The S Sign: A New Radiographic Tool to Aid in the Diagnosis of Slipped Capital Femoral Epiphysis

Background: Children with slipped capital femoral epiphysis (SCFE) are often seen by an array of medical professionals prior to diagnosis. Patients with mild slips, slips with knee pain, or bilateral slips can occasionally present a diagnostic challenge that increases the risk of a delay in diagnosis and associated complications.

Objectives: This study introduces a new radiographic parameter, which we refer to as the S-sign, and analyzes its diagnostic utility on a frog-leg lateral radiograph.

Methods: Twenty observers reviewed the radiographs from 35 patients with SCFE using Klein's line on anteroposterior pelvis radiographs and the S-sign on frog-leg lateral radiographs to diagnose an SCFE. Analysis included diagnostic outcomes and intraobserver and interobserver reliability.

Results: The S-sign was more accurate at identifying an SCFE compared with Klein's line (92.4% vs. 79.2%, respectively). Sensitivity and specificity was greater for the S-sign compared with Klein's line (89.0% and 95.2% vs. 68.3% and 89.0%, respectively). A combination of the S-sign and Klein's line yielded a sensitivity of 96.5% and a specificity of 85.0%. The combination of tests was more diagnostic for an SCFE, compared with using the Klein's line, which was statistically significant (p < 0.001).

Conclusions: With increased awareness of the S-sign and a usage of the combined test, clinicians can more reliably and accurately diagnose an SCFE. Clinicians are more likely to diagnose an SCFE using the combined test, compared with solely relying on Klein's line, which we found to be statistically significant.