Reduced acetabular depth in hip instability in the newborn.

This study assessed the frequency of ultrasonographic hip instability and compared the results with clinical and morphological ultrasonographic evaluations of the hip in the newborn infant. Hip stability in 2016 consecutive births (1074 boys and 942 girls) was assessed clinically and ultrasonographically. Ultrasonography included dynamic stability testing and morphologic evaluations. Compared with ultrasonography, clinical stability testing had a specificity of 0.998 and a sensitivity of 0.667. The incidence of instability was 0.007 for hips and 0.010 for babies. Acetabular depth in 3994 normal hips was 8.3 +/- 0.9 mm, femoral head diameter was 15.4 +/- 1.3 mm, and acetabular bony coverage of the femoral head was 54 +/- 4.3%. Hip instability was significantly associated with a reduced acetabular depth. As the femoral head diameter was within normal in the unstable hips, coverage was significantly reduced.