Patients

Patients with inflammatory low back pain present for two years at most were eligible for inclusion in the study. IBP was defined according to the Calin criteria, which require four of the five following characteristics to be present: insidious onset; onset before the age of 40 years; persistence for at least three months; association with morning stiffness; and improvement with exercise.⁷ Patients could also be included if three of the five criteria were present plus night pain. Preferably, but not obligatorily, patients should have at least one of the following features of spondyloarthritis according to the ESSG criteria: presence of a family member with ankylosing spondylitis; presence or history of psoriasis, inflammatory bowel disease, or uveitis.

The study was approved by the institutional review board and all patients gave their written informed consent.

MRI

An MRI examination of the sacroiliac joints was undertaken using a 1.5 T Philips Gyro Scan ACS-NT. Patients were scanned in the 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. Two slabs were employed—one transverse, positioned cranially to the region of interest to diminish flow artefacts, and one frontally through the bowel and anterior abdominal wall, to diminish motion artefacts from 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).

Inflammation was scored for each sacroiliac joint in the joint space, subchondral bone, bone marrow, ligaments, and joint capsule. 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 (erosions, sclerosis, ankylosis) were scored in joint space, subchondral bone, and bone marrow. Each sacroiliac joint was labelled as showing inflammation or structural changes if these respective features were present in at least one of the areas investigated. Each set of MRIs was scored independently by two observers, who were blind to the patient identity and to the clinical, laboratory, and radiological data. All joints that showed a discrepancy between the readers for inflammation or structural damage, or both, were offered to a third reader. In all, 21 discrepant joints were scored for the assessment of structural change. The final score attributed to the joint was based on a two out of three majority score. A similar process was followed for discrepancies in inflammation. For this purpose 25 discrepant joints were offered to a third reader.

Conventional radiography

Anteroposterior conventional pelvic radiographs were scored independently by two observers—who were not involved in the MRI reading—without knowledge of clinical information, according to the modified New York criteria (from zero (normal) to 4 (complete ankylosis)).¹ In case of discrepancy between the readers, a third reader (who was not involved in the MRI scoring) scored the sacroiliac joint. In all, 42 discrepant sacroiliac joints were offered to the third reader. A final score for each sacroiliac joint was assigned on the basis of the majority score of the three observers.

Analysis

For conventional radiography, the scores were dichotomised. A sacroiliac joint with a majority score of 0 or 1 was considered normal; a joint with a majority score of 2 or more was considered to have radiographic sacroiliitis. For fulfilment of the modified New York criteria we also substituted CR information by MRI information in five different ways so that patients could fulfil the modified New York criteria as follows:

(1) according to the original method based on radiographs;

(2) based on structural changes present on MRI in both sacroiliac joints;

(3) based on inflammation present on MRI in both sacroiliac joints;

(4) based on inflammation and/or structural changes present on MRI in both sacroiliac joints;

(5) based on structural changes on CR combined with inflammation with or without structural changes on MRI in both sacroiliac joints.

If based on radiographs, patients with bilateral grade 2 sacroiliitis, or at least unilateral grade 3 sacroiliitis, were classified as fulfilling the modified New York criteria. If based on MRI, the mere presence of structural damage for definition 2, the mere presence of inflammation for definition 3, or one of both for definition 4 were considered sufficient, and severity or extent of the lesions was ignored. For definition 5 the grading for CR and the presence of inflammation on MRI were combined.

Agreement between structural changes on MRI and sacroiliitis on CR as well as agreement between inflammation on MRI and sacroiliitis on CR was analysed by cross tabulation. This was done both per sacroiliac joint and per classification according to the modified New York criteria. Specificity, sensitivity, and positive and negative predictive values were calculated with conventional radiography as the gold standard.

Patients

The characteristics of the 68 patients included in the study are presented in table 1⇓. Fifteen patients (22%) did not have any of the additional spondyloarthropathy features. Of these 15 patients, seven were HLA B27 positive. Fifty seven patients fulfilled the ESSG criteria, 48 fulfilled the Amor criteria, and 43 fulfilled both sets of criteria.

Adjudication for structural and inflammatory changes on MRI

After the readings by the two observers, there was agreement on the presence of structural change in 17 joints, and on the absence of structural change in 98 joints. Adjudication of the 21 joints where there was a discrepancy in the readings led to a decision that a further three joints should be classed as positive for structural change (all with erosions). One of these was scored positive in a patient in whom the other sacroiliac joint had also been scored positive in the original read; the other two joints were in a single patient, who had been scored as having structural change in both joints by one of the two readers in the original read.

There was agreement on the presence of inflammation in 32 joints, and on the absence of inflammation in 79 joints after the reading by the two observers. Adjudication of 25 joints with a discrepancy for inflammatory changes led to the assignment of inflammation in four additional joints. These joints were all in patients in which both readers already considered unilateral inflammatory changes to be present. Adjudication did not yield additional patients with inflammation.

Adjudication for changes on CR

There was agreement on the presence of sacroiliitis in 29 joints, and on the absence of sacroiliitis in 65 joints. The adjudication of the 42 discrepant joints led to the assignment of eight further joints to the positive group. In four patients adjudication resulted in fulfilment of the modified New York criteria, as these patients showed already grade 2 abnormalities in the contralateral sacroiliac joint.

Abnormalities on MRI

In all, 20 sacroiliac joints in 12 patients showed structural changes on MRI (table 2⇓). Thirty six joints in 22 patients showed signs of inflammation on MRI (table 3⇓). Twelve of these 22 patients also had structural changes on MRI. None of the patients had structural changes on MRI without inflammation.

Abnormalities on conventional radiography

On conventional radiography 37 sacroiliac joints in 23 patients showed radiographic sacroiliitis (28 grade 2, nine grade 3). Fourteen patients fulfilled the radiographic criterion of the modified New York criteria for ankylosing spondylitis: nine patients because of bilateral grade 2 sacroiliitis; four because of bilateral grade 3 sacroiliitis, and the remaining patient because of grade 2 and grade 3 sacroiliitis combined. The remaining nine patients had unilateral grade 2 sacroiliitis.

MRI findings compared with radiographic findings

A comparison between structural changes on MRI and CR is presented in table 2⇑. In all, 12 patients had structural changes on MRI: eight in both joints and four in one joint. Structural changes on MRI were detected in 20 joints, compared with 37 with radiographic sacroiliitis (54%). In two joints that were considered normal on CR, structural changes were scored on MRI. If radiographic sacroiliitis graded according to the modified New York criteria is considered the gold standard, the sensitivity of detecting chronic changes by MRI per sacroiliac joint is 49% (18/37) and the specificity 98% (97/99). Corresponding positive and negative predictive values were 90% and 84%, respectively.

Only eight of the 14 patients (57%) fulfilling the modified New York criteria for ankylosing spondylitis would fulfil the radiographic criterion if this was based on the presence of structural changes on MRI (table 2⇑ (b)), but nine of these 14 patients had signs of inflammation on MRI in both joints and three in one joint (table 3⇑). The two remaining patients showed only structural changes on MRI in one joint.

In 22 of the 37 joints with radiographic sacroiliitis (59%), inflammation was observed on MRI (table 3⇑ (a)). Of the nine patients with unilateral radiographic sacroiliitis, only one had signs of inflammation on MRI (in both sacroiliac joints). Of the 36 joints with inflammation on MRI (17 left, 19 right), radiographic sacroiliitis was detected in 22 (61%; 11 left, 11 right).

If we consider either inflammation or structural changes on MRI as positive findings and compare this with radiographic abnormalities, there is only a small gain compared with the information provided by inflammation alone (table 4⇓). Abnormalities could be identified in two additional joints with MRI, and this appeared to be concordant with the findings on CR. This resulted in two more patients fulfilling the modified New York criteria. Based on the combined information about either inflammation or structural changes on MRI, 11 of the 14 patients (79%) can be identified who fulfilled the modified New York criteria according to CR, and another five patients showing abnormalities on MRI which were not identified on CR. Based on the MRI findings, 16 patients would fulfil the modified New York criteria. If we combine the information obtained by CR with that obtained by MRI, 19 patients would fulfil the modified New York criteria. In addition there were five patients showing abnormalities on MRI in a single joint in patients not fulfilling the modified New York criteria on CR. CR in combination with inflammation on MRI, or CR in combination with inflammation and structural changes on MRI is equally informative.

Summarising the above information, classification according to the modified New York criteria would be justified for eight patients if it was based on MRI for structural changes only, for 14 patients if it was based on structural changes on CR, for 14 (partly) different patients if it was based on inflammation on MRI only, for 16 patients if it was based on inflammation and structural changes on MRI, for 19 if it was based on inflammation on CR combined with MRI, and for (the same) 19 if it was based on inflammation and structural damage on CR combined with MRI. All patients defined as fulfilling the radiographic modified New York criteria according to the various definitions fulfilled both the ESSG and the Amor criteria except for one patient with bilateral inflammation on MRI, but without structural changes on MRI or CR, who did not fulfil any of the spondyloarthropathy criteria. This patient, who presented with arthritis and was HLA-B27 positive, would fulfil the ESSG criteria and the Amor criteria if inflammation on MRI could be substituted for structural changes on CR with a similar weight. In the six patients with structural changes on CR but not on MRI, features other than radiographic ones ensured that they fulfilled the ESSG and Amor criteria.

Conclusions

Conventional radiography can detect structural changes in the sacroiliac joints with greater sensitivity than MRI. However, inflammation on MRI can be found in a substantial proportion of patients with IBP but with hitherto normal radiographs. Applying only MRI for the assessment of both structural changes and inflammation would underestimate sacroiliitis. Assessment of structural changes on conventional radiography followed by assessment of inflammation on MRI in patients with negative findings yields the highest probability of detecting involvement of the sacroiliac joints in patients with recent onset IBP.