In this study, we investigated the prevalence of variant forms of the SIJ using sCT images in patients with clinical suspicion of sacroiliitis. We found these variants to be very common in our study group: 82.8% of the investigated joints demonstrated at least one variant. This prevalence is much higher than in previous studies performed with CT and/or MRI, which reported overall incidences of 25.7–57% [1,2,3, 6, 7, 9]. The most frequent variants were iliosacral complex, bipartite iliac bony plate and crescent iliac bony plate (27.7, 27.2% and 27%, respectively). This is in line with the results of Tok [6] and Cihan [7]. Other authors found other variants to be the most frequent: accessory joint [2, 4, 5], dysmorphic cartilaginous joint [1, 3], or bipartite iliac bony plate [9].

These differences in prevalence can at least in part be explained by differences in study groups. In contrast to previous studies, we did not exclude patients with imaging findings of sacroiliitis [1,2,3,4,5,6,7]. Moreover, our patients were clinically suspected of sacroiliitis. This is in line with the findings of Ziegeler et al. [8], who also reported higher frequencies of atypical joint forms in symptomatic patients than in controls. They found SIJ variants in 80.3% of patients with mechanical SIJ disease, and 44.1% in patients with axSpA [8].

The high incidence of variant SIJ forms could further be explained by differences in imaging technique. The sCT images evaluated in this study consist of a data set with a slice thickness of 0.8 mm, of which reconstruction in any plane is possible. This allows for detection of very small or subtle findings, including variant forms of the SIJ. In contrast, previous CT- and MRI-based studies used different imaging protocols, often with thicker slices (up to 10 mm) and/or fixed slice orientation (e.g., only strictly axial images) [1,2,3,4,5,6,7,8]. Despite our imaging protocol with very thin slices, we found very low prevalences of isolated synostosis and unfused ossification center, similar to previous studies [1,2,3,4,5,6,7,8,9], indicating these are truly rare variants. Finally—in contrast to most other reports [3,4,5,6,7]—we allowed for more than 1 variant to be present in each SIJ, which also contributes to higher frequencies.

SIJ variants were often visible on the conventional MRI sequences as well. The T1-weighted sequence is best suited to discern these variants, as this is the most ‘anatomic’ sequence, allowing the best differentiation between cortical bone and surrounding tissues. T1-weighted spin echo without fat suppression is also the best suited conventional sequence to detect structural lesions of sacroiliitis including erosions and sclerosis [23]. However, in our experience, fewer SIJ variants were detected on the conventional MRI sequences. This is probably at least partially due to the slice thickness of 3 mm for the T1-weighted sequence, versus 0.8 mm for the sCT images. Another factor is the fixed semi-coronal plane of the available T1-weighted images, because in our experience, some variants can readily be seen in the semi-coronal plane of the sacrum (accessory SIJ, iliosacral complex, semicircular defect, isolated synostosis), while other variants are not or very difficult to depict in the (semi-)coronal plane as opposed to the (semi-)axial plane (bipartite iliac bony plate, crescent ilium, unfused ossification center, dysmorphic sacrum).

Some variants were even more frequently found than joints without SIJ variants. The very high prevalence raises the question if these variants—which are often regarded as abnormal—are actually subtypes of the ‘normal’ SIJ. We also found a new variant, never reported before, consisting of a supplementary extension of the SIJ on the posterior-superior side. It is possible that the classification used in the present study is still incomplete. The clinical significance of SIJ variants remains debated. Some authors demonstrated statistically significant associations between SIJ variants and degenerative changes [6, 9], and between SIJ variants and BME [1, 3, 10], whereas others could not find such associations [7]. Variations in SIJ form presumably can alter or aggravate biomechanical stress [1, 4, 6, 8, 9]. It is not clear if SIJ variations can cause symptoms in this way: further research is needed on this matter.

Almost every possible combination of variants was present in our population, but concomitant presence of iliosacral complex and semicircular defect was not found. It is possible that these two variants represent two ends of a spectrum: both essentially exhibit a focal groove in the sacrum, with an accompanying ilial prominence in iliosacral complex, but not in semicircular defect of the sacrum.

Overall, variants were slightly more often found in women than in men (85.8% and 77.8%, respectively), except for iliosacral complex, isolated synostosis and unfused ossification center. This trend is consistent with previous studies, although the difference is much smaller in our population [1,2,3, 6, 8, 9]. The finding that variants were common both in men and women does not support the hypothesis of pregnancy and child birth as an etiological factor in these variants [5].

Isolated synostosis was significantly more frequent with advancing age. On the other hand, semicircular defect and unfused ossification center were more frequent in younger patients—however, these variants were not very frequent in our study group; therefore, results could be due to overfitting. Other authors also found a higher prevalence of several variants in older patients [2, 5, 8]. These findings support the hypothesis that variants are not congenital but rather develop during life [2, 5]. The inversed relationship for unfused ossification center could indicate that these centers can still fuse to the SIJ later in life.

We investigated the relationship between SIJ variants and body weight and found that iliosacral complex was more frequent in patients with higher BMI, but a crescent joint form was more frequent in patients with lower BMI. Demir et al. found slightly different results compared to our study; they found a higher incidence of iliosacral complex in obese patients as well, but they also demonstrated this relationship for accessory SIJ and semicircular defect [5]. This could indicate a role for body weight in the existence and potentially development of the SIJ variants (possibly by altering biomechanical stress), further supporting the hypothesis of an acquired nature of different SIJ variants.

Our study has some limitations. We included patients in a tertiary university hospital, all clinically suspected for sacroiliitis, which can induce selection bias and limit generalizability of our results. Furthermore, we did not use strict definitions or provide measured requirements of the SIJ variants, e.g., depth of sacral defect or bipartite iliac bony plate. Also, it can be challenging to differentiate certain variants from pathologic alterations, for example to differentiate isolated synostosis (consisting of focal bony bridging) from acquired focal bony bridging due to sacroiliitis. This can lead to over- or underdiagnosis of SIJ variants. However, we believe we cannot set a threshold for these variants yet, as the clinical relevance (and thus the potential required size of any variant to induce certain effects) still remains debated. Another limitation is the varying inter-reader agreement. Although it was good to excellent for most SIJ variants, it was only moderate for the variant unfused ossification center. The use of more specified definitions could also lead to better inter-reader agreement. Finally, our study group consisted of a relatively limited amount of patients.