Stability and constraint of the proximal interphalangeal joint.

The kinematics of the intact proximal interphalangeal joint of 12 fresh cadaver index fingers were measured by means of an electromagnetic tracking system. The specimens were then randomly divided into two groups for ligament sectioning in two different sequences and for testing under lateral stress. Lateral stress of the intact proximal interphalangeal joints produced an average of 5 degrees of adduction and 9 degrees of supination motion throughout the arc of flexion/extension. Maximum lateral angulation was 15 degrees under 1 kg of force (30 N cm) applied at the distal end of the middle phalanx. Joint angulation increased to 20 degrees after total sectioning of the collateral ligament. Joint laxity was greatly reduced in full extension, in full flexion, and when the muscles were loaded. The proximal interphalangeal joint remained stable when one half of the collateral ligament was left intact. The results indicate that lateral stability of the proximal interphalangeal joint is provided primarily by the collateral ligament. When the lateral stress test is normal in proximal interphalangeal joint extension, an additional test in 20 to 30 degrees of flexion should be considered to avoid a false-negative result. Angulation greater than 20 degrees is abnormal and indicates a loss of collateral ligament integrity.