Strong, thick, rectangular fibrous band; thickening of deep fascia in front of carpal bones. Converts anterior carpal concavity into carpal tunnel.

Bony Attachments

Two Slips

• Medial superficial slip (volar carpal ligament) — ulnar nerve + ulnar artery pass deep to this slip through Guyon’s canal
• Lateral deep slip — converts trapezium groove into separate canal for FCR tendon

Structures Superficial to Retinaculum (Medial → Lateral)

1. Ulnar nerve
2. Ulnar artery
3. Palmar cutaneous branch of ulnar nerve
4. Tendon of palmaris longus
5. Palmar cutaneous branch of median nerve
6. Superficial palmar branch of radial artery

Structures Deep to Retinaculum (Carpal Tunnel)

• 4 × FDS + 4 × FDP + 1 × FPL = 9 tendons + median nerve = 10 structures total
• Median nerve: most anterior + slightly lateral
• FDS: middle and ring tendons anterior to index and little
• FPL: radial side in radial bursa

Synovial Bursae

Applied Anatomy

Carpal Tunnel Syndrome (CTS)

• Most common entrapment neuropathy
• Causes: tenosynovitis, RA, lunate dislocation, pregnancy, hypothyroidism, acromegaly
• Motor: LOAF muscles (Lumbricals 1+2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis superficial head) → thenar wasting, ape-thumb posture
• Sensory: lateral 3½ digits; thenar eminence spared (palmar cutaneous branch superficial to retinaculum)
• Tests: Phalen’s (wrist flexion 60 s), Tinel’s (percussion over tunnel), carpal compression
• Rx: splinting, steroid injection, surgical decompression

Guyon’s Canal Syndrome

• Boundaries: roof = volar carpal ligament; floor = retinaculum + pisohamate ligament; medial = pisiform; lateral = hook of hamate
• Causes: ganglion, hook of hamate fracture, handlebar palsy
• Features: intrinsic hand weakness (hypothenar, interossei, medial 2 lumbricals, adductor pollicis); dorsal sensation spared (dorsal cutaneous branch arises proximal)

Horseshoe Abscess

• Infection: little finger sheath → ulnar bursa → (if ulnar + radial bursae communicate) → radial bursa → thumb sheath → U-shaped spread

Carpal Tunnel Release Incision

• Along ulnar border of ring finger ray → avoids recurrent (thenar) branch of median nerve

Dupuytren’s Contracture

• Fibrosis of palmar aponeurosis (not retinaculum itself)
• Ring and little fingers most affected; associated with diabetes, alcohol, epilepsy

Lunate Dislocation

• Anterior displacement → acute CTS; requires urgent reduction

Flexor Retinaculum — Definition, Attachments and Slips

Definition

The flexor retinaculum (transverse carpal ligament) is a strong, thick, rectangular fibrous band formed by thickening of the deep fascia in front of the carpal bones.

It bridges the anterior concavity of the carpus, converting it into an osseofibrous tunnel called the carpal tunnel, through which the long flexor tendons of the digits and the median nerve pass.

Dimensions and Orientation

• Rectangular in shape
• Approximately 2–3 cm wide and 3 cm long
• Directed transversely across the wrist

Bony Pillars

The flexor retinaculum is attached to four bony prominences, two on each side, which serve as the four corners of the carpal arch:

Slips of the Flexor Retinaculum

On each side, the flexor retinaculum gives off an additional slip.

Medial Slip (Superficial — Volar Carpal Ligament)

• A superficial slip on the medial side
• Related to the pisiform bone and hook of hamate region
• More superficial than the main flexor retinaculum
• The ulnar nerve and ulnar artery pass deep to this superficial slip but superficial to the main retinaculum through Guyon’s canal (ulnar canal)

Lateral Slip (Deep)

• A deep slip on the lateral side
• Attached to the medial lip of the groove of the trapezium
• Converts the groove on the trapezium into a separate osseofibrous tunnel for the tendon of flexor carpi radialis
• The tendon of FCR therefore passes in its own tunnel in relation to the lateral part of the flexor retinaculum, separate from the main carpal tunnel

Summary

Additional Attachments

The flexor retinaculum gives origin to muscles of the thenar and hypothenar eminences:

• Thenar side (lateral): Abductor pollicis brevis, superficial head of flexor pollicis brevis, opponens pollicis
• Hypothenar side (medial): Abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi

The palmaris longus tendon passes in front of the retinaculum and continues distally into the palmar aponeurosis, which is attached to the distal margin and anterior surface of the retinaculum.

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Relations of the Flexor Retinaculum — Structures Superficial and Deep

Structures Passing Superficial to the Flexor Retinaculum

Six structures pass superficial to the flexor retinaculum from medial to lateral:

Mnemonic (medial to lateral): UUPTPS — ulnar nerve, ulnar artery, ulnar cutaneous branch, tendon of palmaris longus, palmar cutaneous branch of median nerve, superficial palmar branch of radial artery.

Structures Passing Deep to the Flexor Retinaculum (Through the Carpal Tunnel)

Four named structures pass through the carpal tunnel deep to the main flexor retinaculum:

Total within the carpal tunnel: 9 tendons (4 FDS + 4 FDP + 1 FPL) + 1 median nerve = 10 structures.

Structures Not Passing Through the Carpal Tunnel

Arrangement Within the Carpal Tunnel

The structures within the carpal tunnel are arranged as follows:

• Most anterior: Median nerve, just deep to the retinaculum and slightly lateral
• Middle layer: Four FDS tendons, with middle and ring tendons anterior to index and little finger tendons
• Posterior layer: Four FDP tendons
• Radial side: FPL tendon within the radial bursa

Functions of the Flexor Retinaculum

1. Maintains the carpal arch and prevents flattening under load
2. Holds flexor tendons in position at the wrist and prevents bowstringing during flexion
3. Provides pulley action for efficient tendon mechanics
4. Gives origin to thenar and hypothenar muscles
5. Provides attachment to palmaris longus and the palmar aponeurosis

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Synovial Sheaths at the Wrist

Overview

As the long flexor tendons pass through the carpal tunnel, they are enclosed in synovial sheaths that reduce friction. There are two main bursae at the wrist: one for the finger flexors and one for the thumb flexor.

Ulnar Bursa (Common Flexor Synovial Sheath)

Contents: Encloses the tendons of flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP), all eight finger flexor tendons together.

Extent:

• Proximally: extends about 2.5 cm above the proximal border of the flexor retinaculum into the forearm
• Distally: extends to the middle of the palm, corresponding approximately to the level of the metacarpal shafts

Important continuity: The distal medial end of the ulnar bursa is usually continuous with the digital synovial sheath of the little finger, explaining spread of infection between the little finger sheath and the ulnar bursa.

Arrangement: The synovial sac is invaginated by the tendons, producing a parietal layer lining the tunnel and a visceral layer applied to the tendons.

Radial Bursa (Synovial Sheath of Flexor Pollicis Longus)

Contents: Encloses the tendon of flexor pollicis longus (FPL) only.

Extent:

• Proximally: coextensive with the ulnar bursa, extending about 2.5 cm above the flexor retinaculum
• Distally: continues along the thumb to the distal phalanx

The radial bursa is continuous with the digital synovial sheath of the thumb, so infection in the thumb sheath can spread proximally towards the carpal tunnel.

Digital Synovial Sheaths

The flexor tendons of the index, middle, and ring fingers have their own individual digital synovial sheaths within the fibrous flexor sheaths of the fingers.

Extent:

• Proximally: from the heads of the metacarpal bones
• Distally: to the base of the distal phalanx of the respective digit

Important: These sheaths do not normally connect with the ulnar bursa, so infections in these three fingers usually remain confined to their own digital sheaths unless anatomical variation or extension of infection occurs.

Summary Table

Clinical Significance

• Horseshoe abscess: Infection may spread from the little finger digital sheath to the ulnar bursa and, through proximal communication between the ulnar and radial bursae when present, to the radial bursa and thumb sheath. The resulting U-shaped spread is called a horseshoe abscess.
• Tendon sheath infections (tenosynovitis): Kanavel’s four cardinal signs are flexed posture of the digit, fusiform swelling, tenderness along the sheath, and pain on passive extension.
• Safe drainage: Incisions for palmar infections must avoid the palmar cutaneous branch of the median nerve and the digital neurovascular bundles.

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Applied Anatomy of the Flexor Retinaculum

Carpal Tunnel Syndrome

Definition: Compression of the median nerve within the carpal tunnel beneath the flexor retinaculum.

Carpal tunnel syndrome is the most common entrapment neuropathy.

Causes — anything that reduces tunnel volume or increases tunnel contents:

Clinical features:

Motor weakness affects the LOAF muscles supplied by the median nerve in the hand:

• Lumbricals 1 and 2, supplied by digital branches of the median nerve
• Opponens pollicis, supplied by the recurrent branch of the median nerve
• Abductor pollicis brevis, supplied by the recurrent branch and the most reliable thenar muscle to test
• Flexor pollicis brevis, superficial head, supplied by the recurrent branch

Weakness of thenar muscles causes flattening of the thenar eminence and loss of thumb opposition. Long-standing severe compression may produce an ape-thumb posture.

Sensory loss: Paraesthesia, numbness, or pain in the lateral 3½ digits: thumb, index, middle, and lateral half of ring finger.

Palmar skin over the thenar eminence is spared because the palmar cutaneous branch of the median nerve arises above the wrist and passes superficial to the flexor retinaculum, outside the carpal tunnel.

Tests:

• Phalen’s test: Maximum wrist flexion for about 60 seconds reproduces paraesthesia in the median nerve territory.
• Tinel’s sign: Percussion over the carpal tunnel produces tingling in the median nerve territory.
• Carpal compression test: Direct compression over the retinaculum reproduces paraesthesia.

Treatment:

• Conservative: wrist splinting, activity modification, steroid injection
• Surgical: carpal tunnel decompression by division of the flexor retinaculum

Guyon’s Canal (Ulnar Canal) Syndrome

Definition: Compression of the ulnar nerve in Guyon’s canal, the tunnel between the volar carpal ligament and the main flexor retinaculum.

Boundaries of Guyon’s canal:

• Roof: Volar carpal ligament and palmaris brevis
• Floor: Main flexor retinaculum and pisohamate ligament
• Medial: Pisiform
• Lateral: Hook of hamate

Contents: Ulnar nerve and ulnar artery.

Causes: Ganglion, hook of hamate fracture, repetitive compression such as cyclist’s handlebar palsy, and rheumatoid arthritis.

Features: Weakness of intrinsic hand muscles supplied by the ulnar nerve, including hypothenar muscles, interossei, medial two lumbricals, and adductor pollicis. Sensory loss may involve the little finger and medial half of the ring finger; dorsal ulnar hand sensation is often spared because the dorsal cutaneous branch arises proximal to the canal.

Dislocation of the Lunate

• A fall on the dorsiflexed wrist may displace the lunate anteriorly.
• The dislocated lunate can directly compress the median nerve in the carpal tunnel.
• This produces acute carpal tunnel syndrome and requires urgent reduction.

Division of Flexor Retinaculum (Surgical Relevance)

Safe incision line for carpal tunnel release:

• Incision is placed in line with the ulnar border of the ring finger ray.
• This avoids the recurrent (thenar) branch of the median nerve, which commonly exits the tunnel radially and curves back to the 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. It is not a disease of the flexor retinaculum itself, but the palmar aponeurosis is anatomically continuous with the distal region of the retinaculum.

Features:

• Localised thickening and nodularity of the palmar aponeurosis, often beginning medially
• Progressive flexion contracture of the ring and little fingers, with the ring finger commonly affected
• Proximal and middle phalanges are flexed; distal phalanx may be relatively spared
• More common in males; associated with diabetes, alcohol use, epilepsy/antiepileptic therapy, and liver disease

Treatment: Surgical fasciectomy or other specialist treatment when hand function is significantly impaired.

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