Abstract
It is widely accepted that loading is an important factor in temporomandibular joint (TMJ) growth, stability, and function. Disk displacement, disk perforation and remodeling of TMJ bony components have been considered likewise to be a result of excessive or misdirected joint loading. Nevertheless, human TMJ loadings during chewing have not been estimated either directly or indirectly.
In this study, TMJ loading during the power stroke of mastication was investigated electromyographically and biomechanically.
The Electromyographic activities of the medial pterygoid muscles, masseter muscles and anterior temporal muscles during gum and beef jerkey chewing were first measured bilaterally. Then, the position and the angulation of the resultant muscle force vector were biomechanically calculated by using those muscle activities and 3-dimensional cephalometric data. The resultant muscle force vector calculated was used to estimate TMJ loading during the power stroke of mastication and it was compared with the loading during unilateral clenching obtained previously.
The following results were obtained:
1. At the beginning of unilatertal chewing, TMJ loading during the power stroke seemed to be distributed more to the balancing side condyle. However, when the subsequent rythmical chewing began, ‘working-side shift’ of the resultant muscle force vector was observed.
2. At the beginning of unilateral chewing, the resultant muscle force vector was approximately perpendicular to the occlusal plane on the frontal view. However, when the subsequent rythmical chewing began, the vector tilted slightly to the balancing side.
These results indicate that masticatory muscle activities during chewing are controlled so as to achieve two effects: to transmit muscle forces to biting force effectively, and to distribute muscle forces to the bilateral condyles equally.
