Osteofibrous tunnel on the front of the wrist; floor + walls = concave carpal arch; roof = flexor retinaculum.

Flexor Retinaculum Attachments

Two Slips

• Lateral deep slip — attaches medial lip of trapezium groove → separate tunnel for FCR tendon
• Medial superficial slip (palmar carpal ligament) — pisiform attachment; ulnar nerve + ulnar vessels pass deep to this slip (Guyon’s canal)

Contents of Carpal Tunnel

1. Median nerve — most anterior
2. 4 × FDS tendons
3. 4 × FDP tendons
4. 1 × FPL tendon
5. Ulnar bursa (FDS/FDP of medial 3 fingers)
6. Radial bursa (FPL tendon)

FCR does not pass through main carpal tunnel — lies in its own FCR canal (lateral slip).

Structures Superficial to Flexor Retinaculum

• Palmar cutaneous branch of median nerve
• Palmar cutaneous branch of ulnar nerve
• Palmaris longus tendon
• Ulnar nerve + ulnar vessels (deep to palmar carpal ligament)
• Thenar + hypothenar muscles (arise from anterior surface of retinaculum)

Palmar cutaneous branch of median nerve passes superficial to retinaculum → thenar eminence sensation spared in CTS.

Applied Anatomy

• CTS — median nerve compression → pain + paraesthesia in lateral 3½ digits + thenar wasting; thenar eminence sensation spared (palmar cutaneous branch outside tunnel)
• Carpal tunnel release — division of flexor retinaculum; avoid recurrent (motor) branch of median nerve + palmar cutaneous branches

The carpal tunnel is an osteofibrous tunnel on the front of the wrist, formed by the concave anterior arch of the carpal bones (floor and walls) roofed over by the flexor retinaculum.

Flexor Retinaculum — Attachments

Two Slips of the Retinaculum

• Lateral deep slip: attaches to the medial lip of the groove on the trapezium, converting it into a separate tunnel for the tendon of flexor carpi radialis.
• Medial superficial slip (palmar carpal ligament): attaches to the pisiform bone. The ulnar nerve and ulnar vessels pass deep to this slip (not through the main carpal tunnel) — this is the basis of the ulnar tunnel (Guyon’s canal).

Contents of the Carpal Tunnel (Deep to Flexor Retinaculum)

1. Median nerve (most anterior structure; for its termination and division in the hand, see Median Nerve in the Hand and Carpal Tunnel
2. Four tendons of flexor digitorum superficialis
3. Four tendons of flexor digitorum profundus
4. Tendon of flexor pollicis longus
5. Ulnar bursa (enclosing the FDS/FDP tendons of the medial three fingers)
6. Radial bursa (enclosing the FPL tendon)

Note: The tendon of flexor carpi radialis does not pass through the main carpal tunnel — it lies in its own separate compartment, between the flexor retinaculum and its lateral deep slip, within the groove on the trapezium.

Structures Passing Superficial to the Flexor Retinaculum

• Palmar cutaneous branch of the median nerve
• Palmar cutaneous branch of the ulnar nerve
• Tendon of palmaris longus
• Ulnar nerve and ulnar vessels (deep to the palmar carpal ligament/superficial slip)
• Thenar and hypothenar muscles arise from the anterior surface of the retinaculum.

The fact that the palmar cutaneous branch of the median nerve passes superficial to (not through) the retinaculum explains why sensation over the thenar eminence is typically spared in carpal tunnel syndrome.

Applied Anatomy

• Carpal Tunnel Syndrome (CTS) results from compression of the median nerve within this tunnel, producing pain and paraesthesia in the median nerve distribution with thenar wasting, but sparing sensation over the thenar eminence (for clinical features, tests, and management correlation, see Applied Anatomy of Median Nerve and Applied Anatomy and Surgical Incisions of Palmar Spaces).
• Surgical division of the flexor retinaculum (carpal tunnel release) is performed for decompression in CTS; care is taken to avoid the recurrent (motor) branch of the median nerve and the palmar cutaneous branches during the incision.

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