The Reliabilities of Radiographic Measurements of Cervical Sagittal Alignment in Ankylosing Spondylitis

Study Design: This is a prospective study design. Objective: To test the interobserver and intraobserver reliabilities of 5 specific measures of global cervical sagittal alignment in patients with ankylosing spondylitis (AS), and to suggest a better means of measuring cervical sagittal alignment. Summary of Background Data: The intraobserver and interobserver reliabilities of several different methods of measuring cervical lordosis have been reported. However, they have not been studied till yet in patients with AS. Methods: Interobserver and intraobserver reliabilities of 5 specific measures of cervical lordosis were evaluated in patients with AS. Eighty patients with AS were allocated to a nonankylosis or an ankylosis group, and the reliabilities of the Cobb C1-C7, Cobb C2-C7, centroid, posterior tangent, and Ishihara index methods were evaluated. Results: The intraclass and interclass correlation coefficients (ICCs) of all 5 methods were generally high. For the 80 study subjects, ICCs were >= 0.84 (excellent) for all 5 radiographic methods. However, comparison of the ICCs, 95% confidence intervals, and mean absolute differences (MAD) between groups with varying degrees of ankylosis showed that the reliability of lordosis measurements decreased as the severity of ankylosis increased. The 5 methods consistently demonstrated higher ICCs for both interobserver and intraobserver comparisons in the nonankylosis group. However, in the ankylosis group, the Cobb C1-C7 method demonstrated high ICCs for both interobserver and intraobserver comparisons, whereas the other 4 methods had high ICCs only for intraobserver comparisons. The intraobserver MADs were similar for the 5 methods (2.4-3.9), but the interobserver MADs of measurement methods in both groups showed low measurement reliability except for the Cobb C1-C7 methods. Conclusions: This study provides a reliability analysis of different cervical lordosis measurement methods in AS, and shows that the Cobb C1-C7 method provides a reliable means for measuring cervical lordosis in AS.