[Osteotomy healing of the lower jaw and the adaptation of the masticatory muscles after miniplate osteosynthesis in rabbits. II. Response pattern of the masticatory muscles (M. masseter)].

16 operated 4 control animals of both sexes were included in the study following osteotomy of the lower jaw and subsequent miniplate osteosynthesis. In addition, muscle tissue excised from 8 rabbits was evaluated at the time of the operation as additional normal material. The study was conducted over 4, 8, 12 and 24 weeks, after which periods the animals were killed and their left and right masseters examined and compared in histological, histochemical and morphometric terms. The masticatory muscles of the control rabbits displayed a marked sex dimorphism. The males had strikingly large, predominantly type-2 fibres, whereas the fibre diameters in the females were found to differ less clearly, with type-1 fibres being predominant. In terms of muscle regeneration, it was found that following soft-part trauma small localized defects (biopsy) had healed completely after 12 weeks. In the residual parenchyma, signs of adaptation to a changed mastication as a result of the experimental operation (osteotomy of the lower jaw, miniplate osteosynthesis, muscle biopsy) could be identified. Atrophy as a result of non-use was found to occur to a limited extent, whereas the number of type-2C fibres and fibre type transformations were raised significantly. After the surgery, the organization of the impaired muscle fibres was aided by a stabilization and rest positioning of the parts concerned. The muscle fibres reinnervation that followed was supported by functional exercise and nerve impulses.