Relations of the Flexor Retinaculum — Structures Superficial and Deep
Structures Passing Superficial to the Flexor Retinaculum
Six structures pass superficial to (in front of) the flexor retinaculum, from medial to lateral:
| Order (Medial → Lateral) | Structure | Notes |
|---|---|---|
| 1 | Ulnar nerve | Passes through Guyon’s canal (deep to volar carpal ligament, superficial to main retinaculum) |
| 2 | Ulnar artery | Passes through Guyon’s canal alongside ulnar nerve |
| 3 | Palmar cutaneous branch of ulnar nerve | Arises from ulnar nerve above the wrist; crosses superficially |
| 4 | Tendon of palmaris longus | Continues as palmar aponeurosis distally; lies in midline |
| 5 | Palmar cutaneous branch of median nerve | Arises from median nerve ~5 cm above wrist; supplies central palm and thenar skin |
| 6 | Superficial palmar branch of radial artery | Crosses superficially to contribute to superficial palmar arch |
Mnemonic (medial to lateral): UUPTP S — Ulnar nerve, Ulnar artery, Ulnar cutaneous, Tendon of palmaris longus, Palmar cutaneous (median), Superficial palmar branch (radial)
Structures Passing Deep to the Flexor Retinaculum (Through the Carpal Tunnel)
Four structures pass through the carpal tunnel (deep to the main flexor retinaculum):
| Structure | Notes |
|---|---|
| Tendons of flexor digitorum superficialis (FDS) × 4 | Arranged in two rows: tendons to middle and ring fingers anterior to those for index and little fingers |
| Tendons of flexor digitorum profundus (FDP) × 4 | Deep to FDS tendons; enclosed with FDS in ulnar bursa |
| Tendon of flexor pollicis longus (FPL) | Radial side of tunnel; enclosed in radial bursa (separate sheath) |
| Median nerve | Most superficial and lateral of deep structures; lies anterior to FDS tendons |
Total structures through carpal tunnel: 9 (4 FDS + 4 FDP + 1 FPL) tendons + 1 median nerve = 10 structures
Structure NOT Passing Through the Carpal Tunnel
| Structure | Actual Relationship |
|---|---|
| Tendon of flexor carpi radialis | Passes through a separate canal in the lateral slip of flexor retinaculum — not in the main carpal tunnel |
| Ulnar nerve and artery | Pass through Guyon’s canal (between volar carpal ligament and main retinaculum) — not in the carpal tunnel |
| Palmar cutaneous branch of median nerve | Passes superficial to the retinaculum — this is why it is not affected in carpal tunnel syndrome |
Arrangement Within the Carpal Tunnel
The structures within the carpal tunnel are arranged as follows (anterior to posterior):
- Most anterior: Median nerve (just deep to retinaculum, slightly lateral)
- Middle layer: Four FDS tendons (2 anterior: middle + ring; 2 posterior: index + little)
- Posterior layer: Four FDP tendons
- Radial side (separate): FPL tendon in radial bursa
Functions of the Flexor Retinaculum
- Maintains the carpal arch — prevents flattening under load
- Holds flexor tendons in position at the wrist — prevents bowstringing during flexion
- Provides pulley action for efficient tendon mechanics
- Gives origin to thenar and hypothenar muscles
- Provides attachment for palmaris longus → palmar aponeurosis