Flexor Retinaculum of the Hand

The flexor retinaculum of the hand, also known as the transverse carpal ligament, is a strong fibrous band that forms the roof of the carpal tunnel. It stabilizes the flexor tendons and median nerve as they pass from the forearm into the hand, playing a critical role in wrist mechanics and digital flexion.

Location

The flexor retinaculum spans the anterior surface of the wrist, deep to the palmar skin and fascia. It attaches between prominent carpal bones on the radial and ulnar sides, converting the concave carpal arch into a tunnel.

Structure

• Proximal Attachments:
  • Laterally (radial side): Tubercle of the scaphoid and ridge of the trapezium
  • Medially (ulnar side): Pisiform and hook of the hamate
• Orientation: Oblique band running from superolateral to inferomedial across the anterior wrist
• Thickness: Dense, approximately 2–3 mm thick in most individuals; may be thicker in dominant hand

Function

The flexor retinaculum serves several important roles:

• Carpal Tunnel Formation: Forms the roof of the carpal tunnel and maintains the integrity of the passageway for tendons and the median nerve
• Prevents Bowstringing: Keeps flexor tendons close to the wrist during flexion movements
• Force Transmission: Helps anchor the hand during grip and forceful actions
• Muscle Attachment: Serves as an origin point for thenar and hypothenar muscles, including the abductor pollicis brevis, flexor pollicis brevis, and palmaris brevis

Contents Passing Deep to It (Through the Carpal Tunnel)

• Median nerve
• Flexor pollicis longus tendon
• Four tendons of flexor digitorum superficialis
• Four tendons of flexor digitorum profundus

Structures Passing Superficial to It

• Palmar cutaneous branch of the median nerve
• Ulnar nerve and ulnar artery (via Guyon’s canal)
• Palmaris longus tendon (if present)

Innervation

While the flexor retinaculum itself is fibrous and lacks direct innervation, its neighboring structures — especially the median and ulnar nerves — are closely associated and clinically significant.

Blood Supply

• Branches from the radial and ulnar arteries
• Small perforating vessels from palmar arches

Relations

Deep to it: Carpal tunnel contents (median nerve, flexor tendons)
Superficial to it: Skin, superficial fascia, palmaris longus, ulnar neurovascular structures, and palmar aponeurosis

Surface Anatomy

Located approximately 1–2 cm distal to the distal wrist crease, it is not directly visible but may be palpated between the pisiform and scaphoid tubercle. Tenderness here is often elicited in carpal tunnel syndrome.

Development

The flexor retinaculum develops from deep fascial layers of the distal forearm and wrist, organizing into a thickened band during late fetal development to enclose and stabilize the flexor tendons.

Anatomical Variations

• Thickness and width vary between individuals and dominant hands
• May have accessory slips or be partially divided in some people
• Variations may predispose to or protect from carpal tunnel syndrome

Clinical Significance

• Carpal Tunnel Syndrome: Compression of the median nerve beneath the flexor retinaculum causes pain, tingling, numbness, and weakness in the radial 3½ fingers
• Carpal Tunnel Release Surgery: Involves partial division of the flexor retinaculum to decompress the tunnel
• Fractures or Swelling: Distal radius fractures, fluid retention, or synovial thickening can increase pressure under the retinaculum

Comparative Anatomy

In other mammals, the wrist flexor sheath exists but is less specialized. In humans, the well-defined flexor retinaculum reflects the need for precise digital control and the biomechanical demand of an opposable thumb and refined grasp.