Changes in the dimensions of milled mandibular models after mandibular sagittal split osteotomies

Three-dimensional models created by milling machines and stereolithography from the three-dimensional computed tomograms (CTs) have become important in the diagnosis and preoperative planning of some craniomaxillofacial irregularities. In this study three-dimensional milled models were used to verify operative results after patients had undergone bilateral sagittal split osteotomies of the mandible. Fifteen models of patients with mandibular prognathism and 31 with retrognathism were created preoperatively and 6 weeks postoperatively to examine the changes of the osseous anatomical structures in three dimensions. There was a forward shift in the mandibular corpus of between 1 and 3 mm in patients with retrognathism and a backward shift to the same extent in the group with prognathism. In the latter group the intercondylar distance increased by a mean of 2.0 (SD 1.2) mm and by 2.9 (SD 1.0) mm in patients who had had a mandibular advancement operation. Furthermore the distance between the coronoid processes increased by 6.6 (SD 1.8) mm in the retrognathic group and decreased by 1.6 (SD 1.4) mm in the prognathic group. The Bonewill angle increased by 1.7 (SD 0.4)° in the prognathic group and decreased by 1.6 (SD 0.3)° in the retrognathic group. Both increase and decrease of the homolateral gonion angle were accompanied by a contrarotary movement of the contralateral gonion angle. We conclude that remarkable changes in condylar position and anatomy take place after bilateral ramus sagittal split osteotomy.