The radiocarpal joint

The radiocarpal joint is formed between the distal surfaces of the radius and the articular disc, and the scaphoid, lunate and triquetral of the proximal row of carpal bones. It is a synovial joint of the ellipsoid type allowing movement in two planes.

Articular surfaces

Distal surface of the radius and articular disc

The radius and articular disc form a continuous, concave ellipsoid surface, being shallower in its transverse long axis than in its shorter anteroposterior axis(a). The articular cartilage on the radius is divided by a low ridge into a lateral triangular and a medial quadrangular area.

Proximal carpal row

The proximal row of carpal bones presents an almost continuous convex articular surface(b). The three carpal bones are closely united by interosseus ligaments which are continuous with the cartilage on the proximal surfaces of the bones. In the anatomical position, the scaphoid lies opposite the medial radial area and the articular disc, and the triquetral is in contact with the medial part of the joint capsule(b).

Joint capsule and synovial membrane

A fibrous capsule completely encloses the joint. It is attached to the distal edges of the radius and ulna anteriorly and posteriorly. Laterally and medially it is attached to the radial and ulnar styloid processes respectively. Distally the capsule is firmly attached anteriorly and posteriorly to the margins of the articular surfaces of the proximal row of carpal bones. Medially it passes to the medial side of the triquetral, and laterally to the lateral side of the scaphoid. Both the anterior and posterior parts of the capsule are thickened and hence strengthened, while at the sides it blends with the collateral ligaments.

Capsular ligaments

The capsular ligaments are distinct bands of fibres passing between specific bones. As well as strengthening the capsule, their arrangement determines that the hand follows the radius in its movements and displacements.

Dorsal radiocarpal ligament. The dorsal radiocarpal ligament extends from the posterior edge of the lower end of the radius to the posterior surface of the scaphoid, lunate and triquetral(a). Its fibres run downwards and medially, principally to the triquetral, and are continuous with the dorsal intercarpal ligaments.

Palmar radiocarpal ligament. The palmar radiocarpal ligament is a broad band of fibres passing downwards and slightly medially from the anterior edge of the lower end of the radius and its styloid process, to the anterior surfaces of the proximal row of carpal bones(b). Some of the fibres are prolonged and extend to attach to the capitate.

Palmar ulnocarpal ligament. The palmar ulnocarpal ligament is formed by fibres extending downwards and laterally from the anterior edge of the articular disc and the base of the ulnar styloid process to the anterior surfaces of the proximal carpal bones(b).

These anterior and posterior capsular ligaments become taut in extension and flexion of the radiocarpal joint respectively.

Synovial membrane

A relatively lax synovial membrane lines the deep surface of the joint capsule attaching to the margins of all the articular surfaces. It presents numerous folds, particularly posteriorly. Because of the presence of the articular disc of the inferior radioulnar joint and the completeness of the interosseus  ligaments uniting the proximal surfaces of the proximal carpal row, the synovial cavity is limited to the radiocarpal space. Only occasionally does it communicate with the inferior radioulnar joint by a perforation in the articular disc, or with the intercarpal joint when one of the interosseus ligaments is incomplete.

Ligaments

At the sides of the radiocarpal joint, collateral ligaments reinforce and strengthen the joint capsule. They are active in limiting abduction and adduction at the joint. In adduction, the radial ligament becomes taut while the ulnar relaxes; in abduction the reverse occurs.

Radial collateral carpal ligament

The radial collateral carpal ligament passes from the tip of the radial styloid process to the lateral side of the scaphoid, immediately adjacent to its proximal articular surface, and to the lateral side of the trapezium.

Ulnar collateral carpal ligament

The ulnar collateral carpal ligament is a rounded cord attached to the ulnar styloid process above and to the base of the pisiform and the medial and posterior non-articular surfaces of the triquetral below. By its attachment to the pisiform the ligament also blends with the medial part of the flexor retinaculum.

Blood and nerve supply

The arterial supply to the joint is by branches from the dorsal and palmar carpal networks, with venous drainage going to the deep veins of the forearm. Lymphatic drainage of the joint follows the deep vessels.

The nerve supply to the joint is by twigs from the anterior interosseus branch of the radial nerve, the posterior interosseus branch of the radial nerve, and the dorsal and deep branches of the ulnar nerve, with root value C7, 8.

Surface making

The position of the joint is indicated by a line, slightly convex proximally, between the radial styloid process and the head of the ulna, so that the concavity of the radius and articular disc face distally, medially and slightly anteriorly.

Movements

Movements of flexion and extension; and adduction and abduction are possible at the radiocarpal joint. However, each of these is also contributed to by movements between the proximal and distal row of carpal bones at the midcarpal joint.

Flexion and extension

Flexion and extension occur about a transverse axis more or less in the sagittal plane such that the hand moves towards the front of the forearm in flexion and towards the back of the forearm in extension. Flexion is freer than extension and has a maximum range of 50°, whereas extension has a maximum range of 35°. The movements are checked by the margins of the radius, and because the posterior margin extends further distally than the anterior, extension is checked earlier than flexion.

In flexion the scaphoid and lunate move within the concave distal end of the radius so that their proximal surfaces face postero-superiorly. In addition the scaphoid twists about its long axis so that its tubercle becomes less prominent in full flexion. During extension the twisting of the scaphoid about its long axis makes the tubercle more prominent in full extension.

Abduction and adduction

Abduction and adduction, also reffered to as radial and ulnar deviation, are lateral or medial movements respectively of the proximal row of carpal bones in relation to the distal end of the radius. The radial styloid process extends further distally than the ulnar styloid process. Consequently, abduction is more limited at the radiocarpal joint having a range of only 7°, whereas adduction has a range of 30°. In adduction the scaphoid rotates so that its tubercle moves away from the radial styloid process, enabling the lunate to move laterally so that it comes to lie entirely distal to the radius. The triquetral lies distal to the articular disc. In abduction the triquetral moves medially and distally to be clear of the radius; the lunate follows so that its centre lies distal to the inferior radioulnar joint. The movement is limited by impact of the scaphoid tubercle on the radial styloid process.

Accessory movements

An anteroposterior gliding of the proximal row of carpal bones against the radius and articular disc can be produced by firmly gripping the lower end of the radius and ulna with one hand, and the proximal row of carpal bones with the other. Alternate anterior and posterior pressure elicits a palpable gliding movement at the radiocarpal joint. With the same grip, a longitudinally applied force along the line of the forearm pulls the carpal bones away from the radius and articular disc.