The cotyloid fossa (acetabular fossa) is the central non-articular depression of the acetabulum, containing a fat pad (Haversian gland), the ligamentum teres attachment, and the synovial membrane folded over the fat. The articular surface of the acetabulum is a horseshoe-shaped arc around this central fossa. The cotyloid notch at the inferior opening of the fossa is bridged by the transverse acetabular ligament, completing the acetabular rim.
The cotyloid fossa is visualised in hip arthroscopy from the peripheral and central compartment portals. The fat pad in the fossa can become hypertrophied or impinged in femoroacetabular impingement, contributing to synovitis and pain. The ligamentum teres, attaching from the fovea capitis of the femoral head to the fossa margins, can tear producing ligamentum teres tears that are an increasingly recognised cause of hip pain in athletes. Acetabular labral tears at the fossa-articular surface junction extend into the cotyloid notch region.
Traumatic or degenerative tearing of the ligamentum teres at its fossa attachment or midsubstance produces deep hip pain and a clicking sensation with combined hip flexion and rotation, increasingly identified on MRI arthrography or arthroscopy as a cause of unexplained hip pain, managed by arthroscopic debridement or reconstruction.
Hypertrophied or inflamed fat pad in the cotyloid fossa from synovitis or femoroacetabular impingement contributes to deep anterior hip pain during flexion, visible as an enlarged intra-articular fat pad on MRI, debrided arthroscopically when symptomatic.
