| During recent 30 years, many clinical and basic researches have been done in craninofacial distraction osteogenesis. But most of them are concentrated on the principles and acceleration factors of bone formation, patterns of clinical application, etc. Less has been done in the research of surrounding tissues' changes caused by distraction and the relationship between distraction osteogenesis and relapse. In the procedure of distraction osteogenesis, peripheral soft tissues is also tensile with distraction, so the adaptive changes of soft tissues such as nerves; muscles and temporo-mandibular joint have a close relationship with post-operative relapse. Due to the powerful strength and the close correlation with mastication, masticatory muscles become the main factors of the relapse after jaw's movement.In this experiment, in the way of observing the histological changes, relative content of myosin heavy-chain mRNA and changes of the expression of IGF-I in masseter and geniohyoid muscle during distraction osteogenesis of canine's mandible, we can learn about the rebuilding ability of masticatorymuscles and its changing regularity, and preliminarily discuss its remodeling mechanism.Anteroposterior bilateral distraction osteogenesis was performed on 14 mongrel canines, 6 to 8 months of age, using intraoral mandibular distractor. The position of osteotomy lies between the 3rd deciduous premolar tooth and 1st permanent molar. Animals were divided into control group and 5 experimental groups. Each experimental group had 2 canines and the central of masseters and geniohyoid were dissected out in 5th, 10th day in distraction and 2nd, 4th, 8th week after distraction. The control group has 4 canines, which were placed distractor using the same method but weren't progressed distraction. This experiment included three parts:1. The histological changes in muscles of mastication during distraction osteogenesis of mandible in canine.This study was conducted to observe the histological changes through gross specimen inspection and HE staining. During the procedure of distraction, geniohyoid expressed hypertrophy and hyperplasia initially and then restore normal level after distraction, while masseters was influenced rarely. Only in 5th day during distraction, did gently atrophy appear in the masseters probably due to occlusal interference but subsequently disappeared. All the muscles rebuilt to stable structure after accomplish of distraction, which may help the post-operative stability.2. The changes of myosin heavy-chain mRNA in muscles of mastication during distraction osteogenesis of mandible in canineThe mRNA of MHCI, MHCH and GAPDH(homekeeping gene) in masseter and geniohyoid were measured by RT-PCR. The result showed that,in geniohyoid, the MHCImRNA was upregulated and MHCIImRNA was repressed; the amount of MHCI/MHCH and MHCI+MHCIIwere also increased, and then restored normal level at the end of experiment. In masseter there was no significant difference in all of detecting result. This indicated that geniohyoid paralleling with the direction of distraction presents adaptive changes such as muscular hypertrophy and muscle fiber type transformation. 3. Differential expression of IGF-I in muscles of mastication during distraction osteogenesis of mandible in canine.This study was conducted to observe the expression of IGF-I in masseter and geniohyoid during distraction osteogenesis of mandible in canine through the immuno-histocytochemical method. The result indicated that the expression of IGF-I increased initially during distraction, attained climax in the completion of distraction, and then decreased to the normal level in 8th week after distraction, which indicated that the increase of IGF-I might associate with the adaptive changes of masticatory muscles. As a cytokine, IGF-I participated in the procedure of functional remodeling caused by mechanical stimulation through autocrine or paracrine.
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