The Predictability from Skull Morphology of Temporalis and Masseter Muscle Cross-Sectional Areas in Humans

Abstract

To carry out functional simulations of the masticatory system that aim to predict strain magnitudes it is important to apply appropriate jaw-elevator muscle forces. Force magnitude estimation from directly measured muscle physiological cross-sectional area or anatomical cross-sectional area (CSA) is not possible for fossils and skeletal material from museum collections. In these cases, muscle CSAs are often estimated from bony features. This approach has been shown to be inaccurate in a prior study based on direct measurements from cadavers. Postmortem alterations as well as age changes in muscle form might explain this discrepancy. As such, the present study uses CT images from 20 living individuals to directly measure temporalis and masseter muscle CSAs and estimated cross-sectional areas (ECSAs) from bony features. The relationships between CSAs and ECSAs were assessed by comparing mean values and by examining correlations. ECSAs are up to 100% greater than CSA and the means of these variables for each muscle differ significantly. Further, ECSA is significantly correlated with CSA for temporalis but not masseter. Cranial centroid size is only significantly associated with CSA for temporalis. These findings indicate that ECSAs should be employed with caution in simulations of human masticatory system functioning; they do not reflect CSAs and it is plausible that this also applies to studies of closely related living and fossil taxa. When ECSAs are used, sensitivity analyses are required to determine the impact of potential errors.