Torsion in metacarpal bones and bilateral asymmetry.

A comparison of the relative position of the dorsoventral axes of the bases and heads of the metacarpal bones shows that these do not lie in the same plane, and that torsion can therefore be said to exist in these bones. Torsion is such that the heads of the second and third metacarpals appear to be rotated laterally relative to the base, whereas the heads of the fourth and fifth metacarpals appear to be rotated medially. The usual direction of "rotation" of the head of the first metacarpal is to the lateral side in the right hand and to the medial side in the left hand (P less than 0.001). In addition, there are statistically significant differences in the degree of torsion in other metacarpal bones of the right and left sides: torsion in the second and third metacarpals is greater in the right hand (P less than 0.001), while torsion in the fourth and fifth digits is greater in the left hand (P less than 0.001). All these asymmetries represent a "lateral shift" in the degree of torsion in metacarpal bones of the right side. Torsion in the metacarpal bones appears to facilitate efficiency of the grip: it is apparently an important factor in preventing crowding together of the second to fifth digits during flexion.