Background After 20 years of follow up the majority of patients with axial SpA have developed new bone in the spine although disease may remain isolated in the sacroiliac joints (SIJ) without any new bone in the spine. Assessment of these patients may provide new insights into the factors that influence disease progression. A new hypothesis has proposed that MRI can identify a “progressive phenotype” in early SpA characterized by the appearance of fat metaplasia in the SIJ. Fat metaplasia can occur both in subchondral bone and at the site of erosion after resolution of inflammation, this being termed backfill.
Objectives To determine which clinical and MRI lesions in the SIJ are associated with absence of spinal damage after extensive follow up of patients with AS.
Methods In the FORCAST prospective cohort, AS patients (n=431) attending community and academic practices were assessed for clinical and laboratory outcomes every 6 months, radiography every 2 years, and MRI annually. MR scans of the SIJ were assessed by SPARCC SIJ and SSS methods for inflammation and structural features, respectively, by 2 readers and an adjudicator. Absence of spinal damage was pre-specified as no syndesmophytes or ankylosis on cervical and lumbar spine radiographs after ≥10 years from onset of symptoms and for the entire duration of prospective follow up. Patients with and without spinal damage were matched for age and symptom duration and compared by t-test, Mann-Whitney, and Fisher's exact tests. We used univariate and multivariate conditional logistic regression to determine which demographic, clinical, and imaging variables were associated with absence of spinal damage.
Results Mean (SD) duration of symptoms and prospective follow up was 18.0 (7.0) and 2.3 (0.49) years in cases with no damage (n=42) and 17.5 (8.2) and 2.6 (1.4) years in those with damage (n=81). The group with no damage had fewer males (p=0.004), lower CRP (p=0.02), and lower MRI SIJ scores for fat (p=0.03) and ankylosis (p=0.003) while score for SIJ erosion was significantly higher (p=0.02). Definite SIJ ankylosis (SSS score ≥2 by both readers) was evident in 20.7% of cases in the no damage group versus 53.3% of those with damage (p=0.007). Univariate regression indicated that no damage was associated with female sex (p=0.002), lower ASDAS (p=0.048), and lower SSS values for fat (p=0.032), backfill (p=0.014), and ankylosis (p=0.001). Smoking and B27 were not significant. A multivariate logistic regression model that included gender, ASDAS, and MRI SIJ features indicated that lower values for SSS backfill and ankylosis were independently associated with no damage (OR [95%CI]=0.83 [0.69–0.99] and 0.89 [0.82–0.96], respectively). When all definite MRI features (SSS score ≥2 by both readers) were included in the model, definite SIJ ankylosis was significantly less likely in those who did not develop spinal damage (OR [95%CI]=0.24 [0.08–0.70].
Conclusions The absence of radiographic damage in the spine after prolonged duration of symptoms is associated with the lack of fat metaplasia and ankylosis in the SIJ on MRI. This supports the hypothesis that fat metaplasia identifies a progressive phenotype and that its appearance in the SIJ, together with SIJ ankylosis, may predispose to spinal progression.
Disclosure of Interest None declared