Scoring radiographic progression in ankylosing spondylitis: should we use the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) or the Radiographic Ankylosing Spondylitis Spinal Score (RASSS)?

Radiographic damage is one of the core outcomes in axial spondyloarthritis (axial SpA) (including both non-radiographic axial SpA and ankylosing spondylitis (AS)) as recommended by the Assessment of SpondyloArthritis international Society (ASAS) [1]. Cross-sectionally, it is associated with impairment in spinal mobility [2, 3] and longitudinally with functional disability [4], emphasizing the importance of assessment. ASAS recommends routine radiography of the lateral cervical and lumbar spine for assessing damage over time, but radiographs should not be repeated more frequently than every 2 years, unless indicated in individual cases, who might show faster progression [5, 6].

Different scoring methods have been developed to quantify structural damage in axial spondyloarthritis: the Bath AS Radiology Index (BASRI) [7], the Stoke AS Spine Score (SASSS) [8] and a modification of the SASSS, the mSASSS [9]. In a formal comparison, the mSASSS has shown best reliability and sensitivity to change [10]. Consequently, it is the preferred scoring method for assessing structural damage in the spine for use in clinical trials, as endorsed by ASAS and Outcome Measures in Rheumatology Clinical Trials (OMERACT) [11]. The mSASSS assesses the presence of erosions, sclerosis, squaring, syndesmophytes and bridges at the anterior vertebral corners (VCs) of both the cervical and lumbar spine [9].

More recently, a new scoring method, the Radiographic AS Spinal Score (RASSS), has been proposed that includes the lower thoracic vertebrae, under the hypothesis that most progression is found in these segments [12]. Four thoracic VCs are added and the same features are scored as for the mSASSS, though with slightly modified scoring rules.

The usefulness of the RASSS has not been further evaluated thus far. Hence, it is important to compare both scoring methods, in order to establish the preferred method for the assessment of structural damage as an outcome measure.

Outcome measures should be valid in all their aspects. To standardize the nomenclature of validity, the OMERACT filter has been proposed and this includes three aspects: discrimination, truth and feasibility [13]. The main objective of the present study was to compare the mSASSS and RASSS with regard to performance, taking the aspects of the OMERACT filter into account.

A total of 195 patients had at least one radiograph that could be scored (according to the mSASSS and/or RASSS), 64 had a radiograph that could be scored at year 12 and a total of 520 2-year progression intervals throughout the 12-year follow-up period were available Patients had a mean age of 42.8 (SD 12.4) years, mean disease duration since symptom onset of 20.0 (SD 11.6), mean disease duration since diagnosis of 11 (SD 8.7) years, 71% were males and 84% HLA-B27 positive. Baseline demographic, clinical and radiographic characteristics are summarized in Table 2.

The present study shows that the mSASSS remains the most appropriate method for scoring radiographic progression in patients with AS based on feasibility, discrimination and truth aspects of the OMERACT filter.

With regard to feasibility, the 2-year progression scores were available for the RASSS in only 63% of the cases in which mSASSS progression intervals could be calculated. In the paper describing the RASSS, the authors reported an availability of 88% of the progression scores within the first 2 years [12], while in our study only in 59% of the cases a RASSS progression score could be calculated in this first 2-year interval. Furthermore, in our study in one third of the radiographs in which the RASSS could be calculated, only one to two thoracic VCs were accessible, meaning that calculation of the RASSS was based on imputed and therefore non-informative VCs in the lumbar spine. This shows that an important number of radiographs obtained in the Netherlands, France and Belgium does not include the lower part of the thoracic spine, in contrast to what has been suggested for Germany [12]. According to the OMERACT filter, feasibility captures an essential element in the selection of measures, one that may be decisive in determining a measure's success [13]. The worse feasibility of the RASSS compared with the mSASSS jeopardizes its wide use.

The RASSS demonstrated a higher mean progression, but an increase in the variance of the progression scores was also observed, resulting in similar effect sizes between mSASSS and RASSS. A higher mean progression was expected because the RASSS includes four additional VCs compared with the mSASSS, so that the RASSS is by definition almost always higher than the mSASSS. Exceptions are only the cases with erosions in any segment or squaring in the cervical spine, which are scored for the mSASSS, but not for the RASSS. Nevertheless, and comparing the progression scores with the limited data available in the literature for the RASSS, our RASSS progression scores were higher. In the first 2 years, we found a progression in the RASSS of 2.2 (5.0), whereas Baraliakos et al. reported a 2-year progression of 1.6 (2.8) [12]. Also, our mSASSS progression scores were higher compared with the same study (1.9 (4.1) vs. 0.9 (2.5)) [12]. A possible explanation for the difference in progression scores can be the difference in baseline radiographic damage (baseline mSASSS of 8.1 (14.6) in German cohort vs. 10.2 (14.2) in OASIS cohort). It is well known that presence of radiographic damage is a predictor of further and faster progression of radiographic damage [18‐20]. Other literature on RASSS progression scores is currently lacking. However, our mSASSS progression scores can be compared with other available studies. There are reports of 2-year mSASSS progression scores of around 1 mSASSS unit [21‐23], 2.5 units [18] and 2.6 units (extrapolation to a 2-year period of the annual progression rate of 1.3 (2.5), and assuming linearity) [20]. The differences between scores can be attributed to differences in selection of patients, baseline radiographic damage of patients, conditions in which radiographs were read [24] or the method of imputation of missing VCs. The increase in the variance around the progression scores resulted in similar effect sizes for both methods (0.57 for the mSASSS and 0.55 for the RASSS), showing that the higher mean progression of the RASSS is offset by the increased noise. For discrimination, both scoring methods seemed to be reliable, however, the SDC for the mSASSS was slightly smaller compared to the RASSS (2.9 vs. 3.5), suggesting that the measurement error with the RASSS is somewhat higher. Reliability of the RASSS could possibly improve by having an additional and separate radiograph to score the thoracic VCs. This could reduce the parallax associated with extending the view of the lumbar radiograph to include the thoracic VCs, but would on the other hand imply higher costs and radiation for the patients. With regard to the truth aspect of the OMERACT filter, we found that most progression occurred in the cervical segment of the spine (55%), followed by the lumbar spine (29%) and only 16% was found in the thoracic vertebrae. Furthermore, the progression in the thoracic vertebrae was not significantly different from what was expected, if progression throughout the spine would occur in a balanced way. In addition, we showed that new syndesmophytes exclusively occurring in the thoracic spine occurred in a maximum of 2% of the intervals. These data should be interpreted with caution, because a progression score of 2 does not always correspond to a new syndesmophyte, but can also mean twice a score of 1 in two separate VCs, reflecting development of squaring or sclerosis. This shows that the RASSS does not capture more progression occurring in the thoracic vertebrae, as was hypothesized by Baraliakos et al. [12]. In our study, a 2-year progression in the thoracic vertebrae of 0.4 (1.4) out of a total RASSS progression of 2.2 (5.0)) was found. Baraliakos et al. reported a progression of 0.6 (3.3) out of a total RASSS progression of 1.6 (2.8), indicating a higher contribution from the thoracic VCs to the total RASSS. The reason for this discrepancy between both studies is not entirely clear, but availability of the thoracic VCs for scoring can play a role.

Some limitations of the present study should be addressed. Films were obtained throughout a 12-year follow-up, which means that some of them were old and did not have the optimal quality. Nevertheless, this limited the reading of both scoring methods similarly. Furthermore, the findings of this study may not be generalizable to countries where lumbar radiographs routinely include the low thoracic spine, which is not standard procedure in our three countries.

In conclusion, the calculation of RASSS for status or progression of radiographic abnormalities in the spine is frequently impossible or strongly influenced by non-contributory imputation. The effect size of both methods is similar. In comparison to the mSASSS, the contribution of thoracic VCs in the RASSS is negligible, and does not justify the additional scoring efforts. The mSASSS remains the most appropriate measure to assess radiographic damage in patients with AS.