LAUSR

BACKGROUND Ultrasound (US) is the preferred imaging modality for the diagnosis and treatment of infantile developmental dysplasia of the hip (DDH). Currently accepted indices that distinguish normal from dysplastic hips in the coronal plane include percent femoral head coverage (FHC), α angle, and β angle. Recent data suggests that significant user and interscan variability may exist for these metrics. Less studied, however, is potential variability because of patient positioning, specifically coronal flexion versus coronal neutral views. The purpose of this study was to compare standard DDH indices between coronal US views with hips in flexion versus neutral positions, for hips of varying DDH severity. METHODS This retrospective study included normal infants and those treated for different severities of DDH. Coronal flexion and coronal neutral US images from the same study were evaluated at diagnosis, early treatment, start of weaning, and treatment resolution. FHC, α, and β angles were measured on both views at each time point and compared. Inter-rater and intra-rater reliability assessments were performed for all metrics. RESULTS Among the 168 hips in this study (45 normals, 45 Ortolani positive, 17 Barlow positive, and 61 stable dysplasia), median FHC was significantly lower in coronal flexion compared with coronal neutral for normals and all 3 severities of DDH at each time point (mean decrease 8.4%; range 5.5% to 10.9%; P<0.01). Alpha angle also decreased on coronal flexion views, observed for all hip types, but only at certain time points (mean decrease 3.3 degrees; range 0 to 7.5 degrees; P<0.01 to 0.35). β angles demonstrated less variability between views, but also had poor reliability. CONCLUSIONS Coronal flexion and coronal neutral views demonstrated significant differences in FHC and α angle across a spectrum of DDH severities and treatment time points. Flexion views may represent a "baby Barlow" test, revealing subtle instability as evidenced by the significant and consistent decrease in FHC across all hips. Indices measured in flexion, therefore, may represent more stringent criteria for defining normal hips. LEVEL OF EVIDENCE Level III-diagnostic study.