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 canines.
Methods: Three-dimensional models of human mandible were fabricated using different birefringent materials to simulate bone and teeth. The models were identical except for alveolar support conditions: normal and 5 mm reduction from normal. The models had all teeth except for the first premolars bilaterally and were placed to represent post-leveling positions. A 0.016"x0.022" rectangular wire was ligated to the brackets. The wire was serially ligated between canines. Retraction force was applied by power chains between the canine and second molar. Resulting stresses were observed and photographed in a circular polariscope.
Results: With normal support, activation caused stress along the mesial root and distal crest of the second premolar, indicating mesial motion and rotation. With reduced support, the stress along the mesial root of the second premolar was reduced, but the stress at the mesial crest increased considerably. This stress distribution indicates strong mesial rotation of second premolar. Stress on the anterior teeth was similar for both support conditions.
Conclusions: The results indicate that, with increasing loss of periodontal support, the increasing tendency for mesial crown rotation of the premolar needs to be compensated. This may be accomplished by gable bending of the wire.