Femoroacetabular impingement (FAI), especially in adolescent athletes, has received a lot of attention from orthopaedists in the last 15 years. In the May 18, 2016 edition of The Journal of Bone & Joint Surgery, a longitudinal radiographic study by Morris et al. sheds light on how a measurement called the epiphyseal extension ratio (EER) delivers excellent diagnostic accuracy for predicting cam morphology of the femoral head, one of the main causes of FAI.
The authors carefully analyzed at least five consecutive annual hip radiographs from 96 healthy adolescents. Specifically, they measured changes in the anteroposterior alpha angle and the superior EER (the superior epiphyseal extension divided by the femoral head diameter). They found a mean increase in alpha angle and EER between Oxford bone age (OBA) stages 5 and 7/8. The mean EER increased significantly at each stage, with the greatest increase occurring between OBA stages 6 and 7/8.
In this study, the EER showed excellent diagnostic accuracy for predicting a final alpha angle of ≥78, which prior research has suggested is a threshold that predicts an increased risk for developing end-stage hip osteoarthritis. However, as commentator John H. Wedge, MD emphasizes, Morris et al. “do not recommend radiographic screening for this marker.”
Dr. Wedge adds that this study lends credence to the hypothesis that cam deformity develops from chronic impingement before rather than after proximal femoral physeal closure. But perhaps the most interesting messages are in the discussion section, where Morris et al. state that “epiphyseal extension may be a physiologic, protective response to increased physeal shear forces that decreases the risk of progression to SCFE [slipped capital femoral epiphysis].” The authors describe the cam-morphology downside of epiphyseal extension as “the unfortunate long-term consequence of a short-term adaptive response.”