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The hip is a ball-and-socket joint.

• The socket is formed by the acetabulum, which is part of the large pelvis bone.
• The ball is the femoral head, which is the upper end of the femur (thighbone).

A slippery tissue called articular cartilage covers the surface of the ball and the socket. It creates a smooth, low friction surface that helps the bones glide easily across each other during movement.

The acetabulum is ringed by strong fibrocartilage called the labrum. The labrum forms a gasket around the socket, creating a tight seal and helping to provide stability to the joint.

In a healthy hip, the femoral head fits perfectly into the acetabulum.

In FAI, the femoral head and/or the acetabulum have bony prominences (bumps) because either:

• Your hips formed that way at birth
• There is bone overgrowth, or bone spurs

bone overgrowth — called bone spurs — develop around the femoral head and/or along the acetabulum. This extra bone causes abnormal contact between the hip bones and prevents them from moving smoothly during activity. Over time, this can result in tears of the labrum and the breakdown of articular cartilage (osteoarthritis).

Types of FAI

There are three types of FAI: 

• Pincer. This type of impingement occurs because extra bone extends out over the normal rim of the acetabulum. The labrum can be crushed under the prominent rim of the acetabulum.
• Cam. In cam impingement, the femoral head is not round and cannot rotate smoothly inside the acetabulum. A bump forms on the edge of the femoral head that grinds the cartilage inside the acetabulum.
• Combined. Combined impingement means that both the pincer and cam types are present.

(Left) Pincer impingement. (Center) Cam impingement. (Right) Combined impingement.

FAI occurs because the hip bones do not form normally during the childhood growing years. It is the deformity of a cam bone spur, pincer bone spur, or both, that leads to joint damage and pain. When the hip bones are shaped abnormally, there is little that can be done to prevent FAI.

It is not known how many people have FAI. Some people may live long, active lives with FAI and never have problems. When symptoms develop, however, it usually indicates that there is damage to the cartilage or labrum and the disease is likely to worsen.

Because athletic people may work the hip joint more vigorously, they may begin to experience pain earlier than those who are less active. However, exercise does not cause FAI.

The most common symptoms of FAI include:

• Pain
• Stiffness
• Limping

Pain often occurs in the groin area, although it may occur toward the outside of the hip. Turning, twisting, and squatting may cause a sharp, stabbing pain. Sometimes, the pain is just a dull ache.

When symptoms first occur, it is helpful to try to identify an activity or something you may have done that could have caused the pain. You can modify your activities, let your hip rest, and see if the pain will settle down.

Over-the-counter anti-inflammatory medicines, such as ibuprofen and naproxen, may help to relieve the pain.

If your symptoms persist, you will need to see a doctor to determine the exact cause of your pain and provide treatment options. The longer painful symptoms go untreated, the more damage FAI can cause in the hip.

During your first appointment, your doctor will discuss your general health and your hip symptoms. They will also examine your hip.

Impingement Test

As part of the physical examination, your doctor will likely conduct the impingement test. For this test, your doctor will bring your knee up toward your chest and then rotate it inward toward your opposite shoulder. If this re-creates your hip pain, the test result is positive for impingement.

Impingement test.

Reproduced from Armstrong AD, Hubbard MC, eds: Essentials of Musculoskeletal Care, ed 5. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2015.

Surgery can successfully reduce symptoms caused by impingement. Correcting the impingement can prevent future damage to the hip joint. However, not all of the damage can be completely fixed by surgery, especially if treatment has been put off and the damage is severe. It is possible that more problems may develop in the future.

While there is a small chance that surgery might not help, it is currently the best way to treat painful FAI.

As the results of surgery improve, doctors will probably recommend earlier surgery for FAI.

Surgical techniques continue to advance, and some surgeons are using computer-based navigation systems to help plan and guide bone resection (removal) in FAI surgery.