In this expert opinion, two pediatric orthopedic surgeons from Children’s Hospital in Boston discuss femoroacetabular impingement caused by Perthes disease. Perthes disease of the hip (also known as Legg-Calvé-Perthes) occurs when there is a loss of blood supply to the growth center at the top of the femoral head. Without enough blood, the bone dies, degenerates, and collapses.
Children with Perthes disease of the hip may recover fully without further hip problems later. But those patients with growth disturbance of the femoral head and altered shape of the normally round femoral head (top of the thigh bone) may end up with femoroacetabular impingement (FAI) (pinching of soft tissue and bone).
The body is capable of limiting this disease and growing new bone. But in the meantime, the weight of the body on the unstable bone can cause the head of the femur to become more oval-shaped.
That’s a problem because the hip socket is designed to hold the round head of the femur. In fact, the fit is specific and quite tight. That’s what’s needed to provide a stable but moveable hip joint. Without the perfect match-up of femoral head and hip socket, the danger of hip dislocation increases with Perthes disease.
And with the change in shape of the femoral head, there is also a risk of impingement . As the femoral head is pressed down, the femoral neck (between the shaft of the thigh bone and the femoral head) is shortened. There can be a rotation of the bone as well. All these features add to the likelihood of an impingement problem.
How do we know when a child with Perthes hip disease is also experiencing hip impingement? Symptoms of groin pain that is worse with activity or prolonged sitting are the first clues. There could be just stiffness and loss of hip motion without pain. The real tip off is the position of the hip when the symptoms are the worst: internal rotation and flexion.
X-rays will help show what’s going on. The radiologist and orthopedic surgeon look for something referred to as acetabular coverage. This is a view of how much of the femoral head is inside the socket (called the acetabulum). With impingement from Perthes, it is common to see overcoverage (shelf of the socket hangs down over too much of the femoral head).
Other deformities can be seen on X-ray and the physicians use several ways to look for these (e.g., Shenton’s line, cross-over sign, acetabular index). Depth of the socket and presence of rotation of the bones are also assessed. In some cases, it may be necessary to order additional imaging studies such as CT scans or MRIs.
The authors of this article emphasize the need to identify all changes and deformities within the hip complex associated with Perthes disease. They say that successful treatment (especially surgery) depends on understanding all the components that contribute to Perthes hip deformities.
What can be done about femoroacetabular impingement in a child with Perthes hip disease? The surgeon must look at both sides of the hip: the femoral head and the acetabulum (socket). It may be necessary to make surgical corrections of both areas.
On the femoral side, the surgeon may change the length or angle of the femoral neck. The misshapen and enlarged head may have to be corrected, a procedure called osteochondroplasty. This requires surgically dislocating the hip. That sounds pretty dramatic but the authors assure us it can be done safely and is quite effective.
While correcting the deformity that causes impingement, the surgeon will also look for any other areas of soft tissue damage. There may be a tear in the labrum that needs attention. The labrum is a rim of cartilage around the hip socket designed to give the socket a little bit more depth and the hip greater stability inside the acetabulum.
The ultimate goal of surgery for femoroacetabular impingement in the Perthes hip is to improve hip joint motion. Reducing pain and improving joint stability are also important. The surgery can become quite complex when there are numerous changes in the hip to be addressed.
The surgeon must also pay attention to alignment of the involved leg. No sense in making all these corrections to the deformities only to create a leg length difference, loss of joint stability, or abnormal arc of motion.
In summary, children with Perthes hip disease can develop a type of hip impingement as they get older. The effects of the disease in changing the shape of the femoral head contribute to this problem. Surgery to correct the impingement is a possible treatment option. Careful assessment of all deformities and damage present in the hip complex can be done best with surgical dislocation.