The time-averaged inflammatory disease activity estimates the progression of erosions in MRI of the sacroiliac joints in ankylosing spondylitis.

A method to estimate the individual ankylosing spondylitis (AS) patient radiological progression of semi-quantitative magnetic resonance imaging (MRI) changes in the sacroiliac joints has not been described yet, which this study examines. Inflammatory disease activity and MRIs of the sacroiliac joints of 38 patients with recent onset established AS were analyzed at baseline and during follow-up. Sacroiliac MRIs were semi-quantitatively assessed using a modification of the "Spondylarthritis Research Consortium of Canada" (SPARCC) method. In each patient, the annual inflammatory disease activity was estimated by the time-averaged C-reactive protein (CRP; mg/l), calculated as the area under the curve. The mean (SD) CRP decreased from 1.3 (1.8) at baseline to 0.5 (0.6) at follow-up MRI (p < 0.04), which has been performed after a mean (SD) disease course of 2.8 (1.5) years. The mean (SD) annual increase (∆) of SPARCC score from baseline to follow-up MRI was 0.4 (0.4). Baseline individual SPARCC sub-score for bone marrow edema did not statistically significantly correlate with individual ∆SPARCC sub-score for erosions (p = N.S.). The individual AS patient correlation between annual time-averaged inflammatory disease activity and each annual ∆SPARCC sub-scores was only statistically significant for erosions (p < 0.01; r = 0.71). Our results show that bone marrow edema and contrast-medium enhancement at baseline do not relate to the progression of erosions but the calculation of the individual patient annual time-averaged inflammatory disease activity allows to estimate the annual progression of erosions in sacroiliac MRIs of patients with AS.