Alterations of the Temporomandibular Joint in Experimental Posterior Condylar Displacement during Mastication in Rats

Excessive stresses on the temporomandibular joint (TMJ) affect remodeling process of the TMJ. Since movability of TMJ to posterior direction is limited, intermittent posterior condylar displacement may induce excessive stresses on TMJ structures. Objective: The purpose of this study was to test the hypothesis that posterior condylar displacement during mastication due to occlusal dysfunction traumatizes condylar cartilage and joint innervated nerve fibers in rats. Methods: To induce posterior condylar displacement during mastication, guiding appliances were attached to maxillary incisors of twenty-eight 8-week-old male Wistar rats for 4, 7, 14 and 21 days. Sixteen normal rats served as controls. Radiographs of rat skulls and undecalcified sections of TMJ were used to confirm the position of condyles and discs. Decalcified sections were stained with hematoxylin and eosin or processed for immunohistochemistry of growth-associated protein-43 (GAP-43), a marker of growth and regeneration of nerve fibers after injury. The cartilage widths at posterior regions were measured and compared in statistics. Results: The condyles in experimental rats were displaced posteriorly with anterior disc displacement at occluded condition. At posterior regions of the condyles, cells in the proliferative zones were obviously diminished at day 4 and calcified chondrocytes in the hypertrophic zones almost totally vanished at day 7. After day 14, cells in hypertrophic zones tended to increase. The cartilage widths of condyles significantly decreased at day 7, 14 (P<0.01) and day 21 (P<0.05). Moreover, GAP-43 immunoreactive nerve fibers were found at lower posterior attachments of discs at day 7 and extremely at day 14 while no GAP-43 immunoreactive nerve fibers were observed in the controls. Conclusions: These results indicate that excessive stresses due to intermittent posterior condylar displacement cause dysfunctional remodeling of condyles and nerve injury. These suggest that proper occlusion and condylar position are important for maintaining structures and function of TMJ.