Intrinsic muscle contribution to the metacarpophalangeal joint flexion moment of the middle, ring, and small fingers

PURPOSE: To analyze the relative contribution of the intrinsic muscles to the flexion moment potential of the metacarpophalangeal (MCP) joints of the middle, ring, and small fingers and to calculate the moment potential loss occurring with deep motor branch, low, or high ulnar nerve palsy or low median nerve palsy. METHODS: Eleven fresh cadaver hands were used. A small wire sutured to each tendon was connected to an excursion transducer containing a constant-tension spring. The tendon excursion and the MCP joint rotation were measured simultaneously during 10 to 16 cycles of passive flexion/extension cyclic motion. The moment arm was calculated from the tendon excursion-joint rotation curve as the derivative of the curve. The flexion moment potential was calculated by multiplying the moment arm with the known tension fractions of the muscles. RESULTS: When the MCP joint was at 0 degrees of flexion the relative moment potential contributions of the intrinsic muscles to MCP joint flexion were 8%, 13%, and 28% in the middle, ring, and small fingers, respectively. Moment potential losses were 7%, 13%, and 6% in the middle, ring, and small fingers, respectively, in deep motor branch ulnar nerve palsy. In low ulnar nerve palsy the losses were 7%, 13%, and 28%, respectively. In high ulnar nerve palsy they were 7%, 64%, and 82%, respectively. Low median nerve palsy, however, resulted in a moment potential loss at the middle finger MCP joint of less than 2%. CONCLUSIONS: The relative contribution of the intrinsic muscles to the total flexion moment at the MCP joint was different for each finger. The small finger had a large intrinsic contribution, primarily because of the larger moment arms of the hypothenar muscles.