Effects of the adductor pollicis and abductor pollicis brevis on thumb metacarpophalangeal joint laxity before and after ulnar collateral ligament reconstruction

Purpose The intrinsic muscles and ulnar capsuloligamentous structures (UCLS), which consist of the ulnar collateral ligament (UCL), accessory UCL, dorsal capsule, and volar plate of the thumb metacarpophalangeal (MCP) joint are important for controlling the motion and stability of the MCP joint during pinch. The purpose of this cadaveric study was to determine the effects of the adductor pollicis (AdP) and abductor pollicis brevis (APB) on the 3-dimensional MCP joint laxity before transection of the UCLS and after reconstruction of the UCL and repair of the dorsal capsule. Methods Loads were applied to the flexor pollicis longus (FPL) alone, to the AdP and FPL in combination, and to the APB and FPL in combination in 11 cadavers. This was done in the intact joint after the UCLS were transected and after the UCL was reconstructed for flexion angles of 0°, 15°, 30°, and 45°. The spatial positions of the proximal phalanx and the metacarpal of the MCP joint were measured with a 6-degrees-of-freedom digitizing system. Results In the intact joint combined loading of the AdP and FPL did not affect the position of the proximal phalanx. Combined loading of the APB and FPL changed the position of the phalanx from an ulnar to a radial shift and from an ulnar to a radial deviation and it increased pronation. After transection of the UCLS combined loading of the FPL and AdP increased supination of the MCP joint and combined loading of the FPL and APB increased radial shift, radial deviation, and pronation of the joint. Reconstruction of the UCL restored normal laxity to the MCP joint. Conclusions The AdP failed to affect MCP joint motion. The ABP produced a radial shift and radial deviation of the MCP joint and increased pronation of the thumb. Transection of the UCLS increased joint laxity for each of the combined loadings and reconstruction of the UCL restored normal laxity to the MCP joint.