Dynamic Responses in Monkey Cervical Vertebrae during Occlusal Forces

Abstract

     In order to investigate the dynamic response of the cervical vertebrae during occlusion, we electronically stimulated both masseter muscles of anesthetized adult Japanese monkeys, and measured the strain in the lamina of the vertebral arch on both sides of each vertebra when the animals were made to occlude on sticks of 3, 7 or l0mm thickness at either the canine or first molar on one side.
     With occlusion on a stick at the working side canine, the second cervical vertebra served as the focal point in preventing the vertebrae from slanting to one side due to occlusion. In the case of occlusion on the working side first molar, the fourth cervical vertebra was the focal point. However, with occlusion at either the canine or first molar on the non-working side, the third and seventh cervical vertebrae, respectively, served as the focal points.
     With occlusion on a stick at the canine, the upper portion of each cervical vertebra was compressed while the lower portion was under tension. In addition, there was a tendency for the central region on the working side to expand while the non-working side contracted to balance out the distortions in the upper and lower portions. In contrast, with occlusion on a stick at the first molar, there was tension along the body axis on the working side in virtually all regions of the cervical vertebrae; on the non-working side most of the cervical vertebrae showed tension in the horizontal direction. These effects were greater the larger the opening of the mouth.
     A tendency was observed for the stresses to concentration on the non-working side of the sixth cervical vertebra with occlusion at either the canine or the first molar, regardless of the size of the stick. However, because there were a large number of wave forms in the cervical vertebrae that appeared to be dispersing the stresses more than the facial and cranial bones did, it seems that the cervical vertebrae deform in response to their respective position and shape, and ingeniously disperse the stresses.