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Effects of mouth opening degree on masseter muscle and temporomandibular joint of rats

AIM: To investigate the effects of mouth opening degree on masseter muscle and temporomandibular joint in rats so as to find out the most suitable degree of mouth opening in experimental procedures.METHODS: 25 male SD rats aged 12 weeks were randomly divided into 5 groups(n=5).After anesthesia the rats were intervened with different degrees of passive mouth opening(10 mm,15 mm,20 mm and 25 mm) for 0.5 h a day.The control rats received no intervention.Body weight of the rats was recorded during the intervention period.After 7 days of intervention,all rats were sacrificed,the masseter muscles and the temporomandibular joints were taken and examined by histology.RESULTS: No significant change was found among the 10 mm,15 mm and 20 mm groups and the control group in terms of body weight and histological feature(P0.05).However,in 25 mm group the body weight of the rats was lower than that in the other groups from the 2nd day of intervention(P0.05).In addition,exudated red blood cells were found among the muscle fibers,accumulation of inflammatory cells were observed,and a number of cartilage cells were proliferated in the condylar process of the temporomandibular joint.The thickness of the condyl cartilage increased.CONCLUSION: In SD rats mouth opening≥25 mm may induce pathological changes in masseter muscle and temporomandibular joint.