Reliability and reproducibility of interapical distance assessment of the lateral deviation of vertebrae in scoliosis

[Purpose] The purpose of this study was to investigate the interobserver reliability and intraobserver reproducibility of interapical distance (IAD) and to analyze its correlation with the Cobb angle (CA). [Subjects and Methods] IAD, a handy tool for assessment of the lateral deviation of vertebrae with a metric scale, was defined as the horizontal distance between one apical vertebra and its counterpart, the opposite apical vertebra in the case of a double curve and the farthest vertebra in the case of a single curve. Fifty full-length, standing anteroposterior radiographs of "idiopathic scoliosis" were reviewed. Three investigators independently measure the CA and IAD at the same time and remeasured the IAD on the same radiograph a week later. [Results] There was no interobserver difference (reliability) in the measurement of IAD or statistical differences in intraobserver reproducibility for each observer. IAD was well correlated with the CA for each observer (r=0.765, r=0.737, and r=0.764). [Conclusion] IAD is useful when assessing lateral deviation in scoliosis and may be a reliable and reproducible index that is well correlated with the CA, and it can be used as a supplementary measure to describe the overall derangement of scoliosis in the coronal plane.