Postoperative Stability and Three-dimensional Evaluation of Medial Pterygoid Muscle in Skeletal Mandibular Protrusion Cases Undergone Short Lingual Osteotomy

Abstract

In mandibular protrusion cases who undergo short lingual osteotomy（SL）, interference between the distal mandibular segment and medial pterygoid muscle（MPM）is occasionally seen at setback operation. The interference causes posterior deviation of the mesial mandibular segment and affects the postoperative stability. Thus, the relationship between the MPM and the mesial segment was examined three-dimensionally using preoperative CT images.
In this study, 18 cases of mandibular protrusion（7 males and 11 females, average age at first visit: 22.4 years old）who visited Meikai University Hospital were examined. All cases were not affected by any congenital anomalies, and the lateral deviation of Me was within 3mm. Lateral cephalograms were taken immediately before surgery（T1）, less than 5 weeks after surgery （T2）, and more than 1 year after surgery（T3）. CTs were taken at T1. Seven of the 18 cases showed postoperative anterior relapse of more than 2.0mm at Me from T2 to T3（group U）. The relapse of the other 11 cases（group S）was 2.0mm or less. Using DICOM data of CT, MTM was reconstructed three-dimensionally, and the relationship with the mesial mandibular segment was analyzed.
There was no significant difference in the amount of mandibular setback and decrease in SNB angle between group S and group U from T1 to T2. Group U, but not group S, showed posterior deviation of the mesial mandibular segment from T1 to T2. MPM was significantly inclined less laterally in group U than group S at T1, indicating that cases in group U exhibited closer proximity of MPM to the mandibular ramus.
These findings suggest that the proximity of MPM to the mandibular ramus at T1 is one of the risk factors causing interference, and affects the postoperative stability when performing SL. Simulation of MTM, together with the mandible, is informative in predicting postoperative stability at SSRO.