A Method for Quantifying Articular Surface Morphology of Metacarpals Using Quadric Surface Approximation

The shapes of articular surfaces in human metacarpals differ considerably from those in African apes, and such distinctive human morphological characteristics are considered to be strongly linked to dexterous manipulative capabilities that have evolved in the human lineage. However, no detailed studies have quantitatively compared the morphology of the articular surfaces of the metacarpals. We here propose a method for quantifying the orientation and curvature of distal articular surfaces in the 5 metacarpals and proximal saddle surface in the 1st metacarpal via quadric surface approximation. This method utilizes detailed digital models of metacarpals constructed using a 3D noncontact digitizer, and approximates the articular surfaces using a paraboloid to calculate the orientations of the surfaces with respect to the diaphyses and curvature. To demonstrate efficacy of the proposed method, we applied the technique to analysis of the articular surface variation in a total of 98 metacarpals from 10 humans and 11 chimpanzees. The results show that the distal and proximal surfaces of the 1st metacarpal in humans were significantly more axially twisted in a pronating direction; hence, the human thumb pronates and faces the palm and the other fingers with flexion of the first carpometacarpal joint. Further, the head of the 2nd metacarpal in humans is significantly twisted in a pronating direction and the torsion of the metacarpal head gradually shifts toward supination approaching the 5th metacarpals. The proposed method allows quantitative description of such interspecific variations in metacarpals, suggesting its effectiveness for the assessment of the morphological affinities of metacarpals in hominoids.