Patterns of internal bone structure and functional adaptation in the hominoid scaphoid, lunate, and triquetrum
Authors: Bird Emma E. , Kivell Tracy L. , Skinner Matthew M. 
Published: November 2021

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 joints have been loaded during the lifetime of an individual. Using a broad sample of extant great apes and humans (n = 177 total bones), 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 that broad patterns in BV/TV and DA differentiate 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.

Objectives

Functional adaptation in the trabecular and cortical bone of individual wrist bones has been investigated across hominoid species but functional conclusions remain limited. This study examines whether relative patterns in internal bone architecture across multiple carpal bones can be correlated to the known or assumed kinetics and kinematics of the wrist joint in extant hominoids.

Materials and Methods

This study applied a whole‐bone methodology to quantify the internal architecture (cortical and trabecular bone) of the scaphoid, lunate, and triquetrum of suspensory (Pongo sp.), knuckle‐walking (Pan paniscus, Pan troglodytes, Gorilla sp.) and bipedal (Homo sapiens) hominoids (n = 177 total bones).

Results

H. sapiens showed unique patterns in both measured parameters: a decrease in degree of anisotropy (DA) from the scaphoid to the triquetrum with higher bone volume to total volume (BV/TV) in the lunate relative to the other bones. Knuckle‐walking taxa had similar patterns in both parameters: highest mean DA in the lunate and lowest in the triquetrum while significantly higher BV/TV was recorded the triquetrum. Pongo exhibited the same DA pattern as knuckle‐walking taxa but a distinct pattern of continual decrease in BV/TV from scaphoid to triquetrum.

Discussion

Relative differences in the internal bone structure across multiple carpals differentiated locomotor modes in extant hominoids. The triquetrum and lunate are particularly understudied but their importance to differentiating locomotor mode indicates further research is warranted. Establishing patterns across more carpal joints in primates should be a research priority as they will provide critical context to interpreting fossil species represented by single or few carpal elements.