Femoroacetabular impingement (FAI) or hip impingement is a condition where abnormal contact occurs between the upper femur and the acetabulum during motion.
It can be caused by excess bone along the upper femoral neck (CAM type FAI), excess bone along the margin of the acetabulum (Pincer type FAI), a combination of excess bone (Mixed cam-pincer type FAI) or by supraphysiologic ranges of motion. FAI can lead to hip labral tears, cartilage damage and degenerative osteoarthritis. It is a common cause of hip pain in adolescents although it rarely occurs in growing children. Many childhood hip conditions like Perthes and slipped capital femoral epiphysis can lead to the development of FAI.
Pain and limited hip range of motion are the two most common symptoms. The pain is felt when abnormal contact occurs or when tissues become permanently damaged. Pain is typically located in the groin but can occur along the outside of the hip, the back of the hip, the lower back, or in the thigh. FAI pain rarely goes below the knee. The pain tends to occur more with the hip in a flexed and internally rotated position. Many patients find that rotating their hips outward helps to relieve some of the pain. Hip flexion and internal rotation are typically limited.
Physical examination can demonstrate decreased hip flexion and internal rotation. With forced internal rotation of the hip in a flexed position, pain will be elicited which is known as a positive impingement test. Physical examination can also help to differentiate FAI from other common adolescent hip conditions.
Plain x-ray radiographs, AP and axial views, typically demonstrate the excess bone around the hip. The acetabulum may show a "cross-over" sign or excess lateral coverage. The femur may show lack of head-neck offset. Often both the acetabulum and the femoral head-neck junction are abnormal. With more advanced pathology, a calcified labrum, decreased joint space, or cysts may be seen which indicate early osteoarthritis. MRIs are often obtained to help to evaluate the cartilage and labrum as well as to better define the areas of abnormal bone.
The first line of treatment for FAI is activity modification, NSAIDs and a program of hip strengthening which may include physical therapy. Hip stretching exercises are often counterproductive because they may cause further abnormal bony contact. Frequently these measures fail to provide adequate relief and then surgical treatment becomes an option. This can be arthroscopic, open, or a combination of the two. The goal of surgery is to improve the hip's range of motion so that bony abutment does not occur during normal motions. If the abnormal contact can be eliminated then further cartilage and labral damage may be prevented. Any damage to the labrum or cartilage that may have occurred before surgery will be addressed to the extent that it can be; often torn or worn cartilage cannot be "repaired".
One surgical treatment for FAI involves dislocating the femur from the socket. An open surgical dislocation provides access to the entire acetabulum and femoral head and neck. The excess bone along the acetabular rim can be trimmed and the labrum can be repaired back to the newly created rim with suture anchors. The femoral head-neck junction can be inspected and templates can be placed on the femoral head to determine where it is out of round. That area can then be trimmed. The exposure requires a trochanteric flip osteotomy which is later repaired with screws. Weight bearing on the surgical limb is limited for 8 weeks after which therapy is started.
Hip arthroscopy can also be utilized to manage select cases of FAI. Access to the hip is more limited than with the open approach, so not every case can be managed with this technique. The surgery is typically performed as an outpatient procedure. Sometimes, arthroscopy is combined with an open procedure.
Another option for FAI is the reverse periacetabular osteotomy. This involves a series of cuts around the acetabulum and then reorienting the entire socket. This technique is useful in cases of retroversion where trimming the excess anterior bone might lead to instability.
If FAI is detected and treated early, before irreversible cartilage and labral damage occurs, we are hopeful that degenerative osteoarthritis may be delayed or even prevented.