Incisor Torque Expression With Varying Degrees of Twist: An In-vitro Analysis

Objectives: The proper labiolingual inclination of anterior teeth is critical in achieving good overjet, overbite and occlusion. The objective of this study was to compare the torque expression created by different types of archwires undergoing varying degree of torsion in the maxillary incisors brackets.
Methods: Four types of rectangular archwires (0.019x0.025-inches) were investigated . A torque measuring device consisting of a rotating table with two vertical beams to stabilize the archwire was designed. Four incisor standard edgewise brackets (0.022x0.028-inches) were passively bonded on the rotating table with an inter-bracket distance of 6 mm. 10 samples of each archwire type were twisted (loaded) clockwise from a passive state in increments of 5° till 40° and then detwisted (unloaded) in the same manner back to 0°. The same process was followed on the same archwires in the counter clockwise direction. The expressed torque (N-mm) was measured by load cells placed along the long axis of the archwires.
Results: The magnitude of expressed torque decreased significantly when twisted in the counter-clockwise direction. Also, the unloading curves showed significantly smaller magnitudes than the loading curves for all wires and twists. Stainless steel (SS) archwires had the highest torque expression followed by beta titanium (ß-Ti), nickel-titanium (NiTi) and Copper (Cu)-NiTi archwires for any given degree of twist. The clinically acceptable torque of 10-20 N-mm was observed at 20° for SS wires and at 25°, 26° and 28° degrees for ß-Ti, NiTi and Cu-Niti wires respectively. SS wires showed significantly higher torque expression than NiTi and Cu NiTi wires between 10-30° however no significant differences were observed for the same between ß-Ti and NiTi wires.
Conclusions: Torque expression of NiTi and ß-Ti wires were similar but lower compared to SS wires. However, the difference between these wires for the minimum effective clinical torque is about 5-10 degrees