The degradation of metallic materials placed in

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1 ORIGINAL ARTICLE Biodegradation of orthodontic metallic brackets and associated implications for friction Saulo Regis Jr, a Paulo Soares, b Elisa S. Camargo, c Odilon Guariza Filho, c Orlando Tanaka, d and Hiroshi Maruo c Curitiba, Parana, Brazil Introduction: This study aimed to assess the effect of clinical exposure on the surface morphology, dimensions, and frictional behavior of metallic orthodontic brackets. Methods: Ninety-five brackets, of 3 commercial brands, were retrieved from patients who had finished orthodontic treatment. As-received brackets, matched by type and brand, were used for comparisons. Surface morphology and precipitated material were analyzed by optical and scanning electron microscopy and x-ray microanalysis. Bracket dimensions were measured with a measuring microscope. Resistance to sliding on a stainless steel wire was assessed. Results: Retrieved brackets showed surface alterations from corrosion, wear, and plastic deformation, especially in the external slot edges. Film deposition over the alloy surface was observed to a variable extent. The main elements in the film were carbon, oxygen, calcium, and phosphorus. The as-received brackets showed differences (P\0.05) in the slot sizes among brands, and 1 brand showed a 3% increase in the retrieved brackets slots. The frictional behavior differed among brands. Retrieved brackets of 2 brands showed 10% to 20% increases in resistance to sliding. Conclusions: Metallic brackets undergo significant degradation during orthodontic treatment, possibly with increased friction. At present, it is difficult to predict the impact of these changes on the clinical performance of orthodontic components. (Am J Orthod Dentofacial Orthop 2011;140:501-9) The degradation of metallic materials placed in patients has long been a concern of biomaterials science. Laboratory experiments that simulate in-vivo degradation of metal implants have made it possible to estimate the effect of specific parameters but lack the consistency required to represent the complexity of the oral environment. 1-5 Variations in temperature and ph caused by diet, decomposition of foods and cell debris, and oral florae and their by-products are important factors to consider when evaluating the clinical behavior of dental materials and comparing them with other biomaterials. In particular, orthodontic accessories are under masticatory forces and multi-axial loads from the activation of the wire in the bracket slot. 1 From the Pontifical Catholic University of Parana, Curitiba, Parana, Brazil. a Postgraduate student, Graduate Dentistry Program in Orthodontics. b Associate professor, Mechanical Engineering Department. c Associate professor, Graduate Dentistry Program in Orthodontics. d Professor, Graduate Dentistry Program in Orthodontics. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Hiroshi Maruo, Graduate Dentistry Program in Orthodontics, Pontifical Catholic University of Parana, R. Imaculada Conceiç~ao, 1155, Curitiba, Parana, , Brazil; , h.maruo@gmail.com. Submitted, May 2010; revised and accepted, January /$36.00 Copyright Ó 2011 by the American Association of Orthodontists. doi: /j.ajodo In-vivo aged orthodontic components show signs of degradation such as morphologic changes and surface alterations from corrosion, wear, and formation of integuments. 4,6-12 Concerns regarding the clinical impact of these alterations include (1) elements released into the oral environment and their implications on biocompatibility, and (2) impairment of the performance of the orthodontic appliance. 13 Orthodontic metallic materials are usually composed of alloys including several base metals such as nickel, chromium, cobalt, iron, molybdenum, and titanium. Concerning biocompatibility, nickel stands out among the other elements. Carcinogenic, 14 mutagenic, 15 and cytotoxic properties 16,17 have been attributed to it, although no local or systemic clinical condition is clearly related to the forms of nickel used in dentistry. 13 The lone exception is hypersensitivity reactions. 18 Nickel might also elicit periodontal reactions such as gingival hyperplasia in orthodontic patients. This condition is hardly distinguishable from microbial-induced gingival overgrowth. 19 Surface alterations in orthodontic devices might compromise the appliance s esthetics, 20 increase microbial adhesion, 11,21 modify bracket-wire activations such as torque expression, 22 cause fractures during clinical use, 23 and influence the magnitude of friction between the bracket and the wire. 1,20,24 Many situations in an orthodontic 501

2 502 Regis et al practice require sliding teeth through a properly contoured archwire, and friction might cause significant loss of the applied force. 10,24 Many investigations have focused on the frictional behavior of as-received orthodontic materials Nevertheless, the alterations that orthodontic brackets undergo during treatment and their impact on clinical performance, especially friction, are mostly unknown. Therefore, we aimed to evaluate the surface morphology, dimensional stability, and frictional behavior of orthodontic metallic brackets retrieved after full orthodontic treatment, compared with as-received brackets from the manufacturers. MATERIAL AND METHODS The sample consisted of brackets with a slot size of mm ( in), as described by the manufacturer, retrieved from patients who had finished orthodontic treatment in the private practice of 2 experienced orthodontists (E.S.C., O.T.). The brackets were carefully debonded with a direct bond bracket remover pliers and kept in receptacles with distilled water. They were brushed with an electric brush for 10 seconds and rinsed with distilled water to remove any loosely attached integuments. They were then kept in self-sealed sterilizing packs until analysis. The following information was registered: patient identification, date of placement and date of removal of the appliance. Brackets with obvious distortions or calcifications that hindered the engagement of a mm ( in) cross-sectional wire were discarded. A total of 95 brackets of different types (for premolars, canines, and incisors from both arches) were selected. The sample comprised 3 brands: 32 Mini Standard Edgewise stainless steel brackets (American Orthodontics, Sheboygan, Wis), 34 Kirium Edgewise stainless steel brackets (3M Abzil, Sumare, Brazil), and 29 NuEdge preadjusted Roth prescription brackets (TP Orthodontics, LaPorte, Ind) made of copper-chromium alloy. The brackets were retrieved from 7 patients (mean age, 18 years 9 months), with a mean treatment time of 41 months. Stainless steel and nickel-titanium wires used in the orthodontic treatments were ligated with elastomeric and metallic ligatures. A group of brackets as-received from the manufacturers, matched by types and brands to the sample, was used for comparisons. They were submitted to the same procedures as the retrieved specimens. An optical reflected light microscope (BX60; Olympus Optical, Tokyo, Japan) was used to evaluate the surface morphology of the as-received and the retrieved brackets for areas of corrosion, signs of wear, plastic deformations, and adherent materials. The images were acquired in a bright field at various magnifications ( times). Fifteen retrieved brackets, equally distributed among brands, and their as-received counterparts were selected based on reflected light images for analysis in a scanning electron microscope (JSM-6360LV; Jeol, Tokyo, Japan) and an energy dispersive x-ray spectrometer. Secondary electron images and backscattered electron images were acquired at various magnifications ( times) by using a 20-kV accelerating voltage. The images allowed for assessment of the micromorphologic characteristics of the slot surfaces. Areas of interest were submitted to microanalysis for element assessment. Spectra were acquired with the same accelerating voltage at different magnifications and a 100-second acquisition time. The slot sizes (left and right, vertical dimension) and the internal widths between the tie-wings (cervical and occlusal, horizontal dimension) of the remaining 80 retrieved brackets and their as-received counterparts were measured in a measuring microscope (MM-40; Nikon, Yokohama, Japan). Measurements were made by 1 operator using a holder with a wire of a crosssection of mm ( in). The wire was used to position the brackets slots perpendicular to the microscope table. To evaluate the method error, the measurements of 12 specimens were repeated after a 1-week interval. No statistically significant difference was found between repeated measurements according to a paired-samples t test (P ). The sliding resistance analysis with stainless steel wires was conducted on both as-received and retrieved brackets in a universal testing machine (DL-500; EMIC, S~ao Jose dos Pinhais, Brazil), with a device especially designed for this experiment (Fig 1). Test specimens were obtained by bonding the brackets with a cyanoacrylate adhesive to a mm acrylic plate with a holder in a standardized way. This guaranteed that the bracket slots stayed parallel to the testing machine s vertical axis. Stainless steel wire segments (Shiny Bright, TP Orthodontics) with a cross-section of mm ( in) and a length of 11.5 cm were used. The wires were cleaned with 70% alcohol. They were ligated in their middle portion to the brackets with 3.0-mm elastomeric ligatures (Mini Stix noncoated, TP Orthodontics) immediately before the test to standardize the ligation force. Each segment was used only once. Test specimens were mounted in the device and assembled in the test machine. A 300 g weight was attached to the lower extremity of the wire to keep it under tension. The wire was pulled along the bracket at a rate of 5 mm per minute for 1 minute. The force levels were registered by a 10 kgf load cell. The sliding resistance was calculated by averaging the forces registered between the first and fifth millimeters of displacement, October 2011 Vol 140 Issue 4 American Journal of Orthodontics and Dentofacial Orthopedics

3 Regis et al 503 Fig 1. Device with test specimen mounted for the sliding resistance test, seen with greater magnification in detail. disregarding the initial static friction. Since the retrieved brackets were of different types, even for the same brand, it was of no use to consider the mean sliding resistance for each brand, but only the differences between retrieved and as-received brackets. For means of comparison among the different brands, the percentage difference in the sliding resistance between retrieved and as-received brackets was calculated with the following equation: DifSR(%) 5 [(SRretrieved SRas-received)/ SRas-received] 3 100, where DifSR is the percentage difference in sliding resistance, and SR is sliding resistance. Statistical analysis Kolmogorov-Smirnov and Levene tests were used to evaluate the normality of data distribution and the homogeneity of variance, respectively. Only the tie-wings widths and the percentage differences in the sliding resistance variables had nonnormal distributions. Only the as-received brackets sliding resistance had a homogeneous variance. The effect of usage on the tie-wings widths was investigated with the Mann-Whitney test. Slot dimensions were compared among brands and usage Fig 2. Optical microscopy images (bright field, 200 times magnification) of the slots of: A, NuEdge; B, Mini Standard Edgewise; and C, Kirium retrieved brackets. These images illustrate the wear and plastic deformation (arrows) of the external slot edges. conditions (as-received or retrieved) with analysis of variance (ANOVA) and the Games-Howell post-hoc test. The Kruskall-Wallis multiple comparisons test was used to evaluate differences among brands and patients in the percentage difference in the sliding resistance. A pairedsamples t test was used to investigate the effect of usage on sliding resistance for each brand in a 2-tailed model. The level of significance for all tests was set at a RESULTS Surface modifications with signs of corrosion, plastic deformation, and wear were seen in the retrieved brackets (Fig 2) when compared with the as-received brackets during optical microscopic evaluation. Wear American Journal of Orthodontics and Dentofacial Orthopedics October 2011 Vol 140 Issue 4

4 504 Regis et al Fig 3. Scanning electron microscope images of Kirium brackets: A, slot of an as-received bracket (backscattered electron image, 100-times magnification); B, slot of a retrieved bracket (secondary electron image, 100-times magnification); C, in the retrieved brackets, there were smooth milling grooves in the lateral walls of the slot (backscattered electron image, 100-times magnification); D, delamination debris on a retrieved bracket (secondary electron image, 300-times magnification); E, a precipitate of a retrieved bracket containing silica and barium (backscattered electron image, 300-times magnification). and deformation were especially seen on the external edges of the slot bottom surface. The retrieved specimens had film deposition and islands of aggregated materials to a variable extent. The variability occurred even with brackets removed from the same patient, with no individual pattern in the amount of integument formed. Mini Standard Edgewise retrieved brackets slots varied in areas with pits and film deposition or areas with extensive signs of wear and deformation. This last pattern was more frequent in retrieved Kirium brackets. NuEdge brackets had a rough surface, with pits and crevices in a parallel arrangement, in both as-received and retrieved specimens. These originally roughened surfaces, a coating applied by the manufacturer, and the presence of integuments made it difficult to evaluate specific usage-related alterations in NuEdge brackets. The same morphologic patterns were seen in greater detail in the scanning electron microscope images (Figs 3-5). The precipitated film exhibited a dark phase in the backscattered electron images, contrasting with the bright phase of the brackets alloy. This finding suggested the presence of elements with a low atomic number in the film. Carbon, oxygen, calcium, and phosphorus were the main elements detected in the film by energy dispersive x-ray spectrometry microanalysis. However, nitrogen, sulfur, sodium, potassium, and aluminum were eventually found. In some areas, the precipitates masked the topographic features of the underlying alloy surface (Fig 6). Greater magnifications showed details of the arrangement of pits and crevices in the NuEdge specimens (Fig 5, D and E). The coating applied by the manufacturer in the as-received NuEdge brackets was seen in backscattered electron images as a dark-phased, noncontinuous pellicle. The same distribution was not seen in the retrieved specimens, in which the integuments were limited to slots and gaps. The original coating was visually indistinguishable from the biofilm formed during oral exposure. Some materials of atypical composition were found in retrieved brackets, including a 150-mm precipitate containing silica, aluminum, barium, iron, carbon, and oxygen (Fig 3, E). Also included were silver and silica incrustations that had a brighter phase than the cobalt-chromium alloy (Fig 6). A comparison between as-received and retrieved brackets found no significant difference (P.0.05) in the tie-wings internal widths (Table I). Table II presents the slot dimensions according to usage and brands. NuEdge brackets had significantly greater slot dimensions than the other brands (P \0.05). Differences between retrieved and as-received brackets October 2011 Vol 140 Issue 4 American Journal of Orthodontics and Dentofacial Orthopedics

5 Regis et al 505 Fig 4. Scanning electron microscope images of Mini Standard Edgewise brackets: A, slot of an asreceived bracket (backscattered electron image, 100-times magnification); B, slot of a retrieved bracket (backscattered electron image, 100-times magnification); C, slot lateral wall of a retrieved bracket, with plastic deformation (bright arrow) and film deposition (backscattered electron image, 300-times magnification); D, islands of precipitates (300 times magnification) and E, pitting corrosion (dark arrows; 100-times magnification) after intraoral exposure. were seen only for the right slot of the Kirium brackets (P \0.05). Sliding resistance, evaluated separately for each brand according to usage condition by t tests in a 2-tailed model, had P values of 0.054, 0.103, and 0.08 for NuEdge, Mini Standard Edgewise, and Kirium, respectively. Since various bracket types were tested (premolar, canine, and incisor for both arches), it was not viable to calculate the sliding resistance means and compare them among brands. However, the percentage difference in the sliding resistance, as a ratio of sliding resistance alteration between retrieved and as-received brackets, could be compared among brands. The percentage difference in the sliding resistance had mean values of 17.99% (SD, 36.50%) for NuEdge brackets, 13.62% (SD, 34.26%) for Kirium, and 3.10% (SD, 31.82%) for Mini Standard Edgewise. Differences were statistically significant between Mini Standard Edgewise and the other brands (P \0.05). No significant differences were found for the percentage difference in the sliding resistance between patients (P ). DISCUSSION Microscopic analysis suggested that the orthodontic brackets underwent significant alterations in variable ways during treatment. Retrieved orthodontic accessories have shown diverse behaviors in the literature: some had significant alterations, 4,6,9-12,29 and others showed no differences from as-received. 7,8,29-32 These conflicting results might be attributed to the diversity of the materials analyzed (brackets, archwires, headgear components), the time of exposure to the oral environment, and the patient s characteristics. Additionaly, corrosion susceptibility is influenced by the composition of the alloy, its microstructure, manufacturing procedures and their impact on the internal stress of the material, and the formation of a surface passivation film. 33 Since the brackets evaluated in this study had different compositions and brands, dissimilarities in their surface morphology after orthodontic treatment could be related to each alloy s tribological, physical, and electrochemical properties, and to manufacturing procedures. The precipitated film over the retrieved brackets surfaces was also found on orthodontic wires 9,30 and headgear components. 10 The overlap of the calcium and phosphorus distributions (Fig 6) is consistent with crystalline particle formation. This finding was also described by Eliades et al. 9,10 These authors additionally examined the molecular composition of the precipitated film. They found amide, alcohol, and carbonate as the main American Journal of Orthodontics and Dentofacial Orthopedics October 2011 Vol 140 Issue 4

6 506 Regis et al Fig 5. Scanning electron microscope images of NuEdge brackets: A, slot of an as-received bracket (backscattered electron image, 100-times magnification); B, slot of a retrieved bracket (backscattered electron image, 100-times magnification); C, as-received bracket slot with deformation (secondary electron image, 100-times magnification); D, internal edge between the slot and the middle portion of the bracket, showing traces of the mechanical action in the slot (left) in contrast to the rough surface (right) that does not come into contact with the wires (backscattered electron image, 1000-times magnification); E, retrieved bracket slot surface showing the linear distribution of crevices typically seen in NuEdge brackets. Also seen is an area with signs of wear and plastic deformation (secondary electron image, 300-times magnification). organic constituents. They also identified the presence of chlorine, potassium, and sodium in a uniform distribution. Such findings suggest the formation of a biofilm composed of a proteinaceous matrix and scattered crystalline particles. The differing properties of the bracket materials and differing retrieval protocols might account for the dissimilarities in the integuments found in these studies.the cleansing procedures we used were intended to simulate a patient s dental hygiene and to remove loosely attached material. The precipitate containing silica and barium might be attributed to slot contamination by composite resins or adhesives, which have similar compositions. 34 Although mass transfer can occur with the sliding of a metallic surface, the silver-containing incrustations found on retrieved brackets (Fig 6) could not be attributed to any clinical findings. 26 Calcium and aluminum inclusions found on biomedically applied materials were strongly related to corrosion. Other sources of inclusions could be raw-material impurities and inclusions not dissolved in the melt. 35 The new brackets slot dimensions varied among brands, with a peak of mm ( in; 12%) beyond the nominal value in the NuEdge specimens. When evaluating the same dimensions, Oh et al 25 also found differences among brands, although of smaller magnitudes (0.018 mm). The evaluated brackets had acceptable dimensional stability during orthodontic treatment. Only the Kirium retrieved brackets had significant differences compared with their as-received counterparts. Specifically, there was a mm ( in; 3%) increase in a slot size. These findings agree with those of Fischer-Brandies et al, 36 who found indentations and a mm slot widening on stainless steel brackets tested with wire torque activations. They attributed the results to the low stiffness of the bracket material compared with the wires. The same mechanism could be related to the wear and deformation of the slot edges of the retrieved brackets in this study. Alterations caused by debonding were minimized by using a procedure that applied force in the bracket base, preserving the area of interest. Considering the slot variations among brands, the observed changes in used Kirium brackets might not have clinical significance. These results did not allow the rejection of the null hypothesis of no difference in sliding resistance between October 2011 Vol 140 Issue 4 American Journal of Orthodontics and Dentofacial Orthopedics

7 Regis et al 507 Fig 6. MEV and energy dispersive x-ray spectrometry images of a NuEdge bracket with film deposition and silver-containing particles: A, (100-times magnification) backscattered electron images of the slot, where silver particles appear as the brighter phase; B and C, backscattered electron and secondary electron images, respectively, of the marked area in A with greater magnification (1000 times); D, energy dispersive x-ray spectrometry map of cobalt, with the bright dots indicating that element, and chromium had a similar distribution; E, energy dispersive x-ray spectrometry map of calcium demonstrating a greater concentration in the area corresponding to the precipitated film images in B and C, with phosphorus having a similar distribution; F, energy dispersive x-ray spectrometry map of silver. Molybdenum and silica had more uniform distributions, although they were concentrated in the same areas as the silver. Table I. Comparison of the internal horizontal widths of the tie-wings of as-received and retrieved brackets Mean SD Significance* Cervical tie-wings internal width (mm) As-received Retrieved Occlusal tie-wings internal width (mm) As-received Retrieved *Mann-Whitney test results. as-received and retrieved brackets by using a 2-tailed model for each brand separately. However, the percentage difference in the sliding resistance indicates significantly different tendencies when comparing Kirium (13.62%; SD, 34.26%) and NuEdge (17.99%; SD, 36.50%) brackets with Mini Standard Edgewise brackets ( 3.10%; SD, 1.82%). Considering that the percentage differences in the sliding resistance results indicate the sliding resistance changes in each brand, a 1-tailed model could be adopted for statistical testing. In this scenario, by using a t test for paired samples to compare sliding resistance values between as-received and retrieved brackets separately for each brand, the P values were for NuEdge, 0.04 for Kirium, and for Mini Standard Edgewise. Therefore, Kirium and NuEdge brackets had greater mean friction, whereas the Mini Standard Edgewise brackets had no significant alteration. Table II. Heights (mm) of right and left slots according to brand and use n Mean SD Height of left slot NU as-received a NU retrieved a MS as-received b MS retrieved b KR as-received b,c KR retrieved c,d Height of right slot NU as-received a NU retrieved a MS as-received b MS retrieved b KR as-received b,c KR retrieved d Similar letters identify groups without significant differences (P $0.05), according to the Games-Howell multiple comparisons test. NU, NuEdge; MS, Mini Standard Edgewise; KR, Kirium. Although some authors have hypothesized that an increase in friction occurs due to surface alterations, 1,20,24 Berg et al 37 suggested a lubricating effect for salivary pellicles over the sliding surfaces in the oral environment. The percentage difference in the sliding resistance variable had great variability for each brand, with high standard devation values. Oh et al 25 found similar results in an in-vitro setup, where variations in American Journal of Orthodontics and Dentofacial Orthopedics October 2011 Vol 140 Issue 4

8 508 Regis et al the frictional resistances ranged up to 2.5 times among brackets of the same brand. Eliades and Bourauel 1 reported preliminary results of an ongoing study showing that retrieved nickel-titanium and stainless steel wires might have approximately 10% to 20% increases in frictional forces, with great fluctuations over the measured displacement. They attributed this finding to the complexes precipitated intraorally. Wichelhaus et al 38 also verified a significant increase in friction of nickeltitanium wires after clinical use. We found no similar evaluations of retrieved brackets. The frictional test used in this study does not fully represent oral conditions. Second- and third-order inclinations, dental-arch convexity, binding effects between bracket and archwire, and material-related frictional implications were not part of our goals. Nevertheless, this study provides evidence of the effects of clinical use and associated phenomena on orthodontic components classic friction. The different behaviors of the brackets we evaluated raises doubt about which factors determined the observed alterations. Further investigations with other commercial brands, standardized treatment mechanics, and controlled durations of oral exposure might clarify the effect of specific factors on orthodontic accessories friction during clinical use. Our results suggest that care must be taken when extrapolating conclusions obtained in research with as-received materials to long-term clinical scenarios. CONCLUSIONS Clinical use causes surface alterations in metallic orthodontic brackets, with distinct patterns of alterations for different brands. Differences in morphology after use are smaller than those found in as-received brackets among brands. Distinct frictional behaviors were observed for each bracket brand with clinical use. There were 10% to 20% increases between retrieved and asreceived brackets in NuEdge and Kirium, whereas the Mini Standard Edgewise brackets remained unaffected. REFERENCES 1. Eliades T, Bourauel C. Intraoral aging of orthodontic materials: the picture we miss and its clinical relevance. Am J Orthod Dentofacial Orthop 2005;127: Eliades T, Gioka C, Zinelis S, Eliades G, Makou M. Plastic brackets: hardness and associated clinical implications. World J Orthod 2004;5: Cortizo MC, De Mele MF, Cortizo AM. Metallic dental material biocompatibility in osteoblastlike cells: correlation with metal ion release. Biol Trace Elem Res 2004;100: Eliades T, Zinelis S, Eliades G, Athanasiou AE. Nickel content of as-received, retrieved, and recycled stainless steel brackets. Am J Orthod Dentofacial Orthop 2002;122: Kao CT, Ding SJ, Min Y, Hsu TC, Chou MY, Huang TH. The cytotoxicity of orthodontic metal bracket immersion media. Eur J Orthod 2007;29: Widu F, Drescher D, Junker R, Bourauel C. 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9 Regis et al Oh KT, Choo SU, Kim KM, Kim KN. A stainless steel bracket for orthodontic application. Eur J Orthod 2005;27: Kusy RP, Whitley JQ. Effects of surface roughness on the coefficients of frictionin model orthodontic systems. J Biomech 1990;23: Kusy RP, Whitley JQ, de Araujo Gurgel J. Comparisons of surface roughnesses and sliding resistances of 6 titanium-based or TMA-type archwires. Am J Orthod Dentofacial Orthop 2004;126: Cacciafesta V, Sfondrini MF, Scribante A, Klersy C, Auricchio F. Evaluation of friction of conventional and metal-insert ceramic brackets in various bracket-archwire combinations. Am J Orthod Dentofacial Orthop 2003;124: Alcock JP, Barbour ME, Sandy JR, Ireland AJ. Nanoindentation of orthodontic archwires: the effect of decontamination and clinical use on hardness, elastic modulus and surface roughness. Dent Mater 2009;25: Edie JW, Andreasen GF, Zaytoun MP. Surface corrosion of nitinol and stainless steel under clinical conditions. Angle Orthod 1981; 51: Grimsdottir MR, Hensten-Pettersen A. Surface analysis of nickel-titanium archwire used in vivo. Dent Mater 1997;13: Petoumeno E, Kislyuk M, Hoederath H, Keilig L, Bourauel C, Jager A. Corrosion susceptibility and nickel release of nickel titanium wires during clinical application. J Orofac Orthop 2008;69: Lin MC, Lin SC, Lee TH, Huang HH. Surface analysis and corrosion resistance of different stainless steel orthodontic brackets in artificial saliva. Angle Orthod 2006;76: Soderholm KJ, Yang MC, Garcea I. Filler particle leachability of experimental dental composites. Eur J Oral Sci 2000;108: Hastings GW. Implant retrieval: material and biological analysis. Biomaterials 1980;1: Fischer-Brandies H, Orthuber W, Es-Souni M, Meyer S. Torque transmission between square wire and bracket as a function of measurement, form and hardness parameters. J Orofac Orthop 2000;61: Berg IC, Rutland MW, Arnebrant T. Lubricating properties of the initial salivary pellicle an AFM study. Biofouling 2003;19: Wichelhaus A, Geserick M, Hibst R, Sander FG. The effect of surface treatment and clinical use on friction in NiTi orthodontic wires. Dent Mater 2005;21: American Journal of Orthodontics and Dentofacial Orthopedics October 2011 Vol 140 Issue 4