Pelvic Rotation and Tilt Can Cause Misinterpretation of the Acetabular Index Measured on Radiographs

BACKGROUND: Radiographic diagnosis and followup studies of developmental dysplasia of the hip are commonly performed by measuring the acetabular index on radiographs using Hilgenreiner’s method. The outcome of the measurement, however, depends on the orientation of the subject’s pelvis relative to the xray source. The influence of pelvic rotation and tilt on the measurement error has been evaluated separately but not in combination. QUESTIONS/PURPOSES: We asked whether (1) combinations of pelvic rotation and tilt introduced systematic error in acetabular index measurement in a reproducible way, and (2) ratios proposed to evaluate either pelvic rotation (R(rotation)) or pelvic tilt (R(tilt)) are influenced by pelvic tilt and rotation, respectively. METHODS: Radiographic measurements of the acetabular index, R(rotation), and R(tilt) were performed on digitally reconstructed radiographs of one high-resolution three-dimensional CT dataset with various combinations of pelvic rotation and tilt. RESULTS: For rotations and tilt up to 12°, the average systematic errors in the acetabular index varied from −8.8° to 4.5°. Negative and positive error values can be interpreted as underestimations and overestimations of the acetabular index, respectively. Errors in acetabular index measurements were acceptable for R(rotation) values between 1.0 and 2.0 and R(tilt) values between 1.1 and 1.8. CONCLUSIONS: To limit the systematic error in assessing the acetabular index caused by pelvic misalignment, we recommend only radiographs acquired with ± 4° rotation and ± 4° tilt be considered acceptable.