Influence of topographical features on the fluoride corrosion of Ni-Ti orthodontic archwires

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ Esta es la versión aceptada del artículo publicado en: This is a accepted manuscript of a paper published in: Journal of materials science-materials in medicine (2011 DOI: https://doi.org/10.1007/s10856-011-4460-y Copyright: © Springer Nature El acceso a la versión publicada del artículo puede requerir la suscripción de la revista. Access to the published version may require subscription. This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect postacceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s10856-011-4460-y 1 Influence of Surface Pattern on the Fluoride Corrosion of Ni-Ti Orthodontic Archwires Camilo Abalos,a Antonio Paul,b Asunción Mendoza,c Enrique Solano,d and Francisco Javier Gil.e aDepartment of Dental Materials, cDepartment of Pediatric Dentistry and dDepartment of Orthodontics, School of Dentistry, University of Sevilla, Spain. bDepartment of Mechanical and Materials Engineering, School of Engineering, Seville, Spain. eDepartment of Materials Science and Metallurgical Engineering, Polytechnic University of Catalonia, Barcelona, Spain. Abalos,C. Mendoza, A. and Solano, E.: Facultad de Odontología, c/ Avicena s/n, 41009-Sevilla, Spain. Telephone number: +34954481137. Fax number: 0034954481157. E-mail address: [email protected], e[email protected], [email protected] Paul, A.: Escuela Técnica Superior de Ingenieros, Avda. de los Descubrimientos s/n, 42092 Sevilla, Spain. Telephone number: +34954487304. Fax number: +34954460475. E-mail address: [email protected] Gil, FJ: Departamento de Ciencia de Materiales e Ingeniería Metalúrgica, Universidad Politécnica de Cataluña, Diagonal 647, 08028 Barcelona, Spain. Telephone number: +34934016111. Fax number: +34934016210. E-mail address: [email protected]. 2 Abstract Introduction: Different manufacturing processes of Ni-Ti archwires respond differently to corrosion. The influence of surface condition is important. In this study we establish several surface patterns and study their influence in fluoride corrosion. Methods: In a preliminary study we determined four surface patterns (smooth, scratch, dimple and crack) according to the main surface defect. We selected archwires with these patterns (n=40). Immersion tests were performed in artificial saliva+fluorated prophylactic gel (12500 ppm) for 28 days. The surface was characterised by SEM and Laser Confocal Microscopy. Anova test was carried out (α≤0.05). Results: There is an increase in the surface defects and/or roughness of the crack and scratch patterns. Conclusions: There is a relationship between the surface pattern and the extent of fluoride corrosion. That factor should be taken into account in future studies. The increase in defects is independent to roughness. Manufacturing processes that produces surface cracks should be avoided. Key words: NiTi, Orthodontic archwire, Surface pattern, Surface topography, Roughness, Fluoride, Corrosion resistance. Abbreviations: NiTi: Nickel-Titanium alloys; SEM: Scanning Electron Microscope; EDS: Energy Dispersive Spectrometry; LSCM: Laser Scanning Confocal Microscope System; PP: polypropylene; Ra: the arithmetical mean of the absolute values of the scanned surface profile; Rms: the root-mean-square value of the scanned surface profile; Rp: the higher value of the scanned profile; Rv: the lower value of the scanned profile. 1 Introduction Nickel-Titanium alloys (NiTi) near equiatomic composition have wide clinical application in orthodontics due to their excellent mechanical properties [1,2]. They have good biocompatibility in artificial saliva, Ringer’s solution and physiological salt solution [3]. Nevertheless, the corrosion resistance of NiTi alloys decreases in acidic saliva [4-6] and chlorine [7] or fluoride solutions [8-12], which is frequently used in orthodontic treatments for the prevention of dental caries. Fluoride ions can deteriorate the passive film of the titanium oxide surface [9,13-15] by hydrogen absorption [15], decreasing the corrosion resistance of the alloy and hence its biocompatibility [3,5,6]. Factors such as exposure time [15,16], fluoride concentration [8,9,17] and a more acidic pH [15,17] facilitate the surface corrosion of Ni-Ti alloys since these factors are interrelated [16]. A pH dependent critical concentration has even been defined [8,18,19] which can degrade the passive film of superficial oxides. When this happens, the effect of 3 the corrosion is a release of metallic ions [5,8,14,19,20], morphological surface changes [8,15,16] and increases in the roughness [5,8] and friction coefficients [21] among others. Besides the aforementioned factor related to fluorine ions, the wire surface plays an important role in corrosion, although the results published are controversial. Previous studies [5,20,22,23] point to surface defects produced during wire manufacturing as preferential corrosion areas while in other studies they do not [4,9]. Other authors [13] consider the roughness of the archwire to be an indicator of the tendency toward corrosion, although other research [4,5,8,9] did not find such a relationship when including residual stresses and the degree of homogeneity as corrosion accelerators [4,9]. This is to say that different archwire surfaces have a better or worse response to corrosive fluoride. Thus, the archwire manufacturer is a variable itself [4,8], although more research is required [8,9]. The wire surface, surface defects and roughness are related to the manufacturing processes [8] that lead to different surface finishes [24]. The objective of this study is to examine the influence of the surface (surface defects and roughness), as a result of the manufacturing process, in the corrosion of NiTi archwires exposed to a high fluoride concentration but in a way never done before. In previous studies [4,7-9,16,17,19,20] there are 1 to 4 archwires from different manufacturers and the surface finish is related to a specific commercial brand. In our study, we analyze 16 brands from different manufacturers and establish a surface pattern for each wire depending on the predominant defect (crack, scratch, pore, dimple or smooth). We group the wires from different manufacturers according to the surface pattern which corresponds to the different manufacturing processes. Then, we study the influence of each pattern on the fluoride corrosion by means of a surface study (Scanning Electron Microscopy) and roughness (Laser Confocal microscopy) characterization. The hypothesis is that different surface patterns, derived from different manufacturing processes, respond differently to fluoride corrosion. 4 2 Materials And Methods 2.1 Preliminary Study We studied the surfaces of 16 NiTi archwires (0.016x0.022 inch.) from 9 commercial brands in the as-received condition [25] (Table 1). The surface morphology and composition was evaluated by Scanning Electron Microscope (SEM) (Philips XL-30 Philips, Eindhoven, Netherlands) and Energy Dispersive Spectrometry (EDS) (Edax International, Mahwah, U.S.A.); roughness was measured using a Laser Scanning Confocal Microscope System (LSCM) (Leica TCS-SP2, Wetzlar, Germany). We studied three samples from each batch of archwires, each sample coming from a different archwire within the batch. The archwires were classified according to their surface patterns using 18 variables. The Pearson correlation test was performed to evaluate the existence of linear correlations between the variables and the surface patterns. We found that five of these correlated with the pattern: dimples, scratches, cracks, pores and smooth/no defect. As a result we classified the NiTi archwires according to these five patterns (Table 1) depending on the predominant surface defects. The different surface defects are determined in the following way: Crack is a long, deep fissure with irregular edges and variable width (Fig. 1C); Scratch is a long fissure with smooth edges and constant width, in which the bottom is clearly visible (Fig. 1D); Pore is a deep, usually rounded defect; Micropore is a pore with diameter smaller than one micrometer. Dimple is a small hole, an elliptical or rounded depression on the surface similar to the dimples in a golf ball (Fig. 1B). 2.2 Material and Immersion Test Eight NiTi wires with near equiatomic compositions made by different manufacturers were used in this study. Each surface pattern (dimple, scratch, crack, pore and smooth/no defect) corresponds to two different commercial brands (Table 1). The porous surface pattern was eliminated from this study because there was only one manufacturer. For the tests, we used 5 samples of different archwires from the same batch (n=40). The samples were 2 cm segments 5 extracted from the straight part of the arch. Previous to immersion test each specimen was cleaned and degreased for 5 minutes in acetone. The samples were put in a polypropylene (PP) tube (15 ml) with 2 ml artificial saliva for 28 days at 37ºC. Modified Fusayama artificial saliva composition: NaCl (0.4 g/l), KCl (0.4 g/l), CaCl2 .2H2O (0.906 g/l), NaH2PO4 .H2O (0.690 g/l), Na2S.9H2O (0.005 g/l), KSCN (0.3 g/l) and urea (1 g/l) was used. [5,6,9,10] The pH was measured by means of a pH-meter (micropH 2001, Crison Instrument S.A., Barcelona, Spain) giving a value of 5.3. Then they were placed in a 15 ml PP tube with 2 ml artificial saliva + fluoride solution with a pH of 4. Immersion time was 8 minutes, which is equivalent to 4 weeks of tooth brushing for 2 minutes per week. The fluoride solution was prepared by ultrasonic agitation in the PP tube of 5 ml artificial saliva and 5g fluorated prophylactic gel (12500 ppm) (Elmex gel, GABA GmbH, Lörrach, Germany); the mixture was centrifuged for 10 minutes at 2000 rpm. The upper clear liquid without the deposits in the PP tube was used as the immersion test solution and kept at 37ºC. [8] Right after the immersion test the samples were rinsed for 5 minutes in distilled water and 5 minutes in methanol in a stirrer maintained at the testing temperature of 37ºC. 2.3 Surface Characterisation The surface topography of the NiTi archwires was examined before and after the immersion tests. SEM micrographs at low (400x) and high (1600x) magnification were taken of representative degraded areas of the surface. We used the 400x images to measure the surface occupied by each defect (dimples, scratches, cracks and pores) in three different areas of 10000 μm2. The three measurements were then averaged. When small micropores appeared on the surface we used the 1600x images with an area of 400μm2. So, we have 5 samples per brand of wire (n=40) and 3 areas per specimen. The surface roughness of the NiTi archwires was analysed before and after the immersion tests by means of LSCM. The microscope was previously calibrated using a Mitutoyo precision Reference Specimen (Code no. 178-601) with Ra=3.1μm, error coefficient was below 0.3%. The images were processed using the Leica Confocal software ( Leica Microsystems GMBH, Wetzlar, Germany). Micrographs corresponding to 750 linear microns were taken. We randomly 6 selected 5 areas of 10000 μm2 from each topographic image and measured the roughness. Ra, Rms, Rp and Rv values obtained in each area were averaged. Ra and Rms represent the arithmetical mean of the absolute values and the rootmean-square value of the scanned surface profile, respectively. Rp and Rv represent the higher and lower values of the scanned profile. So, for roughness studies we analysed 5 areas from 5 samples of each archwire (n=40). 2.4 Statistical Analysis Mean values and standard deviation of each of the 8 brands and the 4 surface patterns were computed from the data obtained during the surface characterisation. This was done for the as-received archwires as well as after the immersion test. Since the archwires used had different initial values, in order to ensure a valid comparison of statistical results it was deemed appropriate to use the difference between the as-received and post-immersion test values obtained for each wire tested. So, the final data refer to the increases or decreases in the area occupied by the surface defects or surface roughness. Positive values mean surface degradation while zero or negative values indicate the absence of corrosion. Statistical analysis was carried out with an analysis of variance for comparison of mean values, the statistical significance being set at α ≤ 0.05. The data were analysed using SPSS 14.0 statistical software package (SPSS Inc, Chicago, Ill., USA). 3 Results Table 2 shows the results of the surface defect in the as-received archwires. The quantification was done with the surface values of each defect and the total surface defects for each archwire. The incremental value also referred to each surface pattern included for the archwires after the immersion tests in saliva+fluoride. There are significant differences in the pattern with cracks. Figure 1 shows representative SEM images of the changes in the surface of the four patterns after the corrosion tests. In the smooth pattern, we observe pitting 7 corrosion and dark circular spots (Fig. 1A). In the scratch pattern there are also some circular spots (Fig. 1D). In the dimple pattern there is no pitting or discoloration but we see a smoothing of the surface which reduces the number of dimples (Fig. 1B). Finally, in the crack pattern we observe a 6% increase in the surface occupied by the defects mainly due to the increased surface of the cracks (Fig. 1C). The surface roughness data (LSCM) of the archwires in the as-received and post-immersion conditions are expressed in Table 3. There are significant differences for the cracked and scratched patterns where there is a 33% increase in the surface roughness: In the crack pattern Ra changes from 0.3 μm in the asreceived archwires to 0.4 μm in the post immersion samples. In the scratched pattern Ra changes from 0.35 to 0.48μm. Figure 2 shows representative LSCM images of the four patterns studied. 4 Discussion The production process of orthodontic archwires is not disclosed by manufacturers. Nevertheless, it leaves marks on the surface of the wires. If we observe the surface of archwires from different manufacturers with an electron microscope, we will see different surface defects (cracks, dimples, scratches oriented in the axis direction or not, pores, etc.) [4,5,8,9,20,25]. We will also find discolorations, deposits, inclusions and various degrees of roughness [4,9,25]. This is due to the different treatments used throughout the production process (thermomechanical treatments, wiredrawing, polishing, etc.) [4,5,8,20]. These findings show very different surface finishes on the archwires which may not always be of good quality [20,24,25]. In our preliminary studies [25], we grouped the wires according to their predominant surface defect, corresponding different archwires to the same surface pattern and therefore to a similar finishing process. Consequently, we were able to study the influence of these surface patterns. There is a general agreement in the bibliography that the archwire manufacturer has a statistically significant influence on the corrosion resistance [4,5,9,20]. However, there is some 8 controversy to considering the surface defects [4,5,9,20,22,23] and/or the roughness [4,5,8,9] as the causes of the corrosion. These studies compare archwires with different surface finishes, but the authors do not specify their predominant surface pattern. We found defects that clearly influence the corrosion in fluoride, as is the case of cracks. However, other surface patterns did not show any corrosion as is the case of smooth and dimple patterns. The preexisting surface pattern could provide answers about the variability of the corrosion resistance of different NiTi archwires with near equiatomic composition. We think that the surface pattern should be taken into account when comparing different NiTi orthodontic wires. Nevertheless, we agree with other authors [8,9] that more studies are necessary since other factors such as residual stresses [4,9], the amount of residues [20,24,24) and the degree of homogeneity of microstructures[20], etc. can also be involved. In our study, we used an amine fluoride gel solution with a concentration of 1.25 % (12500 ppm) and pH=4 as the immersion media. Corrosion in HF is produced with a critical concentration of 30 ppm that will destroy the TiO2-based passive film on NiTi archwires [26]. This concentration is reached with ≥500 ppm fluoride and pH=4[26] or with 2250 ppm fluoride in neutral solution [9]. Thus, the solution used in our research is justified by its high concentration and acid pH, ensuring its corrosive effect. Different kinds of fluoride solutions (stannous fluoride, sodium fluoride, etc) have different degrees of corrosiveness [18,19], therefore the influence of the fluoride solution on the surface pattern of orthodontic archwires could be the subject of future research. After immersion in fluoride solution, we found that there was an increase in the surface defects in the archwires with crack as surface pattern, unlike those with smooth, scratch or dimple surface patterns. The increased number of defects is caused by an increment in the number and size of the cracks. In previous studies [5,20,22,23] preexisting defects were identified as corrosion sites. In the same way, studies on archwires with polished surfaces find less corrosion [23,27]. Corrosion is due to higher residual stress and/or non-uniform passive film on the surface defects [5]. Repassivation can also be different due to different surface conditions [28]. These preexisting surface defects are related to the production process [5]. Furthermore, other researchers have not found any relationship between the surface defects and the degree of corrosion [4,9]. So, the difference 15 Ramatitan Lite Dentaurum 60648 54.5 / 45.5 Scratch G4 G&H 142669 53.3 / 46.7 Scratch M5 G&H 126755 53.3 / 46.6 Scratch Thermomemoria Leone 08022001 53.9 / 46.1 Scratch Thermaloy RMO A07304 53.4 / 46.6 Scratch Reflex Heat-Activated TPO 1297060 53.2 / 46.7 Scratch Neo Sentalloy GAC H326 53.1 / 46.8 Porous (*) %Ni / %Ti (wt %) (†) Archwires used in the experimental study (‡) %Ni / %Ti / %Cu (wt %) 16 Table 2 Surface area of defects and variation (Δ Def, obtained by subtracting average Total defects) before and after immersion test. Crack pattern shows statistically significant variation. Ni-Ti Archwires MANUFACTURING DEFECTS (In 10.000 μm2) PATTERN Δ Def-SALIVA+F Cracks Pores μPores Scratches* Dimples Total Mean SD Reflex SE 0 0 0.8 2.9 0 3.7 Smooth 39.8 24.6 Nitinol Classic 0 174 1.9 0 25 200.1 Orthonol 0 228 1.5 0 6215 6444.5 Dimple -144 48.0 Align XF 73.7 258 3,3 0 1302 1637 Memoria 577.5 5.1 1.7 12.8 176 773.1 Crack 631.3† 68.0 Titanol SE 264.5 35.6 1.3 0 0 301.4 Tensic 70 0 0 6.8 0.28 77.1 Scratch 51.7 31.6 Nitinol HA 57.9 58.8 7.7 13 138 275.4 (*) μm per 100 μm in linear scratches. (†) Mean significant difference (p < 0.05) 17 Table 3 Surface roughness data and variation before and after immersion test. Scratch and crack patterns show statistically significant variations. Ni-Ti Archwires SURFACE ROUGHNESS AS-RECIEVED (10.000 μm2) SALIVA+FLUORIDE Ra Rms Rv Rp PATTERN Ra Rms ΔRa ΔRms Reflex SE 0.22 0.31 2.31 1.20 Smooth 0.32 (0.13)* 0.45 (0.18) 0.03 (0.01) 0.03 (0.01) Nitinol Classic 0.46 0.61 3.46 2.76 Orthonol 1.32 1.71 8.45 4.53 Dimple 1.01 (0.35) 1.35 (0.47) -0.06 (0.02) -0.08 (0.02) Align XF 0.70 0.95 7.60 4.09 Memoria 0.44 0.60 4.30 2.23 Crack 0,35 (0.13) 0.48 (0.17) 0.13† (0.04) 0.19† (0.06) Titanol SE 0.25 0.37 3.33 1.18 Tensic 0.34 0.47 3.87 1.87 Scratch 0.30 (0.09) 0.40 (0.13) 0.10† (0.03) 0.14† (0.04) Nitinol HA 0.26 0.36 3.14 1.81 * Standard deviations are given in parentheses. † Mean significant difference (p < 0.05)