Applied Anatomy of the Flexor Retinaculum
Carpal Tunnel Syndrome
Definition: Compression of the median nerve within the carpal tunnel (beneath the flexor retinaculum) producing a characteristic clinical syndrome.
Most common entrapment neuropathy in the body.
Causes — Anything that Reduces Tunnel Volume:
| Category | Examples |
|---|---|
| Increased tunnel contents | Tenosynovitis (rheumatoid arthritis), lipoma, ganglion |
| Reduced tunnel size | Malunited Colles’ fracture, dislocated lunate (the lunate displaces anteriorly and directly compresses the median nerve) |
| Fluid retention / oedema | Pregnancy, hypothyroidism, acromegaly, obesity |
| Idiopathic | Most common — degenerative thickening of flexor tendons and synovium |
Clinical Features (LOAF muscles + sensory):
Motor loss — LOAF muscles supplied by the recurrent branch of median nerve (given off just distal to retinaculum):
- Lumbrical 1 and 2
- Opponens pollicis
- Abductor pollicis brevis (most reliable to test)
- Flexor pollicis brevis (superficial head)
Weakness of thenar muscles → flattening of thenar eminence → ape hand deformity (cannot oppose thumb)
Sensory loss: Palmar surface of lateral 3½ fingers (thumb, index, middle, lateral half of ring) — the area supplied by the digital branches of the median nerve
Palmar skin is SPARED — because the palmar cutaneous branch of the median nerve arises ~5 cm above the wrist and passes superficial to the retinaculum — it does not pass through the carpal tunnel.
Tests:
- Phalen’s test: Maximum wrist flexion for 60 seconds → reproduces symptoms (paraesthesia in median nerve territory) — due to increased pressure in carpal tunnel
- Tinel’s sign: Percussion over the carpal tunnel (flexor retinaculum) → tingling in the median nerve territory (positive if nerve is irritable/regenerating)
- Carpal compression test: Direct compression over retinaculum for 30 seconds → paraesthesia
Treatment:
- Conservative: wrist splinting, steroid injection into tunnel
- Surgical: carpal tunnel decompression — division of the flexor retinaculum (incision along the line of the ring finger to protect the recurrent branch of median nerve)
Guyon’s Canal (Ulnar Canal) Syndrome
Definition: Compression of the ulnar nerve in Guyon’s canal — the tunnel between the volar carpal ligament (superficial medial slip of flexor retinaculum) and the main retinaculum.
Boundaries of Guyon’s canal:
- Roof: Volar carpal ligament (superficial medial slip)
- Floor: Main flexor retinaculum and pisohamate ligament
- Medial: Pisiform
- Lateral: Hook of hamate
Contents: Ulnar nerve + ulnar artery
Causes: Ganglion, hook of hamate fracture, repetitive compression (cyclists — “handlebar palsy”), rheumatoid arthritis
Features: Weakness of intrinsic hand muscles (hypothenar + interossei + medial two lumbricals + adductor pollicis); sensory loss over little finger and medial half of ring finger
Dislocation of the Lunate
- Fall on the dorsiflexed wrist displaces the lunate anteriorly
- Dislocated lunate directly compresses the median nerve in the carpal tunnel
- Produces acute carpal tunnel syndrome
- Requires urgent reduction
Division of Flexor Retinaculum (Surgical Relevance)
Safe incision line for carpal tunnel release:
- Incision along the ulnar border of the ring finger ray (line of the ring finger)
- This avoids the recurrent (thenar) branch of median nerve which exits the tunnel radially and curves back to thenar muscles
- The recurrent branch is at risk if the incision is made too radially
Dupuytren’s Contracture
Definition: Progressive fibrosis and contracture of the palmar aponeurosis (not the flexor retinaculum itself, but closely related anatomically — the palmar aponeurosis blends with the distal border of the retinaculum).
Features:
- Localised thickening and nodularity of the palmar aponeurosis, beginning in the medial part
- Leads to progressive flexion contracture of the ring and little fingers (ring most commonly affected)
- Proximal and middle phalanges are flexed; distal phalanx may be spared
- More common in males; associated with alcohol, diabetes, epilepsy, liver disease
Treatment: Surgical fasciectomy when hand function is significantly impaired