The purpose of this study was to investigate the positional changes of the hyoid bone following surgical correction of mandibular prognathism. The materials consisted of cephalograms of 10 patients before surgery and one and five years after surgery. To eliminate the influence of head posture, the position of the hyoid bone was revised on cephalogram with SH and C3H before surgery. The results as follows:
1. In the stable group after surgery, the position of the hyoid bone changed back-and downward. In the relapse group, the hyoid bone position did not change back-and downward. However, there were two cases which the hyoid bone position changed back-and upward, and three cases showed a forward and downward change.
2. The same result was apparently observed in the stable group and relapse group in one year after surgery.
3. In the stable group, the hyoid bone position also changed back-and downward five years after surgery. In the relapse group, the hyoid bone position changed back-and downward in three cases, and returned to the preoperative position in one case.
