Condylar resorption in the TMJ or TMJ-OA has been experienced occasionally in daily orthodontic practice and recognized to induce substantial influences on craniofacial morphology and the treatment outcomes. This study was designed to investigate the mechanisms of TMJ-OA by means of biomechanical, histological and biochemical approaches. Biomechanical study with finite element stress analysis revealed an existence of large compressive stresses in the anterior, middle and lateral areas on the condyle and prominent increases in the compressive stresses in association with vertical discrepancy of the craniofacial skeleton. Such skeletal discrepancy, simulated in growing rats by placing a metal plate on the upper molars, produced a decrease in the thickness of cartilage layers and an increase in the number of TRAP-positive cells, both of which lead to degenerative changes in the articular cartilage of the mandibular condyle. Furthermore, excessive tensile stresses, applied to articular chondrocytes with use of the Flexercell Strain Unit, induced an imbalance between matrix metalloproteinases(MMPs) and tissue inhibitors of matrix metalloproteinases(TIMPs), which is assumed to induce lower resistance to external stimuli and degenerative changes leading to the resorption of bone and cartilage. It is thus shown that excessive or imbalanced mechanical loading on the TMJ components from occlusal and skeletal discrepancies induce various degenerative responses of cartilaginous tissues and articular chondrocytes, leading to the destruction of bone or cartilage in TMJ-OA
