Foundation And Clinical Research Of Removing Mandibular Outer Cortex And Onlay Bone Graft In Craniomaxillofacial Surgery

Osteotomy of mandibular outer cortex was used to treat width and dissymmetry of lower face or as source of autogenous bone graft simultaneously in craniomaxillofacial surgery. This article illustrates some study of mandibular outer cortex and onlay bone graft on basic and clinic area, reports the results of our further basic and clinic research.Animal experiment: 8 minitype pigs with 8-12months age were selected as the experimental animals. The outer cortex of mandibular body and angle was removed in one side; part of outer table of skull was also decorticated. Similar size about 25×10×2mm~3 of mandibular graft or calvarial graft was implanted on each side of the snouts of the animal after volume determining by a volume-displacement method.2 pigs were selected randomly and were killed 12 weeks after bone grafting. Each 1.5×0.5cm~2 specimen was harvested from grafting and donor site of mandibular outer cortex and outer table of skull, histological study(HE stain, scanning electron microscopical, VEGF immunohistochemistry) was performed. The other 6 animals were killed 24 weeks after operation. Gross inspection of the snout was performed at that time and included examining the condition of the grafts. Cast was made using an elastic polyether dental impression material, dental gypsum models were then poured and allowed to harden, the volumes of the gypsum grafts were then determined using the same volumetric technique as performed preoperatively on the bone grafts. Osteotomy of mandibular body perpendicular to inferior border were done every 1cm from mental foramen, thickness of mandibular outer cortex were measured bilaterary on 4 cross-sections been chosed. 3.5×1.5cm~2 bone strut of mandibular angle region was taken bilaterary and send for biomechanical test. Histological observation(HE stain, scanning electron microscopical, VEGF immunohistochemistry) was done to both graft and donor sites of twe type of bone graft. The results showed:①There was no obvious difference compared with outer table of skull in the amout of bone resorption and the histological change. For onlay bone graft to craniofacial area, mandibular outer cortex is a good choice as source of autogenous bone graft.②local defect can have histological repair completely 24weeks after mandibular outer cortex osteotomy. Biomechanical test shows no significant difference at angle area between experimental and normal sides. Outer cortex of mandibular body get thinner after regeneration. Bone tubercle can be found at angle area. Lateral deviation occlusion was found in some animals. Clinical research: There were 52 cases of mandibular outer cortex osteotomy or craniofacial bone graft simultaneously. Using computer tomography(CT) true-up and dissection technique, variables that have been observed and analyzed included: thickness of mandible preoperative, immediate post-surgical, 6 months postoperative; regeneration at 6 months postoperative compaired with immediate post-surgical; volume change of bone graft at different recipient-site. The results show: @Depression could be seen at the area that mandibular outer cortex had been removed for 6 months, especially at the external oblique line region. Average cup depth is 5.85±1.41mm for immediate post-surgical Vs preoperative, 2.30±1.01mm for immediate post-surgical Vs 6months postoperative, 3.64±1.67mm for 6 months postoperative Vs preoperative; compaired with preoperative, volume diminution percentage is 1.7±0.5％to the whole mandible and 55±9％to the area of outer cortex osteotomy at 6 months postoperative.②At the area that mandibular outer cortex had been removed, bone regeneration could be seen, compaired with immediate post-surgical, percentage of bone neogenesis is 84.6±7.3％6 months postoperative. The main regeneration region showed on the rainbow tomograph of differences is angle area.③After onlay graft, bone resorption occurred. To different recipient-site, bone resorption rate is inequally. At mandible area, bone resorption rate is 20.8±7.2％, mainly at lower and posterior border. At anterior part of maxilla, bone resorption rate is 11.2±2.3％. Statistics showed significant difference of resorption rate between the two sites.Conclusion: New bone regenerate and histological structure can gain complete repair after removal of mandibular outer cortex. Depression can be seen and bone gets thinner at the area. Main site of bone regeneration is at angle region. Bone resorption rate of mandibular outer cortex after onlay bone graft has no significant difference to that of outer table of skull and histological changes are similar between them. Bone resorption of mandibular outer cortex after onlay graft is few and variant according to different recipient-site. Conclusion supports the clinical application of mandibular outer cortex as therapeutic tool of facial skeleton aesthetic surgery and autogenous bone graft resource. Quantization of bone resorption rate can guide its clinical use better. The incomplete coincidence of bone resorption rate between animal experiment and clinic research should be study further by improvement of experiment condition and augmentation of sample size.