus located in the transverse plane [17]. In abduction, the foot rotates laterally in the negative direction, while in adduction, the foot rotates in the positive direction of the axis [17], Figure 6. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 7 Figure 6Representation of abduction and adduction movements (top view of the right foot). (Adapted from [13]) In contrast, inversion and eversion movements are carried out along the sagittal axis, located in the frontal plane. In eversion, the foot rotates in the positive direction of the transverse and longitudinal axes (inwards and upwards). In an inversion, the foot rotates in the negative direction of the transverse axis and the positive direction of the longitudinal axis [11], [16] (outwards and upwards), Figure 7. Figure 7Representation of inversion and eversion movements. (Adapted from [18]) Development of a medical device compatible with MRI/CT to measure ankle joint laxity 14 Figure 15Objective tree of the PATD In synthesis, it can be said that the main objectives are the main features related to the device's safety, functionality and versatility. The first, safety, is more on the scope of not hurting the foot (comfort and natural movements), allowing the patient to make the exam without pain. The natural movement side is related to the fact that the movement of the device needs to be as close to the natural movement of the foot as possible. This will make sure that the injury does not worsen. The hygiene part of this objective is to ensure there isn’t any contamination because the medical environment needs to be extremely clean. The second objective, functionality, is perhaps the most important. This objective indicates that the device needs to be durable (not break with any minor misuse) and allow control of the various movements that will be made to the foot because the device aims to test the ligaments, not deteriorate the injury. It also needs to be easy to use because the MRI is extraordinarily costly, so the device has to be easy to operate. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 15 Finally, the PATD has to be versatile. This means it must be compatible with the PKTD and usable for very different people with very different feet, making the device very appealing for customers that can purchase a product that can test both knee and ankle. 3.3. DESIGN REQUIREMENTS The objectives definition is beneficial to understand what is required from the final solution, which also helps to define the project requirements more clearly. This is because these objectives are still unclear from an engineering point of view, and they need to be more specific. In this regard, there were various conversations with professionals from Clínica do Dragão and some visits to the facilities to understand how the exam was realized and the exam environment. This was super important because it allowed us to judge better the conditions in which this device needs to operate. That being said, the design requirements were defined from the objectives tree and the requirements discussed and demanded by Clínica do Dragão. They can be seen in Table 1. Table 1Design requirements. 1 It must be used during a clinical exam 2 It can evaluate the functional condition of the ankle ligament structure 3 It can evaluate the existence of joint instability 4 It must perform movements passively on the foot 5 The movements are controlled by a health technician 6 The device doesn’t affect the characteristic hygiene of the medical environment 7 It can be used in the MRI/CT environment 8 The presence of the PATD doesn’t affect the exam 9 It is compatible with the PKTD 10 It has an intuitive usage process 11 It can be used in the majority of the patients 12 It is quickly prepared for the exam Next, a brief description of each requirement will be presented 1. It must be used during a clinical exam This device must be used during the examination process of a patient with an injury in the ankle joint. It can be used in the diagnosis phase and in the post-operation step to assess the result of the Development of a medical device compatible with MRI/CT to measure ankle joint laxity 16 intervention. It also needs to be capable of testing both the left and right foot, as it is common to compare to injured ankle with the healthy one. 2. It can evaluate the functional condition of the ankle ligament structure To be able to attend to this requirement, the device must be capable of doing all the movements pretended by the customer. 3. It can evaluate the existence of joint instability This requirement is linked to the previous one; however, this one pretends to emphasize the fact that the device needs to assess, or in other words, measure. This way, the device will need to have graduated rules that allow measuring the displacements. 4. It must perform movements passively on the foot This requirement intends to highlight that foot movement needs to be as natural as it can be because if not, the device can hurt and injure the patient. 5. The movements are controlled by a health technician This means that the technician can choose the sequence of movements the patient will be subjected to and can easily control the device. 6. The device doesn’t affect the characteristic hygiene of the medical environment The device should not be made from materials compatible with the medical environment, such as polymers, metals that don’t oxidate, etc. 7. It can be used in the MRI/CT environment The MRI/CT are the best exams available to evaluate the ligaments' condition, and one of the main objectives of this thesis is to combine the physical exam with the imaging exam. This implies that the device must fit MRI and CT devices. 8. The presence of the PATD doesn’t affect the exam. In the MRI environment, materials that interfere with the electromagnetic field created during the exam cannot be used, such as iron, cobalt and nickel. The presence of elements that interfere with the electromagnetic field creates an attraction or repulsion phenomenon that can injury the patient. 9. It is compatible with the PKTD. This device must be compatible with the PKTD. This means that they must link with each other, and this way, they can be sold together. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 17 10. It has an intuitive usage process As the PATD will be used by the technician responsible for the exam and they have education in the field of health, it can be thought that they don t know how to operate mechanical systems. That being said, the device operation must be accessible and intuitive, generating no confusion at the exam moment. 11. It can be used in the majority of the patients This medical device is intended to be commercialized all over the world. For this reason, it must be capable of evaluating very different feet from people of different origins. Also, it needs to be able to test patients with disparate anthropometric measurements. 12. It is quickly prepared for the exam The procedure for performing an MRI examination is already a relatively lengthy process. The technician must interrupt the exam to position the ankle in the desired stance and then capture the image. If the process of preparing the device is too lengthy and complex, the device could lose customers' interest. 3.4. TECHNICAL SPECIFICATIONS DEFINITION The initially defined objectives are sometimes called design specifications, which is not correct because they only manifest affirmations of what the project should accomplish and do not impose either limitation on the mechanical project. After that, the technical specifications were defined based on the design requirements list. These technical specifications translate from the engineering point of view boundary conditions and the functionalities that the device must have. These boundaries should be accurate because if they are too tight, there will be very few solutions, and if they are too wide, the choice of solutions will be challenging. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 18 Table 2Technical specifications 1 Perform the exam for both the left and right foot 2 Abduction movement from 0 to 60 degrees 3 Adduction movement from 0 to 60 degrees 4 Eversion movement from 0 to 60 degrees 5 Inversion movement from 0 to 60 degrees 6 Anterior translation of the astragalus up to 28 mm 7 It must be able to develop the above movements independently 8 It must be able to realize the abduction and adduction movements together with the anterior translation of the astragalus 9 Ankle dorsiflexion/plantar flexion angle adjustment (-15º, 0º and 15º) for foot positioning 10 The leg and foot fixation zone cannot affect motion transmission and cannot cause patient discomfort or pain 11 The fixation mechanism mustn’t affect the foot movement 12 The mechanical system of manual drive 13 Need to fit in the MRI machine 14 Cannot be made of electromagnetic materials 15 Adjustment to the anthropometric needs of each patient 16 Does not cause any discomfort to the patient during the examination 17 PKTD is compatible with lower limb support and positioning These technical specifications are referent to the device as a whole. Still, each component should also obey some geometric and dimensional character requirements so that these components can be integrated with the final device. After this process, where all the needed specifications are defined, the concept design can be started. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 19 3.5. EVOLUTION OF CONCEPTS Given the clinical and technical specifications this device needs, the work started by inspecting which ones the initial device already had, Figure 16. Figure 16PATD version 1.0. The initial device complies with the design requirements. Still, regarding the technical specifications, it falls short of expectations since it does not allow any rotation or adjustment of the dorsiflexion/plantarflexion angle of the foot. It only complies with the movement eversion/inversion and anterior translation of the astragalus. It was also found a problem of fixation of the tibia that is uncomfortable for the patient since, in previous tests with the previous prototype, the patient sometimes does not support the anterior translation test in its entirety (i.e., in the total excursion) due to discomfort at the tibial attachment site. It should also be noted that the eversion and inversion movements are abrupt and must be refined. All of these considerations were found by Clínica do Dragão while testing the PATD version 1.0. Therefore, it started by devising a system that would respond to all the most relevant specifications, allowing all the movements in the technical specifications. Next, an exploded view of the created set is shown in order to identify all the components, Figure 17. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 20 Figure 17Exploded view of version 2.0 of the PATD. Table 3List of components of the 2.0 version of the PATD In this way, an angle adjustment was started to be incorporated to allow the regular position of the foot according to the type of examination. This system consists of parts 5 and 6. Shaft 5 is fixed to piston 8 through the slots. Locks 6, in turn, fit into the extruded hexagon on shaft 5, so they do not rotate either. They only allow adjusting the angle of piece 4, employing its translation. Once a screw crosses the centre of the three parts, and as all Nº of the component Designation 1 Foot support 2 Bushing 3 Rotation shaft 4 PP connection 5 Fixation shaft 6 Block 7 Cylinder cover 8 Piston 9 Vertical cylinder 10 Polymer bearing 11 Nut Development of a medical device compatible with MRI/CT to measure ankle joint laxity 21 the parts tighten, all the components stand together and find themselves immobilized in the pneumatic cylinder. First the entire system is locked in the desired position, Figure 18. Figure 18Exploded view of the set that allows angle adjustment (on the left). Cutaway view of the lilac shaft fitted to the cylinder piston (in the centre). Detail of the angle lock system (right). Then one rotating shaft support was added to the foot support piece so that rotational movement is possible, Figure 19. A sprocket was also added to enable this movement to be driven by a pneumatic cylinder that would be installed later. Figure 19Addition of rotating shaft support and a sprocket. Finally, a polymeric bearing was added to the cylinder responsible for the anterior translation of the astragalus to smoothen the eversion/inversion movement. The toothed wheel installed in the cylinder was also modified, decreasing the toothed module and aiming at smoothing this action. The result is PATD version 2.0, Figure 20. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 22 Figure 20Version 2.0 of the PATD. This solution has, however, a problem. It does not respect the centre of rotation of the foot located in the ankle area. Thus, foot angle adjustment movements, as well as eversion/inversion, do not cause passive movement of the foot as the movement is not the natural movement of the joint, Figure 5. Therefore, this limitation was corrected by changing the component that connects the cylinder shaft to the foot support, Figure 21. Figure 21Version 2.1 of the PATD. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 23 This solution is similar to the previous one, except for part 4. However, solving the centre of rotation of the eversion/inversion movement does not solve the fact that when adjusting the angle of the foot, it does not move according to the centre of rotation of the foot. This makes the whole leg rise when the foot angle is changed. When the leg goes up, as the tibia is fixed, the patient will feel enormous discomfort which may even worsen the patient's injury. Therefore, it was necessary to develop an angle adjustment that allows the angle adjustment according to the foot's natural movement. The developed system is based on creating an arc of circumference from the ankle, Figure 22. Figure 22Sketch used to model the foot angle adjustment. The result is a support with a semi-circular track, and a new component dubbed the 'angle adjustment' also with a semi-circular surface where part 1 slides and allows the adjustment of the foot angle to the angles intended by the examiner, Figure 23. Figure 23Idealized angle adjustment system. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 30 Figure 35Transfer spheres (section view of the midplane of the part). Finally, the gear that will allow the inversion/eversion movement was added, Figure 36. The calculation of the gears is shown is Appendix A. Figure 36Several functions added to the L connection component of PATD version 4.0. One detail was added that would serve as an indicator to measure the inversion/eversion angle that the foot will reach, and another detail that will allow the translation cylinder to be fixed so that it does not rotate along its axis during the inversion movement. Regarding the translation cylinder, only a tiny notch was added to allow immobilization, as described above. Regarding this component, it was tried to minimize the changes made since this component was not designed from scratch but was already in version 1.0. Subsequently, with the cylinder cover and the piston, no design changes were made, and these components present the geometry shown in version 3.0 of the PATD, Figure 37. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 31 Figure 37Details of cylinder-cap-piston assembly (left) and notch added to the cylinder (right). Also, the system, which in the initial versions was for angle adjustment, was kept to connect the piston to connection I. However, this system lost the angle adjustment function. Regarding Connection I, it underwent several changes throughout the design process. Here the final design will be presented and explained. First, it was decided that two bearings would be used to support the shaft that will pass through the central hole in order to give more rigidity to the assembly. The choice of bearings was based on the loads that need to be supported, in this case, 200 N. As each bearing supports 140 N, using two guarantees safe use. Two holes and a slot were also created in order to mount a double-acting cylinder that will be presented later, Figure 38. Figure 38Section view of connection I with the various details added. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 32 After redesigning Connection I, the angle adjustment part was carried out, where this component was reinforced since this component is highly demanded from a mechanical point of view, Figure 39. Figure 39Reinforced angle adjustment component. The foot support underwent minor changes, namely, to allow the introduction of foot fixation tapes. These rips allow adjustment of the tightness of the fixation of the foot. The part was 3D printed to get these locations, and different areas were marked for different feet. For this, some persons were used in order to have a wide range of foot sizes, Figure 40. After this, the part was 3D printed again with these rips, and in this way, the location of these rips was verified. Figure 40Foot support component of the PATD version 4.0. Then, the designing of the shaft began, which will be housed in Connection I and will be responsible for the transmission of movement in the rotation movement. This component seems complex. However, its operation is quite simple. The Angle Adjustment Component and the rotation gear will be connected Development of a medical device compatible with MRI/CT to measure ankle joint laxity 33 to the ends of the shaft. The backstop and the thread will allow the shaft to be solidary with the bearings, Figure 41 and Figure 42. Figure 41Rotation Shaft Component. Figure 42Section view of PATD rotation shaft assembly. As can be seen, it is a very compact design that maximizes the available space without needing to increase the parts' thickness. That being said, the pneumatic cylinder responsible for the rotation movement was designed next. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 34 Figure 43Section view of the double-acting pneumatic cylinder. The slot for the seal will ensure an efficient seal. In contrast, the slots 0º and +/- 15º allow the foot support to be rotated through these certain angles in order to be able to carry out the translation movement with the foot at these angles and thus eliminate the need for the scaffolds (this was in version 1.0 but it didn’t work as intended), Figure 43. There is also a slot for the rod seals so that no air escapes. Next, the part that will connect to the shaft of the double-acting cylinder and allow the foot rotation, called Rotation Rack Support, was designed, Figure 44. Figure 44Rotation Rack Support Component. This component is quite reinforced as it will have to transmit a very high force, which will be applied at a long distance, so the moment created will be very high. The fact that the rack is not in this component is related to the fact that if there is ever any incorrect use that leads to one or more teeth of the rack breaking, the rack can be easily replaced. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 35 Finally, the tibia support was positioned, and the cylinders responsible for the inversion/eversion movement were mounted, Figure 45. Figure 45Positioning of the tibia support and pneumatic cylinders. In the tibia support, slots were built to allow the passage of “Double D” tapes to fix the leg. The eversion cylinders were mounted on the base, and a system similar to the rotation movement was built so that the rack could also be easily replaced. It should be noted that these racks must be made of a less rigid material (lower Young's Modulus) so that any damage occurs to them. The result of all these changes can be seen in Figure 46. Figure 46Preliminary Design of the PATD. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 36 This design is a design close to the end design. However, it is necessary to check the various deformations in the different critical components to obtain the mechanism's viability to work correctly. After this phase, some areas will still be included to place markings that will allow the measurement of the various displacements. Either linear or angular. Material The material that will be used is a very important parameter and Clínica do Dragão has a few options, all of which are resins of polyurethane, Table 7. • PR403, which was the material used in the PKTD. • PR700, which is similar to ABS. • 8051, with or without adding glass fibre. Table 7Different materials available with their main properties PR403 PR700 8051 8051 w/20% GF Yield Strength (MPa) 47 55,9 55,9 70,1 Young Modulus (MPa) 1850 1800 2150 5696 It is obvious that the best material in terms of mechanical resistance would be the 8051 w/20% GF. However, the mechanical simulations that will be carried out will be considered the combination of the worst material properties. This means the properties that will be used for the FEA study will be: 𝜎𝑦=47 𝑀𝑃𝑎 𝐸 = 1800 𝑀𝑃𝑎 This is a conservative approach, and it’s justified since the material will be chosen after finishing the design process. This means that every choice of material will result in a safe choice. Also, it should be said that the manufacturing process will be casting for the device's production while the prototype will be made by 3D printing. The technical sheet of each material can be seen in Appendix B. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 37 4. VALIDATION OF THE PRELIMINARY DESIGN 4.1. MECHANICAL VALIDATION First, it is necessary to determine the critical components that need to be studied. The first components are the gears that will transmit the movement as they need to be stiff and withstand the loads. Secondly, the system responsible for the fixation between the piston and the I connection needs to be studied to prove the robustness of the structure. Finally, a simulation for each of the three types of movement will be performed on the assembly because the device’s complexity makes it very difficult to predict how each part will behave relative to the next. 4.1.1. EVERSION/ INVERSION MOVEMENT GEARS 1. Pre-analysis Before starting the initial phase of the simulation, it is necessary to carry out a pre-analysis. This is intended to perform some calculations by hand in order to verify the simulation results. Remember that since some simplifications will be made, the values calculated here are merely indicative. As verified by the Ludwig equation, the stress must have the value of: 𝜎 = 𝑘𝑣⋅ 𝑤𝑡 𝐹 ⋅ 𝑚𝑡⋅ 𝑌 (1) 𝜎 = 1,074 ⋅181,82 10 ⋅10−3 ∙ 1,5 ⋅ 10−3 ∙ 0,485 ≈27 𝑀𝑃𝑎 2. Geometry The geometry imported into the simulation will be simplified; therefore, only the sprocket and rack will be studied, Figure 47. Figure 47Simplified geometry of the eversion gears. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 38 3. Mesh This point is one of the most important since a weak discretization of the set will inevitably lead to poor and far from real results. Given the low computational power available, the refinement will be carried out specifically in critical areas. In this case, the contact between the teeth of the rack and the wheel, Figure 48. Figure 48Mesh control used in the eversion gear. In the remaining areas, a mesh size of 3 mm was used, Figure 49. Figure 49Mesh size used in the eversion gear. Finally, the quality of the mesh was also verified, and a good mesh must have at least 90% of the elements with an aspect ratio below 5 [61]. In this case, there are only 66 elements with an aspect ratio above 5, so the mesh quality is satisfactory, Figure 50. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 39 Figure 50Aspect ratio above 5 of the mesh used in the eversion gear. 4. Model Configuration In this simulation phase, it is necessary to define the boundary conditions and material properties. Regarding material properties, see Figure 51. Figure 51Material Properties. In terms of boundary conditions, it was defined that the rack would be fixed, and the wheel could only rotate along its axis, Figure 52. Finally, a torque was applied to the centre of the wheel, Figure 53. Since the eversion/reversion movement is performed with a maximum torque of 4000 Nmm [60], when considering a safety factor of 1.5: 𝑇 = 4000 ∙ 1,5 = 6000 𝑁 ∙ 𝑚𝑚 The last important aspect is the contact parameters, Figure 54. In this section, it was considered a friction coefficient of 0,4 as it is the highest possible friction coefficient for plastic on plastic [62]. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 46 Figure 65Numerical results required for the rotation gear. 6. Numerical Results In the specific case of this simulation, the stress result will only be later verified with the one calculated in the pre-analysis. The displacement values will also be seen, given that high stiffness and reliable equipment are intended. Regarding the results in terms of stress, the maximum value recorded was about 28 MPa, while in terms of displacement, the maximum value found was about 0,14 mm, Figure 66. Figure 66The rotation gear's stress (above) and displacement (behind) results. 7. Verification and/or Validation In this last step, some checks will be made to verify the simulation's validity. Firstly, the maximum stress value is confirmed at the expected location, in the contact of the teeth, Figure 67. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 47 Figure 67Maximum stress location for the rotation gear. Regarding the displacement, it seems that the results do not make much sense but looking at the strain, it is clear that although the teeth that are not in contact travel a greater distance, they are not the ones that suffer the most deformation, Figure 68. Figure 68Strain results for the rotation gear. Finally, just check the error between the stress value calculated in the pre-analysis and the one obtained in the simulation: 𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐸𝑟𝑟𝑜𝑟 = 33 −28 33 ∙100 ≈15 % Although the error is substantial, given the simplifications of the Ludwig Equation and the fact that the values are very close in absolute terms, the validity of the result is verified. Also, the mesh near the contact probably should be more refined. However, this error will be considered acceptable due to the limitations of the computing power available. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 48 4.1.3. CONNECTION VALIDATION 1. Pre-analysis Before starting the initial phase of the simulation, it is necessary to carry out a pre-analysis. This is intended to perform some calculations by hand in order to verify the simulation results. Since some simplifications will be made, the values calculated here are merely indicative. This connection is very complex, with multiple bodies and many contacts, making the analysis very complicated. This means there will be no validation because there will not be any hand calculations in this case. 2. Geometry The geometry imported into the simulation will be simplified; therefore, only the main components will be inserted, Figure 69. This reduces the number of elements and computational time. Figure 69Simplified geometry of the connection. 3. Mesh This point is one of the most important since a weak discretization of the components will inevitably lead to poor results. Given the low computational power available, the mesh will have a variable size. Figure 70Mesh parameters used in the connection. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 49 The mesh quality was also verified and is very good, as most elements have an aspect ratio below 3, Figure 70. 4. Model Configuration Now, it is necessary to define the boundary conditions and material properties. The material properties are the same for all the simulations and parts because the device will be produced using the same material. Firstly, a bolt was inserted to tighten the assembly in terms of boundary conditions, Figure 71. Figure 71Bolt connector specs used in the connection. Secondly, the fixation shaft was fixed, and the PP connection part was considered a fixed hinge, only allowing rotation along the fixation shaft axis, Figure 72. Figure 72Boundary conditions used in the connection. Finally, the load was applied [60]. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 50 Figure 73Stress and displacement results of the rotation gear. In this case, no safety factor was used because there was no pre-analysis made or hand calculations, so the objective is to know the most realistic scenario. 5. Numerical Solution In terms of the desired solution, the most important thing is that the stress does not exceed the material's yield stress, and the displacements do not compromise the motion of the different parts of the device, Figure 74. Figure 74Numerical results required for the connection. 6. Numerical Results Regarding the results in terms of stress, the maximum value recorded was above the yield strength since there are some stress concentrations. In terms of displacement, the maximum value found was about 2,5 mm, which should be acceptable, Development of a medical device compatible with MRI/CT to measure ankle joint laxity 51 Figure 75Stress and displacement results for the connection. Figure 76Location of the stress concentration. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 52 7. Verification and/or Validation In this last step, some checks will be made to verify the simulation's validity. Firstly, the maximum stress value is found at an expected location in the contact of the teeth of the block parts teeth and the PP connection's teeth, Figure 76. Regarding the displacement, the displacement increases with the distance to the bolt axis, Figure 75. Finally, the fact that the stress surpasses the yield stress should not lead to failure because only some very localized zones enter the plastic regime. These zones have very sharp edges, and that increases the stress locally. Also, the fact that these stresses are compressive means they shouldn’t cause cracks. Nonetheless, a more straightforward system will be studied. 4.1.4. RACK SUPPORT 1. Pre-analysis Before starting the initial phase of the simulation, it is necessary to carry out a pre-analysis. This is intended to perform some calculations by hand in order to verify the simulation results. This part is quite complex because the cross-section area varies a lot, and the geometry would have to be significantly simplified to a point where the hand calculations would be irrelevant to this analysis. This means there will be no validation because there will not be any hand calculations in this case. 2. Geometry The geometry imported into the simulation will not be simplified since there is not much complexity in this geometry, Figure 77. Figure 77Simplified geometry of the rack support. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 53 3. Mesh This point is one of the most important since a weak discretization of the components will inevitably lead to poor results. Figure 78Mesh parameters used in the rack support. The quality of the mesh was also verified, and it is excellent. Almost every element has an aspect ratio below 3, Figure 78. 4. Model Configuration Now, it is necessary to define the boundary conditions and material properties. The material properties are the same for all the simulations and parts because the device will be produced using the same material. Regarding boundary conditions, firstly, the rack support was fixed in the connection with the piston shaft, Figure 79. Figure 79Boundary conditions used in the rack support. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 54 Secondly, the load was applied. Regarding this movement, there weren’t any typical load values, so the maximum load available was used and a safety factor of 1,5. 𝐹 = 200 ∗ 1,5 = 300 𝑁 Figure 80Load applied to the rack support. 5. Numerical Solution In terms of the desired solution, the most important thing is that the stress does not exceed the material's yield stress, and the displacements do not compromise the motion of the different parts of the device, Figure 81. Figure 81Numerical Results required for the connection. 6. Numerical Results Regarding the results in terms of stress, the maximum value recorded was below the yield strength, about 39 MPa, Figure 82. In terms of displacement, the maximum value found was about 1,6 mm, Figure 83, but in terms of directions, the maximum was 1,5 mm in the Z direction. In the Y direction was about 0,9 mm, which should be acceptable, Figure 84. Also, the fact that the load is much higher than the real one should be considered. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 55 Figure 82Stress results for the rack support. Figure 83Displacement results for the rack support. Figure 84Displacement in Y and Z directions results for the rack support. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 62 4. Numerical Solution In terms of the desired solution, the most important thing is that the stress does not exceed the material's yield stress, and the displacements do not compromise the motion of the different parts of the device, Figure 94. Figure 94Numerical Results required for the assembly of the eversion movement simulation. 5. Numerical Results Regarding the results in terms of stress, the maximum value recorded was well below the yield strength, about 3 MPa, Figure 95. In terms of displacement, the maximum value found was about 0,3 mm, which is an excellent result, Figure 96. Figure 95Results in terms of stress for the assembly of the eversion movement simulation. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 63 Figure 96Results in terms of displacement for the assembly of the eversion movement simulation. 6. Verification and/ or Validation This step is complicated in assemblies with multiple components because the location of the maximum stress or the maximum displacement is very difficult to predict. Nonetheless, as expected, this movement is not the hardest to do from the mechanical point of view, as in version 1.0, this movement was performed relatively easily. 4.1.6. ABDUCTION MOVEMENT 1. Geometry In terms of geometry, some parts irrelevant to the analysis were suppressed, and only the foot support, the angle adjustment and the rotation shaft were considered, Figure 97. Also, the shaft faces were cut in order to impose the boundary conditions where the bearings will be installed. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 64 Figure 97Assembly used for the abduction movement simulation. 2. Mesh In terms of mesh, a variable mesh size was used with a minimum element size of 0.2 mm and a maximum element size of 5 mm. The mesh quality was verified as very good, as about 97% of the elements have an aspect ratio below three, and there are no distorted elements, Figure 98. Figure 98Mesh parameters used in the assembly of the abduction movement simulation. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 65 3. Model Configuration Now the boundary conditions, contact parameters and connectors need to be defined. Firstly, it was added two pin connectors, Figure 99. Figure 99Pin connector parameters used for the abduction movement simulation. Next, the contact parameters were imposed. In this stage, the global interaction between the components was defined with contact, Figure 100. No friction coefficient was considered in order to simplify the simulation. Figure 100Contact parameters used for the abduction movement simulation Development of a medical device compatible with MRI/CT to measure ankle joint laxity 66 Finally, the boundary conditions were defined. The foot support was fixed on one side, and torque was applied to one end of the rotation shaft. The value of this torque was 6000 Nmm, as in the rotation gears movement simulation, Figure 101. Also, the location of the bearings was considered a fixed hinge, Figure 102. Figure 101Load applied to the assembly of the eversion movement simulation. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 67 Figure 102Boundary conditions applied to the assembly of the abduction movement simulation. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 68 4. Numerical Solution In terms of the desired solution, the most important thing is that the stress does not exceed the material's yield stress, and the displacements do not compromise the motion of the different parts of the device, Figure 103. Figure 103Numerical Results required for the assembly of the abduction movement simulation. 5. Numerical Results Regarding the results in terms of stress, the maximum value recorded was below the yield strength, about 42 MPa, Figure 104. The maximum value found in displacement was about 1.2 mm, which is a solid result, Figure 105. Figure 104Results in terms of stress for the assembly of the abduction movement simulation. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 69 Figure 105Results in terms of displacement for the assembly of the abduction movement simulation. 6. Verification and/ or Validation This step is quite tricky in assemblies with multiple components because the location of the maximum stress or the maximum displacement is very difficult to predict. Nonetheless, as expected, the maximum stress is located in the contact of the shaft and the adjustment angle part, as in this zone, there are sharp edges which lead to some stress concentrations. 4.1.7. ANTERIOR TRANSLATION MOVEMENT This simulation is quite tricky due to the complex parts in the connection between the piston and the fixation shaft, as seen in the Connection Validation phase. This simulation doesn’t pretend to validate this connection but simplifies it and, at the same time, gets rid of the stress concentrations and complex parts while still improving the stiffness of the assembly. 1. Geometry In terms of geometry, firstly, it explored one where the connection is made through a hexagonal pin, Figure 106. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 70 Figure 106First geometry used for the anterior translation movement simulation (left). Exploded view (right). 2. Mesh In terms of mesh, a variable mesh size was used with a minimum element size of 0.5 mm and a maximum element size of 3 mm. The mesh quality was verified as very good, as about 99% of the elements have an aspect ratio below three, and there are no distorted elements, Figure 107. Figure 107Mesh parameters used in the first geometry iteration of the assembly of the anterior translation movement simulation. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 71 3. Model Configuration Now the boundary conditions, contact parameters and connectors need to be defined. Firstly, two pin connectors were added, Figure 108. Figure 108Pin connector parameters used in the first geometry iteration of the assembly of the anterior translation movement simulation. Next, the contact parameters were imposed. In this stage, three different contacts were defined. The first was to describe the contact between some components, Figure 109. Figure 109First contact parameters used in the first geometry iteration of the assembly of the anterior translation movement simulation. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 78 This is an acceptable result. However, it should be noted that considering a linear behaviour (which is a very reasonable assumption), the expected displacement for the real load (200 N) is: 𝑅𝑒𝑎𝑙 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 = 4,5 × 200 300 = 3 𝑚𝑚 This is a very respectful result as the load is very high and the materials used are not properly loadbearing. Also, keeping the linear behaviour consideration if the material with the highest Young Modulus is used, then the displacement becomes: 𝑅𝑒𝑎𝑙 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 = 3 × 1800 5696 ≈ 1 𝑚𝑚 Keeping this in mind, it is advised to use the material with the highest mechanical properties because even this material doesn’t present properties of a load-bearing material; however, it is the best option available. 4.1.8. TIBIA SUPPORT 1. Pre-analysis Before starting the initial phase of the simulation, it is necessary to carry out a pre-analysis. This is intended to perform some calculations by hand in order to verify the simulation results. Since the geometry is very complicated, there will not be any pre-analysis as the results would be far from reality due to the simplifications used. 2. Geometry The geometry imported into the simulation will be simplified; therefore, only the sprocket and rack will be studied, Figure 122. Figure 122Simplified geometry of the tibia support. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 79 3. Mesh This point is one of the most important since a weak discretization of the set will inevitably lead to poor and far from real results. A mesh size of 3 mm was used, Figure 123. The mesh quality is very satisfactory as 99,6% of all the elements have an aspect ratio below 3. Figure 123Mesh parameters used in the tibia support. 4. Model Configuration Now, it is necessary to define the boundary conditions and material properties. The material properties are the same for all the simulations and parts because the device will be produced using the same material. In terms of boundary conditions, see Figure 124. Figure 124Boundary conditions and force applied to the tibia support. The base was considered fixed, while in the zone where the tibia seats, a force of 200 N was applied. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 80 5. Numerical Solution In terms of the desired solution, the most important thing is that the stress does not exceed the material's yield stress, and the displacements do not compromise the motion of the different parts of the device, Figure 125. Figure 125Numerical results required for the tibia support. 6. Numerical Results Regarding the results in terms of stress, the maximum value recorded was about 18 MPa, while in terms of displacement, the maximum value found was about 10 mm, Figure 126. Figure 126Stress (above) and displacement (behind) results of the tibia support. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 81 7. Verification and/or Validation In this last step, some checks will be made to verify the simulation's validity. Firstly, the maximum stress value is found at the expected location in the reinforcement backbone, Figure 127. Figure 127Maximum stress location for the tibia support. Regarding the displacement, it seems that 10 mm is a lot; however, this is the maximum value. If we look at the zone where the tibia is supported, it is noticeable that the displacement results are satisfying, Figure 128. Figure 128Displacement detail for the tibia support. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 82 4.2. KINEMATIC VALIDATION Having the device validated from the mechanical point of view will now be validated from the cinematic point of view so that every movement can be realized without any interference. In this chapter, the Motion Study feature of Solidworks was used. Eversion Movement Firstly, the eversion/ inversion movement was performed. This was accomplished as expected for an angle of 60°, Figure 129. Figure 129Motion simulation of the eversion/ inversion movement. Abduction Movement Secondly, the abduction/ adduction movement was simulated. Once again, and as expected, the motion was completed without any hiccups due to the same analysis performed on the concept, Figure 130. Figure 130Motion simulation of the abduction/ adduction movement. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 83 Anterior Translation Movement Finally, the anterior translation movement was simulated. As expected, this movement was performed correctly. This was expected as this was the movement better performed in the original device, Figure 131. Figure 131Motion simulation of the anterior translation movement. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 84 5. PNEUMATIC SYSTEM Once the device is validated, the actuating system needs to be designed. Firstly, this system is a pneumatic one, and the components must operate in the MRI environment. Therefore, they cannot have metals or any magnetic materials in their composition. This means that some of the system's main components may need to be designed. First, it should be understood what the main goals of this system are: ➢ No need to connect/ disconnect the actuators from the supply each time the health technician changes the movement. ➢ Reduce the exam duration and simplify the overall process. To accomplish these goals, the idealized system must have a pneumatic reservoir with the necessary volume to realize the exam. This reservoir is filled outside the MRI room with a compressor and should then be connected to the PATD. There are three cylinders to actuate: the eversion, the abduction and the anterior translation cylinder. The first two are bi-directional, meaning they must always be actuated. The final one only needs to actuate in one motion. The retreat doesn’t need to be actuated. Also, the cylinders' movement should be smooth, so a flow controller is necessary, Figure 132. Figure 132Scheme of the pneumatic system needed. Using the FluidSim software, the following system was designed: two 4-way valves and one 3-way valve. They control the eversion and abduction cylinders, respectively, while the last valve controls the translation cylinder. Also, instead of one flow control and three faucets, it was used three flow controls, Development of a medical device compatible with MRI/CT to measure ankle joint laxity 85 Figure 133. This way allows for a reduction in the number of components and also the space needed to allocate the pneumatic system. Figure 133Pneumatic system designed in FluidSim. Because the cylinders are already designed, the valves are the first components to design in this section. 5.1. VALVE DESIGN A valve is a simple component that consists of two bodies, the main valve (red) and the spool (black), that moves and allows the user to select which hole the air is going to, Figure 134. Figure 1344-way valve, section view. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 86 The spool has six slots for O-rings to prevent the pressurized air from escaping at the contact between the spool and the main body. In position 1, the pressurized air enters hole number 1 and is directed to hole number 2 while the air in the cylinder gets out to the environment through holes 4 to 3. In position 2, the pressurized air enters hole number 1 and is directed to hole number 4 while the air in the cylinder gets out to the environment through holes 2 to 3. The final design can be seen in Figure 135. Figure 1354-way valve final design. Next, the 3-way valve was designed. This valve is simpler as it only has 3 ways and, for that reason, 3 holes, and the spool is much shorter than the previous one, Figure 136. Figure 1363-way valve, section view. The spool has 2 slots for O-rings to prevent the pressurized air from escaping at the contact between the spool and the main body. In position 1, the pressurized air enters hole number 1 and is Development of a medical device compatible with MRI/CT to measure ankle joint laxity 87 directed to hole 2. In position 2, the pressurized air that remains in the cylinder gets out to the environment through holes 2 to 3. The final design can be seen in Figure 137. Figure 1373-way valve final design. 5.2. FLOW REGULATOR DESIGN A flow regulator is a device that allows controlling the volume of air that flows out of this device. As a first design, the simplest flow regulator was designed. This one consists of two bodies (green and yellow) that form a channel where air flows. Then there is a needle (red) that is adjusted just like a screw and allows it to close and gradually open the air-out port, Figure 138. Figure 138Flow regulator, section view. Next, an exploded view with the component designation is shown in Figure 139 and Table 8. Development of a medical device compatible with MRI/CT to measure ankle joint laxity 94 𝐹 𝑖𝑠 𝑡ℎ𝑒 𝑤𝑖𝑑𝑡ℎ 𝑚𝑡 𝑖𝑠 𝑡ℎ𝑒 𝑚𝑒𝑡𝑟𝑖𝑐 𝑚𝑜𝑑𝑢𝑙𝑒 𝑌 𝑖𝑠 𝑡ℎ𝑒 𝑓𝑜𝑟𝑚 𝑓𝑎𝑐𝑡𝑜𝑟 𝑣 ≈ 70 1=70 𝑚𝑚/𝑠 = 0,007 𝑚/𝑠 𝑘𝑣=3,56 +√0,07 3,56 ≈ 1,074 𝑊𝑡= 𝐹𝑡=181,82 𝑌 = 0,485 47 ≤1,074 ×200 × 1,5 10 ×10−3 × 𝑚𝑡× 0,485 𝑚𝑡≥ 1,41 𝑚𝑚~1,5 𝑚𝑚 Development of a medical device compatible with MRI/CT to measure ankle joint laxity 95 APPENDIX BMATERIALS SHEET Development of a medical device compatible with MRI/CT to measure ankle joint laxity 96 Development of a medical device compatible with MRI/CT to measure ankle joint laxity 97 Development of a medical device compatible with MRI/CT to measure ankle joint laxity 98 Development of a medical device compatible with MRI/CT to measure ankle joint laxity 99 Development of a medical device compatible with MRI/CT to measure ankle joint laxity 100 Development of a medical device compatible with MRI/CT to measure ankle joint laxity 101 Development of a medical device compatible with MRI/CT to measure ankle joint laxity 102 Development of a medical device compatible with MRI/CT to measure ankle joint laxity 103 BIBLIOGRAPHY [1] S. 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