Articular size and curvature as determinants of carpal joint mobility and stability in strepsirhine primates.

Theoretical and empirical evidence suggest that limb joint surface morphology is mechanically related to joint mobility, stability, and strength. This study tests hypotheses relating aspects of joint surface shape to joint function by comparing carpal joint size and curvature among strepsirhine primates that differ significantly in their positional behaviors and hand postures: vertical clingers, active arboreal quadrupeds, and slow cautious climbers. Joints that are very mobile are expected to have increased size and curvature of male joint mating surfaces, whereas those that function primarily in weight-bearing are expected to have relatively expanded female joint mating surfaces. Results show that 1) high male joint mating surface curvature is related to increased joint mobility and 2) increased female joint mating surface curvature is related to increased joint stability under loads of different orientation. Arc lengths of both male and female joint mating surfaces do not differ significantly between locomotor groups. Moreover, carpal joint curvature is not significantly correlated with either joint size (arc length) or body size, but carpal joint size and body size are highly correlated with one another. Relative to body size, articular arc lengths scale close to isometry (geometric similarity) both within and among groups. These results suggest that structural changes leading to increased joint mobility involve modifying joint surface curvature, and in the case of the carpal joints do not include altering joint size. Curvature of female joint mating surfaces appears related to variation in load orientation, but not necessarily load magnitude and frequency.