In vitro evaluation of physical vapor deposition coated beta titanium orthodontic archwires
Abstract
Objective:
To determine in vitro the frictional properties, surface morphology, and load deflection rate with looped designs of two newly developed titanium aluminum nitride (TiAlN) and tungsten carbide/carbon (WC/C) physical vapor deposition (PVD) coated beta titanium orthodontic archwires coated with PVD.
Materials and Methods:
Frictional properties with Tidy's protocol, surface evaluation before and after friction testing with the help of scanning electron microscopy (ESEM), and load deflection rate with different orthodontic loops on Instron universal testing machine were evaluated.
Results:
The results clearly indicate reduced frictional properties for WC/C coated archwires when compared with uncoated and TiAlN coated archwires. There were no significant surface alterations upon ESEM evaluation of friction tested archwires. Low load deflection rate was exhibited by both coated archwires, the difference in load deflection rate between the coated and uncoated archwires was statistically significant.
Conclusion:
WC/C coated wires can be recommended for even sliding mechanics due to reduced frictional properties, better surface characteristics, and low load deflection rate compared with TiAlN coated and uncoated archwires.
INTRODUCTION
Titanium alloy archwires with various properties such as shape memory and superelasticity (nickel-titanium—austenitic and CuNiTi), bendability, weldability (beta titanium), and finishing wires (niobium titanium and alpha titanium) are helping orthodontic clinicians at all stages of orthodontic treatment. Meta stable beta titanium alloys used in orthodontics contain about 80% titanium, produce linear forces per unit of deactivation and have substantially more range and higher spring back.1 This alloy wire possessed good formability and weldability, and was smoother in comparison to NiTinol wires. Indeed, beta titanium was almost the perfect wire since its characteristics were so balanced; yet it had a latent flaw. The coefficient of friction was the worst of any orthodontic alloy and demonstrated higher levels of bracket/wire friction than either stainless steel or cobalt chromium wires.2 The clinical implication of this was the slower rates of tooth movement observed during canine retraction and space consolidation with beta titanium wires than with stainless steel and Co-Cr wires. In order to reduce friction and improve esthetic characteristics, various coating methods have been tried over beta titanium archwires, such as ion implantation with diamond-like carbon and nitriding, which have shown limited success.3
Upon exposure to air, several oxides are formed on titanium alloys, including titanium dioxide and titanium oxide, with the titanium dioxide being the most dominant.4 Titanium-based alloy archwires are usually subjected to hydrogen damage, galvanic corrosion, stress corrosion, and microbial corrosion5–8 when they get exposed to various reductive agents such as fluorides from toothpastes, mouthwashes, and gels used for caries protection. Recently, we have introduced two new physical vapor deposition (PVD) coated beta titanium archwires so that the corrosion resistance against fluoride attack is enhanced.9 We could demonstrate excellent corrosion resistance of titanium aluminum nitride (TiAlN) coated beta titanium archwires over tungsten carbide/carbon (WC/C) coated archwires through its electrochemical behavior, surface characteristics, microstructure, and toxicologic evaluation. However, the clinical utility of these archwires was not investigated. The present research is directed as a preliminary study evaluating the frictional properties, surface morphology, and load deflection characteristics in various loop forms of TiAlN and WC/C coated beta titanium archwires in comparison to its uncoated forms.
MATERIALS AND METHODS
Specimen
Preformed rectangular 0.43 × 0.64 mm (0.017 × 0.025 inch) beta titanium orthodontic archwires (77.55% titanium, 11.5% molybdenum, 6% zirconium, 4.5% tin, 0.35% iron) were obtained from Ormco Corp (Glendora, Calif). Cathodic arc physical vapor deposition (CA-PVD) and magnetron sputtering were the PVD methods used to deposit thin films of TiAlN and WC/C, respectively, over beta titanium orthodontic archwires. The uncoated and coated wires were divided into three groups:
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group 1: BTUC (beta titanium in uncoated form),
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group 2: BTCT1 (beta titanium coated with TiAlN), and
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group 3: BTCT2 (beta titanium coated with WC/C).
Physical Vapor Deposition
CA-PVD and magnetron sputtering were used to deposit thin films of TiAlN and WC/C, respectively, on beta titanium orthodontic archwires (groups 2 and 3). In CA-PVD, a high current low voltage arc is connected to the cathode, which acts as a highly energetic emitting area known as a cathode spot. This extremely high temperature results in vaporized cathodic material (titanium and aluminum) travelling at high velocity (10 km · s−1) through a cloud of ions (plasma). Nitrogen is introduced and reaction between titanium, aluminum, and nitrogen generates TiAl nitrides (TiAlN). This TiAlN mixture will travel toward the negatively charged substrates and will condense on its surface. Magnetron sputtering is utilized for the removal of atomized material from a solid (tungsten) due to bombardment of its surface layers by ionized or neutral particles at high impact velocities. Sputtering is performed in a near vacuum. During particle bombardment, a controlled flow of inert gas (carbon) is introduced in order to raise the pressure. A highly negative voltage source is applied to the substrate material which attracts positive ions at high speed.9,10 Coating was performed under 250°C for 4 to 5 hours.
Frictional Characteristics
Six specimens (length of 65 mm) from each group were utilized for the evaluation of frictional properties at the archwire-bracket interface following the test protocol described by Tidy.11 This consisted of a simulated half arch fixed appliance with archwire ligated in position (Figure 1). Four edgewise brackets (Mini Edgewise Nickel-Lite Opti-MIM; Ortho Organizers, Carlsbad, Calif) having slot dimension of 0.022 × 0.028 inches with zero torque and zero angulation were bonded into a rigid Perspex sheet at 8-mm intervals. A space of 16 mm was left at the center for sliding the canine bracket to simulate canine retraction. The archwires were secured using .012-inch elastomeric ligatures (Ormco). The movable canine bracket was soldered with a 12-mm power arm from which weights of 0.5N and 1N were hung to represent the single equivalent force acting at the center of resistance of the tooth root. All tests were conducted in dry condition with an Instron universal testing machine (Model number 1195-5500R, Instron Corporation, Canton, Mass). The movable bracket was suspended from the load cell of the testing machine, while the base plate (Perspex sheet) was mounted on the cross head below. The full-scale load was set at 5 N with a crosshead speed of 10 mm/min. At the start of each test, a trial run was performed with no load on the power arm to check whether there was any binding between the archwire and bracket. Then, a 0.5-N followed by 1-N weight was suspended from the power arm, and the load needed to move the bracket across the central span in apparatus was recorded separately. The load cell reading represents the clinical force of retraction that would be applied to the canine, part of which would be critical friction while the rest would be the translation force on the tooth. The difference between the load cell reading and load on the power arm represents frictional resistance. The coefficient of friction at the archwire-bracket interface was calculated using the appropriate formula,11 
, where P = frictional resistance, F = equivalent force acting at a distance, W = bracket slot width, h = 12 mm, and μ = coefficient of friction.
Citation: The Angle Orthodontist 82, 1; 10.2319/040811-251.1
Cross-sectional Characteristics
The cross-sectional view of the coated archwires was observed by environmental scanning electron microscopy (ESEM; FEI Quanta 200, FEI, Eindhoven, The Netherlands). The straight portions of the specimens were cut to suitable length and vertically mounted on acrylic blocks. The mounted specimens were then subjected to polishing with different grades (120–400) of silicon carbide paper and final polishing with a fine diamond suspension on a polishing buff. The highly polished reflective surfaces thus obtained were washed thoroughly in running water, dried, and then examined under ESEM under low (120×) and higher magnification (500×), and representative micrographs were obtained.
Surface Morphology Before and After Frictional Analysis
Surface characteristics of each specimen of wires, from groups 1, 2, and 3, before and after frictional analysis were studied with the help of ESEM. A 1-cm long specimen, obtained from the area where the canine bracket sliding occurred, of each alloy wire was mounted on studs, which were later placed in the vacuum chamber of the ESEM. The surface was scanned and viewed on the monitor at different magnifications (300× and 1000×), and representative micrographs were obtained.
Load Deflection Rate with Looped Design
The load deflection rate of all the groups of archwires were evaluated after bending the archwire into canine retraction loops.12 Five archwire specimens from each group were used for each looped design (U loop, reverse U loop, and helical loop) formed (Figure 2). They were formed as canine retraction loops with the dimensions matching one side of the dental arch consisting of maxillary canine, first and second premolar, and first molar. The total wire size was 38 mm: the length of the anterior portion was 6 mm and the length of posterior portion was 22 mm. Apart from that 5 mm wire, each side was spared in each specimen, so that the wire could be held securely in the Instron universal testing machine. A total of 45 specimens (five loops in each design for three groups) were used for evaluation with the Instron universal testing machine, the load cells of which extended at 1 mm/min for 2.5 mm. The load at the extension of 2.5 mm for each looped design was tabulated and mean and standard deviation were calculated.
Citation: The Angle Orthodontist 82, 1; 10.2319/040811-251.1
Statistical Analysis
All data obtained were tabulated as mean, standard deviation, and one-way analysis of variance (ANOVA) with Tukey HSD as post-hoc test. Statistical significance was established only if P < .05.
RESULTS
Frictional Characteristics
The coefficient of friction calculated out of load values obtained after friction evaluations for all three groups of archwires was tabulated. Mean and standard deviation were calculated and the data were analyzed with ANOVA followed by post-hoc comparisons. The group 3 archwires exhibited less frictional characteristics when compared with the other two groups upon 0.5 N and 1 N force application. Group 2 archwires were showing more friction when compared with uncoated forms upon 0.5 N force application, but upon 1 N force, they were showing reduced friction compared with uncoated forms. Except for the value comparing groups 1 and 2 upon 1 N force application, all other values were statistically significant (Table 1).
Surface Morphology
The cross-sectional views of the archwires from all groups are shown in Figure 3. The coating as well its varied thickness from each group of specimens was clearly visible from both the low (120×) and higher (500×) magnification images provided. Figures 4 and 5 provide low (300×) as well as higher (1000×) magnification of all three archwire groups before and after frictional analysis. Apart from minor drawing lines, the retraction process has not produced any significant surface abnormalities. The coating remains stable even after sliding of the bracket with loads of 0.5 N and 1 N.
Citation: The Angle Orthodontist 82, 1; 10.2319/040811-251.1
Citation: The Angle Orthodontist 82, 1; 10.2319/040811-251.1
Citation: The Angle Orthodontist 82, 1; 10.2319/040811-251.1
Load Deflection Rate With Looped Design
The load deflection rate of both coated wires (groups 2 and 3) remained to be low when compared with the uncoated form (group 1) in all three looped designs (U loop, reverse U loop and helical loop) tested. The difference between uncoated and coated wires was statistically significant, while the difference in load deflection rate exhibited by coated wires (between groups 2 and 3) was statistically insignificant (Table 2). It was interesting to note that the helical looped design was showing consistently low load deflection rate in all three groups of archwires.
DISCUSSION
The space closure mechanics in orthodontic treatment can be carried out either as frictional or sliding and looped mechanics. The archwires used in frictional mechanics should be able to slide through the brackets easily, while the archwires used in loop mechanics should exhibit low load deflection properties for optimal force application. The newly developed PVD coated archwires were excellent in their corrosion resistance against fluoride attack, showing low load deflection characteristics upon three-point bending test and were biocompatible upon cytotoxic evaluation.9 The heating process of approximately 250°C was not crossing the melting range of the beta titanium alloy and was not found to affect the mechanical properties of the archwires.9 In order to be clinically applicable, these archwires should be able to combat intraoral frictional forces and should be bendable to various looped designs without failure in coating.
Frictional force has long been an important consideration in orthodontic mechanotherapy. It is a well-known fact that any force needed to retract teeth must overcome friction.13 Various methods have been used in vitro to evaluate the frictional resistance of archwires against brackets, of which Tidy's protocol is one.11 This test closely simulates the clinical retraction of a canine and was adopted in this study. This can only be taken as a means of comparing the frictional characteristics of different alloy archwires in similar testing conditions because this does not replicate the exact intraoral environment. The test clearly indicated the superior nature of WC/C coated orthodontic archwires over uncoated and TiAlN coated archwires, exhibiting reduced frictional forces to sliding mechanics. Typically, WC/C coating with magnetron sputtering is carried out at temperatures below 500°C, and allows growth of coatings without significantly affecting the substrate's mechanical properties.14 This coating exhibits a number of favorable tribological properties, ie, low coefficient of friction, excellent adhesion, high hardness, and high residual compressive stresses.15 In recent times, the carbon-based coatings have also attracted attention in the medical arena owing to their good biocompatibility.16 Coatings of TiAlN were deposited using cathodic arc vapor (plasma or arc ion plating) deposition. The high ionization ratio and high deposition rate of cathodic arc process leads to the formation of a very dense and adherent coating imparting improvement in the lifetime of substrate as well as its wear and corrosion resistance.17 While TiAlN coatings improved corrosion resistance,9 the frictional characteristics were very high compared with WC/C coatings. Friction was higher than uncoated specimens upon 0.5N force application, while showing a lower value than uncoated specimens upon 1N force application, but the latter comparison was statistically insignificant.
All three archwire groups exhibited no significant surface changes from its prefrictional evaluation form when observed through ESEM at low and higher magnifications (Figures 4 and 5). The thinnest nature of WC/C coatings (mean value of 1.61 µm9) was further confirmed with the cross-sectional evaluation under ESEM (Figure 3). With all these favorable features, WC/C coated beta titanium orthodontic archwires can be recommended during orthodontic space closure, when sliding mechanics is preferred.
The ability to be formed into various looped archwires was the main advantage of the beta titanium orthodontic archwire combined with its low load deflection properties. So, the prerequisite of any coating done over the beta titanium substrate should improve or at least maintain these properties. Interestingly, the two newly developed TiAlN and WC/C coated beta titanium orthodontic archwires were consistently showing lower load deflection characteristics than its uncoated forms, which was statistically significant. This might be the result of change in its load deflection properties with the etching and heating process, while coating is performed over archwire blanks. The stability of two coatings over the beta titanium substrate upon bending at right angles was demonstrated previously.9 However, orthodontic loop making involves complex bending processes with high chances of archwire fracture due to incorporation of stress-generating areas. Both coated archwires withstood the bending process and the load deflection rates exhibited were also comparable with each other (Table 2).
Increasing the vertical or horizontal dimension of the wire increases its length incorporated into the archwire, which lowers the load deflection rate, helping application of low as well as consistent orthodontic forces. At the same time, closing loops should be stiff enough for the stabilization of arches preventing untoward tooth movements. The load deflection rate evaluation with looped designs showed that low force application was possible with helical design (which incorporates more length of the wire). The same pattern was exhibited by all three groups of archwires such as uncoated, TiAlN, and WC/C coated. Hence, both the coated archwires can be recommended for space closure stage of orthodontic mechanotherapy with looped archwires, where low load deflection rate matters.
The findings obtained in the present study along with the excellent corrosion resistance against fluoride attack, low load deflection rate and excellent biocompatibility demonstrated through a previous study,9 the newly developed TiAlN and WC/C PVD coated beta titanium orthodontic archwires might be useful during the space closure stage of orthodontic mechanics, be it sliding or frictionless loop mechanics.
CONCLUSIONS
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WC/C coated archwires with their thin nature and smooth surface showed low frictional properties when compared with uncoated and TiAlN coated archwires making it ideal for space closure stage of orthodontic mechanics, when sliding mechanics is used.
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No group of archwires, coated or uncoated, exhibited significant surface alterations upon ESEM evaluation after friction testing with 0.5 N and 1 N force application.
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Both, TiAlN and WC/C coated archwires exhibited low load deflection characteristics in comparison with uncoated forms, in looped designs, making it ideal for space closure during frictionless mechanics.
(a) Schematic representation of Tidy's protocol11. (b) Set-up for evaluation of frictional characteristics.
Various looped designs used in the study (U loop in BTUC, helical loop in TiAlN coated archwire, and reverse U loop in WC/C coated archwire in the descending order).
ESEM of cross-sectional view of uncoated and coated archwires in low (120×) and higher (500×) magnifications. Note that what appear as debris in the micrograph are trapped acrylic resin particles.
Lower magnification (300×) scanning electron micrograph of both uncoated and coated beta titanium orthodontic archwires before (BTUC, BTCT1, and BTCT2) and after friction evaluation (BTUCFRI, BTCT1FRI, and BTCT2FRI).
Higher magnification (1000×) scanning electron micrograph of both uncoated and coated beta titanium orthodontic archwires before (BTUC, BTCT1, and BTCT2) and after friction evaluation (BTUCFRI, BTCT1FRI, and BTCT2FRI).
Contributor Notes