Methods: The load systems of two groups of 17 segmental TMA T-loops in six bilateral canine retraction patients were used in this study. Their values before and after each treatment interval were tested on the corresponding maxillary dental casts using a custom-made orthodontic force tester. Both groups had an initial average retraction force of 125 cN. The average initial anti-tipping moment of each group was 6.1 N-mm for controlled tipping and 7.7 N-mm for translation. The other moment and force components were kept minimum. The same spring was then mounted on the cast made after the treatment. The residual load system was measured. The pre- and post-treatment IBDs were recorded for calculating the amount of space closure.
Results: The initial IBDs in this study range from 16.1 to 24.4 mm because of interpersonal difference or incremental tooth displacement. As IBDs decrease due to canine retraction, the averaged Retraction Force Drop per IBD Decrease is 38 cN/mm, a 30% drop per 1 mm IBD decrease (P<0.0001); and the averaged anti-tipping Moment Drop per IBD Decrease is 0.8 N-mm/mm, ~12% drop per 1 mm IBD decrease (P=0.0151). The results indicate that the moment is less sensitive to IBD decrease than the force. Consequently, the average Moment-to-Force (M:F) Ratio Increase per IBD Decrease is 1.9 mm/mm (P=0.0001).
Conclusions: In canine retraction, the retraction force decreases percentagewise more than the anti-tipping moment, which results in the M:F ratio increase. The initial M:F ratios need to be lower than ideal to reach the expected values.
Keywords: Biomechanics and Orthodontics