This is a synovial joint whose bony surfaces are formed proximally by the distal surface of the radius and the attached fibrocartilage, and distally by the scaphoid, lunate and triquetral bones. The triangular fibrocartilage (p. 95), which holds the lower ends of the radius and ulna together, separates the wrist (radiocarpal) joint from the distal radioulnar joint. It does not transmit thrust from the hand. Examine the lower end of the radius. The triangular facet whose apex is the styloid process is for articulation with the scaphoid. The rectangular area next to it is for the lunate. The triquetral is adjacent to the fibrocartilage and the ulnar collateral ligament; only in extreme degrees of adduction may the triquetral make contact with the radius. It is a common fallacy to think that ‘the triquetral only takes part in the joint in extreme adduction’. The fallacy is in thinking that ‘taking part in the joint’ is the same as ‘articulating with the radius’; the triquetral is always part of the joint. A simple capsular ligament surrounds the joint and is thickened on the radial and ulnar sides by collateral ligaments, and in general is much thicker in front than behind. Strong ligaments run transversely from the lower end of the radius to the lunate, forming the palmar radiocarpal ligament.
Movements at the joint are flexion and extension, adduction (ulnar deviation) and abduction (radial deviation). These four movements occurring in sequence produce circumduction. Some of the movement of flexion and extension is always accompanied by similar movement at the midcarpal joint (p. 120). Of the total range of flexion (about 80°), a greater proportion occurs a: the midcarpal joint; in extension (60°), there is a greater proportion at the wrist joint itself. The four movements are carried out by combinations of muscle groups. Thus flexion is produced by flexor carpi radialis and flexor carpi ulnaris as prime movers, by palmaris longus and the flexors of fingers and thumb and abductor pollicis longus. Extension is produced by the radial extensors (longus and brevis) and the ulnar extensors as prime movers assisted on occasion by the extensors of fingers and thumb. Abduction is earned out by abductor pollicis longus and, when the wrist is displaced from the midline, by flexor carpi radialis and the two radial extensors acting together.
Similarly adduction (60°) is brought about by simultaneous contraction of flexor and extensor carpi ulnaris. In the rest position the axis of the metacarpus (third metacarpal) is not in line with the axis of the forearm. With the supinated forearm by the side the axis of the hand is parallel with the humerus, thus compensating for the carrying angle at the elbow. Lateral movements of the wrist are symmetrical about this axis, but they are, ipso facto, not symmetrical about the axis of the forearm, from which the metacarpus can be brought further into ulnar adduction than into radial abduction. In the former movement there is some gliding of the carpal bones across the lower end of the radius but in the latter the carpal bones, rather, rotate each about its own axis.
It is worth noting that the most usual movement of the wrist is one of extension combined with radial abduction, and of flexion combined with ulnar adduction. Hammering in a nail illustrates this movement, and exactly similar movements of the wrist, in the working position of the forearm, occur in a host of everyday acts like eating and drinking, washing and dressing, writing, etc. Indeed, pure flexion-extension and abduction-adduction are unusual movements. Since extension-abduction is an antigravity movement in the normal working position, this may explain the presence of two radial extensors where one serves on the ulnar side, and there is only one flexor each for radial and ulnar sides.