Investigation of vertical mandibular distraction osteogenesis on the masticatory muscles in a ’unilateral hemifacial microsomia like’ defect in the sheep model.

Distraction osteogenesis is a recently developed option for surgical correction of the craniofacial discrepancy but there are few studies that look at the long-term effect of the relapse phenomenon. This study was conducted to look at the bone and muscle response at different consolidation and remodelling periods. Thirty ten-week-old Merino lambs were subjected to this current study. The surgically created defect was performed on the experimental side (right) by superficial masseter myectomy and a condylectomy. The lambs showed a midline shift to the affected side three months later. Marker screws were placed on both sides of the mandible to examine the adjustment of the vertical and horizontal dimension of the mandible. In this present study, the left side was used as the control. The affected mandible was surgically corrected using a vertical ramus distraction osteogenesis protocol. The latency period was 7 days where the device was inactive. Distraction was then performed at 1 mm per day until the distracted gap was 10 mm on a radiograph. Initially, the sheep were divided into 8 groups but later the groups were refined into 6 groups for statistical analysis; Group 1: Immediately post distraction Group 2: Consolidation 2 months and remodelling 1 month Group 3: Consolidation 3 months Group 4: Consolidation 3 months and remodelling 1 month Group 5: Consolidation 3 months and remodelling 2 months Group 6: Consolidation 4 months A ‘hemifacial microsomia like’ defect was successfully created and it was then corrected using vertical ramus distraction. Cephalometric examination showed that the vertical height was significantly increased after the distraction was completed. Examination of the bone and muscle was performed to look at the adjustment of bone structure in relation to relapse. The bone investigation was conducted using radiological analysis, histological analysis and direct measurement of the vertical screws at pre-distraction and at sacrifice as well as histomorphometric analysis. The radiological examination was conducted using cephalometric analysis of the distance between marker screws on both experimental and control sides. Bone histology was investigated on the middle and posterior position of the experimental side. The histomorphometric analysis was conducted using Quantiment analysis software. The muscle responses and adaptation were investigated by measuring: the weight of the masseter and medial pterygoid muscles; length between 6 different points (4 landmarks); cross section and thickness of masseter muscles by ultrasound and by histopathological examination of both masseter and medial pterygoid muscles on the experimental and the control sides. The distracted bone showed a completion of maturation after a three month consolidation period. Bone formation was shown to continue after longer consolidation periods. A two-month consolidation period was insufficient time for the bone to consolidate and stable, as there was evidenced of relapse during this period. There was no…