Synovial hyperplasia is a feature of degenerative temporomandibular joint (TMJ) disease. However, the mechanism by which hyperplasia progresses in the TMJ is unknown. Based on the hypothesis that the oxidative stress generated by mechanical loading causes degenerative changes in the TMJ synovium, we investigated the generation of the highly reactive species, peroxynitrite, and the occurrence of DNA damage in the synovium. After condylar hypermobility of rat TMJs, a marker of peroxynitrite, nitrotyrosine, was localized to the nuclei and cytoplasm of the synovial lining cells and fibroblasts in synovitis-induced TMJ. DNA single-strand breaks were found in the nuclei of the synovial cells only after enzyme treatment, whereas DNA double-strand breaks were not detected. These findings indicate that condylar hypermovement induces the proliferation of synovial cells, and suggest that oxidative stress leads to the progression of synovial hyperplasia via DNA damage of the synovial cells in TMJs after mechanical loading.
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