Proximal radioulnar joint

Articular surfaces

The articular surfaces of the proximal radioulnar joint are the head of radius and the radial fossa of ulna.

Both surfaces are lined with hyaline cartilage. The radial head is circular and convex, while the radial fossa is reciprocally concave.

However, the radial fossa is in touch with only one-fifth of the radial head. For the sake of completeness of this pivot joint, the annular ligament surrounds the radial head and holds it tight against the radial fossa of ulna.

Ligaments and joint capsule

The fibrous capsule of the radioulnar joint attaches to the annular ligament distally, while proximally it is continuous with the capsule of the elbow joint. The synovial membrane attaches to the margins of the articular surfaces and to the annular ligament. It lines the interior surface of the capsule,where it is continuous with the synovial membrane of the elbow joint. Thus, the elbow and proximal radioulnar joints share one continuous synovial cavity.

The proximal radioulnar joint is reinforced by the annular and quadrate ligaments.

Annular ligament

The annular ligament extends from the anterior margin of radial fossa of ulna, encircles the radial head and attaches to the posterior margin of the radial fossa. Thus, the annular ligament helps to create a stable ring within which the radial head can rotate.

The proximal margin of the ligament is fused with the joint capsule, while the distal margin attaches to the neck of radius. The internal surface of the annular ligament is covered by a thin layer of cartilage which is in direct contact with the surface of the radius. The superficial surface is fused with the radial collateral ligament and is one of the attachment sites of the supinator muscle.

The quadrate ligament is a short fibrous band that spans from the superior part of the supinator fossa of ulna to the neck of radius, just proximal to the radial tuberosity.

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Innervation

The proximal radioulnar joint is supplied by the branches of median, musculocutaneous, radial and ulnar nerves.

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Blood supply

The blood supply to the proximal radioulnar joint comes from a periarticular network formed by the radial collateral branch of deep brachial artery, radial and recurrent branches of the radial and common interosseous arteries.

Movements

The proximal radioulnar joint works in a unit with the distal radioulnar joint to enable rotatory movements of the forearm; pronation and supination. When the arm is in the resting position beside the body, the range of motion for pronation is 61–66°, while for supination it is 70–77°

In the proximal radioulnar joint, the head of radius rotates within the ring formed by the radial fossa and annular ligament. The axis of rotation is dynamic and depends on the position of the forearm. When the forearm is in supination, the axis passes through the center of the head of radius proximally, and through the ulnar attachment of the articular disc in the distal radioulnar joint. When the forearm is pronated, the distal point of the axis moves medially, passing through the head of ulna.

Besides rotation of the radial head, the supination and pronation are followed by a sequence of additional movements in the proximal radioulnar joint;

• The superior surface of the radial head rotates against the capitulum of humerus
• The ridge of the radial head glides against the groove between the capitulum and trochlea of humerus
• The head of radius tilts laterally and inferiorly in the transverse plane
• Since the head of radius is ellipsoid on cross-section, its wider axis comes into a transverse, thus displacing the radial head laterally

The proximal radioulnar joint takes a closed packed position at the 5° of supination. The open packed (resting) position occurs when the forearm is flexed at 70° and supinated at 35°. The capsular pattern of the joint is defined and limited by pronation and supination. The proximal radioulnar joint permits the accessory movements of anteroposterior gliding of the radial head against the ulna and the capitulum of humerus.

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Muscles acting on the proximal radioulnar joint

The muscles that act on the proximal radioulnar joint to produce pronation are pronator quadratus and pronator teres. The force of the pronator quadratus is enough for slight movements, while the pronator teres is included in fast movements and movements against resistance.

Supination is produced by the contraction of the supinator muscle when the forearm is extended. For the movements against resistance and/or when the forearm is flexed, the biceps brachii muscle acts as an accessory supinator.