THE ROLE OF THE LATERAL PTERYGOID MUSCLES IN TMJ DISORDERS DURING STATIC CONDITIONS

Intramuscular EMG of the lateral pterygoid muscles (LPM), surface EMG of the temporalis and masseter muscles and force measurements of the t emporomandibular joint (TMJ) were synchronously used to investigate th e biomechanical role of the two heads of the LPM in relation to intern al derangement (ID) of the TMJ. EMG and kinetic analysis of five stati c conditions (resting, protraction, opening, molar and incisor clenchi ng) and three maximum isometric masticatory forces (opening, molar and incisor clenching) were done to compare forces and muscular activity between TMJ ID and control subjects. The analysis of variance results of the integrated linear envelope (LE) EMG showed no significant diffe rences between the two groups for the masseter and temporalis muscles. Therefore, there is no apparent reason to believe that these muscles are hyperactive in TMJ ID. The integrated LE EMG of the SLP was signif icantly lower in the TMJ group during molar clenching (104 mu V+60.0 o ver 159 mu V+68.8 for a p = .020). The superior head of the lateral pt erygoid muscle (SLP) seemed to have lost its diskal stabilizing functi on. The integrated LE EMG signals of the ILP were significantly higher in the TMJ ID group during rest, resisted protraction and incisor cle nching (p = .029, p = .046, p = .031 respectively). The ILP muscle has probably adapted to control the inner joint instability while continu ing its own actions. The ILP muscle seemed to have lost its functional specificity. The results of the isometric forces showed that TMJ ID s ubjects exhibited significantly lower molar bite forces (297.1N over 4 19N, p = .042) confirming that they have less muscle strength and tiss ue tolerance than subjects with healthy masticatory muscle systems. A neuromuscular adaptation could be occurring in the TMJ ID masticatory system affecting muscular actions and forces.