Femoroacetabular Impingement

Femoroacetabular impingement (FAI) syndrome is a motion-related hip disorder in which abnormal contact between the femoral head-neck junction and the acetabular rim causes groin pain, usually in active young adults.

The Warwick Agreement (2016) requires symptoms, clinical signs and imaging findings together: morphology alone, in a person without symptoms, is not a diagnosis. FAI, particularly cam morphology, is associated with later hip osteoarthritis (OA).

FAI occurs when abnormal bony shape causes early contact during hip movement, particularly flexion, adduction and internal rotation.

• •Cam morphology (femoral side): loss of the normal concavity at the head-neck junction, usually anterolateral, seen as a 'pistol grip' on an AP radiograph. The bump shears the labrum and delaminates the adjacent cartilage in flexion and internal rotation. It is more common in young men, and an alpha angle over 60° is a commonly cited threshold
• •Pincer morphology (acetabular side): over-coverage, either focal (acetabular retroversion, with a crossover sign) or global (lateral centre-edge angle over 40°). The labrum is crushed between rim and neck. It is more common in middle-aged women
• •Mixed: both, and the most common pattern in clinical practice

The labrum deepens the socket and maintains the fluid seal. Labral tears are the most common intra-articular finding in FAI and cause catching, clicking and sharp groin pain.

Cam morphology develops in adolescence, while the proximal femoral growth plate is open, in response to high-impact sport. Other risk factors include previous Perthes disease or slipped upper femoral epiphysis (SUFE) and genetic predisposition. Developmental dysplasia of the hip is under-coverage, the opposite of pincer morphology.