Acetabular labrum

At a glance

English

Acetabular labrum

Latin (TA2)

Labrum acetabuli

System

Cartilage

Region

Lower limb

Side

Paired, left and right

About the acetabular labrum

The acetabular labrum is a firm lip of fibrocartilage attached to the bony margin of the acetabulum, the socket of the hip joint. Older texts call it the cotyloid ligament. It is triangular in cross-section: a broad base fixed to the rim, an inner surface that faces the head of the femur and blends with the articular cartilage of the socket, and a thin free edge that grips the femoral head just beyond its widest part. It is two to three millimetres thick, wider and thinner in front.

The ring is not complete on bone. At the bottom of the socket the bony rim is interrupted by the acetabular notch, and here the labrum continues across the gap as the transverse acetabular ligament, so that the combined ring encircles the femoral head entirely. Blood vessels and nerves enter the joint beneath this ligament. The outer surface of the labrum faces the joint capsule, which attaches just beyond it; only the outer, capsular part of the labrum receives a blood supply, so tears in its inner part heal poorly.

By extending the socket beyond the bony rim, the labrum deepens the acetabulum, so that more than half of the femoral head is held within it. It acts as a seal that keeps a film of synovial fluid under pressure inside the joint, which helps lubricate the cartilage, spreads load over a wider area and resists the head being drawn out of the socket. It also absorbs shock, and it carries nerve endings that sense pain and joint position.

Tears usually affect the front or upper front part, where the labrum is thinnest and where the change from labrum to articular cartilage is most abrupt; in people who habitually sit on the floor the back part is more often torn. Many tears develop gradually from repeated twisting, pivoting or extreme hip motion, as in ballet or ice hockey, while others follow a fall or dislocation. Femoro-acetabular impingement is a common cause: an extra bump of bone on the femoral head-neck junction (cam type), an overgrown socket rim (pincer type) or both make the bones pinch the labrum. Tears are best shown by MR arthrography and confirmed by arthroscopy, where they can be repaired or trimmed.

Attachments and function

Attaches from

Its base is attached to the bony rim of the acetabulum all round, except at the acetabular notch.

Attaches to

It bridges the notch as the transverse acetabular ligament; its free edge grips the femoral head, and its outer surface faces the joint capsule.

Function

Deepens the hip socket, seals in synovial fluid under pressure, spreads load and helps hold the femoral head in the acetabulum.

Nerve supply

Free nerve endings in its outer part come from branches of the obturator and femoral nerves and the nerve to quadratus femoris.

Blood supply

Only the capsular periphery is vascular, supplied by the obturator artery and the medial and lateral circumflex femoral and gluteal arteries; the inner part is nourished by synovial fluid.