Outpatient Center Stanford Medicine

Femoroacetabular Impingement (FAI)

(aka: Femoral Acetabular Impingement)

Femoroacetabular impingement or FAI is a condition where the bones of your hip joint come too close and pinch tissue or cause too much friction. Usually, the ball of the hip joint (femoral head) sits on the femoral neck similar to an ice cream sitting on a cone. The pinching and friction occurs when the femoral head and neck contact the socket (acetabulum), creating damage to the hip joint. The pinching or friction may cause damage to the labrum (a fibrous cartilage that lines the outer edge of the socket) and/or the articular cartilage (the white covering over the bony surfaces that results in the very smooth surface gliding of the joint).


FAI generally occurs as two forms: Cam and Pincer. The Cam form describes the femoral head and neck relationship as aspherical or not perfectly round. This loss of roundness contributes to abnormal contact between the head and socket as the hip goes through a range of motion. The Pincer form describes the situation where there is overcoverage of the socket or acetabulum relative to the ball or femoral head. This over-coverage typically exists along the front-top rim of the socket (acetabulum).  The end result is that the labral cartilage gets "pinched" between the rim of the socket and the front part where the femoral head meets the femoral neck. The Pincer form of the impingement is typically the results of "retroversion", where the socket is pointed backwards a bit (rather than the usual situation where it is angled forwards), or where the socket is too deep. Very often, the Cam and Pincer forms exist together. The cause of these bony variations is not known at this time.


FAI is associated with articular cartilage damage and labral tears and may result in hip arthritis at a younger age than usually occurs.   

Who is at risk?

FAI is common in high level athletes, but also occurs in active individuals. While either type of impingement can occur in men or women at any age, most frequently the Cam type of impingement tends to affect young (20s) male athletes, while Pincer tends to occur more commonly in women in their 30s and 40s who are athletically active. Sports associated with FAI include Martial Arts, Ballet, Cycling, Rowing, Golf, Tennis, Soccer, Football, Ice Hockey, Baseball, Lacrosse, Field Hockey, Rugby, Water Polo, and Deep squatting activities such as power lifting.



Your doctor will ask about your hip (your symptoms and how the pain started, for how long, etc) and perform an examination. Your doctor will move your hips and legs in different positions to assess your range of motion and evaluate the positions where your hip hurts. 
To confirm a diagnosis you will likely get XRays of your hip. Often, you may undergo a special type of magnetic resonance imaging (MRI) called magnetic resonance arthrography (MRA).
Magnetic resonance arthrography (MRA) is a noninvasive, non-irradiating imaging technique that uses a magnetic field and radio waves to evaluate your hip.  While XRays show bones well, the MRI is particularly good at showing the non-bony structures of the body, such as the labrum and articular carilage. Further, while XRays are like looking at shadows, the MRI allows evaluation of the tissues around the hip in slices (like slices of bread as opposed to seeing the whole loaf without what is inside) and allows viewing from different views. During magnetic resonance arthrography, dye (contrast material) is injected into the joint space to help make images more clear.  Frequently, local anesthetic (numbing medicine) is added to the contrast material to help determine if the pain is coming from inside the joint.  The MRI will also help eliminate certain causes of non FAI hip pain including avascular necrosis (dead bone) and tumors.
Sometimes your physician may order a CT or CAT scan.  This study can help understand the exact shape of the bones of the hip, but not essential to the diagnosis of FAI.  It is a balance of the relatively large amount of radiation to the pelvis/reproductive region due to the CT scan with the amount of information obtained.  This concern of additional radiation is especially important when 3-D CT scans are performed, which are particularly good at giving the doctor an very realistic perspective of the shape of the bone. 


The underlying problem with FAI is a bony abnormality.  This bony shape will not change with physical therapy or rest.  However, the shape of the bones itself do not cause pain.  Other structures that can be injured with FAI, such as the labrum, or articular cartilage may cause the pain in the hip. Neither the labrum or articular cartilage have much capacity to heal, but sometimes these structures, even when injured do not cause pain or other symptoms.  Thus, for those with symptoms the initial treatment may involve rest and rehabilitation, while those that have symptoms that persist, arthroscopic surgery may be needed.  The long term sequelae of FAI has not been conclusively proven, but there is much evidence that it may be a major cause of premature arthritis of the hip.  It has also not been proven that surgery for FAI will prevent arthritis.  However, removing the offending bone may help reduce further injury to the joint, while also reducing symptoms.  The results of surgery are clearly better when there is no articular cartilage damage.  Thus, most physicians familiar with this problem often recommend early surgical intervention for symptomatic patients with FAI. 


Nonoperative management of FAI can be attempted.  However, it involves a change in lifestyle from active to less active and a commitment to maintaining hip strength. A good physical therapy program focusing on hip strengthening instead of stretching may be beneficial.  Stretching associated with yoga and sometimes physical therapy may make the symptoms worse. Activity modification should involve avoiding activities that take the hip through extreme or full ranges of motion.  Anti-inflammatory medications can also be attempted. 


Surgery for FAI can be performed using hip arthroscopy or open surgery. In hip arthroscopy, the hip is distracted and an arthroscope (a videocamera about the size of a pen) is used to look in the joint to see and treat damage that is found using two to five incisions that are about ¼" in size. Often, all of the components of FAI such as the labral tear, damaged articular cartilage, and bony changes between the ball and socket can be treated with the assistance of the arthroscope. Repair of a torn labrum as well as stimulating new cartilage growth (microfracture) are often possible with the arthroscopic approach. A hip arthroscopy involving labral debridement (no repair) and no bony decompression usually takes less than one hour. A hip arthroscopy involving labral/cartilage repair and FAI decompression usually takes about two hours.  This is done as an outpatient surgery (go home the same day). This is the way it is performed here at Stanford.
The open surgical hip dislocation involves a single long incision (approximately 7 to 10 inches), cutting a bone of the upper thigh, and dislocation of the ball from the socket exposing all parts of the joint.  This exposure allows treatment of labral tears and abnormal contact between the ball and socket.  The open approach can typically be done in a few hours. Pateints usually stay in the hospital for several days after this approach. 

Recovery from Surgery

The patient is on crutches after surgery. Recovery time from most FAI surgical procedures is 4 - 6 months to full, unrestricted activity. Your postoperative activity level will depend on your surgeon's recommendation, the type of surgery performed, and the condition of the hip joint at the time of surgery.  

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