| Objective:Establish an animal model of muscle imbalance of pterygoid muscle to investigate the effect on the subchondral bone of condylar, and assess the role of stress in the mechanism of subchondral bone reconstruction.Methods:Forty-eight-9-weeks-old male rats were randomly divided in to 2 groups: the experimental group and the control group,and each group of 24 SD rats.The SD rats in the experimental group was injected of botulinum neurotoxin A in right lateral pterygoid muscle from extraoral,and in the control group,the same dose of 0.9% normal saline was injected in the same anatomical site of the SD rats. After modeling, the changes of occlusion of SD rats were observed, and the rats were sacrificed at the end of 2th,4th,8th and 12th weeks,three rats in each group during each period. Acquired TMJ condylar and observed its histological changes:1. observed the changes in the volume and texture of the lateral pterygoid muscle by macroscopic observation.2. The condylar were used for HE staining and observed the structual changes of condylar subchondral bones.3. Detected the changes of the mRNA levels of OPG,RANKL,and the ratio of OPG/RANKLmRNA in the condylar subchondral bones with RT-PCR.Results:1. After modeling for 2 weeks and 4 weeks, SD rats in experimental group appeared obvious occlusal disorder including mandibular retrusion or(and) deviation,no bite between maxillary incisors with mandibular incisors, unilateral chewing,and abrasion asymmetry of incisors, especially obvious after modeling for 2 weeks and 4 weeks. After modeling for 8 weeks to 12 weeks,occlusal relations gradually restored, and only a slight mandibular retrusion.2. At 2 and 4 weeks after modeling, compared with the left side, the lateral pterygoid muscle in right side was significantly smaller, the texture of the lateral pterygoid muscle was hard, and it was mainly obvious in the inferior head of the lateral pterygoid muscle. At 8 weeks and 12 weeks after modeling,both in the experimental group and control group,the volume and texture of the lateral pterygoid muscle in the two sides(left side and right side) were basically similar.3. HE staining showed:in the experimental group,at the 2 weeks after modeling,the surface layer of the fibrous layer on the condylar cartilage was stripped off, and the cells in the calcification and hypertrophy zone were reduced. There were haemorrhages in the subchondral bone of the condylar, and the gap between the trabecular bone was filled with a lot of red blood cells, which was included in the calcified layer of cartilage.At 4 weeks after modeling, the trabecular bone of the condylar subchondral bone was significantly reduced, and the area of the trabecular bone decreased obviously in the unit area. Even the appearance of bone resorption such as the uninterrupted trabecular bone and micro fracture of bone was observed.There were a large number of chronic inflammatory cells infiltration in the gap between the trabecular bone, but haemorrhages in the subchondral bone of the condylar was not as obvious as 2 weeks after modeling.At 8 weeks after modeling, the trabecular bone appeared to be restored.At 12 weeks after modeling, the number, area and shape of the subchondral bone of the condyla were basically the same as that of the control group, but the arrangement of the trabecular bone was not as well as the control group.4. The results of RT-PCR showed that the levels of OPG in the experimental group was significantly lower than that in the control group (P<0.05) at 2 and 4 weeks after modeling and higher than the control group (P<0.05) at 8 weeks after modeling, and the difference was statistically significant;At 12 weeks after modeling, the the mRNA levels of OPG was similar to the control group (P>0.05),and the difference was not statistically significant.The mRNA levels of RANKL in the experimental group was significantly higher than that in the control group (P<0.05) at 2 and 4 weeks after modeling, and the difference was statistically significant;At 12 weeks after modeling, the mRNA levels of OPG was similar to the control group (P>0.05),and the difference was not statistically significant. At 8 weeks after modeling, the mRNA levels of OPG was lower than the control group (P>0.05),and at 12 weeks after modeling, was similar to the control group (P>0.05), and botn the differences were not statistically significant.The ratio of OPG/RANKLmRNA in the experimental group was lower than the control group at 2 and 4 weeks after modeling(P< 0.05),and the difference was statistically significant. Compared with the normal group,the decreasing amount of the ratio at 4 weeks after modeling was more than the amount of 2 weeks after modeling. At 8 weekss after modeling, the ratio of OPG/RANKLmRNA was higher than the control group(P< 0.05), and the difference was statistically significant; At 12 weeks after modeling, the ratio of OPG/RANKLmRNA was slightly higher than the control group, However, the difference was not statistically significant(P> 0.05).Conclusions:1. This experiment successfully established the animal model of unbalanced strength of lateral pterygoid muscle through a unilateral injection in lateral pterygoid muscle of SD rats with botulinum neurotoxin.2. The block of lateral pterygoid muscleof the SD rats can induce a significant disturbance of occlusion in the early stage. In the same side, the number,shape and structure of the trabecular bone were abnormal. The disorder of the arrangement was the most obvious in the 2 and 4 weeks after modeling. Compensatory bone repair was found in 8 weeks after modeling, and the remodeling of the bone was relatively stable at 12 weeks.3.The result suggested that the unbalanced strength of the lateral pterygoid muscle can lead to pathological remodeling of the mandibular condylar cartilage.4. The imbalance strength of lateral pterygoid muscle is involved in the pathological remodeling of condylar cartilage through the OPG/RANKL signaling pathway. The ratio of OPG/RANKLmRNA is significant for bone resorption and bone remodeling, and OPGmRNA may play a more active role in the process of remodeling. |
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