Superior Radioulnar Joint

Introduction

Introduction to the Superior (Proximal) Radioulnar Joint

The superior radioulnar joint is one of the two pivot articulations that allow the radius to rotate around the ulna, producing pronation and supination of the forearm. It is formed by the cylindrical head of the radius articulating with the radial notch of the ulna, and is held in place by the strong annular ligament. The joint shares a common synovial cavity with the elbow joint and is therefore an integral part of the elbow complex. Despite its small size, its integrity is essential for the coordinated rotation of the forearm and hand.

This module details the articular surfaces, ligamentous support, capsule, relations, movements, blood and nerve supply, and the clinical pathology associated with the superior radioulnar joint.

Study

1. Articular Surfaces and Classification

The superior radioulnar joint is a pivot (trochoid) synovial joint. The head of the radius is disc‑shaped and presents a smooth articular circumference that rotates within a fibro‑osseous ring. The ring is formed anteriorly and laterally by the annular ligament, and posteriorly and medially by the radial notch of the ulna. The concavity of the radial head (fovea) also articulates with the capitulum of the humerus, integrating the joint functionally with the elbow.

2. Ligaments and Joint Capsule

The annular ligament is a strong, U‑shaped band attached to the anterior and posterior margins of the radial notch of the ulna. It encircles the articular circumference of the radial head, holding it firmly against the ulna while allowing free rotation. The internal surface of the annular ligament is lined by fibrocartilage where it contacts the radius. Inferiorly, the fibrous capsule is reinforced by the quadrate ligament, a thin band running from the inferior border of the radial notch to the neck of the radius.

The joint capsule is continuous with that of the elbow joint. The synovial membrane lines the deep surface of the annular ligament and extends a short distance down the radial neck to form a redundant fold (sacciform recess), which permits the radius to rotate without stretching the membrane.

The radial collateral ligament of the elbow blends with the annular ligament but does not attach directly to the radius; this allows the radius to rotate freely under the lateral epicondyle.

3. Relations

Anterior to the joint lie the brachialis muscle, the biceps tendon (inserting on the radial tuberosity), and the median nerve. The radial nerve and its deep branch (posterior interosseous nerve) pass anterior to the joint and then wind laterally around the radial neck, piercing the supinator muscle. Posteriorly, the anconeus muscle and the common extensor origin cover the joint. The ulnar nerve is posteromedial, not in direct contact.

4. Movements and Biomechanics

The superior radioulnar joint, together with the distal radioulnar joint, allows the radius to pivot around the ulna. During pronation, the distal radius crosses over the ulna while the radial head rotates internally within the annular ligament. In supination, the radius rotates externally back to its parallel position. The axis of rotation runs from the centre of the radial head proximally to the ulnar styloid distally. The biceps brachii and supinator are the primary supinators; pronator teres and pronator quadratus effect pronation.

5. Blood Supply

The joint is supplied by branches of the radial recurrent artery, the interosseous recurrent artery, and small vessels from the cubital anastomosis. Venous drainage accompanies the corresponding arteries.

6. Nerve Supply

Because the superior radioulnar joint shares its capsule with the elbow joint, it receives the same innervation: branches from the musculocutaneous nerve (C5, C6), median nerve (C6, C7), radial nerve (C5, C6, C7), and ulnar nerve (C7, C8, T1). The articular branches to the radial head region are mainly from the posterior interosseous nerve (radial nerve) and the median nerve.

7. Clinical Considerations

Radial head fractures: Commonly caused by a fall on the outstretched hand with the elbow extended and forearm pronated. The Mason classification (types I–III) guides treatment. Displaced fractures can block forearm rotation.

Pulled elbow (nursemaid's elbow): A longitudinal traction injury in young children (typically 1–4 years) causes the radial head to subluxate from under the annular ligament. The child holds the arm in a pronated, slightly flexed position and refuses to use it. Reduction is achieved by supination and flexion.

Essex‑Lopresti injury: A radial head fracture combined with disruption of the interosseous membrane and instability of the distal radioulnar joint. This longitudinal instability requires recognition and may need radial head replacement or reconstruction.

Posterior interosseous nerve injury: Fractures or dislocations around the radial head can damage the deep branch of the radial nerve as it passes through the supinator, causing a purely motor palsy.

Summary

Summary of the Superior Radioulnar Joint

The superior radioulnar joint is a pivot synovial joint formed by the head of the radius and the radial notch of the ulna, stabilised by the annular and quadrate ligaments. It shares its capsule and synovial cavity with the elbow joint. The joint allows pronation and supination, with an axis passing from the radial head to the ulnar styloid. Clinical conditions include radial head fractures, pulled elbow in children, Essex‑Lopresti injuries, and posterior interosseous nerve palsy.