Annular ligament of radius

At a glance

English

Annular ligament of radius

Latin (TA2)

Ligamentum anulare radii

System

Ligament

Region

Upper limb

Side

Paired, left and right

About the annular ligament of radius

At the proximal radio-ulnar joint the disc-shaped head of the radius spins against a small concave facet on the side of the ulna, the radial notch. That notch forms only about a fifth of a circle; the rest of the ring is supplied by the annular ligament (TA spelling anular), a tough band of fibres that wraps around the radial head and closes the collar.

Both ends of the ligament are fixed to the ulna, one to the front edge and one to the back edge of the radial notch, so the band and the notch together make an osteofibrous ring. The ligament is wider at the top than at the bottom, giving it the shape of a shallow funnel: it hugs the radial head above and narrows around the neck below. Because the head is broader than the neck, it cannot slip down through the ring. Behind, its attachment broadens so that it overlaps the notch at both ends. Its inner surface is lined with fibrocartilage, which merges with the notch's own articular cartilage so that the head turns on a continuous smooth surface. Above, it merges with the elbow capsule and with the radial collateral ligament; below, a few fibres pass to the radial neck and hold up a loose fold of synovial membrane. The quadrate ligament, a thickened strip, runs from its lower edge to the radial neck.

The ligament is firm yet flexible enough to accommodate the slightly oval radial head as it rotates during pronation and supination. It keeps the head centred and stops the radius being pulled away from the ulna or displaced downwards.

In young children the radial head is small and the ligament relatively loose. A sudden pull on the outstretched hand, such as swinging a child by the arms or snatching them away from traffic, can make the radial head slip partly out from under the ligament, which becomes trapped in the joint. This is pulled elbow (nursemaid's elbow). The child holds the arm still and refuses to use it, and a gentle manoeuvre of supination and flexion usually reduces it at once.

Attachments and function

Attaches from

The front edge of the ulna's radial notch.

Attaches to

The back edge of the ulna's radial notch, after encircling the head of the radius.

Function

Holds the radial head against the ulna so that it can rotate freely in pronation and supination without displacement.

Nerve supply

Articular branches of the radial, musculocutaneous and median nerves to the elbow region.

Blood supply

The radial recurrent and interosseous recurrent arteries from the anastomosis around the elbow.