The Effect Of Severe Occlusal Attrition And One-visit Occlusal Reconstrution On Masseter Muscle Of Rats

In dental clinical work, the number of patients with severe occlusal attrition（SOA） isincreasing, especially among older people. SOA often leads to the decrease of occlusalvertical distance （OVD）, which can cause a series of changes of stomatognathic system.Occlusal reconstruction is the common prosthodontics method for treating SOA. In thetraditional concepts, the occlusal reconstruction is a systematic therapeutic process, andthe OVD should be increased gradually and slowly. The OVD should be increased2-3mmonce and increased step by step when OVD should be increased more than4mm.Eventually, the increase of OVD should be below interocclusal distance. Our researchteam have dedicated to the study of occlusal reconstruction of SOA for many years. Wehave treated many SOA patients whose OVD lost more than4mm, with one-visit occlusal reconstruction. All these patients satisfied the treatment results. During the follow-upperiod after one-visit occlusal rehabilitation, their masticatory muscles andtemporomandibular joints （TMJs） did not appeared significant discomfort. It conflicts withthe traditional treatment concept. Up to now, the research of the SOA treatment byocclusal reconstruction still stay at the stage of evidence based medicine, lack of sufficienttheoretical bases and statistics data to support how to choose the method of occlusalreconstruction. Therefore, we need to confirm the feasibility of one-visit occlusalreconstruction for treating SOA, and provide reliable experimental theoretical basis forclinical treatment.ObjectiveThe study was to establish rat model with SOA, and then treat SOA rat with one-visitocclusal reconstruction. The morphological changes of masseter muscle will be observedand analyzed at different stage by relevant techniques. It will provide theoretical basis forthe clinical features of SOA and the feasibility of one-visit occlusal reconstruction fortreating SOA.Methods1. The rats were executed to mouth opening passively in terms of different degree ofopening mouth with self-made regulative mouth gag, to investigate the effects of differentmouth opening degree on masseter muscle morphology and body weight.2. With the help of surgical operating microscope, the animal model of SOA wasestablished by decreasing the crown height of the upper molars artificially, progressivelyand mechanically. The changes of masseter muscles caused by SOA were analyzed atdifferent time-points by cryotomy, ultramicrotomy and biochemical detection technology.3. With two-step impression technique, the occlusal splints were made and thenadhered to the surface of the upper molars by resin bonding technology, to establish theanimal model of one-visit occlusal reconstruction for treating SOA. The changes ofmasseter muscles caused by one-visit occlusal reconstruction were analyzed at differenttime-points by cryotomy, ultramicrotomy and biochemical detection technology.Results 1. Compared to the control group, with the distance of passive mouth-opening at25mm,0.5h per day, the histological morphology of the masseter muscle of the SD ratswere changed significantly after7days, but not in10mm-,15mm-,20mm-groups.2. The changes of microstructure and ultrastructure of the masseter muscle wereinduced by SOA, expressing the ultrastructural lesions primarily. Structural damage ofmitochondria is regarded as typical feature, accompanied by the increase of themitochondria Ca²⁺content. However，these changes only appeared at early stage of thestudy, and confined in the isolated mitochondria, without extensive damage. Theorganizational structure recovered on the42th day.3. One-visit occlusal reconstrution can lead to the damage of the masseter muscle.The number of mitochondria was increased by the third days; mitochondrial vacuolardegeneration occurred by the seventh days, coupling with the changes of cell nucleus andmyofibrils. Mitochondrion Structures recovered on the28th day.Conclusion1. The mouth opening distance of rat should be20mm in follow-up study. Meanwhile,the operation time should be less than0.5h.2. In the present study, the high-emulational SOA animal model was successfullyestablished. Morphological changes can be found in the masseter muscle in the form ofgrowth remodeling, not pathologic changes.3. On this basis, the animal model of one-visit occlusal reconstruction for treatingSOA was also established. Morphological changes can be found in the masseter muscle inthe form of growth remodeling, not pathologic changes.