The wrist comprises eight carpal bones that articulate with the radius proximally and the metacarpals distally. Motions include flexion, extension, radial deviation, and ulnar deviation. The carpals are divided into a proximal row of four bones and a distal row of four bones (Fig. 12–1). The proximal row, from radial to ulnar surfaces, includes the scaphoid (navicular), lunate (semilunar), triquetrum (triangular), and pisiform. The distal row, from radial to ulnar surfaces, includes the trapezium (greater multangular), trapezoid (lesser multangular), capitate (os magnum), and hamate (unciform). The pisiform lies adjacent to the volar surface of the triquetrum and does not articulate with the forearm bones or with any of the remaining carpal bones.
The bony anatomy of the wrist (mc = metacarpal).
Of the forearm bones, only the radius articulates with the carpal bones. The ulna has a nonosseous fibrocartilaginous union with the triquetrum and the radius, known as the triangular fibrocartilage complex (TFCC). The ulna articulates with the radius at the distal radioulnar joint (DRUJ). An interosseus membrane, dorsal and palmar radioulnar ligaments, and the TFCC stabilize this joint. Injury to the bones or ligaments of the DRUJ may significantly affect wrist mechanics, lead to subluxation or dislocation, and if not treated, may result in long-term problems.
The ligaments of the wrist are considered extrinsic if they join the carpal bones to the radius, ulna, or metacarpals, and intrinsic when they link the carpal bones to one another. The ligaments of the wrist are also classified as dorsal, volar, or interosseus. The volar ligaments are stronger than their dorsal counterparts and provide the greatest stability. Injury to these ligaments results in carpal instability and will be considered later in this chapter.
Many important neurovascular structures pass through Guyon's canal formed by the pisiform and the hook of the hamate (Fig. 12–2). The deep branch of the ulnar nerve and artery supply the three hypothenar muscles, the interossei, the two ulnar lumbricals, and the adductor pollicis. A fracture to either the hamate or the capitate may result in neurovascular bundle damage and subsequent impairment of normal function. The median nerve lies in close proximity to the volar surfaces of the lunate and the capitate and may be injured following a fracture.
The several important neurovascular structures of the wrist are confined within the Guyon's canal.
It is essential to understand the relationship between the tendons and the carpal bones. The tendon of the flexor carpi ulnaris virtually engulfs the pisiform in its attachment. The close proximity of the flexor carpi radialis to the tubercle of the trapezium is also noteworthy. Trapezium fractures may result in tendon damage with subsequent pain during normal motion.