Magnetic resonance imaging changes of sacroiliac joints in patients with recent-onset inflammatory back pain: inter-reader reliability and prevalence of abnormalities

Ankylosing spondylitis (AS) is a chronic rheumatic condition, characterized by inflammation of the axial skeleton, particularly the sacroiliac (SI) joints. Patients fulfil classification criteria for AS if characteristic radiological changes of the SI joint are present, together with defined clinical symptoms and findings [1]. AS belongs to the group of seronegative spondyloarthritides (SpA). The European Spondylarthropathy Study Group has developed classification criteria for SpA [2].

Sacroiliitis is a characteristic feature of AS and is frequently found in patients with SpA, although it is not obligatory. Patients with sacroiliitis experience chronic low back pain with an inflammatory pattern that often begins in young adulthood. Because chronic low back pain is common in the population, sacroiliitis is often not considered as a cause of back pain. Besides, early sacroiliitis is often not visible on conventional radiographs, or is difficult to interpret, which may lead to a long delay in establishing a diagnosis. Frequently, a mean duration of more than eight years between the start of symptoms and the diagnosis of AS is reported [3, 4]. Such a delay is increasingly unwarranted because of the availability of effective treatment. Magnetic resonance imaging (MRI) is an imaging modality that may shorten the delay between the start of symptoms and a classifying diagnosis of AS or SpA since MRI can detect inflammation early [5–7]. Algorithms for diagnostic purposes have recently been proposed in which MRI of the SI joints was attributed a prominent place [8]. In order to judge whether MRI is helpful in making an early diagnosis, the psychometric properties of assessing inflammation and structural changes by MRI should appropriately be tested in patients with very early disease and not only in those with advanced AS. The aim of this study was to evaluate whether MRI could reliably assess inflammation and structural damage of SI joints in patients with short-term inflammatory back pain.

One of the important aims of this study was to establish whether inflammation and structural changes on MRI could reliably be assessed. In order to allow a detailed judgement, and to trace redundancies, we decided to score inflammation and structural changes per site and per type of lesion. It can be concluded that the agreement between both readers about the presence or absence of pathological findings on MRI was reasonable, especially for inflammation at sites were it was most prevalent. With agreement levels mostly around 85% for the presence of inflammation overall and at different locations, it seems sufficiently high to justify a conclusion of inflammation made by one observer in clinical practice. Expectedly with reference to the population under study, the prevalence of chronic changes on MRI was low. Because of this low prevalence of structural changes, the reliability of scoring these changes is more difficult to assess. This similarly applies to the assessment of inflammation in the joint space, capsule and ligaments. Notwithstanding this limitation, the overall agreement for the different sites of the joint was comparable, with a possible exception for inflammation in the joint capsule. An explanation may be that the delineation of the joint capsule is poorly defined, which may give rise to misinterpretations.

Another important finding in this study was that it is probably sufficient to look for bone marrow oedema and/or subchondral inflammation. The contribution of other sites of the joint to make a diagnosis of inflammation was only marginal. We found only one patient in whom inflammation was restricted to joint capsule and ligaments.

A few studies reported agreement with respect to lesions found on MRI examination of the SI joints, but none of the studies was performed in patients with recent onset inflammatory back pain. Bigot and colleagues proposed 11 criteria referring to both the synovial and the fibrous part of the SI joint that point to sacroiliitis, and showed a good intra-observer and inter-observer reliability (a kappa value of 0.89 for detecting bone marrow oedema) [10]. However, this was a study in 22 SpA patients with established disease, in which grade 2 radiological sacroiliitis according to the New York criteria was present in 80% of the SI joints. Puhakka and colleagues have proposed a scoring system for MRI abnormalities of the SI joints, which distinguished inflammatory activity as well as joint damage [11]. Inter-observer reliability in this study, which included 41 patients with SpA, of whom 20 patients had grade 2 sacroiliitis or more of at least one SI joint on radiography, was importantly lower (a kappa value of 0.47 for bone marrow enhancement, and of 0.67 for joint space enhancement) as compared with our study. Finally, Docherty and colleagues found a kappa value of 0.63 for inter-observer agreement with respect to inflammation on MRI in a study of 20 patients with established or suspected sacroiliitis on radiographs, but contrast administration was not performed [12]. Our results are largely in accordance with the published literature, although comparability is limited due to the differences in study population, with the prevalence of the abnormalities largely influencing kappa values.

In this cohort of inflammatory back pain of less than two years duration, inflammation in the SI joints on MRI could be detected in about one third of the patients (22/68). Moreover, one sixth of patients already showed signs of structural changes on the MRI scan (11/68 patients). Although the number of patients with structural changes is low, this finding indicates that MRI might be a useful tool in the assessment of patients with early inflammatory back pain. It is the amount of variation in the outcome of interest (inflammation and/or structural changes) rather than the number of patients under study that is important in judging whether the sample size is sufficient to test reliability. As long as all kinds of abnormalities are covered, it is possible to test reliability even in situations like this, with only 11 patients showing abnormalities. Undoubtedly, however, the likelihood of covering all kinds of abnormalities will increase by increasing patient number. The real value of MRI will therefore be ascertained in future, by following the patients longitudinally and obtaining more data, which will occur in this cohort. The development and choice of an appropriate scoring system for sacroiliitis on MRI to be used in clinical studies and trials will be subject of interest in an ongoing ASAS-OMERACT working group [13].

MRI can reliably detect inflammation and structural changes in SI joints in patients with early inflammatory back pain. Assessing bone marrow and/or subchondral bone enhancement suffices to detect inflammation. Inflammation in the joint space, the joint capsule and the ligaments hardly contributes to this detection, because it is associated with inflammation in the bone marrow and/or subchondral bone. About one-third of patients with recent-onset inflammatory back pain show inflammation, and about one-sixth of patients show structural changes in at least one SI joint, indicating that MRI might be a useful tool to diagnose sacroiliitis in patients with inflammatory back pain.