EP1.48 Comparison of 2D versus 3D measurements of Lateral Center Edge Angle in Patients with Femoroacetabular Impingement Syndrome and Hip Instability: A study of 754 hips

Abstract

Abstract Purpose The lateral center edge angle (LCEA) is a radiographic measure used to assess acetabular coverage. Prior literature suggests CT values are higher than 2D radiographs, though reasons remain unclear. This study aims to compare LCEA between standing AP radiographs and 3D CT, and to determine whether differences between modalities are influenced by acetabular orientation and morphology. Methods This retrospective cohort study included patients with femoroacetabular impingement syndrome (FAIS) and hip instability. Inclusion required preoperative standing AP pelvis radiographs and 3D CT reconstructions. Exclusion criteria included prior ipsilateral hip surgery, Tönnis grade >1, and missing imaging. LCEA was measured using Ogata and Wiberg methods on standing AP radiographs and compared to CT-derived LCEA at the 12 o’clock position. Pelvic rotation and tilt were measured using obturator foramen width (OFW) difference and pubic symphysis-sacrococcygeal distance (PSSC). OFW difference was calculated between right and left foramina at their widest points. Differences (Δ = x-ray - CT) were calculated. Nonparametric testing included a Friedman test with Wilcoxon signed-rank post hoc comparisons. Spearman correlations evaluated relationships between morphology variables and Δ for both Wiberg and Ogata. Multivariable linear mixed-effects regression with a random intercept for patient ID was used to evaluate independent predictors of Δ (Ogata - CT), accounting for bilateral hips. Results A total of 754 hips from 611 patients (age 33.5±10.5 years, BMI 25.5±4.6, 73% female) were included. CT-derived LCEA values were higher than both Ogata and Wiberg measurements (31.3° vs 20.8° and 27.7°, respectively; p<0.001), with all pairwise differences significant. Acetabular version demonstrated the strongest association with Δ for both methods (Spearman r≈ -0.32 to -0.36, p<0.001). The difference between Ogata and Wiberg (sourcil-bone edge difference) was moderately correlated with Δ (Ogata - CT) (r=0.46, p<0.001). From the mixed-effects model analysis, acetabular version, sourcil-bone edge difference (Ogata - Wiberg), acetabular coverage, Tönnis angle, PSSC, and age were associated with Δ (Ogata - CT). Accounting for bilateral hips, the model explained 50% of the variance between Ogata and CT LCEA (conditional R2=0.50). Lower acetabular coverage (β= -0.12) and higher Tönnis angle (β=0.09) were associated with larger differences (p<0.05). Sex (β= -0.35, p=0.36) and pelvic rotation (OFW difference; β=0.07, p=0.15) were not significant predictors. Conclusion CT-derived LCEA was higher than standing x-ray measurements. Differences between modalities were partially explained by acetabular version, coverage, age, pelvic tilt, and sourcil-bone edge difference, but these factors accounted for only ~50% of the variability. Differences between Ogata and CT LCEA were greater in hips with lower coverage and higher Tönnis angle, suggesting caution when interpreting radiographic LCEA in unstable hips. Future studies should evaluate LCEA across multiple 3D CT clockface positions and supine AP views to better characterize differences between radiographic and CT-based measurements.