Internal stabilization in the thumb.

An analysis of the thumb as an articular chain, with the biarticular tendon displacement used as a model, is presented. Comparison of the extensor pollicis longus/flexor pollicis longus (EPL/FPL) tendon displacement ratios at the three joints, as determined from cadaver studies, shows that the thumb will be unstable, with a persistent tendency to extend at the carpometacarpal (CMC) joint and flex at the interphalangeal (IP) joint when it is under the exclusive control of the EPL and FPL muscles. In addition, according to the model, with stabilization of the CMC joint, the MCP joint will tend to extend and the IP joint will flex. Clinical data from thumbs of leprosy patients with thenar paralyses largely confirm these predictions and the validity of this approach in studying the functional behavior of the thumb.