CHANGES IN THE DIMENSIONS OF MILLED MANDIBULAR MODELS AFTER MANDIBULAR SAGITTAL SPLIT OSTEOTOMIES

Three-dimensional models created by milling machines and stereolithogr aphy from the three-dimensional computed tomograms (CTs) have become i mportant in the diagnosis and preoperative planning of some craniomaxi llofacial irregularities. In this study three-dimensional milled model s mere used to verify operative results after patients had undergone b ilateral sagittal split osteotomies of the mandible. Fifteen models of patients with mandibular prognathism and 31 with retrognathism were c reated preoperatively and 6 weeks postoperatively to examine the chang es of the osseous anatomical structures in three dimensions. There was a forward shift in the mandibular corpus of between 1 and 3 mm in pat ients with retrognathism and a backward shift to the same extent in th e group with prognathism. In the latter group the intercondylar distan ce increased by a mean of 2.0 (SD 1.2) mm and by 2.9 (SD 1.0) mm in pa tients who had had a mandibular advancement operation. Furthermore the distance between the coronoid processes increased by 6.6 (SD 1.8)mm i n the retrognathic group and decreased by 1.6 (SD 1.4) mm in the progn athic group. The Bonewill angle increased hy 1.7 (SD 0.4)degrees in th e prognathic group and decreased by 1.6 (SD 0.3)0 in the retrognathic group. Both increase and decrease of the homolateral gonion angle were accompanied by a contrarotary movement of the contralateral gonion an gle. We conclude that remarkable changes in condylar position and anat omy take place after bilateral ramus sagittal split osteotomy.