Skeletal Changes and Stability Following Various Orthognathic Surgical Procedures for Correction of Mandibular Retrognathia

The improvement of diagnostic methods, treatment planning, orthodontic preparation of the patients and techniques of orthognathic surgery within the last decade have evolved to allow optimal correction of the facial disproportion resulting from structural abnormalities of the jaws. Nevertheless, relapse has been a major problem. Multiple factors are associated with or contribute to postsurgical instability and the resulting relapse. The results of recent investigations have shown that there is considerable relapse following surgical correction of mandibular retrognathia more than any other orthognathic surgical procedure. Therefore, the aim of this thesis was to evaluate skeletal stability following various orthognathic surgical procedures for correction of mandibular retrognathia. The following surgical procedures were assessed in this thesis; pedicled advancement genioplasty, sagittal split advancement osteotomy, bimaxillary osteotomy with internal rigid fixation and bimaxillary osteotomy with non-rigid fixation. Bimaxillary osteotomy cases had simultaneous maxillary impaction and sagittal split advancement osteotomy. The cases studied in this thesis were treated at either Canniesburn Hospital, West of Scotland Regional Plastic & Maxillofacial Unit, U.K. or at Michigan University Hospital, Ann Arbor, U.S.A. Lateral cephalometric radiographs were used to evaluate surgical and post-surgical changes. It has been shown in the literature that conventional cephalometry is inappropriate tool for morphometric analysis. The limitations of conventional cephalometric technique had induced stasis in the understanding of facial form and its changes. The recently introduced methods of morphometric analysis, Finite Element and Euclidean Distance Matrix analyses have seemed to overcome most of the problems associated with cephalometric analysis. The methods allow assessment of surgical changes independently to any plane of orientation or registration e.g. Sella-Nasion or Frankfort plane and do not require any cephalometric superimposition. These methods in addition to conventional cephalometric analysis were used to evaluate surgical stability in the different group of cases considered in the thesis. Advancement genioplasties were performed by horizontal osteotomy to provide chin prominence. Three inter-osseous wires were used for the stabilisation of the symphysis segments. This study has shown that the standard advancement genioplasty produced excellent results. Bone stability was generally very good irrespective of any other concomitant type of orthognathic surgery which was carried out. In the group of cases who had bimaxillary osteotomy with rigid internal fixation at Canniesburn Hospital, the maxillray osteotomy was more stable when compared to mandibular advancement. The mandibular postsurgical changes at 6 month follow up demonstrated clockwise rotation with posterior settling. This was mainly secondary to the posterior condylar displacement during surgery. No further changes were detected at one year follow up. The cases who had bimaxillary osteotomy at Michigan, with rigid internal fixation were more stable. At 6 months follow up, the mandible showed less posterior settling than Canniesburn cases. The condyles were in their presurgical anatomical position which may have enhanced mandibular stability. However, in comparison with a similar group of cases treated at Canniesburn, the magnitude of mandibular advancement was smaller, with less presurgical facial deformity. In cases who had bimaxillary osteotomy with non-rigid fixation, at Canniesbum Hospital, the maxilla was generally stable similar to other groups. However, mandibular stability was different in comparison with the rigid fixation group treated at the same centre