Three-dimensional digital analysis of musteloid masticatory muscle architecture.

Abstract

Masticatory muscle architectural properties (i.e., fascicle length [FL] and physiological cross-sectional area [PCSA]) relate to an animal's diet. However, the traditional gross dissection approach to evaluating them destroys the three-dimensional relationships between and within muscles. Recent advances in three-dimensional analytical approaches, particularly diffusible iodine-based contrast-enhanced computed tomography (DiceCT), have been used to evaluate, in primates, both traditional and in situ variables, including tortuosity and fascicular orientation-which can inform the interpretation of a muscle's position on the length-tension curve and its vector of pull, respectively. To evaluate this method's applicability in carnivorans, we used DiceCT to analyze the masticatory musculature of seven dietarily diverse members of the superfamily Musteloidea. Digitally reconstructed muscle masses corresponded to gross dissection values from conspecifics in all but one specimen, a discrepancy we attribute to intraspecific body size variation. Digitally derived FL and PCSA values also broadly agreed with previously reported dissection-based values. No architectural variables (FL, PCSA, or muscle mass) showed consistent correlations with diet in this small sample. As seen in primates, fascicular orientation revealed an anterior-vertical-posterior division among musteloid masseteric components. Tortuosity values also displayed functionally coherent trends: abductors exhibited the lowest tortuosity at occlusion, while the temporalis components showed the highest, aligning with hypothesized gape-dependent activation sequences, for example, the "triplet" motor pattern. These findings demonstrate that DiceCT reliably reproduces traditional architectural variables while revealing additional aspects of muscle structure that relate to important functional activation patterns supporting the broader utility of DiceCT for comparative musculoskeletal research across Mammalia.