Effects of extensor pollicis longus transposition and extensor indicis proprius transfer to extensor pollicis longus on thumb mechanics

PURPOSE: The spatial relationship of the extensor pollicis longus (EPL) to the thumb carpometacarpal (CMC) joint may be altered by its transposition from the third dorsal wrist compartment and by subcutaneous extensor indicis proprius (EIP) to EPL tendon transfer. Changes in tendon position could alter thumb function. This study examined changes in the EPL adduction moment arm after EPL tendon transposition from its extensor compartment or EIP transfer. METHODS: The EPL adduction moment arm at the thumb carpometacarpal joint was determined under 4 tendon conditions: (1) intact extensor pollicis longus, (2) transposed extensor pollicis longus, (3) extensor indicis proprius to extensor pollicis longus tendon transfer through an extensor retinacular pulley, and (4) extensor indicis proprius tendon transfer through a subcutaneous route. Each tendon condition was tested in 2 wrist positions: neutral and 40 degrees of flexion. RESULTS: The wrist neutral/flexion moment arms for the 4 tendon conditions, in millimeters, were 9.2/7.3, 3.6/1.2, 8.3/5.1, and 4.8/1.0. CONCLUSIONS: EPL transposition produces a significant decrease of its adduction moment arm at the thumb CMC joint, an effect exacerbated by wrist flexion. The moment arm mechanics of the pulley and subcutaneous EIP tendon transfer resemble those of the intact and transposed EPL, respectively. Diminution of the adduction moment arm could impair thumb function, especially adduction.