What proportion of patients with cam-type femoroacetabular impingement is due to slipped capital femoral epiphysis?

Slipped capital femoral epiphysis is a condition that occurs mainly during adolescence. In this condition the femoral head rotates downwards and backwards in relation to the femoral neck, so that the anterior and superior offset ratio is reduced. This produces the characteristic configuration of the femoral head, in which there is a femoral hyperostosis visible on the anteroposterior, but more clearly on the lateral, radiograph. This abnormality in the shape of the femoral head causes early impingement of the head-neck junction against the labrum and against the acetabulum more generally, particularly in movements of the hip through a large range. It has been shown that this condition can cause pain in the hip in the medium term and lead to early wear of the joint in the long term. Femoroacetabular impingement is generally a well-known cause of pain in the hip, particularly in people who play sport. It has also been shown that femoroacetabular impingement is associated with the development of osteoarthritis in the long term. Other causes of the abnormal morphology of cam-type femoroacetabular impingement are, however, Legg-Calve-Perthes disease, fracture of the femoral neck and other causes that are considered far less common.
The question that remains unanswered in the literature is what proportion of patients with cam-type femoroacetabular impingement have slipped capital femoral epiphysis as the underlying cause. In an attempt to answer this question, Murgier and colleagues published a study in The Bone and Joint Journal in which they developed a new radiographic measurement, named the lateral view head neck index, and used it to detect deformity arising from slipped capital femoral epiphysis. The authors compared the measurements in a patient group made up of 75 patients and 96 hips, and compared the results with a healthy group of 108 healthy hips in 54 patients.
The authors concluded that a substantial number of patients with cam-type femoroacetabular impingement showed deformity of the slipped capital femoral epiphysis type. This result therefore supports the hypothesis that slipped capital femoral epiphysis is a more common condition than we suppose and presents in many subclinical forms.