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The Sphericity Deviation Score: Is It a Useful and Reproducible Parameter to Assess the Femoral Head Sphericity in Perthes Disease?

Year: 2026

Type: article

Abstract: Background: The sphericity deviation score (SDS) is a continuous parameter to describe femoral head shape in patients with Perthes disease. The aims of this study were to evaluate the reproducibility of SDS and its association with the 3-group Stulberg classification. Methods: Anteroposterior and frog-leg lateral radiographs of 41 healed Perthes hips were assessed. Six observers with varying levels of experience, independently performed SDS measurements on two occasions, 2 weeks apart. Associations between SDS and the modified 3-group Stulberg classification were tested using ANOVA or Kruskal-Wallis tests and Spearman's rank correlation coefficient (rho). Intra- and interobserver agreement was assessed with Bland-Altman 95% limits of agreement (LoA), and reliability was analyzed by intraclass correlation coefficients (ICC). Results: < .001). Conclusions: SDS is a complex parameter that did not consistently differentiate between femoral head shapes when used by inexperienced observers. Even among experienced pediatric orthopaedic researchers, SDS showed marginal interobserver agreement. The method should be used only by experienced observers in scientific studies. Key Concepts: (1)The Sphericity Deviation Score (SDS) correlates significantly with the Stulberg classification only when measured by experienced pediatric orthopaedic researchers.(2)SDS is a complex and time-consuming parameter that often failed to distinguish clearly between adjacent Stulberg categories (e.g., round vs. ovoid hips).(3)Among inexperienced observers, the limits of agreement (LoA) of SDS measurements were unacceptable and the reliability (ICC) was poor.(4)SDS should be used only by experienced observers in scientific studies. Even among experienced researchers, interobserver LoA was marginal, indicating that further methodological refinements are desirable. Level of Evidence: III, retrospective cohort study.

Source: Journal of the Pediatric Orthopaedic Society of North America

DOI: 10.1016/j.jposna.2026.100385