EP4.12 MRI Versus CT-Based Three-Dimensional Reconstruction for Quantifying Cam Morphology with Alpha and Omega Angles

Abstract

Abstract Background Cam morphology of the proximal femur is three-dimensional (3D), yet it is commonly summarized using single-plane alpha-angle measurements. Omega angle is an angle that quantifies the circumferential arc of cam deformity around the femoral head–neck junction. 3D Magnetic Resonance Imaging (MRI) may allow radiation-sparing characterization of cam morphology, but comparison with Computed Tomography (CT) remains important before MRI can be used more confidently as a standalone osseous imaging modality. Purpose To compare alpha-angle and omega-angle measurements derived from 3D MRI and CT reconstructions of the same hip in patients undergoing preoperative evaluation for femoroacetabular impingement. Study Design Retrospective comparative imaging study with interobserver reliability analysis. Methods Preoperative CT and MRI studies obtained within 0 to 4 months were paired by patient and side. MRI was performed on a 3 Tesla system using multiplanar turbo spin echo sequences and 3D Volumetric Interpolated Breath-hold Examination (VIBE) acquisition; CT was performed with 1.0-mm slices and high-resolution bone reconstruction. 3D reconstruction was performed using AGFA IMPAX. Radial alpha angles were recorded at 7 clockface positions from 9:00 through 3:00. The omega angle was derived from the contiguous arc of positions with alpha angles greater than 55 degrees. Mean alpha angle across clockface positions and mean omega angle were compared between modalities using paired t tests. Pearson correlation, Bland-Altman analyses, and interobserver reliability testing using intraclass correlation coefficients were also performed. Results Nineteen matched hips were analyzed. Mean alpha angle was 47.3 +/- 7.4 degrees on CT and 47.8 +/- 8.2 degrees on MRI (mean paired difference, CT - MRI, -0.5 degrees; p = 0.38; r = 0.95). Mean omega angle was 39.5 +/- 42.5 degrees on CT and 41.1 +/- 42.7 degrees on MRI (mean paired difference, CT - MRI, -1.6 degrees; p = 0.67; r = 0.93). Bland-Altman analysis showed alpha-angle limits of agreement from -5.6 to 4.5 degrees and omega-angle limits of agreement from -32.4 to 29.3 degrees. Radial alpha-angle profiles were similar between modalities, with peak values in the anterosuperior region. Interobserver reliability was excellent for the 12:00 alpha angle (ICC = 0.987; 95% CI 0.975-0.993) and omega angle (ICC = 0.972; 95% CI 0.947-0.985). Conclusion In this paired preoperative pilot dataset, MRI-based three-dimensional reconstructions produced alpha-angle and omega-angle measurements similar to CT, with strong correlations, no significant differences in paired means, and excellent interobserver reliability for the assessed measurements. Alpha angle demonstrated closer agreement between MRI and CT, whereas omega angle showed wider limits of agreement, reflecting its reliance on a 55° alpha-angle threshold to define deformity extent. These findings support MRI as a radiation-sparing option for three-dimensional cam quantification, although acceptable intermodality differences for extent-based metrics such as omega angle require further definition alongside correlation with intraoperative findings.