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What Is Femoroacetabular Impingement (FAI)?

The hip joint

The hip joint consists of a ball (the femoral head) and socket (the acetabulum). The ball and socket are covered with cartilage that acts as a cushion between the two structures during normal activities. The cartilage (or white covering) also helps to allow for the ball and socket to glide smoothly during activities such as walking or getting out of a chair.   

The labrum is a C-shaped structure that is attached to the acetabulum to help create a suction seal for the hip, keeping the joint fluid in the hip and providing additional support for the hip joint itself. The joint fluid helps keep the cartilage lubricated so that the ball and socket can glide smoothly, similar to the way the engine oil in your car keeps its moving parts lubricated. 

There is an envelope around the ball and socket that is called the capsule. It helps support the hip and keep the joint fluid within the hip. Also part of this capsule are strong ligaments, which help to provide support for the hip joint.

  • What is FAI?

    Femoroacetabular impingement, or FAI, occurs when there is abnormal contact between the acetabulum (the socket) and the femoral head/neck. This contact most often occurs when the hip is flexed up and typically results in decreased range of motion and pain. Over time, this abnormal contact can lead to tears involving the labrum. If left untreated, this can cause degenerative arthritis of the hip that will require a hip replacement.

  • What causes FAI?

    FAI is caused by excessive bone growth on the femoral head or femoral neck. When the hip is flexed, this excessive bone hits the acetabular labrum and causes a labral tear. This is called CAM-impingement.  The other type of FAI, called pincer-impingement, occurs when excessive bone forms on the socket, resulting in over-coverage of the femoral head. Think of this condition as similar to being in a crowded subway and being unable to move around without bumping into something. Frequently, there is excessive bone on both the femur and the acetabulum, resulting in a mixed-impingement condition. 

    Hip impingement is caused by several varying factors. There is a genetic component, but its role is not fully understood. Activity during adolescence likely plays a role in the development of the femur and acetabulum. Patients who participate in activities with repetitive hip flexion such as ballet, soccer or ice hockey are at a higher risk. Repetitive hip flexion causes contact between the femoral head/neck and the acetabulum, which can cause additional bone to form on the femoral head.  

    Up to 30 percent of patients without pain or symptoms have evidence of excessive bone on their femurs, and about 70 percent of patients without pain in their hips have a labral tear seen on MRI. Thus, treatment usually is pursued only in cases where pain is severe enough to interfere with the patient’s normal activities.

  • What are the symptoms of FAI?

    Because the hip joint is located in the groin, patients with FAI will often report groin pain. The pain in the groin is typically worse in the sitting position, making it difficult to sit for car rides, movies or in the classroom. If the labrum is torn or there is a tear in the cartilage, the patient may have locking or catching in the hip. 

  • How is FAI diagnosed?

    FAI is caused by a combination of excessive bone on the femur and acetabulum as well as soft tissue damage, and X-rays and MRI can help confirm the diagnosis. To better visualize the labrum and cartilage, the physician may use a special type of MRI, called an arthrogram, which involves injecting dye into the hip to look for the presence of a tear.

  • Treatment

    Treatment for FAI starts conservatively with rest, activity modification and anti-inflammatories such as ibuprofen or naproxen. Physical therapy is prescribed to help strengthen hip muscles and improve the support of the hip. A numbing medicine and corticosteroid injection into the hip joint can help reduce pain and inflammation in patients whose hip is extremely irritated.

    If these initial conservative treatment options do not improve the pain and function of the hip, your physician may pursue surgical treatment options. For FAI, this is usually addressed with a hip arthroscopy, or sometimes with a surgical hip dislocation. Learn about treatment for FAI.