Carpal tunnel changes and median nerve compression during wrist flexion and extension seen by magnetic resonance imaging.

Magnetic resonance imaging was done on the wrists of 14 volunteer subjects with the wrists in the neutral position, then flexed and extended at 45 degrees. Computer enhanced cross-sectional areas were measured on each subject at the level of the hook of the hamate. In the neutral position the distance between confining structures around the median nerve is 2.0 +/- 0.2 mm in the anteroposterior direction, and the mean cross-sectional area of the carpal tunnel is 1.52 cm2. With the wrist in the extended position the distance available for the median nerve measures 2.2 +/- 0.4 mm and the cross-sectional area increases to 1.75 cm2. When the wrist is flexed, the distance available for the median nerve between the flexor tendons and transverse carpal ligament decreases to 1.1 +/- 0.4 mm, and the cross-sectional tunnel area decreases to 1.36 cm2. Flexion of the wrist and/or fingers both produces a palmar rearrangement of the flexor tendons creating potential compression of the median nerve. The nerve responds to these forces by becoming interposed in various positions between the superficial flexor tendons.