Articular constraint, handedness, and directional asymmetry in the human second metacarpal.

The hypothesis that functional adaptation of joint surfaces to mechanical loading occurs primarily through change in mass, density, and structure of subarticular trabeculae (the "articular constraint" model) is investigated through an analysis of directional asymmetry among three separate bone compartments in the human second metacarpal. Measures of midshaft cross-sectional geometry, osteometry of the distal epiphysis, and subarticular trabecular microarchitecture of the distal epiphysis (assessed by high-resolution microcomputed tomography) were determined for 29 paired male and female metacarpals from a well-preserved nineteenth-century Euro-Canadian historic cemetery sample. For each measure, asymmetry was quantified using both mean-difference and confidence-interval methods. Both methods found a significant right-hand bias for measures of structural strength in midshaft geometry, as has been previously noted for this sample. Articular size, however, exhibits a right-hand bias only with regard to mediolateral, and not dorsopalmar, dimensions, a result that may reflect directional asymmetry in hand breadth at the distal palmar arch. The most striking asymmetries occur for subarticular trabecular microarchitecture. The right metacarpal head exhibits greater bone volume fraction, bone surface density, trabecular number, connectivity, and a more platelike rather than rodlike structure. These outcomes confer greater resistance to both axial compressive and shear strains for the metacarpal head at the metacarpophalangeal arthrosis. In all, these results confirm and extend previous research documenting structural asymmetries and limb dominance and are consistent with the concept of articular constraint. They also suggest a morphological signal through which functional asymmetry associated with handedness in fossil hominins may be investigated.