Carpal Tunnel

Introduction

Introduction to the Carpal Tunnel

The carpal tunnel is a narrow fibro‑osseous passageway located at the volar aspect of the wrist. It serves as the conduit for the median nerve and the long flexor tendons of the forearm to enter the palm. The rigid boundaries of the tunnel mean that any increase in its content volume or decrease in its cross‑sectional area can lead to compression of the median nerve, producing the common and clinically important carpal tunnel syndrome. An intimate knowledge of the tunnel’s anatomy is essential for understanding the symptoms of median nerve entrapment and for performing safe surgical decompression.

This module describes the boundaries, contents, fascial compartmentalisation, relationships, variations, and clinical relevance of the carpal tunnel.

Study

1. Boundaries of the Carpal Tunnel

The carpal tunnel is formed by an arch of carpal bones covered by a thick fibrous band, the flexor retinaculum. The concave palmar surface of the carpus forms the floor and walls of the tunnel, while the flexor retinaculum spans the arch as its roof.

Floor and walls: From medial to lateral, the bony floor consists of the pisiform, triquetrum, hamate (hook), capitate, trapezoid, and trapezium (tubercles). The tubercles of the scaphoid and trapezium form the lateral (radial) margin; the pisiform and hook of the hamate form the medial (ulnar) margin.

Roof: The flexor retinaculum (transverse carpal ligament) is a strong band of deep fascia that attaches medially to the pisiform and hook of hamate, and laterally to the tubercle of the scaphoid and the tubercle of the trapezium. Proximally it is continuous with the antebrachial fascia; distally it merges with the palmar aponeurosis. The palmaris longus tendon inserts into the retinaculum and the palmar aponeurosis.

2. Contents of the Carpal Tunnel

The carpal tunnel transmits the median nerve and nine tendons: four tendons of flexor digitorum superficialis (FDS), four tendons of flexor digitorum profundus (FDP), and the single tendon of flexor pollicis longus (FPL).

The median nerve lies most superficially within the tunnel, immediately deep to the flexor retinaculum. It is accompanied by a small median artery in some individuals (persistent median artery). The FDS tendons lie deep to the nerve, with the tendons to the middle and ring fingers superficial to those for the index and little fingers. The FDP tendons lie deepest, in a single row against the carpal bones. The FPL tendon lies in the radial part of the tunnel, lateral to the median nerve.

All tendons are invested in synovial sheaths: the FDS and FDP tendons are contained within a common ulnar bursa (flexor digitorum synovial sheath); the FPL tendon has its own radial bursa. These sheaths reduce friction during movement.

3. Structures Passing Superficial to the Flexor Retinaculum

Several structures pass superficial to the flexor retinaculum and are therefore spared in carpal tunnel syndrome. The palmar cutaneous branch of the median nerve arises in the distal forearm, runs over the retinaculum, and supplies the thenar skin. The palmaris longus tendon lies in the midline, anterior to the retinaculum. The ulnar nerve and artery pass through Guyon's canal, a separate tunnel formed by the palmar carpal ligament, pisiform, and hook of hamate, superficial and medial to the carpal tunnel. The palmar cutaneous branch of the ulnar nerve runs over the retinaculum to the hypothenar skin.

4. Anatomical Variations

A persistent median artery may accompany the median nerve through the tunnel and can cause dynamic compression. The median nerve may bifurcate high (bifid median nerve), and a persistent median artery often lies between the two nerve bundles. Anomalous muscles such as a proximal belly of the first lumbrical, or an accessory flexor digitorum superficialis muscle slip, may enter the tunnel and increase its content pressure. A small Gantzer muscle (accessory head of flexor pollicis longus) may also contribute.

5. Clinical Anatomy of Carpal Tunnel Syndrome

Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy, caused by chronic compression of the median nerve within the carpal tunnel. Risk factors include repetitive wrist motion, pregnancy, diabetes mellitus, hypothyroidism, rheumatoid arthritis, and acromegaly. Symptoms include paraesthesia in the lateral three and a half digits, nocturnal pain, and thenar atrophy with weakness of thumb opposition in advanced cases.

Provocative tests: Phalen's test (wrist flexion for 60 seconds reproduces symptoms); Tinel's sign (tapping over the tunnel elicits tingling); Durkan's compression test (direct pressure over the tunnel provokes symptoms).

Surgical release (open or endoscopic) divides the flexor retinaculum to decompress the median nerve, taking care to avoid injury to the recurrent motor branch and palmar cutaneous branch of the median nerve.

Summary

Summary of the Carpal Tunnel

The carpal tunnel is a fibro‑osseous canal bounded by the carpal bones and the flexor retinaculum. It transmits the median nerve and nine flexor tendons. Any condition increasing tunnel pressure can compress the median nerve, producing carpal tunnel syndrome—a sensory and motor neuropathy of the radial three and a half digits and thenar muscles. Knowledge of the tunnel’s boundaries, contents, and surface anatomy is essential for diagnosis and surgical treatment.