Carpal bones

At a glance

English

Carpal bones

Latin (TA2)

Ossa carpi

System

Bone

Region

Hand

In order

From the scaphoid bone to the hamate bone

Own pages

Scaphoid bone

About the carpal bones

The wrist, or carpus, is built from eight short bones set in two rows of four. Counted from the thumb side towards the little finger, the proximal row holds the scaphoid, lunate, triquetrum and pisiform, and the distal row holds the trapezium, trapezoid, capitate and hamate. The proximal row faces the radius and the articular disc that sits under the head of the ulna, forming the radiocarpal (wrist) joint; the distal row meets the bases of the metacarpals. Between the two rows runs the midcarpal joint, which shares much of the movement of the wrist.

Each carpal bone is roughly cuboidal, with smooth cartilage-covered facets wherever it touches a neighbour and rough non-articular areas on its palmar and dorsal faces for ligaments. Together they form an arch that is convex on the back of the hand and hollow on the palm. Two pillars of bone frame this hollow on each side: the scaphoid tubercle and the trapezium tubercle at the thumb end, and the pisiform with the hook of the hamate on the little-finger side. The flexor retinaculum stretches between these pillars, turning the hollow into the carpal tunnel, through which the median nerve and the long flexor tendons of the fingers and thumb reach the hand.

The bones differ in size and job along the two rows. The capitate, in the centre of the distal row, is the largest and the first to begin ossifying in infancy; the pisiform, a sesamoid bone embedded in the flexor carpi ulnaris tendon, is the smallest and the last. The trapezium carries a saddle-shaped facet for the first metacarpal, which gives the thumb its wide range, while the trapezoid and capitate are held almost rigidly against the second and third metacarpals to form a stable central column for the hand. Ossification centres appear in a predictable order through childhood, so wrist radiographs are used to estimate bone age.

Clinically the scaphoid matters most. It bridges the two rows, is the carpal bone most often broken, usually in a fall on an outstretched hand, and its blood supply enters mainly at the distal end, so a fracture across the waist can leave the proximal fragment without blood and lead to avascular necrosis or non-union. Tenderness in the anatomical snuffbox is the classic sign. The lunate is the bone most often dislocated, and it can press on the median nerve when it does. The hook of the hamate may break in golfers and racket players, and the ulnar nerve and artery pass close to it in Guyon's canal.