Cross-section dimensions and mechanical properties of esthetic orthodontic coated archwires

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1 ORIGINAL ARTICLE Cross-section dimensions and mechanical properties of esthetic orthodontic coated archwires Dayanne Lopes da Silva, a Claudia Trindade Mattos, b Eduardo Franzotti Sant Anna, c Ant^onio Carlos de Oliveira Ruellas, c and Carlos Nelson Elias d Rio de Janeiro, Brazil Introduction: There has been continuing interest in the development and use of esthetic and effective orthodontic archwires. The aims of this study were to evaluate the inner alloy core dimensions of 4 brands of as-received esthetic coated wires and their mechanical properties before and after 21 days of oral exposure, compared with conventional stainless steel and nickel titanium wires. Methods: Four groups (Ortho Organizers, Carlsbad, Calif; TP Orthodontics, LaPorte, Ind; Orthometric, Beijing, China; and Trianeiro, Rio Claro, S~ao Paulo, Brazil) of orthodontic archwires were tested. Five properties were evaluated: inner wire dimensions, modulus of elasticity, modulus of resilience, maximum deflection force, and load deflection curve characteristics. Images of the transverse sections from the specimens were made with a stereoscope. The inner alloy core dimensions of each wire were measured by using Image Pro Plus software (version 4.5; Media Cybernetics, Silver Spring, Md). All specimens were tested in a universal testing machine in a 3-point bending test. Results: Coated wires of the Ortho Organizers and Trianeiro groups showed greater reductions in their inner alloy core dimensions and produced lower loading and unloading forces and lower modulus of elasticity, modulus of resilience, and maximum deflection force values than did their control wires. Inner alloy core dimensions and the mechanical behavior of coated wires practically did not differ from the control wires in the TP Orthodontics and Orthometric groups. Conclusions: The reduction on the inner alloy core dimensions to compensate for the coating thickness seems to be the variable responsible for greater changes in the mechanical properties of esthetic coated wires. (Am J Orthod Dentofacial Orthop 2013;143:S85-91) Much progress has been made in the development of esthetic clear and translucent brackets for use in labial orthodontics. 1 However, the most effective wires continue to be manufactured from efficient metal alloys. They have the flexibility, strength, and chemical resistance needed for orthodontic purposes. Nevertheless, they are clearly visible to observers, and patients with esthetic brackets might resist their use. a PhD student, Department of Orthodontics, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil. b Substitute professor, Department of Orthodontics, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil. c Professor, Department of Orthodontics, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil. d Professor, Department of Mechanical Engineering and Materials Science, Instituto Militar de Engenharia, Rio de Janeiro, Brazil. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Ant^onio Carlos de Oliveira Ruellas, Av. Professor Rodolpho Paulo Rocco, 325, Ilha do Fund~ao, Rio de Janeiro, Rio de Janeiro, CEP: , Brazil; , antonioruellas@yahoo.com.br. Submitted, May 2012; revised and accepted, September /$36.00 Copyright Ó 2013 by the American Association of Orthodontists. Metallic archwires coated with colored polymers or inorganic materials are currently the solution to this esthetic problem. 2 Materials used in coating are polymers such as synthetic fluorine-containing resin or epoxy resin composed mainly of polytetrafluoroethylyene, which is used to simulate tooth color. 3 The process of applying this coating to the archwire includes some surface treatment on the wire and the use of clean compressed air as a transport medium for the atomized polytetrafluoroethylyene particles to coat the wire. The set is further heat treated in a chamber furnace. 4 The mechanical properties of metallic archwires could be affected during this process and by possible changes on their inner alloy core dimensions to compensate for the thickness of the coating layer. The mechanical properties of orthodontic archwires can be assessed by using a 3-point bending test, which evaluates the load-deflection properties, considered the most important parameters to determine the biologic nature of tooth movement, 5 and provides information on the behavior of wires when subjected to deflection in the horizontal and vertical directions. 6,7 No studies S85

2 S86 Silva et al Table I. Characteristics of the archwires used in the study Group Manufacturer Cross-section size (in) Composition Coating surface I Ortho Organizers, Carlsbad, Calif Control: Pro Form Shiny Bright Stainless steel (CrNi) - Coated: Tooth Tone Plastic Coated All surfaces II TP Orthodontics, LaPorte, Ind Control: Shiny Bright Stainless steel (CrNi) - Coated: Aesthetic Shiny Bright Labial III Orthometric, Beijing, China Control: Flexy Super Elastic NiTi - Coated: Esthetic Flexy Super Elastic Labial IV Trianeiro, Rio Claro, S~ao Paulo, Brazil Control: NiTi wire NiTi - Coated: Coated wire NiTi All surfaces CrNi, Chromium-nickel; NiTi, nickel-titanium. have been conducted to examine the inner alloy core dimensions of esthetic coated stainless steel and nickel-titanium archwires and their mechanical properties after oral exposure. The aims of this research were to investigate the inner alloy core dimensions of as-received esthetic coated archwires and their mechanical properties (3-point bending test) before and after oral exposure, compared with conventional stainless steel and nickel-titanium archwires. MATERIAL AND METHODS Sample size calculations were performed for each test. A sample size of at least 5 wire segments per group was required to detect a difference of of an inch in the dimensions between the coated and the control wires, with a standard deviation of , a significance level of 5%, and a test power of Based on the results from a pilot study, a sample size calculation was performed showing that at least 5 as-received and 10 postclinical wire segments would be enough to detect a 10% difference in the mechanical properties tested in this study with a significance level of 5% and a test power of Four brands of esthetic coated archwires (Ortho Organizers, Carlsbad, Calif; TP Orthodontics, LaPorte, Ind; Orthometric, Beijing, China; and Trianeiro, Rio Claro, S~ao Paulo, Brazil) were tested, and their respective control counterparts (conventional stainless steel and nickel-titanium archwires) were evaluated (Table I). The sample included 176 wire segments at least 20 mm in length. Eighty wire segments were from asreceived archwires (5 specimens of each kind of wire used to measure the inner alloy core dimensions, and 5 were used in the 3-point bending test), and 96 segments (12 specimens of each kind of wire used in the 3-point bending test) were retrieved after 21 days in oral cavities. The retrieved wire segments were obtained from 12 subjects. Ethical approval was obtained for this investigation from the ethics in research committee of the Institute of Public Health Studies from the Federal University of Rio de Janeiro in Brazil (process ). Informed consent form was signed by all patients who met the following criteria: good oral hygiene, no caries, and no systemic changes. Two metal brackets (Morelli, Sorocaba, S~ao Paulo, Brazil) were bonded to teeth on each hemiarch with a distance between them of at least 20 mm. The coated wire segment was tied into the slot, and its control wire segment was tied juxtaposed to the upper base of the brackets wings by using stainless steel in ligatures (Morelli) and ligature pliers (Fig 1). A split-mouth design was used in all groups. The groups were allocated randomly to each hemiarch by using random number tables. The generator and executor of the randomization were separate persons (C.T.M. and D.L.S.). Oral hygiene instructions were given, and the patients used the same types of toothbrush, toothpaste, and dental floss throughout the study. The subjects were told not to use any other oral agents, including oral irrigators or antimicrobial mouthrinses. In every participant, the specimens were placed and removed by the same operator (D.L.S.). The subjects were not aware of which wire was placed in each quadrant in the mouth. Since the colors of the coated wires differed slightly from brand to brand, it was not possible to blind the operator to the types of coated wire. After 21 days, the wire segments were removed and individually placed in an ultrasound cleaner (Cristofoli; Campo Mour~ao, Parana, Brazil) immersed in a multiuse detergent (Amway, Ada, Mich) for 30 minutes, so that organic debris could be removed. The same procedure was used for the as-received specimens. After in-vivo clinical exposure, all samples were subjected to mechanical testing. April 2013 Vol 143 Issue 4 Supplement 1 American Journal of Orthodontics and Dentofacial Orthopedics

3 Silva et al S87 Fig 1. Orthodontic appliances in the oral environment: A, frontal view; B, side view. C, Control wire segment; E, esthetic coated wire segment. Fig 2. Stereoscope images of inner alloy core height and width (3 measurements of each kind): A, group I (Ortho Organizers) coated wire; B, group I control wire. Original magnification, 45 times. Five as-received wires were taken from each group and included, with their transverse section facing up, in plastic tubes (height, 10 mm; diameter, 5 mm) filled with light-curing composite resin (Monolok 2 ; Rocky Mountain Orthodontics, Denver, Colo). Then the samples were ground in a water-cooled polishing machine (Politriz DP-9U2; Struers A/S, Copenhagen, Denmark) with 360-, 400-, 600-, and 1200-grit abrasive papers under refrigeration. The images of the transverse section from each specimen were made with a stereoscope at 45 times magnification (model SZ61; Olympus, Tokyo, Japan). The inner alloy core height and width (3 measurements of each kind) of each wire were measured by using Image Pro Plus software (version 4.5; Media Cybernetics, Silver Spring, Md) (Fig 2). A total of 60 readings, including height and width, were measured in each group. The wires were tested according to a 3-point bending test (ISO 15841). The span length of 10 mm, the crosshead speed of 6.0 mm per minute, and the radii of the fulcrum and the indenter of 0.1 mm were used as suggested by the ISO standardization for orthodontic wire mechanical tests. The load was applied with a 50-N load cell in a universal testing machine (model DL 10000; Emic, Parana, Brazil) to the middle of the specimens, and the flexural strength values were recorded with Tesc software (Emic). Deflection was carried out with a centrally placed indenter moved by a computercontrolled stepper motor. Five as-received and 12 postclinical specimens for control and coated wires of each group were tested for a deflection of 3.1 mm, the force was registered, and the hysteresis curves produced on the chart s recorder were compared. For stainless steel wires, the testing was performed at room temperature, and for nickel-titanium wires at 37 C. The modulus of elasticity was calculated in the linear portion of force-deflection curves of each specimen using the equation American Journal of Orthodontics and Dentofacial Orthopedics April 2013 Vol 143 Issue 4 Supplement 1

4 S88 Silva et al Table II. Means, standard deviations, and P values of independent-samples t test between control and coated wires in each group for wire dimensions (in) Measured inner alloy core dimensions Control 3 coated Stated dimensions Height Width Height Width Group Height Width Mean 6 SD Mean 6 SD P value P value I Control \0.001 \0.001 Coated II Control Coated III Control \0.001 Coated IV Control \0.001 \0.001 Coated Table III. Comparisons of modulus of elasticity, modulus of resilience, and maximum deflection force of as-received and postclinical control and coated wires in each group Modulus of elasticity (GPa) Mean 6 SD Modulus of resilience (N.mm) Mean 6 SD Maximum deflection force (N) Mean 6 SD Group As-received Postclinical As-received Postclinical As-received Postclinical I Control (10.0)aA (7.12)bA 4.15 (0.11)aA 5.30 (0.18)bA (0.23)aA (0.27)bA Coated (4.21)aB (6.31)aB 3.72 (0.09)aB 4.44 (0.25)bB (0.29)aB (0.34)bB II Control (2.54)aA (6.05)bA 2.63 (0.05)aA 5.45 (0.42)bA (0.28)aA (0.50)bA Coated (5.83)aA (13.34)bA 2.70 (0.04)aA 3.34 (0.17)bB (1.04)aA (0.33)bB III Control (10.50)aA (3.59)aA (0.29)aA (1.09)aA (0.19)aA (0.53)bA Coated (13.54)aA (5.57)aA (1.23)aA (0.57)aA (0.75)aA (0.44)aA IV Control (2.89)aA (4.70)bA (0.53)aA (0.35)bA (0.07)aA (0.24)aA Coated (3.38)aB (6.41)aB (1.05)aB (0.20)aB 9.77 (0.17)aB 9.97 (0.23)aB a and b, Different letters mean a statistically significant difference (P\0.05) between as-received and postclinical samples in each group (same line). A and B, Different letters mean a statistically significant difference (P \0.05) between control and coated wires of each kind, as-received or postclinical (same column). E5 PL3 4BH 3 F ; where P is load, L is span length, B is wire width, H is wire height, and F is deflection. Resilience was measured by the modulus of resilience and expressed as the amount of elastic strain energy per unit of volume. This modulus was computed by integrating the area under the linear portion of the stress deflection diagram with OriginPro software (version 8.0; OriginLab, Northampton, Mass). Statistical analysis A standard statistical software package (version 16.0; SPSS, Chicago, Ill) was used for data analysis. The Kolmogorov-Smirnov test was applied to verify the normality of the results, and the independent-samples t test was used to compare the coated and control wires of each brand and to compare the as-received and postclinical samples of each kind of wire. RESULTS Measured inner alloy core and nominal cross-section dimensions for as-received coated and control wire segments used in this investigation are shown in Table II.In general, most measured values for coated wires were smaller than the nominal sizes given by the manufacturers, and there was a significant difference between the control and the coated cross-section dimensions. April 2013 Vol 143 Issue 4 Supplement 1 American Journal of Orthodontics and Dentofacial Orthopedics

5 Silva et al S89 Fig 3. Average force-deflection curves for each group of as-received and postclinical samples. Group II (TP Orthodontics) was the only one that practically maintained the nominal sizes, and there was no significant difference between the control and coated cross-section dimensions in this group. Coated wires from group III (Orthometric) showed an average reduction of only in in their stated width. There were further reductions in both height and width of coated wires in groups I (Ortho Organizers) and IV (Trianeiro), when compared with the stated dimensions. Table III shows the mean values, standard deviations, and comparisons of modulus of elasticity, modulus of resilience, and maximum deflection force for control and coated wire segments in each group, both for asreceived and postclinical specimens. As-received and postclinical coated wires in groups I (Ortho Organizers) and IV (Trianeiro) showed lower modulus of elasticity, modulus of resilience, and maximum deflection force values when compared with their controls. No significant differences in modulus of elasticity and maximum deflection force values between the control and coated wires were observed in group II (TP Orthodontics). However, lower modulus of resilience values were observed in the as-retrieved coated wire segments in this group when compared with their controls. Group III (Orthometric) showed no significant differences between the control and coated wires in the mechanical properties evaluated in all times tested. The representative bending plots for as-received and postclinical coated and control wires of each group are shown in Figure 3. The bending curves for as-received and postclinical control and coated wire segments in groups II (TP Orthodontics) and III (Orthometric) were similar. However, there were considerable general differences in bending deformation behavior for the control and coated wire segments in groups I (Ortho Organizers) and IV (Trianeiro). In these groups, coated wires produced lower loading and unloading forces than did the control wires. DISCUSSION The performance of an archwire depends on the wire material and its cross-sectional geometry. Smaller American Journal of Orthodontics and Dentofacial Orthopedics April 2013 Vol 143 Issue 4 Supplement 1

6 S90 Silva et al archwires are selected to ensure lower forces in the initial stage of fixed appliance mechanotherapy, but they result in inadequate control of tooth movement, since there would be much play between the wire and the bracket. 8 In the final stages, it is necessary to obtain better engagement between the wire and the bracket with larger wires for better control of tooth movement. Thus, wire dimension is a critical and important component in force delivery, but manufacturers differ in their abilities to produce wires accurately Among the wires measured, those from TP Orthodontics were the only ones that practically maintained the nominal sizes, and there was no significant difference between control cross-section dimensions and coated inner alloy core dimensions. In general, most inner alloy core dimensions measured for the coated wires were smaller than the nominal sizes given by the manufacturers, and there was a significant difference between the control and coated cross-section sizes. The archwires with coating on all surfaces from groups I (Ortho Organizers) and IV (Trianeiro) showed the greatest reductions in their inner alloy core height and width compared with the stated dimensions (Table II). In one of the 2 groups with coating only on the vestibular surface (group III, Orthometric), the inner alloy core width was different from the stated width (Table II). Coated wires can have smaller inner alloy core dimensions to compensate for the thickness of the coating layer. That is why both height and width might be affected when all surfaces receive a coating layer, and width is sometimes affected when only the vestibular surface is coated. However, clinicians must be aware that manufacturers generally state the cross-section dimensions of the archwires, including the coating thickness, and not the real size of only the stainless steel or nickel-titanium material. Therefore, these archwires are not expected to have a mechanical behavior similar to an uncoated archwire with the same dimensions. Changes in the stated wire dimensions might influence torsional clearance and stiffness. 9,10 Many orthodontists are likely to use the nominal wire size as their guide in some clinical situations. The results from this study highlight the importance of considering larger nominal wire size than would normally be selected when using archwires with all surfaces coated. Because the amount of torsional clearance between wire and bracket depends on the slot and wire dimensions, as well as the degree of wire rounding, an undersized wire results in poorer fit in the bracket slot and might lead to less control during tooth movement. 12 The advantages of a 3-point bending test are the close simulation to clinical applications and its ability to differentiate wires with superelastic properties. 5 In addition, it offers a high degree of reproducibility that facilitates comparisons between studies. 13 This test produced load-deflection diagrams consisting of an upper loading curve and a lower unloading curve. The loading curve represents the force needed to engage the wire in the bracket, whereas the unloading curve represents the forces delivered to the teeth during treatment stages. 14 The modulus of elasticity corresponds to the elastic stiffness or the rigidity of the material. 15 Increased values indicate stiffer wires. 11 Modulus of resilience is the ability of a material to store energy when deformed elastically and to deliver it when the strain is removed. 15 Resilience tells the orthodontist how much potential energy can be recovered from the wire. In this study, for comparisons, the wire configuration and the alloy used in the manufacture of the control and coated archwires in each group were identical. These results show differences between the stated and the measured dimensions mainly in the coated wires. Furthermore, there are variabilities with respect to mechanical properties in the control and coated wires. Asreceived and postclinical coated wires in groups I (Ortho Organizers) and IV (Trianeiro) showed lower modulus of elasticity, modulus of resilience, and maximum deflection force values when compared with their controls (Table III). The analysis of the bending curves in these groups showed that coated wires produced lower loading and unloading forces than did the control wires (Fig 3). These archwires with a coating layer on all surfaces showed greater reductions in their inner alloy core dimensions; this could explain their different mechanical behavior. These results agree with those of Elayyan et al, 2 where coated wires also showed lower loading and unloading forces compared with uncoated wires, and the authors suggested that it was almost certainly because of the decrease in the size of the active nickel-titanium wires in them. However, the authors did not measure the cross-section size of the wires and evaluated only 1 type of archwire. On the contrary, inner alloy core dimensions of coated wires in group II (TP Orthodontics) practically did not differ from the control wires. In group III (Orthometric), only a decrease in width, where the coating was applied, was noted. Both groups had a coating layer only on the archwire s labial surface. The bending curves for these groups were similar when comparing as-received and postclinical control and coated wires (Fig 3). The major point an orthodontist should consider when selecting a particular wire size is the stiffness of the wire or its load-deflection rate. These differences in the mechanical properties between coated and control wires are probably due to deviations from stated sizes, or variations in material properties and the method of April 2013 Vol 143 Issue 4 Supplement 1 American Journal of Orthodontics and Dentofacial Orthopedics

7 Silva et al S91 applying the coating layer on the wires; this might have been heat treatment. Small changes in cross-section size produce large changes in the load deflection. 11 Ideally, manufacturers should keep inner alloy core dimensions the same and apply a coating layer on the labial surface of the wire that does not compromise its actual size, as shown in group II (TP Orthodontics). In other words, it would be appropriate for orthodontists to have at their disposal coated esthetic archwires with their real metallic dimensions matching their stated dimensions and thus having the same properties as identical solid stainless steel or nickel-titanium wires. Moreover, a coating layer only on the labial surface, which would not be in contact with brackets, could probably provide better bracket-wire engagement with no interference from this layer. CONCLUSIONS 1. Groups with a coating layer on all surfaces showed greater reductions in their inner alloy core dimensions to compensate for the thickness of the coating layer. These as-received and postclinical coated wires produced lower loading and unloading forces and lower modulus of elasticity, modulus of resilience, and maximum deflection force values than did their control wires. 2. The mechanical behavior of the groups whose inner alloy core dimensions practically did not differ from the control wires was similar when comparing the asreceived and postclinical control and coated wires. 3. The reduction of the inner alloy core dimensions seems to be the variable responsible for greater changes in the mechanical properties of coated archwires, along with variations in the materials properties. REFERENCES 1. Russell JS. Aesthetic orthodontic brackets. J Orthod 2005;32: Elayyan F, Silikas N, Bearn D. Mechanical properties of coated superelastic archwires in conventional and self-ligating orthodontic brackets. Am J Orthod Dentofacial Orthop 2010;137: Ramadan AA. Removing hepatitis C virus from polytetrafluoroethylyene-coated orthodontic archwires and other dental instruments. East Mediterr Health J 2009;9: Husmann P, Bourauel C, Wessinger M, J ager A. The frictional behavior of coated guiding archwires. J Orofac Orthop 2002;63: Krishnan V, Kumar K. Mechanical properties and surface characteristics of three archwire alloys. Angle Orthod 2004;74: Kapila S, Sachdeva R. Mechanical properties and clinical applications of orthodontic wires. Am J Orthod Dentofacial Orthop 1989;96: Walker M, White R, Kula K. Effect of fluoride prophylactic agents on the mechanical properties of nickel-titanium-based orthodontic wires. Am J Orthod Dentofacial Orthop 2005;127: Juvvadi SR, Kailasam V, Padmanabhan S, Chitharanjan AB. Physical, mechanical, and flexural properties of 3 orthodontic wires: an in-vitro study. Am J Orthod Dentofacial Orthop 2010;138: Meling T, Ødegaard J, Meling E. On mechanical properties of square and rectangular stainless steel wires tested in torsion. Am J Orthod Dentofacial Orthop 1997;111: Meling T, Ødegaard J. On the variability of cross-sectional dimensions and torsional properties of rectangular nickel-titanium arch wires. Am J Orthod Dentofacial Orthop 1998;113: Burstone CJ. Variable-modulus orthodontics. Am J Orthod 1981; 80: Meling T, Ødegaard J, Meling E. A theoretical evaluation of the influence of variation in bracket slot height and wire rounding on the amount of torsional play between bracket and wire. Kieferorthopadische Mitteilungen 1993;7: Wilkinson PD, Dysart PS, Hood JA, Herbison GP. Load-deflection characteristics of superelastic nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 2002;121: Segner D, Ibe D. Properties of superelastic wires and their relevance to orthodontic treatment. Eur J Orthod 1995;17: Brantley WA. Orthodontic wires. In: Brantley WA, Eliades T, editors. Orthodontic materials: scientific and clinical aspects. Stuttgart, Germany, and New York: Thieme; p American Journal of Orthodontics and Dentofacial Orthopedics April 2013 Vol 143 Issue 4 Supplement 1