Objectives Accurate measurement of syndesmophyte development and growth in ankylosing spondylitis (AS) is needed for studies of biomarkers and of treatments to slow spinal fusion. We tested the longitudinal validity and sensitivity to change of quantitative measurement of syndesmophytes using CT.

Methods We performed lumbar spine CT scans on 33 patients with AS at baseline, 1 year and 2 years. Volumes and heights of syndesmophytes were computed in four intervertebral disk spaces. We compared the computed changes to a physician's ratings of change based on CT scan inspection. Sensitivity to change of the computed measures was compared with that of the modified Stoke AS Spinal Score (radiography) and a scoring method based on MRI.

Results At years 1 and 2, respectively 24 (73%) and 26 (79%) patients had syndesmophyte volume increases by CT. At years 1 and 2, the mean (SD) computed volume increases per patient were, respectively 87 (186) and 201 (366) mm³. Computed volume changes were strongly associated with the physician's visual ratings of change (p<0.0002 and p<0.0001 for changes at years 1 and 2, respectively). The sensitivity to change over 1 year was higher for the CT volume measure (1.84) and the CT height measure (1.22) than either the MRI measure (0.50) or radiography (0.29).

Conclusions CT-based syndesmophytes measurements had very good longitudinal validity and better sensitivity to change than radiography or MRI. This method shows promise for longitudinal clinical studies of syndesmophyte development and growth.