Background/Purpose: Detecting
sacroiliitis on plain pelvic X-rays is known to be difficult, resulting in
large variability regarding presence/absence of radiographic sacroiliitis. In
addition, the number of patients with radiographic sacroiliitis in a cohort of
patients with axial SpA can be expected to (slightly) increase or remain stable
over time but certainly not decrease. We have investigated the change of pelvic
X-ray abnormalities over time in the Assessment of SpondyloArthritis
international Society (ASAS) validation cohort.

Methods: In the ASAS
study, 975 patients with either chronic back pain (>3 months) of unknown
origin beginning <45 years of age or undiagnosed peripheral arthritis,
and/or enthesitis, and/or dactylitis were assessed at baseline. From these, 565
patients were followed and reassessed at follow-up [mean follow-up time 4.44
years (SD: 1.01)]. Patients with radiographs of the pelvis (X-SI) available at
baseline and follow-up were included in this analysis (n=357). Readings were
performed locally at both time points, either by the same or by a different
reader. Positive cases were defined as definite radiographic sacroiliitis
(grade ≥2 bilaterally or grade 3–4 unilaterally) according to the
modified New York criteria (mNY).

Results:
In total, 357 patients with
follow-up visit had also baseline radiographs available. The mean age at
baseline was 33.8 years (SD: 10.8), 47.9% were men, 50.2% had active
inflammation of the sacroiliac joints on MRI and 48.7% were HLA-B27 positive.
The proportion of patients fulfilling the radiographic mNY at baseline was
17.4% (62/357), whereas at follow-up 22.4% (80/357) of the patients fulfilled
these criteria (table). However, more than half of the patients (36/62; 58.1%)
with positive baseline X-SI were graded negative at follow-up. Moreover, 54/295
(18.3%) became positive at follow-up. However, given the percentage of the
patients ‘becoming negative’, it is hard to decide what the real rate of
progression is.

Conclusion: Our results
confirm that radiographic sacroiliitis, defined by the mNY, is a poorly
reliable method. Consequently, this method is difficult to use as an outcome
measure to define progression in a cohort of patients, with and without
treatment, although paired reading may improve the result.