Pubofemoral distances correlate to acetabular morphology and sonological instability in screening for hip dysplasia

Aims: The present study seeks to investigate the correlation of pubofemoral distances (PFD) to alpha angles, and hip displaceability status, defined as femoral head coverage (FHC) or FHC during manual provocation of the newborn hip < 50%.Methods: We retrospectively included all newborns referred for ultrasound screening at our institution based on primary risk factor, clinical, and PFD screening. alpha angles, PFD, FHC, and FHC at follow-up ultrasound for referred newborns were measured and compared using scatter plots, linear regression, paired t-test, and box-plots.Results: We included 2,735 newborns, of whom 754 received a follow-up hip ultrasound within six weeks of age. After exclusion, 1,500 hips were included for analysis. Sex distribution was 372 male and 380 female, and the mean age at examination was 36.6 days (4 to 87). We found a negative linear correlation of PFD to alpha angles (p < 0.001), FHC (p < 0.001), and FHC during provocation (p < 0.001) with a 1 mm increase in PFD corresponding to a -2.1 degrees (95% confidence interval (CI) -2.3 to -1.9) change in alpha angle and a -3.4% (95% CI -3.7 to -3.0) change in FHC and a -6.0% (-6.6 to -5.5) change in FHC during provocation. The PFD was significantly higher with increasing Graf types and in displaceable hips (p < 0.001).Conclusion: PFD is strongly correlated to both alpha angles and hip displaceability, as measured by FHC and FHC during provocation, in ultrasound of newborn hips. The PFD increases as the hips become more dysplastic and/or displaceable.