The superior radioulnar joint

The articulation is between the head of radius rotating within the fibro-osseous ring formed by the radial notch of the ulna and the annular ligament.

Head of the radius

The beveled circumference of the head of the radius is covered by hyaline cartilage continuous with that on its upper concave surface, so forming a smooth surface for articulation with the ulna and annular ligament(a). The anterior, medial and posterior parts of the circumference tend to be wider than the lateral part, for direct articulation with the ulna. The head of the radius tends not to be circular but is slightly oval, with the major axis lying obliquely anteroposteriorly. The major and minor axes have a length ratio of approximately 7:6.

Radial notch

The hyaline-covered radial notch is continuous with the trochlear notch of the ulna on its lateral side, being separated from it by a blunt ridge(a,c). It forms approximately one-fifth of the articular fibro-osseous ring, and is therefore concave anteroposteriorly but almost flat vertically.

Annular ligament

The flexible annular ligament forms the remaining four-fifths of the articular surface which encircles the head and neck of the radius. Its flexibility enables the oval head of the radius to rotate freely in pronation and supination. The ligament is a strong, well-defined band attached to the anterior and posterior margins of the radial notch of the ulna. Posteriorly the ligament widens where it attaches to adjacent areas of the ulna above and below the posterior margin of the notch. The diameter between its lower borders is narrower than that above(c), so it cups in under the head of the radius and acts as a restraining ligament preventing downward displacement of the head through the ring.

Superiorly the annular ligament is supported by the firm fusion of the radial collateral ligament and the blending of the lateral part of the fibrous capsule of the elbow joint in front and behind. Inferiorly a few loose fibres attach the ligament to the neck of the radius beyond the epiphyseal line. These fibres are too loose to interfere with movements at the joint, but give some support to a dependent fold of synovial membrane. The upper part of the ligament is lined with fibrocartilage continuous with the hyaline cartilage of the radial notch. The lower part of the ligament is lined with synovial membrane.

Joint capsule and synovial membrane

The superior radioulnar joint is continuous with the elbow joint and consequently shares the same joint capsule. The synovial membrane associated with the elbow part of the joint space attaches to the upper margin of the fibrocartilage lining of the annular ligament. From the lower border of the fibrocartilage, and lining the lower part of the annular ligament, the synovial membrane extends below the lower border of the ligament to hang as redundant fold which has a loose attachment to the neck of radius. The membrane lies on the upper surface of the quadrate ligament, which limits and supports it, and passes medially from the radius to attach to the lower border of the radial notch of the ulna. The redundancy of the synovial membrane below the annular ligament accommodates to the twisting of the membrane that accompanies rotation of the radius.

Ligaments

Although the annular ligament provides an important support for the head of the radius, it is not sufficient by itself to provide the only support to the superior radioulnar joint, because of its need to change shape with rotation of the radius. Indeed, this constant need to accommodate to the changing orientation of the head of the radius may lead to stretching of the ligament. Consequently, there are additional structures which provide support to the joint: the quadrate ligament and the interosseus membrane.

Quadrate ligament

The quadrate ligament stretches from the lower border of the radial notch of the ulna to the adjacent medial surface of the neck of the radius proximal to the radial tuberosity. Its fibres run in a criss-cross manner between the two bones, so that, irrespective of the relation of the radius to the ulna, some fibres are always under tension. The overall tension within the ligament thus remains constant in all positions of pronation and supination. Its two borders are strengthened by fibres from the lower border of the annular ligament.

Blood and nerve supply

The arterial blood supply to the superior radioulnar joint is by branches from vessels supplying the lateral part of the elbow joint; namely the middle and radial collateral branches of the profunda brachii, and the radial and interosseus recurrent branches from the radial and common interosseus arteries respectively. Venous drainage is by similarly named vessels draining eventually to the brachial vein. Lymphatic drainage is by vessels traveling with the arteries to small nodes associated with the main arteries and then to the lateral group of axillary nodes.

The nerve supply to the joint is by twigs from the posterior interosseus branch of the radial nerve, the musculocutaneous and median nerves, with a root value of C5, 6 and 7.

Surface marking and palpation

The line of the superior radioulnar joint can be palpated posteriorly. Having identified the head of the radius in the depression on the posterolateral aspect of the elbow, a vertical groove between the radius and ulna can be felt medially. This is the position of the joint line. During pronation and supination, the head of the radius can be felt rotating against the ulna.

Relations

Anteriorly the joint is crossed by the tendon of biceps passing to its attachment of the radial tuberosity; posteriorly is the fleshy belly of anconeus . Medial to the tendon of biceps lies the brachial and then the radial artery from above downwards.

Stability

The joint has a reasonable degree of inherent stability. However, in children, the head of the radius may be pulled from the confines of the annular ligament in traction dislocation. Tears of the annular ligament will also result in dislocation at the joint.

Movement

The main movement that occurs at the superior radioulnar joint is rotation of the head of the radius within the fibro-osseous ring of the annular ligament and radial notch of the ulna(a). The movement is probably limited by tension developed in the quadrate ligament.

In addition to this principal movement, there are four other movements related to the joint. These are:

1)      Rotation of the superior concave surface of the radial head in relation to the capitulum of the humerus;

2)      The beveled ridge of the radial head glides in contact with the capitulotrochlear groove of the humerus;

3)      The head of the radius is displaced laterally because the major axis of the oval head comes to lie transversely(a);

4)      The plane of the radial head becomes tilted laterally and inferiorly during pronation due to the radius moving obliquely around the ulna(b).

Accessory movements

Gripping the head of the radius between the thumb and index finger, it can be moved anteroposteriorly with respect to both the ulna and the capitulum.