RESUMO
The aim of this study was to investigate the features of the loading vectors of the temporomandibular joint (TMJ) generated during bilateral static bites at the molars and at the premolars, and to determine the major factors affecting the difference between the two loading vectors. We computed the subjects' estimated and theoretical minimum TMJ loadings under the two different bite conditions by applying the subjects' bite-force and electromyographic (EMG) data to a two-dimensional (2D) standard model of the jaw based on a rigid-body spring model of the TMJ. For a molar bite, (1) the estimated loading vector was not equal to its theoretical minimum; (2) the TMJ-loading/bite-force ratio, describing the proportion of TMJ loading, was relatively small, 0.477 on average; and (3) the estimated loading vector pointed in the direction of the central part of the articular disk's intermediate zone. For a premolar bite, on the other hand, (1) the estimated loading vector was nearly equal to its theoretical minimum; (2) the TMJ-loading/bite-force ratio was relatively large, 0.904 on average; and (3) the estimated loading vector pointed at the superior portion of the articular disk's intermediate zone. The differences between the TMJ-loading vectors for molar and premolar bites originated primarily from changes in the bite-point location.