Muscle and joint forces under variable equilibrium states of the mandible.

It is well established that subjects without molars have reduced ability to comminute foods. However, epidemiological studies have indicated that the masticatory system is able to functionally adapt to the absence of posterior teeth. This supports the shortened dental arch concept which, as a prosthetic option, recommends no replacement of missing molars. Biomechanical modeling, however, indicates that using more anterior teeth will result in a larger temporomandibular joint load per unit of bite force. In contrast, changing bite from incisor to molar position increases the maximum possible bite force and reduces joint loads. There have been few attempts, however, to determine realistic joint loads and corresponding muscular effort during generation of occlusal forces similar to those used during chewing with intact or shortened dental arches. Therefore, joint and cumulative muscle loads generated by vertical bite forces of submaximum magnitude moving from canine to molar region, were calculated. Calculations were based on intraoral measurement of the feedback-controlled resultant bite force, simultaneous electromyograms, individual geometrical data of the skull, lines of action, and physiological cross-sectional areas of all jaw muscles. Compared to premolar and canine biting, bilateral and unilateral molar bites reduced cumulative muscle and joint loads in a range from 14% to 33% and 25% to 53%, respectively. During unilateral molar bites, the ipsilateral joints and contralateral muscles were about 20% less loaded than the opposing ones. In conclusion, unilateral or bilateral molar biting at chewing-like force ranges caused the least muscle and joint loading.