Torque Angle: mid and most-prominent point non-coincidence effect

The torque angle for fixed orthodontic appliances is the angle between the middle of the labial surface of the clinical crown and a perpendicular line dropped on the occlusal plane. In a sample of orthodontic patients we reported that there was only 80% coincidence of the mid- and most-prominent points for upper centrals incisors and 50% coincidence for upper lateral incisors and canines (Smith et al, 2005). This may be clinically important as Meyer and Nelson (1978) showed that a 3mm vertical deviation of bracket position on a mandibular first premolar caused a torque angle discrepancy of 15°. Objectives: to measure the differences in torque angles when calculated from both the mid- and most prominent points of maxillary anterior teeth. Methods: Impressions were taken of 20 upper left and right central and lateral incisors and canine teeth. The resultant study models were then cut with a diamond saw to separate out the teeth which were then sectioned down the facial axis of the clinical crown into two halves. Images were acquired of the cut surfaces using image analysis and the torque angles measured according to Andrews (1972). The most-prominent point torque angle was then subtracted from the mid-point torque angle to determine the torque angle discrepancy. Results: Table 1. Difference between mid- and most-prominent points torque angles.

Conclusion: The marked differences shown in this study in torque angles on upper anterior teeth are likely to be clinically relevant.