Scaling of internal joint distance in the elbow of small- to medium-sized mammals: Implications for range of motion analyses.

Cartilage thickness in mammalian joints increases with higher body masses. Contradicting previous studies found this increase to be positive allometric or negative allometric. Since approaches like computational modelling of animal locomotion, muscle moment arms, and joint dynamics rely on estimates of joint spacing (JS), it is important to accurately estimate an animal's cartilage thickness based on body mass. Here, we measured ex vivo internal joint distances (IJDs) or bone-to-bone distances on CT scans of fresh cadaveric forelimbs in a sample of small- to medium-sized mammals. IJDs were measured in the humero-ulnar and humero-radial joint. We find IJDs to scale isometrically in both joints across the entire sample, with positive allometric tendencies only within a subsample of cursorial species and only in the humero-ulnar articulation. The previously reported positive allometry can be linked to a cursorial sampling bias, and negative allometry results from a size constraint, acting on larger mammals than sampled here. Additionally, the IJDs were not affected by limb poses (i.e., flexed to extended, supinated, pronated). In rats and guinea pigs of varying sizes, we observed intraspecific isometric scaling with slight positive trends. This suggests that theoretically greater absolute forces-resulting from increased body mass with similar posture-only marginally contribute to relatively thicker cartilage in small- to medium-sized mammals. Further, we conducted a "range of motion" (ROM) analysis in the humero-ulnar joint of rats, guinea pigs, and maras, thus species of increasing body mass and level of cursoriality. ROM was assessed with varying estimated JS. Differences in the results are most biologically reasonable when allowing all six degrees of freedom (DOF, three rotational and three translational). Mobility decreases with increasing body mass and level of cursoriality, facilitated by increased restriction of movement to a single axis of flexion and extension. Such a trend persisted regardless of whether JS thresholds were estimated using intraspecific or interspecific regression models. The results suggest that variations in elbow mobility are less influenced by applied JS than by the morphological characteristics of the bones forming the joint. This observation has implications for future comparative studies of mammalian elbow function. Still, more research is needed to separate body mass, degree of cursoriality, or locomotor type as factors for elbow mobility.