Femoroacetabular Impingement (FAI), commonly called hip impingement, is a mechanical condition where the bones of the hip joint clash abnormally during movement. This conflict arises from structural mismatches between the thigh bone (femoral head and neck) and the hip socket (acetabulum). The underlying cause is a structural deformity of the bone shapes, not a problem with surrounding muscles or ligaments. This bony abnormality leads to friction and premature contact, especially during deep flexion or rotation.
The Core Problem Bony Abnormalities
The hip is designed as a highly congruent ball-and-socket joint, where the round femoral head fits perfectly into the cup-shaped acetabulum, allowing for smooth, wide-ranging motion. FAI arises when extra bone growth (a bone spur) disrupts this ideal fit, meaning the “ball” and “socket” no longer glide seamlessly.
The abnormal bone contacts cause friction, particularly during activities requiring a large range of motion. Over time, this repetitive contact damages the soft tissues within the joint, specifically the labrum and the smooth articular cartilage. This damage leads to pain, limited activity, and the early onset of hip osteoarthritis.
Cam Type Impingement
Cam impingement is a type of FAI caused by an abnormally shaped femoral head and neck, often described as a “square peg in a round hole.” This involves an extra bump of bone where the femoral head meets the femoral neck, making the head non-spherical and typically affecting the anterolateral aspect.
During hip movement, especially flexion and internal rotation, this non-round section rotates into the acetabulum. The bump jams against the rim of the hip socket, causing a shearing force that pushes the articular cartilage away from the underlying bone. This damages the joint lining and tears the labrum. Cam-type impingement is the most frequently identified form of FAI and is common in young, athletic males.
Pincer Type Impingement
Pincer impingement is caused by an abnormality of the hip socket (acetabulum), which has excessive coverage over the femoral head. The term “Pincer” describes the socket’s rim extending too far, creating an overhang that decreases the clearance required for the femoral neck to move freely.
The excess coverage may be a localized issue, such as acetabular retroversion (where the socket is angled slightly backward), or a generalized condition where the socket is too deep. When the hip is flexed, the socket rim pinches the labrum between itself and the femoral neck. This repeated crushing force results in labral injury, which can lead to degeneration or bruising. Many FAI cases are “Combined,” featuring both a Cam deformity on the femur and a Pincer deformity on the acetabulum.
Developmental Factors in Bone Shape Formation
The abnormal bone shapes that cause FAI are not present at birth but develop during skeletal maturation, particularly in late childhood and adolescence. This is the period when the growth plates (physes) are still open and actively growing. Researchers agree that the Cam deformity is a developmental adaptation to physical activity during this time.
Repetitive, high-impact forces placed upon the hip during growth influence the remodeling of the soft bone. The stress from extreme hip motion, such as deep flexion and rotation, stimulates extra bone growth at the proximal femoral physis. This leads to the formation of the characteristic bony bump at the femoral head-neck junction.
Studies show a strong correlation between the volume and type of sport participated in during adolescence and the development of the Cam morphology. Athletes in sports involving frequent, high-force hip movements (ice hockey, soccer, and basketball) are at a higher risk for developing the deformity. Training more than three times a week has been associated with a greater likelihood of developing the abnormal shape.
The severity of the Cam deformity appears to have a “dose-response relationship” with the level of sporting activity; the more loading the hip experiences during adolescence, the larger the bony prominence that forms. While physical activity is the dominant factor, certain pre-existing childhood hip conditions, like Slipped Capital Femoral Epiphysis, can also result in the abnormal shape. Genetic predisposition is considered a contributing factor to FAI.