Anatomy of the flexor retinaculum.

For an accurate definition of the anatomic limits of the carpal tunnel, 26 cadaver upper extremities were studied by gross (10), histologic (3), and radiographic (13) methods. The mean proximal limit of the central portion of the flexor retinaculum was 11 mm distal to the capitate-lunate joint, and the mean distal limit of the distal portion was 10 mm distal to the carpometacarpal joint of the third metacarpal. Carpal tunnel width at the hook of the hamate (20 mm) was significantly smaller than its proximal (24 mm) or distal (25 mm) extent. The flexor retinaculum extended from the distal aspect of the radius to the distal aspect of the base of the third metacarpal. We redefined the palmar boundary of the carpal tunnel to include three continuous segments of flexor retinaculum: the thin proximal segment composed of thickened deep investing fascia of the forearm; the transverse carpal ligament; and the distal portion of the flexor retinaculum, composed of an aponeurosis between the thenar and hypothenar muscles. In light of recent operative procedures that divide only the transverse carpal ligament, this study provides an anatomic basis for a more extensive release.