Anatomical variation of the sacroiliac joint carries an increased risk for erosion and bone marrow oedema in axial spondyloarthritis

Anatomical variation of the sacroiliac joints - what the rheumatologist should know

PURPOSE OF REVIEW

Anatomical variations of the sacroiliac joints (SIJ) pose challenges in the diagnosis of axial spondyloarthritis (axSpA). Increased reliance on magnetic resonance imaging (MRI) for early detection has led to concerns about specificity, as anatomical variants can mimic inflammatory changes. This review highlights common SIJ variations and their implications for rheumatologists interpreting imaging findings.

RECENT FINDINGS

Recent studies emphasize the high prevalence of SIJ anatomical variations, particularly in females, and their potential to influence imaging interpretation. Variations such as crescent-shaped ileum, intraarticular dysmorphisms, and accessory joint facets can lead to bone marrow edema and sclerosis, mimicking sacroiliitis. Additionally, lumbosacral transitional vertebrae alter SIJ biomechanics, potentially exacerbating symptoms in axSpA patients. Advances in MRI and computed tomography (CT) imaging protocols provide improved differentiation between anatomical variants and true inflammatory changes.

SUMMARY

Recognizing SIJ anatomical variations is crucial for avoiding misinterpretation of imaging findings and overdiagnosis of axSpA. MRI protocols incorporating additional imaging planes and CT correlation can enhance diagnostic accuracy. Awareness of these variations can refine patient management strategies, ensuring appropriate treatment for inflammatory and biomechanical SIJ pathologies.

Impact of combining magnetic resonance images and radiographs on radiographic grading of sacroiliitis and classification in axial spondyloarthritis: a reliability study

To investigate the reliability of sacroiliac radiographic grading based on the modified New York criteria, both through grading pelvic radiographs alone and in combination with sacroiliac magnetic resonance imaging and the influence of alone and multimodal interpretation on the categorization of radiographic and non-radiographic axial spondyloarthritis (r-axSpA and nr-axSpA). Radiographs of the SI joints from patients with axSpA were graded by two rheumatologists independently in two sessions (radiographs only and MRI + radiographs) and followed by consensus sessions with the participation of a radiologist. Overall, anteroposterior pelvic radiographs and T1 weighted MR scans of the sacroiliac joints from 138 patients with axSpA were eligible. Intra- and inter-reader reliability was fair to moderate for both techniques regardless of whether they were examined alone or in conjunction with MRI, however confidence levels of readers were relatively higher in the MRI + radiographs combined technique compared to radiographs only. Regarding the classification, both readers' intra and inter reader agreements were moderate to substantial. The kappa values and agreement were notably higher for the combined reading technique. In the consensus sessions, 10.2% of patients were recategorized. Merging the information from MRI scans with pelvic radiographs yielded no notable enhancement in the accuracy of radiographic grading of SI joint. Categorizing axSpA as either radiographic or non-radiographic based on pelvic radiography falls short in reliability even after combining with MR images and may likely call for a more reliable definition, axSpA with or without structural changes, relying on more advanced imaging techniques than radiographs.

Current and future role of CT and advanced CT applications in inflammatory arthritis in the clinic and trials

Computed tomography (CT) has traditionally been underutilized in the imaging of inflammatory arthritis due to its limitations in assessing soft tissue inflammation and concerns over radiation exposure. However, recent technological advancements have positioned CT as a more viable imaging modality for arthritis, offering high specificity and sensitivity in detecting structural bone changes. However, advances in ultra-low-dose CT protocols and AI-driven image reconstruction have significantly reduced radiation exposure while maintaining diagnostic quality. Dynamic CT and spectral CT techniques, including dual-energy CT (DECT), have broadened CT's application in assessing dynamic joint instabilities and visualizing inflammatory changes through material-specific imaging. Techniques such as CT subtraction imaging and iodine mapping have enhanced the detection of active soft-tissue inflammation, virtual non-calcium reconstructions, and the detection of bone marrow edema. Possible CT applications span various forms of arthritis, including gout, calcium pyrophosphate deposition disease (CPPD), psoriatic arthritis, and axial spondyloarthritis. Beyond its diagnostic capabilities, CT's ability to provide detailed structural assessment positions is a valuable tool for monitoring disease progression and therapeutic response, particularly in clinical trials. While MRI remains superior for soft tissue evaluation, CT's specificity for bone-related changes and its potential for integration into routine arthritis management warrant further exploration and research. This review explores the current and emerging roles of CT in arthritis diagnostics, with a focus on novel applications and future potential.

Influence of sacroiliac joint variation on clinical features of axial spondyloarthritis: a comparative analysis

OBJECTIVES

Anatomical variation of the sacroiliac (SI) joints is common and specific variants are associated with erosions and bone marrow oedema on imaging. Our investigation aims to evaluate whether anatomical variations influence the clinical presentation of axial spondyloarthritis (axSpA).

METHODS

In this propensity score matched post hoc analysis documented clinical data from four prospective clinical cohorts was assessed. Classification of back pain as inflammatory (=IBP), human leucocyte antigen-B27 positivity, family history, disease activity according to Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), symptom duration, elevated acute phase reactants, peripheral and extramusculoskeletal manifestations were evaluated. Statistical analyses were done using (generalised) linear models, t-tests, χ2 tests and analysis of variances. Multiple testing was corrected according to Bonferroni.

RESULTS

A total of 165 patients (86 women) were included. Atypical SI joints, defined by the presence of accessory joint facets, iliosacral complex or crescent-shaped ilii on MRI, were identified in 61 out of 165 patients with axSpA. Disease activity, assessed by BASDAI and symptom duration were similar in both groups (adjusted ß=-0.118 (95% CI -0.713, 0.476), p=0.696 and 120.0 (107.4) vs 116.5 (98.3) months, p=0.838, respectively). There was no significant difference in IBP between the groups (adjusted OR=0.614 (95% CI 0.274, 1.377), p=0.236). Sex-stratified analysis revealed no statistically significant results.

CONCLUSION

Our analysis suggests that clinical phenotypes do not significantly differ between patients with axSpA with and without atypical joints.

Development of international consensus on a standardised image acquisition protocol for diagnostic evaluation of the sacroiliac joints by MRI: an ASAS-SPARTAN collaboration

BACKGROUND

A range of sacroiliac joint (SIJ) MRI protocols are used in clinical practice but not all were specifically designed for diagnostic ascertainment. This can be confusing and no standard diagnostic SIJ MRI protocol is currently accepted worldwide.

OBJECTIVE

To develop a standardised MRI image acquisition protocol (IAP) for diagnostic ascertainment of sacroiliitis.

METHODS

13 radiologist members of Assessment of SpondyloArthritis International Society (ASAS) and the SpondyloArthritis Research and Treatment Network (SPARTAN) plus two rheumatologists participated in a consensus exercise. A draft IAP was circulated with background information and online examples. Feedback on all issues was tabulated and recirculated. The remaining points of contention were resolved and the revised IAP was presented to the entire ASAS membership.

RESULTS

A minimum four-sequence IAP is recommended for diagnostic ascertainment of sacroiliitis and its differential diagnoses meeting the following requirements. Three semicoronal sequences, parallel to the dorsal cortex of the S2 vertebral body, should include sequences sensitive for detection of (1) changes in fat signal and structural damage with T1-weighting; (2) active inflammation, being T2-weighted with fat suppression; (3) bone erosion optimally depicting the bone-cartilage interface of the articular surface and (4) a semiaxial sequence sensitive for detection of inflammation. The IAP was approved at the 2022 ASAS annual meeting with 91% of the membership in favour.

CONCLUSION

A standardised IAP for SIJ MRI for diagnostic ascertainment of sacroiliitis is recommended and should be composed of at least four sequences that include imaging in two planes and optimally visualise inflammation, structural damage and the bone-cartilage interface.

Reporting Sacroiliac Joint Imaging Performed for Known or Suspected Axial Spondyloarthritis: Assessment of SpondyloArthritis International Society Recommendations

Whereas previous projects attempted to standardize imaging in patients with axial spondyloarthritis (axSpA), few studies have been published about the need for specific details regarding the image acquisition and lesions that may be less familiar to general radiologists. This work reports consensus recommendations developed by the Assessment of SpondyloArthritis International Society (ASAS) that aim to standardize the imaging reports in patients suspected of having or with known axSpA. A task force consisting of radiologists and rheumatologists from ASAS and one patient representative formulated two surveys that were completed by ASAS members. The results of these surveys led to the development of 10 recommendations that were endorsed by 73% (43 of 59) of ASAS members. The recommendations are targeted to the radiologist and include best practices for the inclusion of clinical information, technical details, image quality, and imaging findings in radiology reports. These recommendations also emphasize that imaging findings that indicate differential diagnoses and referral suggestions should be included in the concluding section of the radiology report. With these recommendations, ASAS aims to improve the diagnostic process and care for patients suspected of having or with known axSpA.

Year in Review in Axial Spondyloarthritis Clinical Research and Guidelines: SPARTAN 2023 Annual Meeting Proceedings

PURPOSE OF REVIEW

To highlight high impact clinical publications in spondyloarthritis from May 2022 to April 2023 that were summarized and presented at the SPARTAN annual meeting in May 2023.

RECENT FINDINGS

Publications included updated guidelines on management of axial spondyloarthritis (axSpA) by ASAS-EULAR and development of a modified Juvenile Spondyloarthritis Disease Activity Index (JSpADA). Definitions were published for MRI lesions of the spine in axSpA and active and structural sacroiliac (SI) joint lesions in juvenile spondyloarthritis (JSpA). Anatomic variants of the sacroiliac joint were commonly detected using synthetic CT derived from pelvis MRI images. Anti-CD74 antibodies hold promise as a diagnostic biomarker for axSpA; however, the mechanism of such antibody development seems unrelated to gastrointestinal inflammation. High impact clinical publications in spondyloarthritis addressed lab-based and imaging biomarkers, outcome measures, and updated management guidelines.

The sacroiliac joint across ages - what is normal?

The anatomy of the sacroiliac joint (SIJ) is complex with wide variations inter-individually as well as intra-individually (right versus left) and a frequent occurrence of anatomical variants. Besides, the joints are subject to strain, which may elicit non-inflammatory subchondral changes such as bone marrow edema (BME), sclerosis, and fat deposition simulating inflammatory SIJ changes. Furthermore, normal physiological changes during skeletal maturation can make interpretation of SIJ magnetic resonance imaging in children challenging. Knowledge about the wide range of normal findings is therefore important to avoid misinterpretation of findings as pathological. This review describes the current knowledge about normal SIJ findings across all ages.

Association of anatomical variants of the sacroiliac joint with bone marrow edema in patients with axial spondyloarthritis

OBJECTIVE

To determine the prevalence of sacroiliac joint variants in patients with axial spondyloarthritis (axSpA) using MRI-based synthetic CT images and to evaluate their relationships with the presence of bone marrow edema, as this may potentially complicate diagnosing active sacroiliitis on MRI in patients with suspected axSpA.

METHODS

172 patients were retrospectively included. All patients underwent MRI because of clinical suspicion of sacroiliitis. The diagnosis of axSpA was made by a tertiary hospital rheumatologist. Two readers independently determined the presence of bone marrow edema and the presence of one or more of the nine known sacroiliac joint (SIJ) variants.

RESULTS

SIJ variants were common in axSpA patients (82.9%) and the non-SpA group (85.4%); there were no significant differences in prevalence. Bone marrow edema was frequently found in axSpA (86.8%) and non-SpA patients (34%). AxSpA patients with SIJ variants (except for accessory joint) demonstrated 4 to 10 times higher odds for bone marrow edema, however not statistically significant. The more variants were present in this group, the higher the chance of bone marrow edema. However, some multicollinearity cannot be excluded, since bone marrow edema is very frequent in the axSpA group by definition.

CONCLUSION

SIJ variants are common in axSpA and non-SpA patients. SIJ variants were associated with higher prevalence of bone marrow edema in axSpA patients, potentially due to altered biomechanics, except for accessory joint which may act as a stabilizer.

Sex-specific diagnostic efficacy of MRI in axial spondyloarthritis: challenging the 'One Size Fits All' notion

OBJECTIVES

Sex-specific differences in the presentation of axial spondyloarthritis (axSpA) may contribute to a diagnostic delay in women. The aim of this study was to investigate the diagnostic performance of MRI findings comparing men and women.

METHODS

Patients with back pain from six different prospective cohorts (n=1194) were screened for inclusion in this post hoc analysis. Two blinded readers scored the MRI data sets independently for the presence of ankylosis, erosion, sclerosis, fat metaplasia and bone marrow oedema. Χ2 tests were performed to compare lesion frequencies. Contingency tables were used to calculate markers for diagnostic performance, with clinical diagnosis as the standard of reference. The positive and negative likelihood ratios (LR+/LR-) were used to calculate the diagnostic OR (DOR) to assess the diagnostic performance.

RESULTS

After application of exclusion criteria, 526 patients (379 axSpA (136 women and 243 men) and 147 controls with chronic low back pain) were included. No major sex-specific differences in the diagnostic performance were shown for bone marrow oedema (DOR m: 3.0; f: 3.9). Fat metaplasia showed a better diagnostic performance in men (DOR 37.9) than in women (DOR 5.0). Lower specificity was seen in women for erosions (77% vs 87%), sclerosis (44% vs 66%), fat metaplasia (87% vs 96%).

CONCLUSION

The diagnostic performance of structural MRI markers is substantially lower in female patients with axSpA; active inflammatory lesions show comparable performance in both sexes, while still overall inferior to structural markers. This leads to a comparably higher risk of false positive findings in women.

HLA-B27 status and inflammatory MRI lesions of the sacroiliac joints: a post hoc analysis in patients without axial spondyloarthritis

OBJECTIVE

The assessment of inflammatory and structural lesions in the sacroiliac joint (SIJ) is crucial in axial spondyloarthritis (axSpA). HLA-B27 status plays an important role in axSpA diagnosis and has been linked to MRI lesion burden in the general population. We aimed to investigate the sex-specific influence of HLA-B27 status on inflammatory and structural MRI findings in patients with low back pain of non-inflammatory origin.

METHODS

This post hoc analysis included 139 non-axSpA patients (90 women) with chronic low back pain. Two readers scored MRIs of the SIJ for the presence of sclerosis, erosion, fat metaplasia, bone marrow oedema (BMO) and ankylosis. Frequencies and extent of lesions were compared regarding the HLA-B27 status using χ2 tests and t-tests. Regression models to assess the sex-dependent influence of HLA-B27 on lesion burden were computed.

RESULTS

HLA-B27 was positive in 33 women (36.7%) and 23 men (46.9%). The overall occurrence of all SIJ lesions did not differ in HLA-B27 negative and positive individuals. There were no significant differences in the extent of lesions considering the HLA-B27 positivity, for erosion (mean sum score (MSS) of 0.91 vs 0.48; p=0.144), sclerosis (MSS 1.65 vs 1.88; p=0.576), fat metaplasia (MSS 0.56 vs 0.27; p=0.425), BMO (MSS 0.75 vs 0.59; p=0.460) and ankylosis (MSS 0.06 vs 0.04; p=0.659).

CONCLUSION

HLA-B27 status has no significant influence on the occurrence and extent of SIJ lesions in patients with low back pain of non-inflammatory origin in either men or women.

Impact of sex and gender on axSpA diagnosis and outcomes

Axial spondyloarthritis (axSpA) was historically considered a disease of men, largely due to the recognition of a more severe, progressive phenotype, ankylosing spondylitis (AS; or radiographic axSpA, r-axSpA) aiding the clinical diagnosis [1,2]. Data demonstrating the near equal prevalence of axSpA in women only started to emerge in the last decades, highlighting intrinsic differences in disease phenotype, and clinical and imaging characteristics between sexes, which partly explain the issue of underdiagnosis in women. Similar to the evolving understanding of spondyloarthritis and the diseases that term describes, the concepts of gender and sex also warrant further clarification to accurately assess their potential role in disease pathophysiology and phenotypic expression. This narrative review delves into the most recent evidence from the literature on the true prevalence of sex differences in axSpA, and the impact of sex and gender on diagnosis, disease characteristics and treatment response in this, still underserved, chronic disease.

Sex Disparities of the Sacroiliac Joint: Focus on Joint Anatomy and Imaging Appearance

The sacroiliac joint (SIJ) is an anatomically complex joint which, as a functional unit with the pelvis and spine, is of decisive biomechanical importance for the human body. It is also a commonly overlooked source of lower back pain. Like the entire bony pelvis, the SIJ exhibits major sexual dimorphisms; thus, the sex-dependent evaluation of this joint is becoming increasingly important in clinical practice, both anatomically with joint shape variations and biomechanical differences as well as in terms of image appearance. The influence of the SIJ shape, which differs in women and men, is crucial for the different biomechanical joint properties. These differences are important in the development of joint diseases at the SIJ, which shows a specific difference between the sexes. This article aims to provide an overview of sex disparities of the SIJ regarding different anatomical and imaging appearances to further understand the insights into the interplay of sex differences and SIJ disease.

Anatomical variation of the sacroiliac joints: an MRI study with synthetic CT images

BACKGROUND

Synthetic computed tomography (sCT) images are magnetic resonance imaging (MRI)-based images, generated using artificial intelligence. This study aimed to determine the prevalence of anatomical variants of sacroiliac joints (SIJ) on sCT images and the correlation with age, sex and body weight.

METHODS

MRI of the SIJ including sCT images of 215 patients clinically suspected for sacroiliitis were retrospectively analyzed. The presence of anatomical variants of the SIJ was assessed. Age, sex and body mass index at the time of the MRI were recorded.

RESULTS

SIJ variants were found in 82.8% (356/430) of the evaluated joints. The most frequent variants were iliosacral complex (27.7%), bipartite iliac bony plate (27.2%) and crescent iliac bony plate (27%). One new variant was identified, consisting of an accessory facet of the SIJ on the superior side. Overall, SIJ variants were slightly more frequent in women (85.8% vs. 77.8%), but iliosacral complex was significantly more frequent in men. Isolated synostosis was more prevalent with advancing age, in contrast to semicircular defect and unfused ossification center. The occurrence of iliosacral complex was associated with higher BMI, while crescent iliac bony plate occurred more in patients with lower BMI.

CONCLUSION

Over 80% of patients in this study, who were all suspected of sacroiliitis, had at least one SIJ variant. These variants may actually represent subtypes of the normal SIJ. sCT enables detection of very small or subtle findings including SIJ variants.