Middle Radioulnar Joint

Introduction

Introduction to the Middle Radioulnar Joint (Intermediate Radioulnar Union)

The middle radioulnar joint is not a synovial articulation but a fibrous syndesmosis that connects the shafts of the radius and ulna. It consists of the interosseous membrane of the forearm and the oblique cord. These structures maintain the spatial relationship between the two bones, transmit forces from the radius to the ulna, and provide extensive surfaces for muscle attachment. The integrity of the middle radioulnar union is essential for normal forearm rotation and longitudinal stability. Disruption of this syndesmosis, as seen in Essex‑Lopresti injuries, leads to proximal migration of the radius and wrist dysfunction.

This module covers the attachments, orientation, biomechanical functions, relations, and clinical significance of the interosseous membrane and oblique cord.

Study

1. Components of the Middle Radioulnar Union

The middle radioulnar union consists of two fibrous structures: the oblique cord and the interosseous membrane. The oblique cord is a small, inconstant fibrous band that runs from the lateral side of the ulnar tuberosity to the radius, just distal to the radial tuberosity. It runs in the opposite direction to the majority of the interosseous membrane fibres. The interosseous membrane is a broad, thin, and strong sheet of collagen fibres that spans between the interosseous borders of the radius and ulna.

2. Interosseous Membrane – Attachments and Fibre Orientation

The interosseous membrane is attached medially to the interosseous (medial) border of the ulna and laterally to the interosseous (medial) border of the radius. It is composed predominantly of collagen fibres that run obliquely downwards and medially from the radius to the ulna. A few fibres run in the opposite direction (downwards and laterally from ulna to radius), and a distal thickening called the distal oblique bundle may be present. The membrane is broader in the middle third of the forearm and becomes thinner proximally and distally.

The central band of the interosseous membrane is the thickest and most structurally important part. It is a key ligamentous component that resists proximal migration of the radius.

3. Openings of the Interosseous Membrane

The interosseous membrane has two major openings. The proximal opening, located just below the oblique cord, transmits the posterior interosseous vessels from the anterior to the posterior compartment. The distal opening, near the wrist, transmits the anterior interosseous vessels from the anterior compartment to the posterior compartment. These perforations are clinically important because they allow communication of infection or fluid between the compartments.

4. Biomechanical Functions

The interosseous membrane transmits axial loads from the radius to the ulna. When a force is applied at the wrist (e.g., during a fall on the outstretched hand), about 80% of the load is transmitted through the radius at the wrist. The interosseous membrane transfers a significant portion of this load from the radius to the ulna, so that at the elbow, the load is shared more equally between the two bones. This protects the radiocapitellar joint from overload. The oblique cord and the proximal interosseous membrane also limit excessive supination.

The membrane also maintains the interosseous space during pronation and supination, preventing the radius and ulna from bowing apart or crossing over excessively. It acts as a fulcrum for rotation, keeping the axis of movement stable.

5. Relations and Muscle Attachments

The interosseous membrane provides an extensive origin for the deep muscles of the forearm. Anteriorly, the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus arise from it. Posteriorly, the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis take origin from its dorsal surface. The anterior interosseous nerve and artery lie directly on the anterior surface of the membrane; the posterior interosseous nerve and artery lie on its posterior surface.

6. Blood Supply

The interosseous membrane receives its blood supply from the anterior and posterior interosseous arteries via small nutrient branches. The venae comitantes drain into the deep veins of the forearm.

7. Nerve Supply

The interosseous membrane is innervated by the anterior interosseous nerve (from the median nerve) and the posterior interosseous nerve (from the radial nerve), both of which lie directly on its surfaces. These nerves provide proprioceptive and nociceptive fibres.

8. Clinical Considerations

Essex‑Lopresti injury: A fall on the outstretched hand can cause a radial head fracture, disruption of the central band of the interosseous membrane, and injury to the distal radioulnar joint. This longitudinal radioulnar dissociation leads to proximal migration of the radius and wrist pain. Early recognition is critical because late reconstruction is difficult.

Compartment syndrome: The interosseous membrane forms the deep boundary between the anterior and posterior compartments of the forearm. Swelling or haematoma in one compartment can be confined by the membrane, contributing to increased compartment pressure. Fasciotomy must release both compartments.

Forearm fractures: Fractures of both the radius and ulna (both‑bone forearm fracture) disrupt the interosseous membrane, leading to instability. Anatomical reduction and plate fixation are necessary to restore the interosseous space and rotational alignment.

Cross‑union (synostosis): After severe forearm trauma, heterotopic ossification can bridge the interosseous membrane, causing a radioulnar synostosis that blocks pronation and supination.

Infection spread: The perforations in the interosseous membrane allow pus or fluid to track between the anterior and posterior compartments, so infections may present in both volar and dorsal aspects.

Summary

Summary of the Middle Radioulnar Joint

The middle radioulnar union is a syndesmosis formed by the interosseous membrane and oblique cord. It connects the radius and ulna, transmits loads from wrist to elbow, provides muscle origins, and maintains the interosseous space during rotation. The central band of the interosseous membrane is crucial for longitudinal stability. Disruption in Essex‑Lopresti injuries causes proximal radius migration, and the membrane is relevant in compartment syndromes, fractures, and synostosis.