Carpals (8 bones)

1. Overview

The carpal bones are a group of eight small, irregularly shaped bones that form the wrist (carpus) and connect the forearm to the hand. These bones are arranged in two rows—proximal and distal—and articulate with each other, the radius, and the metacarpals. The carpal bones contribute to the flexibility and strength of the wrist and serve as a stable yet mobile base for hand movement. They are vital for transmitting forces and allowing a wide range of motion while maintaining wrist stability.

2. Location

The carpal bones are located in the wrist region, situated between the distal end of the radius and ulna proximally, and the metacarpal bones distally. They are organized into:

• Proximal row (lateral to medial): Scaphoid, Lunate, Triquetrum, Pisiform

• Distal row (lateral to medial): Trapezium, Trapezoid, Capitate, Hamate

The mnemonic “Some Lovers Try Positions That They Can’t Handle” helps recall the order from lateral (thumb side) to medial (pinky side).

3. Structure

Each carpal bone is composed of cortical bone externally and cancellous (spongy) bone internally. The structural characteristics include:

• Scaphoid: Boat-shaped; largest bone of the proximal row; articulates with the radius.

• Lunate: Crescent-shaped; centrally located; articulates with the radius and capitate.

• Triquetrum: Pyramid-shaped; articulates with lunate and pisiform.

• Pisiform: Pea-shaped sesamoid bone lying over the triquetrum; embedded in the tendon of flexor carpi ulnaris.

• Trapezium: Saddle-shaped; articulates with the first metacarpal; key to thumb mobility.

• Trapezoid: Wedge-shaped; articulates with the second metacarpal.

• Capitate: Largest carpal bone; centrally located; articulates with the third metacarpal.

• Hamate: Characterized by a hook-like projection (hamulus); articulates with the fourth and fifth metacarpals.

4. Function

The carpal bones work collectively to:

• Allow wrist movement: Facilitate flexion, extension, radial deviation, and ulnar deviation of the wrist joint.

• Transmit force: Distribute loads from the hand to the radius and ulna during grip and weight-bearing activities.

• Enable thumb opposition: Especially through the trapezium, allowing fine manipulation.

• Support tendons: Serve as pulleys and surfaces for tendon gliding, especially within the carpal tunnel.

5. Physiological role(s)

The carpal bones contribute to broader functional systems in the upper limb:

• Dexterity: Provide a mobile yet stable platform for finger movement and tool use.

• Muscle attachment: Serve as anchor points for muscles controlling the wrist and hand.

• Ligamentous integrity: Connect with strong ligaments (e.g., scapholunate, radiocarpal) for wrist stability.

• Neurovascular protection: The concavity formed by the carpal arch creates the carpal tunnel, through which the median nerve and tendons pass.

6. Clinical Significance

The carpal bones are frequently involved in fractures, dislocations, and compressive syndromes:

• Scaphoid fracture:

  • Most commonly fractured carpal bone; typically due to a fall on an outstretched hand (FOOSH).

  • Prone to avascular necrosis due to poor blood supply, especially to the proximal pole.

• Lunate dislocation:

  • Can compress the median nerve and lead to carpal instability or acute carpal tunnel syndrome.

• Carpal tunnel syndrome:

  • Compression of the median nerve beneath the transverse carpal ligament, often caused by swelling or structural narrowing involving carpal bones.

• Arthritis:

  • Osteoarthritis or rheumatoid arthritis can affect intercarpal joints, reducing wrist mobility and causing pain.

• Kienböck’s disease:

  • Avascular necrosis of the lunate, leading to progressive collapse and wrist dysfunction.

• Hook of hamate fracture:

  • Often seen in sports (e.g., golf, baseball); may compress the ulnar nerve and affect grip strength.