Objective: A previous study evaluated the response of canine drive in an adult model with reduced periodontal support. However, the differences in the effects of orthodontic forces between the multi root tooth and the single root tooth have not been completely elucidated. The purpose of this study was to photoelastically evaluate the effects of space closure mechanics in second premolar extraction case with reduced periodontal support situations. 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" diameter wire was ligated to the brackets. A retraction force was applied to the first molar by means of power chains between the canines and first molars. The resulting stresses were observed and photographed in a circular polariscope. Results: With decreasing periodontal support, somewhat lower stress developed at the mesial crest of the first molar. However, increased stress was noted at the mesial crest and distal apex of the first premolar, indicating a tendency for distal tipping. Reduction in periodontal support had a lesser effect on the first molar. Conclusions: The results indicate that space closure in situations with reduced periodontal support may require more finishing procedures after closure.

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