Femoro-acetabular impingement is a common cause of hip pain in the young adult and has only been identified since the early 1990s. Hip arthroscopy for its treatment has been around for the last decade however in more recent years is exponentially being used. Not all procedures are successful which is why it is important to understand what causes impingement and which patients are suitable for this operation. To achieve the best outcome the correct indication for performing this procedure is essential.
What causes hip impingement? The most common cause for hip impingement is an abnormal shape of the femoral head. The normal femoral head is spherical which enables a full range of movement without impingement. When the femoral head is aspherical or cam shape it caused impingement on deep flexion of the hip. Here the cam abuts against the edge of the acetabulum damaging the labrum and joint cartilage. This commonly results in labral tears. The labrum is a triangular cartilage attached circumferentially to the acetabular rim producing a seal around the femoral head. It is thought that this repeated micro damage through multiple cycles of deep flexion produces osteoarthritis in the long term.
Another cause of hip impingement is where the acetabulum excessively covers the femoral head. This is called a pincer lesion. This can be focal or global in nature. It produces the same effect as a cam lesion. Here there is impingement between the bony femoral neck and the edge of the acetabulum. In the same way recurrent impingement produces damage to the cartilage and possible long-term risk of osteoarthritis.
There are other less common causes of hip impingement such as extra-articular impingement. However most commonly there tends to be a combination of pincer and cam lesion mentioned above.
Patients can present with groin pain and mechanical symptoms such as clicking or clunking of the joint. The pain is made worse on deep flexion activities such as squatting and sitting in a low chair. Rising up from a sitting position too provokes hip pain. Examination of the patient often reveals a positive impingement test. This test essentially produces deep flexion, adduction (bringing the leg to the midline) and internal rotation of the hip joint. Other movements of the hip are less painful.
The ideal investigations for this condition is a plain x-ray and an MRI arthrogram of the hip joint. On the x-ray there can be evidence of the aspherical femoral head. It is useful to compare the affected joint with the normal joint to rule out reduction of joint space indicating osteoarthritis. The MRI scan is useful in identifying cartilage lesions such as a labral tear. It can also highlight turning off the articular cartilage. Evidence off early osteoarthritis can be noted easily. Some prefer to do a CT scan of the hip joint which shows the bony abnormality better.
There are many options for treating this condition such as physiotherapy, hip injections and hip arthroscopy. I always recommend a trial of physiotherapy to begin with. If unsuccessful a hip injection is beneficial to confirm that the pain stems from the hip joint and nowhere else. Following this a hip arthroscopy can be considered to repair the torn cartilage and reshape the femoral head or acetabulum.
It is important to note as mentioned above all that not all patients are suitable for hip arthroscopy. Poor prognostic indicators are early arthritis, reduced joint space, age over 50, joint hyperlaxity and hip dysplasia.