Effect of femur position on the angular measurement of slipped capital femoral epiphysis.

SUMMARY: A femur model was used to simulate a slipped capital femoral epiphysis (SCFE). Wedges were inserted into a cut in the femoral neck to create SCFEs of 25, 50, and 75 degrees. The effects of femur position on the angular measurement of SCFE were studied by taking radiographs of the SCFE at preselected positions of rotation and abduction, which were ensured by a holding device. The views selected were the cross table lateral (CTL), Billing lateral, and frog lateral (FL) projections. FL views were obtained at 22.5, 45, and 67.5 degrees of abduction. The rotational positions were 20 degrees internal, neutral, and 15, 30, 45, and 60 degrees external. Slip angles were measured from these radiographs. The CTL and FL projections accurately portrayed any severity of an SCFE at all positions except for 45 and 60 degrees of external rotation. The Billing view falsely portrayed a severe SCFE at all positions of rotation. The most accurate representation of an SCFE is seen on either the CTL or FL view when the external rotation of the femur does not exceed 30 degrees.