Femoroacetabular Impingement Measurements Obtained From Two-Dimensional Radiographs Versus Three-Dimensional–Reconstructed Computed Tomography Images Result in Different Values

Purpose: To compare the reliability and accuracy of radiographic measurements obtained from 2-dimensional (2D) radiographs and 3-dimensional (3D)-reconstructed computed tomography (CT) images in the assessment of femoroacetabular impingement syndrome (FAIS). Methods: Consecutive patients with FAIS from January 2018 to December 2020 were identified and included in this study. Two fellowship-trained surgeons and 2 fellows performed blinded radiographic measurements. Lateral center-edge angle (LCEA) and Tönnis angles were measured on anteroposterior pelvic radiographs, and alpha angles were measured on frog lateral radiographs. Reliability coefficients for individual measurement accuracy were performed using the Cronbach alpha and intra- and inter-rater intraclass correlation coefficients (ICCs). Composite measurements for LCEA, Tönnis angle, and alpha angle were compared with the corresponding 3D value using paired sample t-tests. Results: Fifty-three patients with FAIS with standardized 2D radiographic and 3D-reconstructed CT imaging were included. All reliability metrics met thresholds for internal reliability. Inter-rater ICCs for LCEA, Tönnis angle, and alpha angle were (0.928, 0.888, 0.857, all P < .001). When we compared 2D radiographic measurements with 3D-reconstructed CT values, there was a significant difference in the LCEA for 2 authors: surgeon 1 (mean [M] = –9.14, standard deviation [SD] = 5.7); t(52) = –11.6, P < .001, and surgeon 2 (M = –5.9°, SD = 4.7); t(52) = –9.2, P < .001. Significant differences were seen for Tönnis angle for 2 authors: fellow 2 (M = 3.9°, SD = 5.6); t(52) = 5.1, P < .001, and surgeon 2 (M = –2.6°, SD = 4.1); t(52) = –4.6, P < .001. Alpha angle measurements compared to the 3D-reconstructed alpha angle at 2 o’clock was significantly different for 3 authors: fellow 1 (M = 11.9°, SD = 16.2); t(52) = 5.3, P < .001; fellow 2 (M = 10.4°, SD = 18.6); t(52) = 4.1, P = .002; and surgeon 2 (M = –6.5°, SD = 16.2); t(52) = –2.9, P = .005. Positive mean values indicate 2D radiographic measurements overestimated 3D reconstruction values and negative mean values indicate underestimation. Conclusions: The use of 2D radiographs alone for preoperative planning of FAIS may lead to inaccuracies in radiographic measurements. Level of Evidence: Level, III retrospective cohort study.