The Hyperalgesia Of Masticatory Muscles Induced By Acute And Chronic Occlusal Interference In Female Rats

Temporomandibular disorders (TMD) encompasses a number of problems involving the masticatory muscles or the temporomandibular joints and the soft tissues surrounded, which is frequently occurred more in women than in men. It is mainly characterized by temporomandibular joints clicking, orofacial pain and jaw movement disturbance, etc. Orofacial pain and TMD can be associated with pathologic conditions or disorders related to somatic and neurologic structures. These disorders are belong to non-dental pain in the orofacial region, and considered to be a subclassification of musculoskeletal disorders. Till now, there are massy dispute between occlusal disfunction and the reasons that orofacial pain related to TMD and masticatory systems. Female Sprague-Dawley rats were used in this research. First of all, establish animal models with acute and chronic occlusal interference on their left side. And then, mechanical pressure pain threshold was used to observe the hyperalgesia responses of temporalis and masseter muscles. Finally, to observe the proportional changes of myosin heavy chain (MHC) in masseter muscles of both sides by immunohistochemistory.1 Methods1.1 Establishment of occlusal interference in rat model1.1.1 Acute occlusal interference model establishment:The left maxillary second molar of rat was added with a 0.5mm thick and 1.0 mm wide metal pinhead for 15 days. We apply ligature wire to fix the metal pinhead to the occlusion of the second left upper molars, and then we built the acute disordered occlusion which individual posterior tooth is higher than normal occlusion. Check it everyday in order to ensure that the metal pinhead is still existed till 15 days later.1.1.2 Chronic occlusal interference model establishment:An elastic rubber (3 M Unitek, 1/8#) was inserted between the first and second left upper molars of rat for 15 days. Put it into the ginginva side as far as possible in order to avoid contacting with the opposite teeth, and clip the elastic rubber as short as possible so that it could eliminate the functional activity of the soft tissue nearby. Under the elastic effect of elastic rubber, second and third left upper molars were made as anchorage in order to push the first left upper molar move mesiocclusion which could push the first molar anteriorly about 0.8 mm. And so was formed the chronic disordered occlusion. Check it everyday to keep the elastic rubber in the space for 15 days.1.2 GroupsThirty animals were divided into five groups randomly.①Acute occlusal interference group(mentioned above);②Chronic occlusal interference group(mentioned above);③Acute occlusal removal interference group: It is the same as the acute occlusal interference group which the left maxillary second molar of rat was added with metal pinhead for 10 days, after that the appliance was removed.④Chronic occlusal removal interference group: To operate according to chronic occlusal interference group that an elastic rubber was inserted between the first and sencond left upper molars of Sprague-Dawley rat for 10 days. And then the rubber band was taken off.⑤Blank control group: No treatment was carried out on the rats.1.3 Behavior measurementAccording to Ren's method reported in 1999, the mechanical sensitivity is determined by a head-withdrawal response to the probing of von Frey filaments. Then we could confer the pain score before and after occlusal interferences.1.4 Immunohistochemistory detectionThe paraffin sections were processed by the avidin-biotin complex ABC method to assess the influence of occlusal interferences on the expression of myosin heavy chain (MHC) isoforms and regularity of distribution in deep and superficial parts of the masseter muscles.2 Results2.1 Behavior resultThe pain score of acute interference group was significantly higher than that of chronic occlusal interference group, and both of them were higher than that of blank control group (P<0.05). The pain score of muscles follows: ipsilateral temporalis > ipsilateral masseter muscle > contralateral temporalis (P<0.05). But it is not obvious in contralateral masseter muscle. Ipsilateral temporalis and masseter muscles in acute occlusal interference group steps up from the second and the third day after exerting interference, but the chronic occlusal interference group is increasing from the fourth and the fifth day. After that the pain score is continuing raised and arrive to peak time in a week, fall back gradually in the end. Acute occlusal removal interference group back to the level of control group is quicker than acute occlusal interference group (P<0.05). But chronic occlusal removal interference group is always slower than continue chronic interference till the fourteenth day (P<0.05). The pain score of contralateral temporalis is similar to this tendency, but it is a little bit weak. The reason why chronic occlusal removal interference group is always slow is possibly because the molar which back to its original place after antedisplacement became to be the second occusal interference. And acute occlusal interference group return earlier is possibly because the molar which antelocation simply is abrasion partly for compensation mechanism, so that it reliefs itself.2.2 Immunohistochemistry resultThere were significant differences between control group and chronic occlusal interference group, which mainly appearanced that the proportion of MHCⅡin masseter muscles is expressly increased after occlusal interference exerted (P<0.05). There is significant difference between acute and chronic after occlusal interference for fifteen days which the expression of positive cell population is more obvious in chronic occlusal interference group (P<0.05). It was not quiet obviously in the both sides and in different time-seven and fifteen days (P>0.05), so it did in each locations (P>0.05).3 Conclusions3.1 Behavior aspectBoth acute and chronic occlusal interference could lead masticatory muscles hyperalgesia, and there existed some differences for pain behavior between the acute and the chronic occlusal interference. Acute occlusal interference might be more seriously, and the peak time of acute occlusal interference was earlier than chronic occlusal interference. Remove occlusal interference might alleviate the pain sensitivity of masticatory muscles.3.2 histology aspectAfter occlusal interferences exerted, the proportion of MHCⅡin all is increased, and the proportion of MHCⅠis decreased. Furthermore, the expression of MHCⅡin chronic occlusal interference is much more than acute occlusal interference.