O2142: Radiographic Analysis of the Positional Tolerance of Hip Dysplasia Parameters in Antero-Posterior Pelvic Images.

Background:

Background: Diagnosing developmental hip dysplasia in pediatric patients largely relies on radiographic measurements taken from AP view pelvic x-rays. However, these images, and consequently their measurements, are subject to positioning factors such as rotation or pelvic tilt. This study aims to assess the impact of imperfect AP pelvic x-rays on measurements commonly used to assess for acetabular coverage and hip dysplasia.

Methods:

Methods: We retrospectively studied 20 CT scans without pelvic pathology. 3D reconstructions were created and rotated through a series of five anatomical landmarks in the craniocaudal axis as well as five points of rotation in the mediolateral axis, with a simulated radiograph in each position to create a total of 25 images per patient. The “ideal” image was designated as having the coccyx centered between the pubic tubercles and the tip of the coccyx in line with the superior margin of the pubic symphysis. Sharp’s angle, lateral center-edge angle (LCEA), Tonnis angle, and femoral head extrusion index (FHEI) were measured on all images.

Results:

Results: We included one patient for each year of age (8-17 years) and both sexes. Intraclass correlation coefficients (ICC) showed high levels of congruence between measurements made by independent raters for LCEA (ICC = 0.78), Sharp’s Angle (ICC = 0.85), Tonnis Angle (ICC = 0.68), and FHEI (ICC = 0.73). Differences in LCEA, Sharp’s Angle, and Tonnis Angle measurements had a maximum deviation of 5.2°, 3.3°, and 3.3° respectively, when comparing the mean at the most rotated positions versus the ideal position. The upper limit for discrepancies in FHEI was measured at 4.8%. Rotating an x-ray around the mediolateral axis resulted in larger discrepancies compared to the craniocaudal axis. Increasing the severity of misalignment also led to an increase in measurement differences from an ideal image. However, dysplasia parameters measured at the extremes of our image manipulation all had mean values within one standard deviation of the mean of an ideal pelvic x-ray. Sharp’s angle demonstrated the highest level of tolerance to malpositioning.

Conclusion:

Conclusions: Improper positioning of pelvic x-rays can impact the radiographic assessment of hip dysplasia parameters. While the means differences are generally tolerable, some cases did have large differences. This study offers guidance on the amount of positional tolerance of common parameters for measuring acetabular coverage.