Carpal bones

From lateral (thumb side) to medial, the proximal row comprises the scaphoid, lunate, triquetrum and pisiform; the distal row the trapezium, trapezoid, capitate and hamate. The proximal row (except the pisiform, a sesamoid lying on the triquetrum) articulates with the radius and the articular disc at the wrist (radiocarpal) joint; the distal row articulates with the metacarpals at the carpometacarpal joints. Between the rows lies the midcarpal joint, which contributes much of wrist flexion and radial–ulnar deviation. Adjacent bones are joined by intercarpal ligaments; the scapholunate and lunotriquetral ligaments are key stabilisers.

The carpus is concave anteriorly. The flexor retinaculum (transverse carpal ligament) spans the arch from the scaphoid tubercle and trapezium tubercle laterally to the pisiform and hook of hamate medially, converting it into the carpal tunnel, which transmits the median nerve and nine tendons (four of flexor digitorum superficialis, four of flexor digitorum profundus and flexor pollicis longus). The ulnar nerve and artery pass superficial to the retinaculum, through Guyon’s canal between the pisiform and hook of hamate.

Carpal ossification follows a predictable sequence used to estimate bone age: capitate and hamate in the first year, then triquetrum (≈3 y), lunate (≈4 y), scaphoid, trapezium and trapezoid (≈5–6 y) and finally the pisiform (≈9–12 y). All carpals are cartilaginous at birth.