Effects Of Mandibular Setback On The Inferior Alveolar Nerve And Temporomandibular Joint

The sagittal split ramus osteotomy(SSRO) and intraoral vertical ramus osteotomy(IVRO) are the main surgical procedures used to correct mandibular excess. But there is still controversial that which one is the preferred treatment especially as to the surgical techniques on the inferior alveolar nerve(IAN) and temporomandibular joint(TMJ).Rigid internal fixation(RIF) has been used routinely in SSRO, as it can promote bone healing, restore early oral function, and reduce relapse. Some surgeons have applied a popular technique of placing 2 or 3 bicortical screws to fix the proximal and distal segments. Others advocate using monocortical screws and a plate to fix the two segments. Neurosensory deficit after SSRO is one of the most common complications. A number of clinical investigations have reported neurosensory deficits after SSRO associated with the use of various fixation techniques. These may involve traction on the IAN during operation, and direct injury to the nerve when the ramus is split and the screw holes are drilled, compression of the bony segments on the IAN. Effects of monocortical and bicortical osteosynthesis in SSRO on the IAN are an important subject to illuminate. SSRO and IVRO are bound to influence the original structure and function of TMJ, since they alter the mandibular anatomic structure and jaw relation. Changes of condylar position are the major factor inducing the remodeling of TMJ. However its significance hasn't final conclusion that in what way condylar displacement after operation is, and the relationship between orthognathics and temporomandibular disorders (TMD) hasn't been claritied.Based on the above knowledges, this study is to provide with scientific basic for the clinical application of IVRO, and monocortical and bicortical fixation of SSRO, using sensory function of IAN, radiographic, and histo-morphological evaluation.In this study, an animal model of correcting mandibular excess by these two methods was established in adult Macaca Malutta. Then the effects of the two operations were evaluated by sensory nerve action potential(SNAP) of IAN, radiographic, and histo-morphological examinations. The results of this study will provide valuable informations for the application of SSRO and IVRO.The results of this study show that:1. An animal model of correcting mandibular prognathism by SSRO and IVRO was successfully established in adult rhesus monkeys.2. Both fixations can injury the IAN during the SSRO through SNAP testing and histology, but the monocortical fixation had less damage than bicortical fixation. On the other hand, the former bring earlier restoration than the latter. 3. Changes in condylar position were quantified by three-dimensional computed tomography(3-D CT) preoperatively and postoperatively. Histological examination of the TMJ specimens was performed 12 weeks after operation. IVRO group showed a significant anteroinferior displacement of the condyle and thickened cartilage layer and endochondral ossification were seen in the condyles after operation. The adaptive remodeling may have favorable effects on TMJ of those patients with preoperative TMD. Slight posterior displacement and lateral tilting of the condyle and only minimal alteration in articular cartilage were noted after operation in SSRO group.