KINEMATICS OF THE NORMAL TRAPEZIOMETACARPAL JOINT

Motion of the trapeziometacarpal joint was studied in 12 hands from fr esh human cadavera. By use of a magnetic tracking system, a full range of motion of the first metacarpal was analyzed with respect to a defi ned trapezial coordinate system. The traces of the reference points on the head and base of the first metacarpal were monitored, and the ins tantaneous centers of rotation were calculated. During circumduction, the reference points on the head and base followed elliptical paths bu t in opposite directions. The average instantaneous center of circumdu ction was at approximately the center of the trapezial joint surface. In flexion-extension, the axis of rotation was located within the trap ezium, and the path of the head was identical to the path of the base. In abduction-adduction, the axis of rotation was located distal to th e trapezium within the base of the first metacarpal, and the base and head moved in opposite directions. There was no single center of rotat ion; rather, instantaneous motion occurred reciprocally between these centers of rotation within the trapezium and metacarpal base in the no rmal thumb. This changing instantaneous center of rotation results in a unique pattern of motion which is related to congruent, tightly cons trained joint surfaces of two reciprocal saddle joints and to precisel y positioned extraarticular ligaments.