Carpal tunnel

The carpal tunnel is a fibro-osseous tunnel on the palmar surface of the carpal bones which transmits a number of tendons and the median nerve from the forearm into the hand.

Surface markings are, proximally, the distal wrist crease; distally, the hook of the hamate bone.

The recurrent thenar branch of the median nerve, the motor branch to the thenar emminence, leaves the median nerve in or beyond the carpal tunnel, then curves back over the flexor retinaculum to reach the thenar emminence. It is endagered in carpal tunnel surgery if the incision over the carpal tunnel is mede too far laterally (thumb side).

The base and walls of the carpal tunnel are a concave bony channel formed by the carpal bones.

The flexor retinaculum, or transverse carpal ligament, forms the roof of the carpal tunnel. It attaches to the scaphoid tubercle and ridge of the trapezium laterally, and to the pisiform and hook of the hamate laterally.

From superficial to deep, the structures encountered in exposing the carpal tunnel, are:

• Skin
• Palmaris longus tendon merging with palmar aponeurosis
• Palmaris brevis muscle
• Flexor retinaculum

The median nerve is immediately deep to the retinaculum.

The carpal tunnel transmits:

• Flexor carpi radialis tendon, in its own synovial sheath
• Median nerve
• Flexor digitorum superficialis (flex. dig. sublimis) tendons to the four fingers. All four tendons travel in a common synovial sheath. The tendons to the middle and ring fingers lie superficial to the tendons to the index and little fingers
• Flexor digitorum profundus tendons, lying side-by-side. The index finger tendon is separate; the others lie close together.

It is important because the median nerve can be compressed within the carpal tunnel in:

• Carpal tunnel syndrome
• Wrist dislocations
• Fractures around the wrist