Background and Objective: Resistance to sliding is a combination of friction and binding. When the angle between the bracket and the wire exceeds the critical angle, binding occurs. The influence of angulation is confounding because of certain inherent physical characteristics of materials. The present study was carried out to evaluate the effect of wire- bracket angulation on resistance to sliding of NiTi orthodontic wires.

Materials and Methods: In this in vitro study, 60 straight pieces of 0.016-inch NiTi wires were used along with 60 standard system 0.018 stainless steel brackets. The friction test was carried out at three different angles of 0°, 5° and 10° to simulate the 2nd-order bends between the bracket and the wire. The wires were assessed by SEM. One-way ANOVA, and Tukey tests were used for data analysis.

Results: Statistical analysis showed an increase in friction between the bracket and the wire with an increase in the angle between them (P<0.001). The mean of frictional forces at three angles between the wires and the brackets was 1.52±0.16 N. The differences in frictional forces between 0º and 10º, and 5º and 10º angles were significant (P<0.001 and P<0.007, respectively).

Conclusion: The present study revealed a relationship between the bracket and wire angulation and the friction between them. Increasing angulation up to 5º is not significant, but in upper degrees this increase is significant. This matter should be considered in orthodontic treatment to reduce complications and treatment duration.