Correlation between acetabular index at 3 and 12 months of age: a longitudinal radiographic study of 228 neonates treated for 6 or 12 weeks with the von Rosen splint for developmental dysplasia of the hip

BACKGROUND AND PURPOSE: Developmental dysplasia of the hip (DDH) affects around 1.5% of newborns in Sweden with few late detected cases (0.12 per 10,000). The most common treatment for DDH in Sweden is with the von Rosen splint, with radiographs at 3 and 12 months of age. Little is known about the remodeling of acetabular dysplasia following treatment initiated in the neonatal period. We aimed to examine the correlation between the acetabular index (AI) at 3 and 12 months. METHODS: We included 228 patients with early detected DDH with dislocatable hips (Barlow) and dislocated hips (Ortolani), treated with the von Rosen splint at Skåne University Hospital 2003-2019. The treatment length was 6 weeks for 96 children and 12 weeks for 132 children. We calculated the correlation between AI at 3 and 12 months using Pearson correlation (r) and the mean difference, both with 95% confidence intervals (CI). RESULTS: The correlation between AI at 3 and 12 months was moderate, r = 0.43 (95% confidence interval [CI] 0.35-0.50), with changes in AI that differed widely. The mean AI was 23.9° (CI 23.5-24.3) at 3 months and 24.9° (CI 24.6-25.3) at 12 months with a difference of 1.0° (CI 0.6-1.3). CONCLUSION: The correlation between AI at 3 and 12 months was moderate, with non-clinical difference for both 6 and 12 weeks of treatment. The small increase in mean AI was most likely explained by a low AI at 3 months after 12 weeks of treatment.