Radiographic measurement reliability of lumbar lordosis in ankylosing spondylitis

Intraobserver and interobserver reliabilities of the several different methods to measure lumbar lordosis have been reported. However, it has not been studied sofar in patients with ankylosing spondylitis (AS).

We evaluated the inter and intraobserver reliabilities of six specific measures of global lumbar lordosis in patients with AS. Ninety-one consecutive patients with AS who met the most recently modified New York criteria were enrolled and underwent anteroposterior and lateral radiographs of whole spine. The radiographs were divided into non-ankylosis (no bony bridge in the lumbar spine), incomplete ankylosis (lumbar spines were partially connected by bony bridge) and complete ankylosis groups to evaluate the reliability of the Cobb L1–S1, Cobb L1–L5, centroid, posterior tangent L1–S1, posterior tangent L1–L5, and TRALL methods.

The radiographs were composed of 39 non-ankylosis, 27 incomplete ankylosis and 25 complete ankylosis. Intra- and inter-class correlation coefficients (ICCs) of all six methods were generally high. The ICCs were all ≥0.77 (excellent) for the six radiographic methods in the combined group. However, a comparison of the ICCs, 95 % confidence intervals and mean absolute difference (MAD) between groups with varying degrees of ankylosis showed that the reliability of the lordosis measurements decreased in proportion to the severity of ankylosis. The Cobb L1–S1, Cobb L1–L5 and posterior tangent L1–S1 method demonstrated higher ICCs for both inter and intraobserver comparisons and the other methods showed lower ICCs in all groups. The intraobserver MAD was similar in the Cobb L1–S1 and Cobb L1–L5 (2.7°–4.3°), but the other methods showed higher intraobserver MAD. Interobserver MAD of Cobb L1–L5 only showed low in all group.

These results are the first to provide a reliability analysis of different global lumbar lordosis measurement methods in AS. The findings in this study demonstrated that the Cobb L1–L5 method is reliable for measuring the global lumbar lordosis in AS.