| Tumor and tumor-like diseases is one of the main reasons of acquired deformity and defects in maxillofacial surgery.They often cause anisomerous deformity and irregular defects of the tissue,which often result in disfigurement and dysfunction.It is important for maxillofacial surgeons to find ways to resolve the problem.Repairing the irradiated facial bone defects,especially caused by tumors,has been a problem for oral and maxillofacial surgeons for a long time. Iliac and fibula transplantation have been proved successful approachs by surgeons. But these techniques also resulted in inreversible defects of other bones.Nowadays, distraction osteogenesis(DO) has become a regular way in reconstruction of craniofacial deformity,but seldom recorded in use in irradiated bone defects. Recently, experiments showed DO should also be a potential way to repair the irradiated mandible. However,there are still many problems about DO in irradiated mandible,such as the character of the new bone,the adaptability of the inferior alveolar nerve (IAN), et al.In this study, we intended to establish an animal model of distraction osteogenesis in the irradiated dog mandible for further study in this field. And with this kind of model, the scope of the irradiation doses that can achieve successful DO and the electrophysiological changes of the irradiated IAN were also studied.Three parts were included in this study.Part 1 Establishment of an animal model of distraction osteogenesis in the irradiated dog mandible and the study of its characteristicsTo explore the character of distracted bone in the irradiated dog mandible and the corresponding change of the irradiated inferior alveolar nerve(IAN) in order to establish an animal model. Eight Chinese dogs were used in this study. Six dogs received a unilateral irradiation from 60Co(group R) in the mandible with a total dose of 22.8Gy in four 5.7Gy fractions (biologically equivalent to 50Gy/25 fractions).The other two dogs without irradiation served as control (group C). Bilateral corticotomy was made 3 months after completion of irradiation. After a 1-week latency period,bone distraction was activated at a rate of 0.5mm twice daily for 10 days, followed by a consolidation phase of 8 weeks. New bone underwent radiographic,histological and single photon emission computed tomography (SPECT) analysis. One dog was excluded from the study due to anaesthetic in group R. After 8 weeks of consolidation, no difference was found between the percent area of new bone in both groups. New bone was more mature and organised in group C than in group R.SPECT analysis showed there are active osteogenic activity in group R. We established the animal model of distraction osteogenesis in the irradiated dog mandible.Part 2 Feasibility of distraction osteogenesis in the dog mandible under different radiographic dosesTo test the safety doses of distraction osteogenesis(DO) in the irradiated dog mandible.14 Chinese dogs were used in this study. They were randomly divided into 4 groups.Group A,B,C own 4 dogs respectively served as experimental groups while group D own 2 dogs served as control group.Group A received a unilateral irradiation from 60Co in the mandible with a total dose of 22.8Gy in four 5.7Gy fractions (equivalent to 50Gy/25 fractions).Group B receieved a unilateral irradiation from 60Co in the mandible with a total dose of 24.8Gy in four 6.2Gy fractions (equivalent to 60Gy/30 fractions). Group C receieved a unilateral irradiation from 60Co in the mandible with a total dose of 27.2Gy in four 6.8Gy fractions (equivalent to 70Gy/35 fractions). Bilateral coticotomy was made 3 months after completion of irradiation. After a 1-week latency period, bone distraction was activated at a rate of 0.5mm twice daily for 10 days, 2 dogs sacrificed just after distraction and the other 2 dogs sacrificed after 8 weeks of consolidation in group A,B,C while 1 dog sacrificed just after distraction and another dog sacrificed after the same consolidation period in group D. New bone underwent radiographic, histological histomorphometric analysis. All dogs were successfully distracted in this study.After distraction,osteogenic activity was detected in group A and group B.After 8 weeks of consolidation, no difference was found between the percent area of new bone in group A and group B.New bone formation was not evident in group C. New bone was more mature and organised in group D than in group A and B. Larger chondroid islands were found evident in distracted bone of group D. It seems that DO can be feasible in previously irradiated dog mandible, 50Gy and 60Gy are the safety doses, 70Gy should be cautiously selected.Part 3 Experimental study of distraction osteogenesis in the irradiated dog mandible :the electrophysiological changes of IANWe proved the feasibility of DO under different radiographic doses through histological analysis and gross specimens in former experiment,but it is not enough for IAN. Another important criteria of IAN is its electrophysiological alternation. We explored the electrophysiological changes of IAN during DO. 12 Chinese dogs were randomly divided into 3 groups--group A,B,C.Each group own 4 dogs respectively.Group A received a unilateral irradiation from 60Co in the mandible with a total dose of 22.8Gy in four 5.7Gy fractions (equivalent to 50Gy/25 fractions) within two weeks.Group B receieved a unilateral irradiation from 60Co in the mandible with a total dose of 24.8Gy in four 6.2Gy fractions (equivalent to 60Gy/30 fractions) within two weeks. Group C receieved a unilateral irradiation from 60Co in the mandible with a total dose of 27.2Gy in four 6.8Gy fractions (equivalent to 70Gy/35 fractions) within two weeks. Bilateral coticotomy was made 3 months after completion of irradiation. After a 1-week latency period bone distraction was activated at a rate of 0.5 mm twice daily for 10 days, followed by a consolidation phase of 8 weeks. During the experiment, 8 time-points was chosed to examine, record and analyze the evoked potentials(EP) of IAN:pre-irradiation,post-irradiation,pre-distraction,the sixth day amid distraction, post-distraction, the 2th, 4th, 8th week during consolidation.The neural electricity tester (Type NDI-200P+,Shanghai Haishen electronic medical Co,Ltd ) was adopted to each group to carry out the electrophysiologic analysis. At each time-point,one needling electrode was stabed in foramen mentale as activate electrode,while another needling electrode stabed in the mandibular skin 1cm away from the activate electrode as reference electrode.Near the foramenmandibulae,we stabed in receive electrode and 2cm away from it another needling electrode was used as indifferent electrode.The grounding electrode was placed in the anterior leg.39.8mA as activate current,the evoked potential was recorded. The action potential(AP) of IAN showed corresponding change during the irradiation and distraction process in group A and B,but group C did not got integrated AP. They are coincident to the change of the bone.
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