A Novel Classification System for Slipped Capital Femoral Epiphysis Based on the Radiographic Relationship of the Epiphyseal Tubercle and the Metaphyseal Socket

Recent studies have suggested that the epiphyseal tubercle serves as a fulcrum for rotation in slipped capital femoral epiphysis (SCFE). However, radiographic evidence of the rotational mechanism is limited. In this study, we describe a novel radiographic staging system for SCFE based on the anatomic relationship between the epiphyseal tubercle and the metaphyseal socket. METHODS: We reviewed the cases of 469 patients with SCFE who were treated at our institution between 2000 and 2017. SCFE was classified according to our proposed staging system using the preoperative lateral radiograph. Normal hips were considered to be Stage 0. In Stage 1, the tubercle is concentric within its metaphyseal socket, which is enlarged and may present peritubercle radiolucency. In Stage 2, there is evidence of eccentricity of the tubercle, which remains in contact with the posterior wall of the socket. In Stage 3, the tubercle and metaphyseal lucency reach the posterior cortex of the femoral neck. A complete dislodgment of the tubercle from the metaphysis is present in Stage 4. Intra- and interobserver agreement, and correlations between the staging system and the traditional classifications of severity, stability, and chronicity, were estimated. RESULTS: The distribution by stage was as follows: 2% of the hips were classified as Stage 0, 19% were Stage 1, 48% were Stage 2, 15% were Stage 3, and 16% were Stage 4. The staging system had excellent intraobserver (κ = 0.89 [95% confidence interval (CI) = 0.83 to 0.96]) and interobserver agreement (κ = 0.87 [95% CI = 0.72 to 1.00]). There was a high correlation between the staging system and SCFE severity as assessed by the Southwick angle (r = 0.77 [95% CI = 0.73 to 0.82]; p < 0.001). There was a moderate correlation between the staging system and the Loder classification of stability (r = 0.55 [95% CI = 0.48 to 0.62]; p < 0.001) and a negligible correlation with the classification of chronicity (r = 0.19 [95% CI = 0.10 to 0.28]; p < 0.001). CONCLUSIONS: The proposed staging system for SCFE is highly reliable and correlates well with the severity of SCFE based on the degree of displacement, with moderate correlation shown for stability. This new staging system helps in understanding the rotational mechanism of SCFE, warranting further investigation to determine its clinical application. CLINICAL RELEVANCE: The novel classification has the potential for the identification of hips that demonstrate subtle SCFE or are at pre-slip stage, or those at risk for osteonecrosis of the femoral head, failure of fixation, or slip progression.