Comparison of Mechanical Properties of Plain and Reversed Curve Nickel-Titanium Archwires

Abstract

In orthodontics, nickel-titanium wires are used for teeth alignment and leveling. For leveling the curve of Spee, reversed curve archwires are often used to increase the vertical force needed to correct a deep bite.Objectives: The aims of this study were to investigate and compare the mechanical properties (unloading force, stiffness, springback, and surface hardness) of the pre-formed plain and reversed curved NiTi archwires.Materials and Methods: NiTi wires of dimensions 0.016x0.022 inch were divided into two groups, Group 1 - plain and Group 2- reversed curve NiTi archwires. For each type of the archwire, load-deflection curve obtained from a three-point bending test, performed by a Texture Analyser (TA.XT.plus, Stable Micro System, United Kingdom) with 5 kg load cell at room temperature, was used to analyze unloading force, springback, and stiffness. Surface hardness was measured by Vickers micro-hardness tester. Kruskal-Wallis test was used to analyze the variables of this study.Results: The results showed that the unloading force of each deflection point of the reversed curve NiTi archwire was more than the plain archwire. The means of unloading force, stiffness, and springback were 2.42 N, 2.76 N; 0.28 N/mm, 0.49 N/mm; and 2.94 mm, 2.98 mm for the plain archwire and reversed curve NiTi archwire, respectively. The properties of reversed curve NiTi archwire were significantly higher (p<0.05) than the plain NiTi archwire, except the springback. The surface hardness of the plain archwire was significantly higher (p<0.05) than reversed curve NiTi archwire in each segment.Conclusion: The reversed curve NiTi archwire had more unloading force and stiffness than plain NiTi archwire. For the correction of deep curve of Spee in orthodontic treatment, clinicians must be aware of the vertical force needed during intrusion of lower incisors or the wires should be used in the later leveling and aligning stage.