Femoro-acetabular impingement

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The metal probe is being used to lift up the labrum to look inside the hip joint. In the foreground is the prominence of bone typical of cam-type FAI.

What is femoro-acetabular impingement?

Femoro-acetabular impingement or FAI is a relatively recently recognised condition that usually affects young adults. They describe intermittent severe aching groin or hip pain, sometimes spreading into the thigh, buttock or lower back. Sometimes this pain has come on suddenly, as a result of a specific injury, but more often it gradually gets worse over several months. The pain is usually brought on by exercise, physical work or sport, or by sitting or driving. Sometimes the symptoms include pain with specific movements, or sharp, catching or locking pain, or even giving way of the hip.

What causes FAI?

Impingement occurs between the ball (femur) and socket (acetabulum) of the hip joint. Two types have been recognised: in cam impingement, a bulge at the junction of the femoral head and neck rubs against the cartilage lining the acetabulum; in pincer impingement a prominent anterior rim of the acetabulum blocks normal movement of the femur. Cam impingement is more common in young men, and pincer in athletic middle-aged women, but they often co-exist. Both may lead to inflammation, labral tears, or damage to the smooth articular cartilage that lines the acetabulum. In most cases it is not known why people have the bone shapes that lead to FAI, but FAI patients are probably symptomatic because they have a particular combination of shape variants along with certain activities.

How can FAI be identified?

FAI is one of many possible diagnoses in young people with hip and groin pain. The history is often very suggestive, and examination findings of restricted movement and positive impingement testing are helpful. Conventional X-rays of the hip may be normal. Magnetic resonance arthrography (MRA) is much more sensitive for cartilage damage and labral tears than plain MRI, and can be used to identify impingement shapes. 3-D surface reconstructed CT provides the best impression of all aspects of hip shape contributing to FAI.

How is FAI related to osteoarthritis?

Damage to the labrum and articular cartilage leads to significant pain and instability and is strongly implicated in the subsequent development of arthritis. Continued impingement seems delaminate and destroy acetabular cartilage, often starting at the antero-superior rim and then spreading throughout the hip and across to the femoral head. It seems likely that FAI is an important factor in the development of hip osteoarthritis in a large proportion of those people who eventually need hip replacement.

How can FAI be treated?

Activity modification or physiotherapy to strengthen hip muscles cannot cure FAI, but may relieve symptoms temporarily. Surgical treatment aims to correct the shape variations that cause impingement, either by open surgery or by arthroscopic techniques. Open surgery involves an extensive approach with trochanteric osteotomy and dislocation of the hip, while the more recently developed arthroscopic technique is performed through several 1cm incisions. This operation typically involves:

Such surgery usually takes from two to four hours, but patients are typically able to begin physiotherapy, including static cycling, on the same day.

Is arthroscopic or open surgery better for FAI?

There is no clear answer to this. Open surgery is a major operation requiring hip dislocation—most patients stay in hospital for several days and use crutches for three months. Arthroscopic surgery is technically difficult, requires special training and equipment, and may not be suitable for the most extensive and circumferential disease. But when it is suitable, arthroscopic surgery is effective, can often be done as a day case or 23 hour stay, and patients can begin a rehabilitation programme immediately.