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Study design and population

MM Mary Lucy Marques
NS Nuno Pereira da Silva
DH Desirée van der Heijde
RS Rosalinde Stal
XB Xenofon Baraliakos
JB Juergen Braun
MR Monique Reijnierse
CB Caroline Bastiaenen
SR Sofia Ramiro
FG Floris A van Gaalen
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We used data from the Sensitive Imaging in Ankylosing Spondylitis (SIAS) study, a multicentre 2-year prospective cohort of patients with r-axSpA recruited in two centres (Leiden, the Netherlands and Herne, Germany). This cohort was previously described in detail.15 For the present study, patients could be included if they had both ldCT and MRI at baseline, and an additional ldCT after 2 years of follow-up. Of note, the unit of analysis was the vertebra. Therefore, a maximum of 50 vertebrae per vertebral level (C3–L5), from 50 patients could be included, which was considered appropriate according to a sample size calculation focusing on inter-reader reliability (measurements of two readers, using predefined intraclass correlation coefficient of 0.80 or higher with a 95% CI ±0.1).18

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