Many articles have been written about the condition affecting the hip known as femoroacetabular impingement (FAI). But the current definition is not specific enough to be used when planning clinical trials.

Therefore, the orthopedic surgeons at The Children's Hospital of Philadelphia (CHOP) have taken on the task of defining this condition with clinical trials in mind. Secondly, they reviewed what is known about the relationship between femoroacetabular impingement (FAI), the later development of hip osteoarthritis, and the eventual need for a hip replacement.

Simply stated, femoroacetabular impingement (FAI) refers to some portion of the soft tissue around the hip socket getting pinched or compressed. Femoroacetabular tells us the impingement is occurring where the femur (thigh bone) meets the acetabulum (hip socket).

There are several different types of impingement. They differ slightly depending on what gets pinched and where the impingement occurs. Anatomic abnormalities of the femur and/or the acetabulum predispose the person to damage of the soft tissue structures inside the joint. Vigorous, repetitive hip motion creates abnormal contact and collision that result in hip pain.

The pain comes on slowly at first. Sitting for long periods of time (prolonged hip flexion) really aggravates the situation. The affected individual may lose hip motion (especially hip flexion and internal rotation). An X-ray will show the changes in the hip typically associated with femoroacetabular impingement (FAI).

The authors propose that in order to include someone in a clinical trial studying femoroacetabular impingement (FAI), there should be three elements consistently present in all participants. These are: 1) symptoms of FAI, 2) positive hip impingement test pain with hip flexion and internal rotation), and 3) imaging findings seen with FAI (e.g., increased alpha angle, bone cysts from impingement, increased lateral center-edge angle).

What about FAI and osteoarthritis? Is there a connection? Well, yes and no. Yes, because people with FAI very often develop osteoarthritis but no because not everyone with FAI develops osteoarthritis and not everyone with osteoarthritis has FAI. The next question that comes up is: which came first? The osteoarthritis and then the FAI or the FAI and then the osteoarthritis? The answer to this question is difficult to determine because osteoarthritis can reshape the bones looking much like femoroacetabular impingement.

The best way to find out is to study young people and follow them through time. X-rays of their hips taken early on can help identify the subset of participants in studies who do, indeed, have FAI. Comparisons can be made over time between those with FAI and those without. Long-term studies can sort out who develops osteoarthritis and what the potential risk factors might be (including FAI).

What has been reported in studies so far? It looks like one particular impingement type (called cam-type impingement) is more likely to lead to tears of the labrum (fibrous rim of cartilage around the hip socket). The cam-type of impingement occurs when the round head of the femur isn't as round as it should be. It's more of a pistol grip shape. It's even referred to as a pistol grip deformity. The femoral head isn't round enough on one side (and it's too round on the other side) to move properly inside the socket.

There is some new evidence that the cam-type impingement is the result of participation in high-impact sports. Adolescents (teens) develop this condition as the bones mature. In other words, they aren't born this way. It appears as though the repetitive motion of the sports activity reshapes the hip resulting in a femoroacetabular impingement (FAI).

Several studies reviewed by the authors present information showing that cam-type deformities are linked with osteoarthritis. But people with cam-type deformities and osteoarthritis are not more likely to end up with a total hip replacement than people without this type of femoroacetabular impingement.

The goal of future studies is to find out who is at risk for developing femoroacetabular impingement that will lead to osteoarthritis and hip replacement. Finding ways to prevent femoroacetabular impingement (and osteoarthritis) would be a natural outcome of these findings. It is possible that risk factors for those who develop osteoarthritis early in life are different than risk factors for adults who develop osteoarthritis in mid- to late-life. This must be investigated as well.

The authors suggest long-term studies should be done following young people through the years. This type of study will give some insight into the natural history (what happens over time) with femoroacetabular impingement (FAI) and osteoarthritis. Variables such as genetic influences, activity levels, and types and severity of anatomic abnormalities should be looked at closely for their role in developing significant FAI and osteoarthritis.

Reference: Wudbhav N. Sankar, MD, et al. Femoroacetabular Impingement: Defining the Condition and Its Role in the Pathophysiology of Osteoarthritis. In Journal of the American Academy of Orthopaedic Surgeons. July 2013. Vol. 21. Supplement 1. Pp. S7-S15.