Reliability of the Modified Method for Sphericity Deviation Score Using Only the Involved Hip Radiographs in the Patients With Legg-Calve-Perthes Disease

Background: The sphericity deviation score (SDS) is a validated radiographic outcome measure that quantifies the severity of the femoral head deformity in the healed stage of Legg-Calve-Perthes disease (LCPD). The current method requires radiographs of both hips regardless of unilateral involvement to standardize radiographic magnification. As LCPD affects the unilateral hip in 85% to 90% of cases, the current method imposes unnecessary radiation exposure to most of the patients and having to exclude patients from research studies who only had unilateral hip radiographs. We thus modified the SDS method to use unilateral hip radiographs. The purpose of this study was to investigate the reliability of the modified SDS method using the radiographs containing only one hip. Methods: This retrospective study involved 40 patients with LCPD with unilateral involvement in the healed stage of LCPD. We modified the SDS measurement method by using the distance between the teardrop and the lateral acetabulum for magnification correction and providing a clear anatomic description of reference points on the femoral head. Three independent observers performed measurements using radiographs containing the affected hip only (modified method) and both hips (conventional method). The intraclass correlation (ICC) estimates were calculated. To verify clinical relevance, the correlation of the SDS with the Stulberg classification and hip range of motion (ROM) were also investigated. Results: Measurements using the modified SDS showed excellent inter-/intra-observer ICCs (0.903 to 0.978). The ICCs between the modified and conventional methods were also excellent: 0.940 to 0.966 within the same observer and 0.897 to 0.919 between different observers. The modified SDS showed moderate-to-strong correlations with the Stulberg classification (Spearman ρ=0.650) and hip ROM (Pearson r=-0.661). Conclusion: The modified SDS measurement method showed excellent inter-/intra-observer reliability and moderate-to-strong correlations with the Stulberg classification and hip ROM. This method will help reduce unnecessary radiation exposure in patients with unilateral LCPD and prevent the exclusion of patients with unilateral radiographs from future research studies. Level of Evidence: Level III - diagnostic study.