Biomechanics of Space Closure with Reduced Alveolar Support

Objectives: The biomechanical effects of anchorage loss of first molar and moving of first premolar has been studied in an adult model with reduced periodontal support. However, all 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, moving of first premolar and anterior teeth 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" rectangular wire was ligated to the brackets. A retraction force was applied to the first molar by means of power chains between the first premolars and second molars. The resulting stresses were observed and photographed in a circular polariscope. Results: With decreasing periodontal support, somewhat higher of stress developed at the mesial crest of the first molar. However, increased stress was 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 effect on the first molar. And somewhat higher of stress developed at the crest of the anterior teeth. Conclusion: The results indicate that space closure in situations with reduced periodontal support may require more torque control anterior teeth procedures before closure.