Background: The false profile radiograph assesses acetabular coverage in prearthritic hip conditions. Precise rotation of this radiograph is difficult to obtain, so the clinician must interpret radiographs with nonstandard pelvic rotation or tilt, despite limited evidence of how this may affect the anterior center edge angle measurement.
Questions/purposes: (1) Does pelvic rotation alter the measurement of the anterior center edge angle on false profile views? (2) Does pelvic tilt alter the measurement of the anterior center edge angle on false profile views? (3) Is there an objective way to assess appropriate pelvic rotation for the false profile view?
Methods: Eight cadaver hips (four female, four male; one hip randomly selected per pelvis) were included in the study. Hips with degenerative changes, evidence of previous fracture or trauma, or previous surgical intervention were excluded. Specimens were between 68 to 92 years of age (median, 76 years). The specimens were fixed to a custom jig, and radiographs were taken at 5° intervals of rotation (45-85°) and 5° intervals of pelvic tilt (+10° to -10°). The primary outcome variable, anterior center edge angle, was measured for each rotation and tilt.
Results: Every degree increase in pelvic rotation toward a true lateral resulted in 0.18° increase in the anterior center edge angle (95% confidence interval [CI], 0.07-0.29; p = 0.002). For every degree increase in pelvic tilt, the anterior center edge angle increased 0.65° (95% CI, 0.5-0.8; p < 0.001). We verified that standard pelvic rotation of 65° for a false profile radiograph was present when the space between the femoral heads is 66% to 100% of the diameter of the femoral head being imaged.
Conclusions: This study shows that the anterior center edge angle increases as pelvic tilt increases, with a 6° increase in anterior center edge angle for each 10° increase in pelvic tilt. Since the false profile radiograph is obtained standing, the patient should be counseled to avoid adopting a forced posture, ensuring the radiograph remains an accurate functional representation of the patient's anatomy. In contrast, pelvic rotation did not influence the anterior center edge angle by an important margin, and while we recommend that radiographs continue to be obtained with standardized pelvic rotation, aberrant pelvic rotation will likely not result in a clinically meaningful difference in anterior center edge angle measurements. In the future, studies to identify the specific regions of acetabular anatomy that constitute the radiographic measurement of the anterior center edge angle would enhance current understanding of the associated radiographic anatomy, and consequently improve the ability of the surgeon to treat the specific area of pathology.
Clinical relevance: In practice, the clinician should pay close attention to pelvic tilt, as a 10° change in tilt may cause 6° of change in the anterior center edge angle. However, false profile radiographs obtained within ± 20° of the targeted 65° of rotation will result in less than 4° change in the anterior center edge angle.