A re-evaluation of the constrained lever model in the primate feeding system

The constrained lever model (CLM) predicts that the jaw adductor resultant muscle forces (RMFs) must pass through a 'triangle of support' (ToS) to prevent temporomandibular joint (TMJ) distraction during biting. The CLM defines distracting forces as perpendicular to the plane of the ToS, but the orientation of the ToS varies both within and between individuals based on bite point, gape, and differences in the height of the TMJ. We compare TMJ distractive forces estimated using a ToS plane versus a fixed, horizontal plane criterion across a range of gapes using muscle moments and forces for the three major jaw adductor muscles in 97 morphologically diverse primate species. At occlusion, 80% of the species experienced stabilizing compressive forces under a horizontal plane criterion, but only 44% of species had an RMF inside the ToS. This mismatch indicates that predictions of TMJ distraction and joint stability are highly dependent upon the comparison plane, which is challenging for comparisons between primates with varying TMJ heights, and consequently, ToS orientations. Joint stability increased with gape but varied little with taxonomy and across diet categories. These results provide strong evidence that the CLM is a poor predictor of the joint stability when the TMJs are elevated. These findings suggest future applications of the CLM should either focus on taxa with TMJs near the occlusal plane or calculate joint reaction forces directly to assess joint stability in mammals with elevated TMJs.