Axial inclination and cuspal articulation in relation to force distribution

Abstract

Occlusal stress purely in the long axis of the tooth rarely exists. Even the apparent vertical pressure of the mandible in centric occlusion has an anterior component due to the hinge axis of the mandible. The mesial axial inclination of the posterior teeth also helps convert “vertical” pressure into torque. The line of force of an occlusal load often falls mesial to the center of rotation of the tooth. Thus occlusal pressure is converted into a mesial torque. A cusp to central fossa relation in centric occlusion is the closest we can come to stress on the long axis of the teeth. In the bicuspid articulation, the upper lingual cusp articulates in the distal fossa of the lower; distal torque often develops. The axial inclination of these teeth can completely change the direction of the torque produced. Good centric occlusal contact results in buccal torque on the upper teeth and lingual torque of the lower teeth. The amount of torque is usually less than when there is poor centric contact. In any case, torque is also produced due to the buccal axial inclination of the upper teeth and the lingual axial inclination of the lower teeth. This is added to the torque resulting from the centric occlusal articulation. Occlusal pressure in centric occlusion results in torque production from several possible sources: (1) the anterior component of the hinge axis of the mandible, (2) the mesial torque produced by the mesial axial inclination of the posterior teeth, (3) buccal and lingual torque produced by the buccal and lingual axial inclination of the upper and lower teeth, respectively, (4) occlusal contact in relation to the center of rotation of the tooth, and (5) incline plane relation in centric occlusion produces torque. All of these torque-producing factors act at one time or another when the teeth are in contact. At any given moment, various torques may be acting at one time to produce a resultant torque. As the teeth function and change their relationship, the resultant torque keeps changing. To say that this is a dynamic system is quite an understatement. Some of these torques play a part in the phenomena of mesial drift. They are: (1) the anterior component of the hinge axis of the mandible (active in all occlusal contact), (2) the mesial axial inclination of the posterior teeth, (3) the incline plane relation in centric position, (4) the incline plane relation in eccentric positions, and (5) interproximal wear (indirect association with mesial drift). The physiologic forces influencing the periodontal support are so complicated that this discussion only scratches the surface. We feel that studies of muscle action and individual habit patterns should be correlated with occlusal forces. Perhaps then a better understanding of the stresses that are distributed to the periodontal structures could be obtained.