Radioulnar joints

Location: Between the radius and ulna of the forearm: one joint just below the elbow (proximal), one just above the wrist (distal), with the interosseous membrane joining the shafts between them.

Bones taking part

• Radius
• Ulna

Articulating surfaces

• Proximal: the rim (circumference) of the head of the radius with the radial notch of the ulna and the inner surface of the annular ligament
• Distal: the head of the ulna with the ulnar notch of the radius
• Middle: the interosseous borders of the radius and ulna, joined by the interosseous membrane (no articular surfaces)

Structural

Proximal and distal: synovial joints (pivot type). Middle radioulnar union: fibrous joint (syndesmosis) formed by the interosseous membrane.

Functional

Proximal and distal: diarthroses (freely movable). The interosseous membrane: amphiarthrosis (slightly movable).

Synovial subtype

Pivot (trochoid) — at both the proximal and distal radioulnar joints.

Axes

Uniaxial: the radius rotates about an axis running from the head of the radius above to the head of the ulna below.

Capsule & cavity

Proximal: no capsule of its own; it lies inside the capsule of the elbow joint and shares its synovial cavity. Distal: a separate, loose capsule (thin superiorly), strengthened by the palmar and dorsal radioulnar ligaments; its synovial membrane extends upward between the radius and ulna as the sacciform recess.

• Pronation and supination: turning the hand palm-down and palm-up, used for turning a key, a doorknob or a screwdriver, and for bringing food to the mouth.
• The interosseous membrane transmits force from the hand and radius to the ulna and then to the humerus.
• The articular disc keeps the distal radius and ulna together and separates the ulna from the carpal bones.

Stability vs mobility: Proximally, the annular ligament is the key stabiliser; in young children the radial head is small and can slip out of it. Distally, the articular disc together with the palmar and dorsal radioulnar ligaments holds the joint. The interosseous membrane binds the shafts along their length. The arrangement keeps the radius and ulna together while letting the radius rotate freely.

• Pulled elbow (subluxation of the radial head) in young children: a sudden pull on the forearm draws the radial head out of the annular ligament.
• Monteggia fracture-dislocation: a fracture of the ulnar shaft with dislocation of the radial head at the proximal radioulnar joint.
• Galeazzi fracture-dislocation: a fracture of the radial shaft with dislocation of the distal radioulnar joint.
• Fractures of the distal radius (for example Colles fracture) can tear the articular disc or disrupt the distal radioulnar joint, causing pain on rotating the forearm.
• Because pronation and supination need both joints and both bones, a fracture of either forearm bone can limit rotation even when the elbow and wrist are uninjured.

• Name three radioulnar unions: proximal (synovial pivot), middle (interosseous membrane, fibrous syndesmosis), distal (synovial pivot).
• State that the proximal joint shares the elbow's capsule, while the distal one has its own capsule and articular disc.
• Explain that at the proximal joint the radius spins, and at the distal joint the radius moves around the ulna; the ulna stays essentially still.
• Name the prime movers: pronator quadratus for pronation; supinator and biceps brachii (with the elbow flexed) for supination.
• Describe the articular disc's attachments and its role in separating the ulna from the carpus.
• Explain the fibre direction of the interosseous membrane and why it transmits force from radius to ulna.

• Saying pronation and supination happen at the elbow joint.
• Saying the ulna rotates around the radius; it is the radius that moves around the ulna.
• Forgetting the interosseous membrane is a joint (fibrous syndesmosis).
• Giving the proximal radioulnar joint its own capsule; it shares the elbow capsule.
• Attaching the annular ligament to the radius; it is attached to the ulna and surrounds the radial head.
• Mixing up Monteggia (ulnar fracture + radial head dislocation) and Galeazzi (radial fracture + distal radioulnar dislocation).