Effect of Various Degrees of Anterior Disc Displacement on the Biomechanical Response of the Masticatory System

Anterior disc displacement (ADD) is one of the most prevalent temporomandibular disorders (TMD). It was widely recognized that occlusal factors could affect temporomandibular joint (TMJ). However, the impacts of ADD on the biomechanical environment of TMJ and occlusion are still unclear. This study aimed to describe the effects of ADD on the masticatory system, including TMJ and occlusion. The finite element model (FEM) was constructed based on the medical images of a healthy adult male. The complete skull, masticatory muscles, TMJs, and related ligaments were included. Three FEMs with different degrees of ADD were constructed with disc-condyle angles of 10 deg, 20 deg, and 30 deg. The muscle forces corresponding to intercuspal clenching (ICC) were applied as the loading condition. Four models were conducted: normal, mild, moderate, and severe ADD. It was found that the overall stress distribution was relatively consistent across the four models. The contact stress on the TMJ and occlusion in severe ADD was visibly different from the other three models. In addition, the contact stress on the condyle gradually increased with the increasing occlusal strength. Abnormally high-stress concentration began to appear on the condyle at 30% muscle strength. Moderate ADD was more of a transitional stage. Compared to mild and moderate ADD, severe ADD had visibly effects on the stress response of the TMJ and the entire mandible (including occlusion), such as abnormally high stresses of the condyle, stress concentration on the second molar, and prone to disc extrusion and anterior slippage during high-strength occlusion.