On inferior alveolar nerve function after sagittal split osteotomy of the mandible

Abstract: Osteotomies of the mandible are designed to adjust improper jaw positions. While vertical ramus osteotomy can be used for mandibular setback but not for mandibular advancement, sagittal split osteotomy can be used to move the lower jaw in both directions and this surgical technique also permits mobility of the lower jaw immediately after surgery. The versatility of sagittal split osteotomy has made it widely used although it is relatively frequently followed by various degrees of neurosensory disturbance ("numbness") in the patient's lower lip and chin. Reports of the incidence of such neurosensory disturbance vary widely in the literature, and the aetiology of the condition has been debated. The possible impact of neurosensory disturbance on a patient's impression of the treatment result has only been vaguely described. This thesis is based upon observations from a patient material consisting of 1,034 orthognathic surgery patients, with a follow-up period of 2 years. The sensitivity of the lower lip and chin was analysed among the 818 who had undergone mandibular surgery. While vertical ramus osteotomy and genioplasty were relatively infrequently followed by neurosensory disturbance, sagittal split osteotomy was followed by such neurosensory disturbance in the lower lip and chin in 40% of the operated sides. Half of these disturbances were mild, the other half more pronounced. Additional genioplasty tended to worsen the neurosensory function after sagittal split osteotomy, but to a lesser degree than expected. Various other parameters were studied with respect to their possible correlation to neurosensory disturbance after sagittal split. It was found that increasing patient age was followed by impaired recovery of the neurosensory function. The surgeons' routine influenced the sensitivity outcome, while neurosensory disturbance was only weakly correlated to other parameters studied, such as type of split technique, degree of intraoperative nerve encounter, degree of mandibular movement, and type of osteosynthesis. The soft tissue dissection prior to the bone cut was suggested to be partly responsible for some of the neurosensory disturbances observed. The bed-side-type of sensitivity evaluation in the large patient material was compared with more sophisticated, objective sensitivity assessments and was found to be quite reliable. While increasing age of the patients resulted in increasing incidence of neurosensory disturbances, it was also found that the impact of such neurosensory disturbances was greater in older patients than in young ones. The youngest three quarters of the patients rated the outcome of their treatment equal regardless of existing or nonexisting neurosensory disturbance. In the oldest quarter (aged over 35) patients who demonstrated a reduced sensitivity of their lower lip and chin rated their final treatment result significantly lower than did those with normal sensitivity. The possible effect of a dissection trauma on the nerve function was studied in an animal experimental model, The signal substances galanin and substance P were examined in the rat trigeminal ganglion after compression injury to the mandibular nerve and after transection of the nerve. It was found that the ganglion responses to both types of nerve injury were identical, indicating that a dissection compression of the nerve might result in severe damage. Conclusions: Sagittal split osteotomy was followed by neurosensory disturbance of the lower lip and chin in 40% of the operated sides. Half of these were mild, the other half more pronounced. Older patients suffered from neurosensory disturbances more often than young ones, and were also more disturbed by the condition than younger patients were. The presently utilised dissection technique may negatively influence the neurosensory function and is proposed to be changed.