Forces in the metacarpophalangeal joint due to elevated fluid pressure--analysis, measurements and relevance to ulnar drift.

The mechanics of the metacarpophalangeal joint are analyzed, with a view to understanding why the finger moves in the direction of ulnar drift when the intra-articular pressure in the joint is increased. The first step is the in situ measurement of stiffness of the various tissues surrounding the joint, and the data show that (a) the two collateral ligaments are the major component and (b) the ulnar ligament is the stiffer. This latter result, combined with finding the centre of pressure at the base of the phalanx, reveals how increased intra-articular pressure produces ulnar deviation. Flexion, the other component of ulnar drift, is produced because of the volar side attachment of the ligaments. The mechanics are evaluated quantitatively by the measurement of forces at the finger tip for various pressures; the data agree reasonably well with predictions using the measured joint properties in a simplified mechanical model of the joint.