A comparison of jaw muscle cross-sections of long-face and normal adults.

Long-face subjects have smaller maximum molar bite forces than do normal individuals. This has been attributed both to differences in moment arms and size of the jaw muscles. In this study, a comparison was made between the mid-belly cross-sectional areas of the jaw muscles of 13 long-face and 35 normal adults by means of serial MRI scans. The subjects were selected on the basis of anterior lower face height as a percentage of anterior total face height. These and other cephalometric variables were measured from lateral radiographs. In the long-face group, the cross-sectional areas of the masseter, medial pterygoid, and anterior temporal muscles were, respectively, 30%, 22%, and 15% smaller than in the control group. By a discriminant analysis and a multivariate analysis of variance, these differences were found to be significant (p less than 0.001). The findings of this study hint that differences in the sizes of the jaw muscles of long-face and normal subjects might explain, in part, the observed differences in maximum molar bite force.