A method for defining carpometacarpal joint kinematics from three-dimensional rotations of the metacarpal bones captured in vivo using computed tomography

The carpometacarpal (CMC) joints of the hand facilitate motion of the metacarpals and are critical for functional tasks, especially tasks involving precision grasping. Despite their importance, only limited data describing metacarpal kinematics exist. In this short communication, we quantified in vivo metacarpal kinematics from a single subject and used these data to develop the kinematic functions necessary to add rotational degrees of freedom at the CMC joints to a biomechanical model of the hand. Computed tomography (CT) was used to capture three-dimensional rotations of the metacarpal bones of the ring and little fingers of the subject in seven different static postures, chosen to position the fourth and fifth metacarpals throughout a functional range of motion. The CT images were manually segmented, yielding digital surface representations of the metacarpals in each posture. From the surfaces, principal axes of rotation were defined by calculating orthonormal bases of the surface vertices. The three-dimensional rotations of the fourth and fifth metacarpals were quantified in each posture about their principal axes, relative to a designated reference posture. For both metacarpals, ranges of rotations were computed about the principal axes across all seven postures. From the processed data, single axes and angles were calculated that were equivalent to the three dimensional ranges of motion. Finally, kinematic functions were defined that enabled modeling of the formation of a metacarpal arch by movement of the fourth and fifth CMC joints as one degree of freedom, coupled to a single generalized coordinate.