A new radiological index for assessing asphericity of the femoral head in cam impingement.

Femoroacetabular cam impingement is thought to be a cause of premature osteoarthritis of the hip. The presence of cam malformation was determined in 2803 standardised anteroposterior (AP) pelvic radiographs from the Copenhagen Osteoarthritis Study by measuring the alpha (alpha) angle and the triangular index, a new measure of asphericity of the femoral head. In addition, the alpha-angle and the triangular index were assessed on the AP and lateral hip radiographs of 82 men and 82 women randomly selected from patients scheduled for total hip replacement (THR). The influence of varying femoral rotation on the alpha angle and the triangular index was also determined in femoral specimens under experimental conditions. From the 2803 radiographs the mean AP alpha-angle was 55 degrees (30 degrees to 100 degrees ) in men and 45 degrees (34 degrees to 108 degrees ) in women. Approximately 6% of men and 2% of women had cam malformation. The alpha-angle and triangular index were highly inter-related. Of those patients scheduled for THR, 36 men (44%) and 28 women (35%) had cam malformation identifiable on the AP radiographs. The triangular index proved to be more reliable in detecting cam malformation when the hip was held in varying degrees of rotation. The combination of the alpha-angle and the triangular index will allow examination of historical radiographs for epidemiological purposes in following the natural history of the cam deformity.