FORCES EXERTED ON THE HUMAN MANDIBLE BY THE MUSCLES OF OCCLUSION

¹ Department of Anatomy, Dalhousie University, Halifax, Canada

Previous estimations of forces of occlusive muscles by examination of cadavera have been very scanty; results have not been concordant; and no attention has been paid to possible variation from one person to another. The object of the present investigations was to determine directions, relative amounts, and approximately absolute amounts (possible variations, rather than averages) of forces exerted by the masseter (superficial and deep parts), internal pterygoid, and temporal (anterior and posterior parts).

Both sides of twelve heads of dissecting-room subjects were examined, ages being from 22 to 87 years, five specimens being females and seven males (temporal was measured on only three specimens). The method included determination of cross-sectional area, perpendicular to all muscle fibers, by inking cut surfaces, imprinting on paper, tracing on kodaloid, cutting out and weighing; estimation of absolute force on a basis of 10 kg. per sq. cm.; resolution of forces into components acting upward, forward or backward, laterally or medially. Total forces (both sides) in three heads completely examined were approximately 308 kg., 273 kg., and 283 kg.

By the method of moments, forces upward and forward were referred to the mandibular second molar. Upward forces at this tooth were comparable with certain gnathodynamometer results obtained from living persons. The forward component, there, exceeded the upward. Possible ultimate combinations of various forces were not analyzed, but tables are given to facilitate reference of forces to other points and combination of forces in other ways. Confidence in the method is strengthened by the agreement of certain results with clinical opinion; e.g., the greater average force associated with better molar dentition; and a tendency for right muscles to be stronger than left muscles. The direction, outward or inward, of the resultant transverse pull depended largely on skull shape.

In most specimens, angles between muscles and eye-ear plane were in the size-order: superficial masseter ( least), internal pterygoid, deep masseter; but this relationship did not always hold, especially between pterygoid and deep masseter.

The authors gratefully acknowledge grants, for technical assistance, made by the Dental Faculty from the Research Fund placed at the disposal of the Faculty by the Dominion Dental Council of Canada.