A Modification of Kleinʼs Line to Improve Sensitivity of the Anterior-Posterior Radiograph in Slipped Capital Femoral Epiphysis

BACKGROUNDRadiographs can diagnose slipped capital femoral epiphysis (SCFE) on the anterior-posterior (AP) pelvis view and the frog-leg lateral view of the hips. On the AP radiograph, the lack of intersection between a line drawn parallel to the superior edge of the femoral neck (Kleinʼs Line) and the epiphysis confirms a slip. Despite broad knowledge of the Kleinʼs Line principle, application is difficult and inaccurate, especially in mild cases. On the frog-leg lateral radiograph, Southwick head/shaft angle and Wilson percent epiphyseal displacement commonly quantify the slip. Here, we set out to evaluate the intraobserver and interobserver reliability and the efficacy of these methods. METHODSFive separate observers on 2 separate occasions evaluated 30 AP and 30 frog-leg lateral radiographs of patients with unilateral SCFE for head/shaft angle, percent epiphyseal displacement, and width of epiphysis lateral to Kleinʼs Line. RESULTSWe calculated the minimum difference required for 2 measurements to be considered different with 95% confidence (“minimum agreement difference”). For head/shaft angle, the intraobserver minimum agreement difference was ±7.27 degrees and the interobserver ±8.80 degrees; for percent epiphyseal displacement, the intraobserver was ±7.18% and the interobserver was ±7.27%; and for width of epiphysis lateral to Kleinʼs Line, the intraobserver was ±1.98 mm and the interobserver ±2.16 mm. For each of these measures, the slipped hips significantly differed from the control hips (P