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How Many People Have Hip Deformity Leading to Arthritis?

There is a condition of the hip called femoroacetabular impingement (FAI) that can lead to early hip osteoarthritis. In an effort to prevent arthritis, there are some experts who suggest routine screening for this problem and early treatment.

But the cost of performing X-rays and/or MRIs on everyone may not yield cost-effective results. That's why these Canadian researchers tested 200 adult volunteers (ages 21 to 50) for the presence of femoroacetabular impingement (FAI). None of them had any previous hip problems.

Impingement 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.

The goal of the study was to determine the prevalence of the Cam-type of FAI in the general adult population. Prevalence refers to how many people have a particular condition at any one point in time. The cam-type of impingement is the most likely to set up conditions ripe for joint wear and tear. This type 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 tilt or 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.

The result is a shearing force on the labrum and the articular cartilage, which is located next to the labrum. The labrum is a dense ring of fibrocartilage firmly attached around the acetabulum (socket). It provides depth and stability to the hip socket. The articular cartilage is the protective covering over the hip joint surface. This abnormal contact between the femur and acetabulum is the leading cause of labral tears and degenerative hip arthritis.

The authors tried to establish what might be considered normal in terms of the femoral head-neck contours. Knowing what falls within the range of normal limits helps identify those who are outside those parameters. Measures used to assess the volunteers for the presence of Cam-type femoroacetabular impingement (FAI) included hip internal rotation, positive impingement sign, and contour/angle of the femoral head-neck junction. MRIs were used to provide information on hip contours and angles. With 200 volunteers enrolled in the study, there were a total of 400 hips to report data on.

A study like this that recruits volunteers from the general population and checks to see how many have femoroacetabular impingement is helpful. By studying people who have no symptoms but who have the problem, it is possible to see over time whether or not they will develop arthritis. It also helps find risk factors for impingement. In this study, they specifically looked at age, gender (male versus female), body-mass index (a measure of obesity), and ethnicity. These potential factors may put people at increased risk for impingement and then for going on to develop arthritis later.

Most of what they found out in this study supports results reported from other studies. For example, there were about 14 per cent of the participants who had femoroacetabular impingement. Other studies report a prevalence of 15 per cent. Men were affected by this particular (Cam) type of femoroacetabular impingement (FAI) much more often than women (25 per cent of men compared with only five per cent of women).

The authors point out that the prevalence of hip osteoarthritis in the general population is only around eight per cent. The much higher 14 to 15 per cent prevalence rate of femoroacetabular impingement (FAI) suggests something else is going on here. There may be genetic or environmental factors contributing to the development of FAI that haven't been discovered yet.

There were some significant findings from the measurements taken of each volunteer when compared with their MRI results. The elevated angle measured on X-ray (called the alpha angle) wasn't diagnostic of femoroacetabular impingement by itself. (Though it was a predictor of hip pain and joint cartilage damage).

But when combined with restricted hip internal rotation, the angle could be used to predict impingement. A positive impingement sign (a test performed by applying manual pressure through the hip) was a reliable indicator of a problem with the labrum (rim of cartilage around the hip socket).

The key finding in this study was the high rate of asymptomatic femoroacetabular impingement in the general adult population. Asymptomatic means they were not aware of the problem and were not having any symptoms from it. Further studies are needed to finish what these researchers started: finding out what alpha angles are normal and what the cutoff angle is to diagnose femoroacetabular impingement.

More studies are also needed to see if early diagnosis and treatment really can reduce the progression of this condition to osteoarthritis. The question of whether or not early testing on all adults is cost-effective was not answered by this research.

References: Kalesha Hack, MD, et al. Prevalence of Cam-Type Femoroacetabular Impingement Morphology in Asymptomatic Volunteers. In The Journal of Bone & Joint Surgery. November 2010. Vol. 92. No. 14. Pp. 2436-2444.

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