Patterns of internal bone structure and functional adaptation in the hominoid scaphoid, lunate, and triquetrum

The morphology of the proximal carpals (scaphoid, lunate, triquetrum) are linked to the range of motion (ROM) at the radiocarpal and midcarpal joints. While the relationship between ROM and habitual locomotor mode is well established, it has yet to be investigated whether relative patterns of internal bone architecture reflect the kinematics and kinetics at the proximal row. As internal bone is known to model its structure to habitually incurred forces, internal architecture has the potential to provide insight into how a joint(s) have been loaded during the lifetime of an individual. Using a broad sample of extant great apes and humans (n=177 total), this study investigates whether relative differences in the bone volume to total volume (BV/TV) and degree of anisotropy (DA) across the scaphoid, lunate and triquetrum correlate with the presumed force transfer and biomechanics of the hominoid wrist. Results reveal broad patterns in BV/TV and DA differentiated hominoids by their predominant locomotor mode. The human pattern suggests the lunate may be the most highly strained bone within the proximal row. Both knuckle-walking taxa (Gorilla, Pan) exhibited similar architectural patterns suggesting they are adapted to resist similar forces in this region of the wrist. The relatively high DA across all Pongo carpals suggests it may have more stereotypical wrist loading than commonly assumed. Finally, the distinctly low DA in the triquetrum across all taxa suggests force transfer via the synapomorphic Triangular Fibrocartilage Complex may leave a distinctive signature in the internal bone architecture that requires further investigation.