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Hip impingement, also called femoroacetabular impingement, or FAI, occurs when moving the hip joint causes too much friction, or rubbing. This often causes pain in the groin and difficulty moving the joint, and can sometimes lead to arthritis. These symptoms are often not apparent during the early stages of the condition, so FAI often goes undiagnosed for years. When it does appear it may be classified in one of two forms: cam and pincer. The impingement may be congenital, or present at birth, or develop over time, and is often due to a combination of genetic predisposition and environment.
The hip joint, formally called the acetabulofemoral joint, is a ball and socket joint between the femur and the acetabulum. The femur is the large bone that spans from the knee to the hip and the acetabulum is an inwardly curved portion of the pelvis. The ball end of the femur fits into the socket-like shape of the acetabulum, allowing the leg to make rotational movements. In a healthy joint, the bones glide painlessly with the help of lubricating synovial fluid and cartilage that acts as a buffer between the femur and the pelvis. If either of the bones is deformed, however, it can cause friction, destruction of the cartilage, and pain.
A cam hip impingement occurs when the head of the femur is not shaped correctly, usually because of a lesion on the bone. This may cause discomfort in forward motion, such as when bringing the knee to the chest or riding a bike. This condition causes abnormal force on the acetabulum and wear on the acetabular cartilage. A cam hip impingement can occur for congenital or developmental reasons. A trauma can cause a deformity in the bone, or a patient may suffer from coxa vara, slipped capital femoral epiphysis (SCFE), or avascular necrosis (AVN).
AVN occurs when the blood vessels that serve the bone are damaged, causing it to deteriorate. Injury, chronic steroid use, and alcoholism can contribute to this condition. Coxa vara occurs when the angle between the ball of the femur and the shaft of the bone becomes too narrow, causing the leg to shorten and weight to be distributed unevenly. Normally, the ball sits on the end of a neck that protrudes at an angle of greater than 120 degrees from the end of the femur. SCFE affects children when the ball of the femur slips off the neck of the bone, causing severe pain and stiffness.
A pincer hip impingement arises in patients whose acetabular rim grows out over the femoral head, or ball. This creates a point of contact between the acetabulum and the ball of the femur, which impedes movement and causes a pinching of the cartilage. Pincer FAIs are frequently found in patients that have an especially deep or irregularly angled hip socket. Often, a patient will have both a cam and pincer hip impingement.
A doctor may use a combination of methods to diagnose FAI. The physician will look at the patient’s history, listen to the symptoms, do a physical exam, and usually take an X-ray. An MRI (magnetic resonance imaging) may also be ordered to rule out other causes of stiffness and pain. If the patient is diagnosed with a hip impingement, the doctor may try to surgically correct the shape and orientation of the joint. If the damage to the cartilage is severe, however, a hip replacement may be the best option.