Inferior Radioulnar Joint

Introduction

Introduction to the Inferior (Distal) Radioulnar Joint

The inferior radioulnar joint, also known as the distal radioulnar joint (DRUJ), is a pivot‑type synovial joint that allows the distal end of the radius to rotate around the fixed head of the ulna during pronation and supination of the forearm. Together with the superior radioulnar joint and the interosseous membrane, it forms the tripartite mechanism of forearm rotation. The stability of the DRUJ is provided primarily by the triangular fibrocartilage complex (TFCC) and the interosseous membrane, rather than by bony congruence. Disruption of this joint is common in wrist injuries and can lead to chronic pain, instability, and loss of forearm rotation.

This module details the articular surfaces, the TFCC and its components, the joint capsule, relations, movements, neurovascular supply, and the clinical syndromes affecting the distal radioulnar joint.

Study

1. Articular Surfaces and Classification

The distal radioulnar joint is a uniaxial pivot (trochoid) synovial joint. It is formed by the convex articular surface of the ulnar head (the seat) and the concave ulnar notch of the distal radius. Unlike the proximal radioulnar joint, the ulna is the fixed pivot point and the radius moves around it. The articular surfaces are covered by hyaline cartilage. The joint cavity is L‑shaped, extending proximally between the radius and ulna and distally over the ulnar head to communicate with the radiocarpal joint in some individuals via a perforation in the articular disc.

2. Triangular Fibrocartilage Complex (TFCC)

The TFCC is the primary stabilising structure of the DRUJ. It is a complex of fibrocartilage and ligaments that spans the interval between the distal radius and the base of the ulnar styloid, separating the DRUJ from the radiocarpal joint. Its components include:

Articular disc: A triangular fibrocartilaginous plate attached at its base to the inferior margin of the ulnar notch of the radius and at its apex to the base of the ulnar styloid and the fovea of the ulnar head. It articulates proximally with the ulnar head and distally with the lunate and triquetrum.

Palmar radioulnar ligament: The anterior (volar) part of the TFCC, running from the palmar edge of the ulnar notch of the radius to the palmar base of the ulnar styloid. It tightens in pronation.

Dorsal radioulnar ligament: The posterior part, from the dorsal rim of the ulnar notch to the dorsal base of the ulnar styloid. It tightens in supination.

Ulnar collateral ligament: A thickening connecting the ulnar styloid to the triquetrum and pisiform.

Ulnocarpal meniscus homologue: A variably present fibrocartilaginous slip between the ulnar styloid and the triquetrum.

Subsheath of the extensor carpi ulnaris tendon: The ECU tendon sheath blends with the TFCC and contributes to dorsal stability.

3. Joint Capsule and Synovial Cavity

The fibrous capsule of the DRUJ is relatively loose to allow rotation. It attaches to the margins of the articular surfaces and is reinforced by the palmar and dorsal radioulnar ligaments. The synovial membrane lines the capsule and extends proximally into the recess between the radius and ulna (sacciform recess). In about 30–40% of individuals, a perforation exists in the articular disc, allowing communication between the DRUJ and the radiocarpal joint.

4. Relations

Superficially, the DRUJ is crossed by the extensor tendons of the fifth and sixth compartments (extensor digiti minimi and extensor carpi ulnaris). Medially, the ulnar styloid process is palpable subcutaneously. The dorsal sensory branch of the ulnar nerve crosses the dorsum of the joint. The ulnar artery and nerve lie palmar to the joint, passing into Guyon's canal.

5. Movements and Biomechanics

During pronation, the distal radius rotates medially and crosses over the ulna, the palmar radioulnar ligament tightens, and the ulnar head moves relatively dorsally. In supination, the radius rotates laterally back to a parallel position, the dorsal radioulnar ligament tightens, and the ulnar head moves slightly volarly. The axis of rotation runs from the centre of the radial head to the fovea of the ulnar head. The TFCC stabilises the DRUJ during rotation and also transmits approximately 20% of the axial load from the carpus to the ulna (in neutral ulnar variance).

6. Blood Supply

The DRUJ and TFCC receive their blood supply from branches of the anterior interosseous artery, the posterior interosseous artery, and the dorsal and palmar carpal arches. The peripheral (vascular) portion of the articular disc has a limited blood supply; the central portion is avascular.

7. Nerve Supply

The nerve supply to the DRUJ comes from the anterior interosseous nerve (median nerve branch), the posterior interosseous nerve (radial nerve branch), and the dorsal and palmar cutaneous branches of the ulnar nerve. These provide proprioception and nociception.

8. Clinical Considerations

TFCC tears: Common after a fall on the outstretched hand or forced rotation of the wrist. They cause ulnar‑sided wrist pain, clicking, and DRUJ instability. Classified by Palmer into traumatic (type I) and degenerative (type II) tears.

DRUJ instability: Can result from dorsal dislocation (more common) or volar dislocation. Dorsal dislocation presents with the ulnar head prominent dorsally and inability to supinate. Volar dislocation is rare, with loss of pronation and a palpable volar ulnar head.

Galeazzi fracture‑dislocation: A fracture of the distal third of the radius with associated disruption of the DRUJ. This is unstable and requires surgical fixation of the radius and reduction of the joint.

Ulnar impaction syndrome: Positive ulnar variance (ulna longer than radius) leads to chronic impaction of the ulnar head against the lunate and triquetrum, causing degenerative TFCC tears and chondromalacia.

Essex‑Lopresti injury: Disruption of the interosseous membrane and DRUJ instability associated with a radial head fracture; the DRUJ component must not be overlooked.

Summary

Summary of the Inferior Radioulnar Joint

The DRUJ is a pivot joint between the distal radius and ulnar head, stabilised primarily by the triangular fibrocartilage complex (TFCC). The articular disc, palmar and dorsal radioulnar ligaments, and ECU sheath are key components. The joint allows the radius to rotate around the ulna during pronation and supination. Lesions include TFCC tears, DRUJ instability, Galeazzi fractures, ulnar impaction syndrome, and Essex‑Lopresti injuries. Knowledge of its anatomy is essential for evaluating ulnar‑sided wrist pain.