A Mechanical Limitation on the Position of the Jaw Muscles of Mammals: The One-Third Rule

Abstract

Resultant force of the mammalian jaw elevating muscles acts on the horizontal jaw ramus closer to the craniomandibular joint than to the anterior teeth,.a condition which reduces the effectiveness of the muscle force. If the resultant muscle force were located more anterior than a point approximately one-third of the jaw length from the jaw joint, the biting side ramus would tend to rotate laterally about its long axis during lateral incisor or canine biting. This rotation would be resisted by the ligaments of the craniomandibular joint so that the forces acting at each end of the jaw would tend to twist the bone in opposite directions. This twisting of the jaw ramus should be avoided because bone is weakest in torsion. Despite the difficulties in estimating muscle action lines, empirical observations suggest that this theoretical one-third limit for the anterior location of the resultant force of the musculature, that will preclude most of the torsional forces, is usually not exceeded; the resultant force of the jaw muscles is in general restricted to the posterior third of the ramus.