Patients with inflammatory low back pain present for two years at most, and without a confirmed rheumatologic diagnosis, were eligible for this study. Inflammatory back pain was defined according to the Calin criteria . Inflammatory back pain by these criteria is defined if at least four of the five following characteristics are present: insidious onset, onset before the age of 40 years, persistence for at least three months, association with morning stiffness, and improvement with exercise. Patients also could be included if three out of five of these criteria were present plus night pain. Preferably, but this is not obligatory, patients should have at least one feature of SpA according to the European Spondylarthropathy Study Group criteria: presence of a family member with AS, and presence or history of psoriasis, inflammatory bowel disease (IBD) or uveitis.
The study was approved by the institutional review board and all patients gave written informed consent.
Magnetic resonance imaging
A MRI examination of the SI joints was performed using a 1.5 Tesla Philips Gyro scan ACS-NT (Philps, Best, The Netherlands). Patients were scanned in a supine position using a Synergy-spine coil as the surface coil. We chose a coronal oblique scan plane parallel to the length of the sacrum and two slabs: one transversal slab was positioned cranially to the region of interest, to diminish flow artefacts; and one was positioned frontally through the bowel and anterior abdominal wall, to diminish motion artefacts of breathing and bowel movements. The following sequences were used: T1-weighted spin echo (SE), short-tau inversion recovery (STIR), T2-weighted fast SE with fat saturation, and T1-weighted SE with fat suppression after the intravenous administration of contrast medium (gadolinium diethylenetriaminepentate, 0.1 mmol/kg body weight).
Different relevant MRI findings with regard to sacroiliitis were identified from the literature; a differentiation was made between inflammatory changes and structural changes and the different localization of these changes. Pathological changes of interest were defined as inflammation and structural changes including erosions, sclerosis and ankylosis. Regions of interest were the subchondral region, the bone marrow, the joint capsule, the joint space and the retro-auricular ligaments.
Firstly, in different sessions, MRI scans were reviewed and scored together by two observers (LHD and RW) and discrepancies in scoring were extensively discussed. After these training sessions, inter-reader reliability was assessed for a small subset of MRI scans. As the reliability appeared sufficiently high, each MRI was thereafter independently scored by these two observers, who were blind for the patient identity and for clinical, laboratory and radiological data. Findings were graded as 0 (absent), 1 (minimal), 2 (moderate) and 3 (extensive). Inflammation was scored per SI joint in the subchondral region (the region adjacent to the cortical lamella, extending 0.5 cm into the bone marrow cavity), the bone marrow, the joint capsule (the transition of the joint space to para-articular soft tissue), the joint space (defined as the space between the cortical lamellae) and the retro-auricular ligaments. Inflammation was defined as a low signal intensity on T1, with enhancement after gadolinium administration, and/or high signal intensity on STIR and/or T2 fast SE. Inflammation in ligaments was defined as areas of low signal intensity running through high signal intensity tissue on T1, which reflects interosseous ligaments crossing juxta-articular fatty tissue.
Structural changes were scored per SI joint, and included erosions (an irregularly delineated joint space on T1), sclerosis (low signal intensity on T1, STIR and T2 fast SE, without enhancement after gadolinium administration) and ankylosis (the disappearance of the joint space in all sequences).
Inflammation and sclerosis were scored on the iliac and sacral side of both SI joints separately. Erosions and ankylosis were scored for the entire left and right SI joints. Active inflammation was defined as inflammation in at least one of the joint regions (subchondral bone, bone marrow, ligaments, joint capsule, joint space) and the presence of structural damage as erosions, sclerosis and/or ankylosis per SI joint.
Agreement between both MRI readers with respect to inflammation (per site) and chronic changes (sclerosis, erosions and ankylosis) was analysed by cross-tabulation, by concordance and discordance rates, and by kappa statistics (unweighted Cohen's kappa).