Biomechanics of First Molar Mesial Movement with Reduced Alveolar Support

Objective: A previous study evaluated the response of anchorage loss of first molar in an adult model with reduced periodontal support. The purpose of this study was to photoelastically evaluate the effects of space closure mechanics in reduced periodontal support situations with first molar anchorage loss using rectangular wire and serial tying between first premolars.

Methods: Three-dimensional models of human mandible were fabricated using different birefringent materials to simulate bone and teeth. The models were identical except for the following alveolar support conditions: normal and 5 mm reduction from normal. The models had all teeth except for the second premolars bilaterally and were placed to represent post-leveling positions. Appropriate brackets with 0.018” x 0.025" slots were bonded on the teeth. A 0.016"x0.0022" diameter wire was ligated to the brackets. This wire was serially ligated between first premolars. A retraction force was applied to the first molar by means of power chains between the canine and first molar. The resulting stresses were observed and photographed in a circular polariscope.

Results: With decreasing periodontal support, somewhat higher of stress developed at the distal of the first molar and distal apex of canine. However, increased stress was noted at the distal apex of canine and distal apex of the first premolar, indicating a tendency for distal tipping. These situations were caused from bowing of wire by distal tipping of canine. Reduction in periodontal support had a lesser effect on the first molar. And somewhat higher of stress developed at the crest of the frontal teeth.

Conclusions: The results indicate that space closure in situations with reduced periodontal support may require more torque control frontal teeth procedures and bend for first premolar and first molar corresponding of wire bowing before closure.