Reproducibility of Acetabular Landmarks and a Standardized Coordinate System Obtained from 3D Hip Ultrasound.

Two-dimensional (2D) ultrasound detection of developmental dysplasia of the hip (DDH) is limited by variation in acetabular appearance and alpha angle measurements, which change with position of the ultrasound probe. Three-dimensional (3D) ultrasound captures the entire acetabular shape, and a reproducible "standard central plane" may be generated, from two landmarks located on opposite ends of the acetabulum, for measurement of alpha angle and other indices. Two users identified landmarks on 51 3D ultrasounds, with ranging severity of disease, and inter- and intra-observer reproducibility of landmark and "standard plane" locations was compared; landmarks were chosen within 2 mm, and the "standard plane" rotation was reproducible within 10° between observers. We observed no difference in variability between alpha angles measured on the "standard plane" in comparison with 2D ultrasound. Applications of the standardized 3D ultrasound central plane will be to fuse serial ultrasounds for follow-up and development of new indices of 3D deformity.