A modification of Klein's Line to improve sensitivity of the anterior-posterior radiograph in slipped capital femoral epiphysis.

BACKGROUND: Radiographs 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 (Klein's Line) and the epiphysis confirms a slip. Despite broad knowledge of the Klein's 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.

METHODS: Five 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 Klein's Line.

RESULTS: We 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 Klein's 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<0.001 in all cases). Our analysis of the classical definition of Klein's Line--the lack of intersection between Klein's Line and the epiphysis as a sign of a slip--identified only 40.3% of slips. By modifying Klein's Line such that we measure the width of epiphysis lateral to Klein's Line, we improved sensitivity to 79% if a difference of 2 mm between hips indicated a slip.

CONCLUSIONS: On the basis of our findings, head/shaft angle and percent epiphyseal displacement provide reliable and efficacious means of radiographic SCFE diagnosis. The classic definition of Klein's line fails to identify 60% of slips. We propose a modification of Klein's Line to improve sensitivity.