[A biomechanic study on the relapse after sagittal split and oblique osteotomy of Ramus]

The relapse after orthognathic surgery was frequently reported in recent years. But up to now researches concerning bio-mechanic mechanism of relapse are rare. The purpose of this study, by imitating the operation on monkeys, was to examine the biomechanical properties and histological characteristics after operation in order to investigate the mechanism of the relapse after orthognathic surgery. 20 rhesus monkeys divided into two groups were subjected to bilateral sagittal split and oblique osteotomy, respectively. The broken strength, stress and strain of the operated sites were examined with the use of biomechanical methods, and the healing scab was observed using histological method and SEM at 3, 6, 12, 24 weeks after operation. The results obtained demonstrated that bone healing and biomechanical properties reached certain level after 6 to 12 weeks' recovery in both operation groups; the biomechanical properties and histological characteristics recovered faster in sagittal split group than in oblique osteotomy group. Histological examination also demonstrated similar results. From the data above it is suggested that the higher relapse rate in sagittal split may more closely related to the mechanical effect different from the scab healing, although 8 weeks' fixation may meet the requirement of clinical treatment, accessory fixation method should be adopted after the removal of inter-maxilla fixation.