Boundaries and contents
The carpal tunnel is bounded by bone on three sides and by the flexor retinaculum in front. The carpal bones form a transverse arch that is concave anteriorly. The retinaculum bridges that arch like a bowstring and turns the groove into a closed tunnel.
The flexor retinaculum
The flexor retinaculum is a thick band of deep fascia, also called the transverse carpal ligament. It attaches medially to the pisiform and the hook of the hamate, and laterally to the tubercle of the scaphoid and the tubercle of the trapezium. Its proximal edge lies at the level of the distal wrist crease. It holds the long flexor tendons against the carpus so they do not bowstring when the wrist flexes. It also anchors the thenar and hypothenar muscles, which arise partly from its anterior surface.
What passes through
Ten structures pass through the tunnel:
- Median nerve, the most superficial structure, lying just deep to the retinaculum on the lateral side.
- Four tendons of flexor digitorum superficialis (FDS), with the tendons to the middle and ring fingers lying anterior to those for the index and little fingers.
- Four tendons of flexor digitorum profundus (FDP), lying deepest, side by side on the floor.
- Tendon of flexor pollicis longus (FPL), on the lateral side.
The FDS and FDP tendons share the common flexor sheath (ulnar bursa). The FPL tendon has its own synovial sheath (radial bursa). The tendon of flexor carpi radialis does not run in the main tunnel: it passes through a separate compartment formed by a split in the lateral attachment of the retinaculum, grooving the trapezium.
Relations: structures outside the tunnel
Several structures cross the front of the wrist superficial to the flexor retinaculum, and the examiner expects them named. They are not affected when the median nerve is compressed in the tunnel.
- Ulnar nerve and ulnar artery pass through Guyon's canal (the ulnar canal), between the pisiform and the hook of the hamate, superficial to the retinaculum.
- Palmaris longus tendon, where present, runs over the retinaculum and blends with the palmar aponeurosis.
- Palmar cutaneous branch of the median nerve arises in the distal forearm and passes superficial to the retinaculum to supply the skin of the central palm and thenar eminence.
- Palmar cutaneous branch of the ulnar nerve supplies the skin over the hypothenar eminence.
- Flexor carpi ulnaris inserts into the pisiform, medial to the tunnel.
The median nerve in the tunnel
The median nerve enters the hand through the tunnel and divides at its distal edge into a recurrent motor branch and digital branches. The recurrent (thenar) branch curls back around the distal border of the retinaculum to supply abductor pollicis brevis, opponens pollicis and the superficial head of flexor pollicis brevis. Its origin varies: it may arise within the tunnel and pierce the retinaculum, which puts it at risk during surgery.
The digital branches carry sensation from the palmar surface of the thumb, index, middle and lateral half of the ring finger, including the nail beds. They also supply the first and second lumbricals.
| Median nerve branch | Relation to retinaculum | Territory |
|---|---|---|
| Palmar cutaneous branch | Superficial to it | Skin of central palm and thenar eminence |
| Recurrent motor branch | Usually arises at its distal edge | Abductor pollicis brevis, opponens pollicis, flexor pollicis brevis (superficial head) |
| Palmar digital nerves | Beyond its distal edge | Lateral three and a half digits; lumbricals 1 and 2 |
Clinical relevance
Carpal tunnel syndrome is compression of the median nerve in this rigid space, and it is the commonest entrapment neuropathy of the upper limb. Anything that swells the contents or narrows the tunnel raises the pressure: pregnancy, hypothyroidism, rheumatoid synovitis, diabetes, acromegaly and malunited distal radius fractures.
- Symptoms are tingling, numbness and pain in the thumb, index and middle fingers, typically worse at night.
- The palm is spared because the palmar cutaneous branch runs superficial to the retinaculum.
- Late signs are wasting of the lateral thenar eminence and weakness of thumb abduction, tested against resistance with the thumb pointing to the ceiling.
- Provocation tests include Tinel's sign (tapping over the nerve) and Phalen's test (sustained wrist flexion).
Carpal tunnel decompression divides the retinaculum along its ulnar side, through an incision in line with the radial border of the ring finger. Staying ulnar protects the recurrent motor branch and the palmar cutaneous branch, which both lie radially.