Acetabular labrum

Acetabular labrum is a C shaped fibrocartilaginous structure which acts to deepen the acetabulum and increase contact between the pelvis and the femoral head. Its exact biomechanical role remains to be fully elucidated. Its opening is anteroinferiorly at the site of the acetabular notch. Here it is bridged by the transverse ligament (thus forming the acetabular foramen beneath it). Elsewhere it is attached to the margins of the acetabulum.

The labrum is thickest posterosuperiorly and widest anterosuperiorly. It is triangular in cross-section. The fibrocartilage is is arranged in three distinct layers:

1. external surface: circumferentially orientated layer with radial reinforcing filaments.
2. middle layer: dense lamellar collagenous layer
3. articlar surface: randomly oriented fibrillar layer with chondrocytes

The capsule of the hip joint attaches to the margins or immediately adjacent to the acetabulum and transverse ligament. Superiorly the capsule's attachment is removed from the labrum by a few milimeters forming the perilabral sulcus. Anteriorly and posteriorly the attachment of the capsule is much closer to the base of the labrum, and thus the perilabral sulcus is commensurately smaller.

Radiographically the labrum is best evaluated with MR arthrography, and should appear as a uniformly low signal triangular structure, although signal at the base of the labrum is variable. Additionally intrasubstance heterogeneous signal is more frequently seen in older patients.

Labral pathology contributes to hip pain and the development of osteoarthritis of the hip. Labral lesions are more frequently seen in patients with 'abnormal' hip morphology including:

• cam-type and pincer type femoroacetabular impingement
• acetabular dysplasia