Epiphyseal Angulation and Related Spatial Orientation in Slipped Capital Femoral Epiphysis: Theoretical Model and Biomechanical Explanation of Varus and Valgus Slip

The management of slipped capital femoral epiphysis (SCFE) is controversial. Surgical decision-making is based regularly on the chronicity, stability, and severity of the slip. The purpose of this study was to determine the true angulation and spatial orientation of the epiphysis in hips with SCFE and contralateral hips. METHODS: Eighteen hips in 18 patients with SCFE were included in the analysis. Trigonometric calculations, based on angle measurements using 2 conventional radiographs in planes that are perpendicular to each other, were used to determine the angulation of the epiphysis and its orientation in space. RESULTS: The mean absolute epiphyseal obliquity of the SCFE hips was 56.2° and the spatial orientation was 36.5°. The mean obliquity of the contralateral side was 34.0°, with a related spatial orientation of 16.8°. The maximum error can reach up to 9.9° (or 41%) when comparing the calculated angles with the angle measurements on radiographs. CONCLUSIONS: On standard radiographs, the epiphyseal angulation in SCFE is consistently underestimated. As a consequence, the assigned classification of some patients may be 1 severity group too low, which impacts the value of traditional severity classification for surgical decision-making. The analysis of the spatial orientation of the slip with the concomitant direction of the resultant shear can partially explain varus and valgus slip in SCFE. LEVEL OF EVIDENCE: Diagnostic Level IV. See Instructions for Authors for a complete description of levels of evidence.