Coated and Uncoated Nickel-Titanium Archwires: Mechanical Property and Surface Topography Response to a Simulated Oral Environment with and without Bracket-Related Load Deflection Induced Stress
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This study examined the effect of exposure to bracket-related load deflection on the mechanical properties and surface characteristics of esthetic coated and uncoated nickel titanium archwires. Two types of coated archwires were tested: one with a rhodium ion coating and another with a polymer coating. Corresponding uncoated wires of the same size were also tested. These four different wires were divided into treatment groups based on exposure to bracket-related load deflection and storage in PBS at 37±1°C. A three-point bend test in DI water at 37±1°C was performed on specimens at three time points: 4 weeks, 8 weeks, and 12 weeks. Unloading forces at 1, 2, and 3 mm of deflection were recorded. After 12 weeks exposure, three representative specimens from each treatment group along with untested wires were viewed using scanning electron microscopy (SEM) to provide a qualitative surface topography analysis. Results of the present study showed that no significant differences (p > 0.05) in unloading force were present in wires exposed to bracket-related load deflection. However, significant differences were observed between coated and uncoated wire types (p < 0.05). Rhodium-coated wires exhibited significantly increased unloading forces compared to their corresponding uncoated wires. Conversely, polymer-coated wires exhibited significantly decreased unloading forces compared to their corresponding uncoated wires. This may be due to the process of manufacturing the coated archwires or due to differences in the underlying wire itself. SEM analysis revealed that for the rhodium-coated wires, rhodium precipitated out after 12 weeks exposure to PBS at 37±1°C. Furthermore, the degree of this precipitation appeared to be more severe in wires exposed to bracket-related load deflection. Polymer-coated wires showed a complete loss of coating in areas where the archwire contacted orthodontic brackets. This did not occur in wires not exposed to bracket related load deflection. The change in mechanical properties, specifically unloading forces, of coated archwires compared to uncoated archwires of the same size could be clinically significant in either increasing or decreasing rate of tooth movement. In addition, coating instability could cause increased friction in the archwire-bracket interface, which could also decrease efficiency of orthodontic tooth movement.
Table of Contents
Introduction -- Materials and methods -- Results -- Discussion -- Conclusions -- Literature cited -- Appendix