Biomechanics of Anchorage Loss with Reduced Alveolar Support

Objective: A previous study evaluated the response of anchorage loss of first molar canine drive in an adult model with reduced periodontal support. However, the effects of orthodontic forces in situations with varying degrees of anchorage loss have not been completely elucidated. 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. 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. The teeth 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. A retraction force was applied to the first molar by means of power chains between the first premolar and first molars. The resulting stresses were observed and photographed in a circular polariscope. Results: With decreasing periodontal support, somewhat higher levels of stress developed at the mesial crest of the first molar. However, increased stresses were noted at the distal crest and mesial apex of the first premolar, indicating a tendency for distal tipping. Reduction in periodontal support had a lesser impact on the first molar. In contrast, higher stress developed at the crest of the anterior teeth. Conclusions: Orthodontic space closure in situations with reduced periodontal support may require more torque control anterior teeth procedures before closure.