[Postoperative evaluation of mandibular prognathism corrected by sagittal splitting osteotomy]

The purpose of this study is to examine short-term postoperative stability of mandibular prognathism subjected to the surgical-orthodontic treatment. Eight skeletal Class III patients (7 females and 1 male) corrected by sagittal splitting ramus osteotomy after preoperative orthodontic treatment. The stable occlusions at six months after the orthognathic surgery were evaluated. Cephalometric radiographs and cephalometric laminagraphs were used as materials. The results were as follows: 1) Surgical changes: Pogonion was repositioned by 9.8 mm in an average. Overjet was increased of 9.1 mm in an average. Condyles were displaced antero-inferiorly in 3 cases. 2) Changes during intermaxillary fixation: Antero-superior rotation of the proximal segments were found. Region of chin moved inferiorly. Upper and lower incisors were inclined lingually. Variation of lower incisor inclination was larged. Condyle was displaced. 3) Changes from immediately before the release of intermaxillary fixation to the first month thereafter: Anterior-displacement of pogonion and antero-superior rotation of the proximal segment were found. Upper and lower incisors were inclined labially and lingually, respectively. Overjet was decreased. 4) Changes from the 1st to the 6th month after the release of intermaxillary fixation: Antero-superior rotation of the proximal segment and anterior displacement of pogonion were found to a slight extent. Upper incisor presented labial inclination and condyle tended to return to its preoperative position. The above results revealed that the aspect of relapse presented different changes between intermaxillary fixation period and after the release thereof. It was suggested to be ascribable to the setting up of the posterior margin of the distal segment in a position susceptible to the influence of soft tissues at skeletal fixation and to the extension of medial pterygoid muscle adherent to the distal segment. Therefore, appropriate retention of Class III elastics after the release there of were judged to be useful as countermeasures against relapse. In order to obtain postoperative stability, appropriate positioning of the proximal segment and countermeasures to the stress of medial pterygoid muscle during operation remain to be required further.