We have twin boys who are both active in different sports. It turns out that they also both have a slightly misshaped hip. For one boy this has caused all kinds of hip problems, groin pain, and lost playing time on his soccer team. The other boy doesn’t seem affected at all. How come?

With impingement, the soft tissues around the joint get caught between the femur and the hip socket. There are several different types of impingement. They differ slightly depending on what gets pinched and where the impingement occurs.

Femoroacetabular impingement occurs when abnormal hip anatomy is aggravated by repetitive movements of the hip. There could be a slightly off center placement of the hip in the socket or a femoral head that isn’t perfectly round that is contributing to the problem.

Or the hip socket may be too deep for the size of the femoral head or the rim of the hip socket is too prominent. Sometimes the angle of the femoral neck is bent or twisted just a tad from normal. There could be a separate piece of bone called the os acetabulum along the front rim of the hip socket. Any of these morphologic changes can lead to impingement.

The labrum, a fibrous rim of cartilage around the hip socket is the most likely area to get pinched. Add repetitive motion and you get repetitive pinching or compression until the labrum starts to fray and tear.

No one knows for sure just yet why some athletes with this problem are affected while others seem not to notice a problem. With your sons, it could be there is just enough difference in the shape of one boy’s hip that he is spared the painful loss of motion and function. There could be differences in the degree of pinching that is going on or the tension of the soft tissues, muscles, and tendons around the joint.

Studies show that some high level athletes with abnormal hip joints never develop problems. Who does develop femoroacetabular impingement and how to predict if/when it should be treated are areas where further study is needed.