Correlations between the alpha angle and femoral head asphericity: Implications and recommendations for the diagnosis of cam femoroacetabular impingement

Abstract

Objective To determine the strength of common radiographic and radial CT views for measuring true femoral head asphericity. Patients and Methods In 15 patients with cam femoroacetabular impingement (FAI) and 15 controls, alpha angles were measured by two observers using radial CT (0°, 30°, 60°, 90°) and digitally reconstructed radiographs (DRRs) for the: anterior-posterior (AP), standing frog-leg lateral, 45° Dunn with neutral rotation, 45° Dunn with 40°external rotation, and cross-table lateral views. A DRR validation study was performed. Alpha angles were compared between groups. Maximum deviation from a sphere of each subject was obtained from a previous study. Alpha angles from each view were correlated with maximum deviation. Results There were no significant differences between alpha angles measured on radiographs and the corresponding DRRs (p = 0.72). Alpha angles were significantly greater in patients for all views (p ≤0.002). Alpha angles from the 45° Dunn with 40° external rotation, cross-table lateral, and 60° radial views had the strongest correlations with maximum deviation (r = 0.831; r = 0.823; r=0.808, respectively). The AP view had the weakest correlation (r = 0.358). Conclusion DRRs were a validated means to simulate hip radiographs. The 45° Dunn with 40° external rotation, cross-table lateral, and 60° radial views best visualized femoral asphericity. Although commonly used, the AP view did not visualize cam deformities well. Overall, the magnitude of the alpha angle may not be indicative of the size of the deformity. Thus, 3D reconstructions and measurements of asphericity could improve the diagnosis of cam FAI.

Automated 3D quantitative assessment and measurement of alpha angles from the femoral head-neck junction using MR imaging

“…The intra-class correlation coef cient (ICC), with 95% con dence intervals (CI), was used to evaluate the intra-and inter-rater reliability of manual alpha angle measures. The agreement between manual and automatic alpha angle measurements was assessed using linear regression and Pearson's correlation coefcients, with categorization of signi cant values of r = 0.60-0.79 as strong and 0.80-1 as very strong correlations (Harris et al 2014). Bland-Altman plots (Bland and Altman 1986) were used to visualize the agreement in alpha angle measures between the manual and automatic approaches.…”

The influence of radiographic viewing perspective and demographics on the critical shoulder angle

“…The DRR has recently been validated as a surrogate for clinical radiographs that can be generated from controlled perspectives with respect to the CT image stack. 15 Furthermore, little is known about how the CSA varies in populations with otherwise healthy shoulders as a function of demographic factors like glenoid size, gender, or side.…”

Quantitative comparison of cortical bone thickness using correspondence-based shape modeling in patients with cam femoroacetabular impingement

“…24–26 Correspondence-based methods are ideal because they do not determine, a-priori, the ideal shape that the structure should conform to. We previously used SSM to demonstrate that cam femora are significantly different in shape compared to controls, and established principal component analysis (PCA) modes that captured the variance in shape.…”