Sacroiliac joints: anatomical variations on MR images

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.

Sacroiliac joint MRI for diagnosis of ax-SpA: algorithm to improve the specificity of the current ASAS MRI criteria

OBJECTIVE

To compare sacroiliac joint (SIJ) lesions on MRI in women with versus without axial spondyloarthritis (ax-SpA) and establish an algorithm to determine whether such lesions are due to ax-SpA.

METHODS

This retrospective comparative study assessed bone marrow edema (BME), sclerosis, erosions, osteophytes, and ankylosis at the SIJ in two groups of women, one with and another without ax-SpA. Sensitivity and specificity were calculated for combinations/characteristics of lesions, using rheumatologists' assessment with assessment of spondyloarthritis international society (ASAS) criteria as the gold standard for diagnosis of ax-SpA.

RESULTS

Compared to women without ax-SpA, women with ax-SpA had more BME (61% vs 17%, p < 0.001), sclerosis (40% vs 22%, p < 0.001), erosions (35% vs 5%, p < 0.001), and ankylosis (2% vs 0%, p = 0.007), but less osteophytes (5% vs 33%, p < 0.001). The ASAS MRI criteria yielded 59% sensitivity and 88% specificity, while a new algorithm achieved 56% sensitivity and 95% specificity using the following criteria: no osteophytes at the SIJ and either (i) BME at the SIJ with at least one dimension ≥ 8 mm or (ii) at least one erosion at the SIJ.

CONCLUSIONS

We recommend the following pragmatic algorithm for MRI diagnosis of ax-SpA in women: no osteophytes at the SIJ and either (i) BME at the SIJ with at least one dimension ≥ 8 mm or (ii) at least one erosion at the SIJ. The false positive rate when using the new algorithm (3.3%) is less than half than when using the ASAS MRI criteria (7.7%); thus, its application in clinical practice could reduce overdiagnosis and prevent overtreatment of ax-SpA.

CLINICAL RELEVANCE STATEMENT

The developed algorithm has a false-positive rate that is less than half than when using the ASAS MRI criteria (3.3% vs 7.7%), thus its application in clinical practice could reduce overdiagnosis and prevent overtreatment of axial spondyloarthritis.

KEY POINTS

• Compared to women without axial spondyloarthritis (ax-SpA), women with ax-SpA had a significantly higher prevalence of bone marrow edema (BME), sclerosis, erosions, and ankylosis, but a significantly lower prevalence of osteophytes. • A new algorithm for positive ax-SpA based on sacroiliac joint MRI was developed: no osteophytes at the sacroiliac joint (SIJ) and either (i) BME at the SIJ with at least one dimension ≥ 8 mm or (ii) at least one erosion at the SIJ. • We recommend this new algorithm for diagnosis of ax-SpA in women, as it has a significantly better specificity than the assessment of spondyloarthritis international society (ASAS) MRI criteria and less than half the false positive rate; thus, its application in clinical practice could reduce overdiagnosis and prevent overtreatment of ax-SpA.

Analysis of the sacroiliac joint vacuum phenomenon in adolescent thoracic idiopathic scoliosis (Lenke types 1 and 2)

The sacroiliac joint (SIJ) is the largest axial joint in the human body, and the SIJ vacuum phenomenon (SIJ VP) is a common finding in computed tomography studies of the abdomen, pelvis, and lumbosacral spine in adults, with the incidence increasing with age. Adolescent idiopathic scoliosis (AIS) is an abnormal spinal curvature that appears during adolescence and places abnormal stress on the SIJs. This retrospective observational study aimed to investigate the incidence of the SIJ VP in thoracic AIS (Lenke types 1 and 2). Sixty-seven patients with AIS (age: 12-19 years) and 76 controls (age: 11-19 years) were retrospectively analyzed to investigate SIJ VP, subchondral bone cysts, and SIJ degeneration (Eno classification: type 0, no degenerative change; type 1, mild degenerative changes; type 2, substantial degenerative changes; and type 3, ankylosis). SIJ degeneration was defined as type ≥ 2. The association between SIJ VP, cysts, SIJ degeneration, and sagittal/coronal spinopelvic alignment was assessed. SIJ VP (59% vs. 35.5%, P < .01), cysts (32.8% vs. 1.3%, P < .01), and SIJ degeneration (3.2% vs. 2.6%, P = .823) differed significantly between the 2 groups. There were 0 cases of SIJ ankylosis (Eno classification type 3) in both groups. The VP was not correlated with lumbar lordosis, sacral slope, or Cobb angle. All lumbar modifier type C belonged to the VP present group, whereas none to VP absent group. Our results suggest an association between AIS and SIJ VP and SIJ cysts. SIJ VP and SIJ cysts in AIS may be caused and accelerated by abnormal mechanical stress on SIJ due to spinal deformity.

Novel insights into the anatomy and histopathology of the sacroiliac joint and correlations with imaging signs of sacroiliitis in case of axial spondyloarthritis

For a better understanding of the pathophysiology of spondyloarthropathy (SpA), a detailed anatomical description of the sacroiliac joint is required because sacroiliitis is the earliest and most common sign of SpA and an essential feature for the diagnosis of ankylosing spondylitis. Beyond the anatomy, the histopathology of sacroiliac entheses and immunological mechanisms involved in sacroiliitis are crucial for a better understanding of disease causation. In this narrative review, we discuss the core anatomical, histological, and immunohistological observations involved in the development of sacroiliitis, focusing particularly on imaging-based information associated with sacroiliitis. Finally, we try to answer the question of whether at the sacroiliac joint, enthesitis precedes synovitis and subchondral bone changes in SpA.

Anatomical Variants of the Sacroiliac Joint

Anatomical variants are frequently encountered when assessing the sacroiliac joints (SIJ) using magnetic resonance imaging. When not located in the weight-bearing part of the SIJ, variants associated with structural and edematous changes can be misinterpreted as sacroiliitis. Their correct identification is necessary to avoid radiologic pitfalls. This article reviews five SIJ variants involved in the dorsal ligamentous space (accessory SIJ, iliosacral complex, semicircular defect, bipartite iliac bony plate, and crescent iliac bony plate) and three SIJ variants involved in the cartilaginous part of the SIJ (posterior dysmorphic SIJ, isolated synostosis, and unfused ossification centers).

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

OBJECTIVES

To assess the impact of joint shape variations on inflammatory lesions on SI joint MRIs in patients with axial spondyloarthritis (axSpA).

METHODS

A total of 1194 patients from four different prospective cohorts were evaluated, with 684 (57.3%) having sufficient imaging data for inclusion (379 axSpA, 305 controls). All images were evaluated for joint form, erosion, sclerosis, fat metaplasia and bone marrow oedema (BMO) by two independent readers. Logistic regression analyses were used to assess the association of joint form and lesions on imaging for axSpA patients and controls.

RESULTS

Atypical joint forms were common in both axSpA (43.5% [154/354]) and control patients (44.2% [134/303]); both intra-articular variants and a crescent joint shape were significantly more common in axSpA patients (18.4% vs 11.6% and 11.0% vs 5.3.%, respectively; P < 0.001). The axSpA patients with intra-articular joint form variants had 2-fold higher odds of exhibiting erosions [odds ratio (OR) 2.09 (95% CI 1.18, 3.69)] and BMO [OR 1.79 (95% CI 1.13, 2.82)]; this association was not observed in controls. Accessory joints increased the odds for sclerosis in axSpA patients [OR 2.54 (95% CI 1.10, 5.84)] and for sclerosis [OR 17.91 (95% CI 6.92, 46.37)] and BMO [OR 2.05 (95% CI 1.03, 4.07)] in controls.

CONCLUSIONS

Joint form variations are associated with the presence of inflammatory lesions on SI joint MRIs of axSpA patients. This should be taken into consideration in future research on the interplay of mechanical strain and inflammation in axSpA.

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.

Diagnostics of Sacroiliac Joint Differentials to Axial Spondyloarthritis Changes by Magnetic Resonance Imaging

The diagnosis of axial spondyloarthritis (axSpA) is usually based on a pattern of imaging and clinical findings due to the lack of diagnostic criteria. The increasing use of magnetic resonance imaging (MRI) of the sacroiliac joints (SIJ) to establish the diagnosis early in the pre-radiographic phase has resulted in a shift in the paradigm with an increasing frequency of axSpA diagnoses and a changed sex distribution. Non-radiographic axSpA affects males and females nearly equally, whereas ankylosing spondylitis predominantly occurs in males. The MRI-based increasing frequency of axSpA in women is mainly due to the presence of subchondral bone marrow edema (BME) on fluid-sensitive MR sequences, which may be a non-specific finding in both women and men. Due to the somewhat different pelvic tilt and SIJ anatomy, women are more prone than men to develop strain-related MRI changes and may have pregnancy-related changes. Awareness of non-specific subchondral BME at the SIJ is important as it can imply a risk for an incorrect SpA diagnosis, especially as the clinical manifestations of axSpA may also be non-specific. Knowledge of relevant MRI and clinical features of differential diagnoses is needed in the diagnostic workout of patients with suspected axSpA considering that non-SpA-related SIJ conditions are more common in patients with low back or buttock pain than axSpA sacroiliitis. The purpose of this review was to present current knowledge of the most frequent differential diagnoses to axSpA sacroiliitis by MRI taking the clinical characteristics into account.

Prevalence of accessory sacroiliac joint anatomy and associated clinical features

OBJECTIVE

The accessory sacroiliac joint (ASIJ) is the most common sacroiliac joint anatomical variant; however, its literature-reported prevalence is inconsistent. Previous CT-based studies of the ASIJ have used thick axial slices, which may not adequately detail ASIJ anatomy. The aims of this study are to (1) evaluate ASIJ prevalence and radiographic features in a large age- and sex-balanced cohort using thin-section CT and (2) determine associations between ASIJ anatomy, patient features, and treatment strategies.

MATERIALS AND METHODS

Thin-section CTs (0.75 to 2.00 mm) of the pelvis from 800 patients were reviewed by two musculoskeletal radiologists. Degree of degenerative change and ankylosis at ASIJs were detailed. The EMR was used to capture demographics, lower back or sacroiliac joint symptoms, and treatments.

RESULTS

The ASIJ was present in 25.8% of patients and bilateral in 53.3% of those with any ASIJ. ASIJs were more common at the S2 than S1 neural foramen level (75.7% and 27.2%). There was a statistically significant difference between age and presence of any ASIJ anatomy (mean (SD) 69.0 (19.8) with ASIJ versus 55.9 (22.1) years without ASIJ). Degenerative changes and ankylosis were found in 93.5% and 20.3% of ASIJs, respectively. There was a higher odds ratio of having received a sacroiliac joint corticosteroid injection in those with ASIJ anatomy.

CONCLUSION

Radiologists should be familiar with the ASIJ and consider its age-related association, propensity to show ASIJ degenerative change, and ability to serve as a potential pain generator. Steroid injections may be considered for diagnostic and therapeutic purposes.

Sacroiliac joint beyond sacroiliitis-further insights and old concepts on magnetic resonance imaging

The sacroiliac joint (SIJ) is an amphiarthrosis composed of a posterior syndesmosis and an anterior cartilaginous portion, with limited yet present mobility. Its main function is to transmit the load from the axial skeleton to the lower limbs and vice-versa; it is susceptible to early mechanical and degenerative changes which are much more common than inflammatory sacroiliitis. Magnetic resonance imaging (MRI) has increasingly been used to evaluate these changes, and while subchondral bone marrow edema (BME) is a common finding related to both, care must be taken when applying the ASAS research MRI definition for sacroiliitis without considering lesion BME topography, size and depth, concomitant structural damage and, of course, the clinical picture. In this review, we will discuss the anatomy and biomechanics of the SIJ, the noninflammatory causes of SIJ subchondral BME, and how these concepts combined can be used to increase our diagnostic confidence.

Imaging of Structural Abnormalities of the Sacrum: The Old Faithful and Newly Emerging Techniques

The sacrum and sacroiliac joints pose a long-standing challenge for adequate imaging because of their complex anatomical form, oblique orientation, and posterior location in the pelvis, making them subject to superimposition. The sacrum and sacroiliac joints are composed of multiple diverse tissues, further complicating their imaging. Varying imaging techniques are suited to evaluate the sacrum, each with its specific clinical indications, benefits, and drawbacks. New techniques continue to be developed and validated, such as dual-energy computed tomography (CT) and new magnetic resonance imaging (MRI) sequences, for example susceptibility-weighted imaging. Ongoing development of artificial intelligence, such as algorithms allowing reconstruction of MRI-based synthetic CT images, promises even more clinical imaging options.

Differences in topographical location of sacroiliac joint MRI lesions in patients with early axial spondyloarthritis and mechanical back pain

Background

Early diagnostics of axial spondyloarthritis (axSpA) remains a challenge. Traditional imaging one-plane sacroiliac joint (SIJ) MRI assessment is used. By introducing a two-plane assessment system, the objective was to analyse the differences in SIJ MRI changes in early axSpA compared with changes in patients with mechanical back pain (MBP) by exploring the differences in volume and location.

Methods

MRIs in the early diagnostic state of 25 axSpA patients (mean age 31.3 years) and 59 MBP patients (mean age 32.3 years) were included. The MRIs were assessed by two readers regarding the distribution of bone marrow edema (BME) in 14 joint portions and structural changes in six joint portions in addition to SIJ anatomical variations and lumbar spine disc degeneration.

Results

AxSpA patients had a significantly higher overall BME sumscore (volume) of 25.1 compared to MBP patients 6.8, p < 0.005. The MBP group had the highest prevalence (66%) and sumscore (5.7) in the middle anterior sacrum. The axSpA group had significantly higher prevalence and sumscores in all joint portions except the three cartilaginous anterior sacral joint portions, including the ligamentous compartments (prevalence 40–60% compared to 8–15%, p both < 0.005). The axSpA group had also a significantly higher prevalence of erosions and fatty marrow disposition, but there were no differences in the prevalence of anatomical variations except the bipartite iliac bony plate.

Conclusions

AxSpA patients demonstrated a widespread distribution of both inflammatory and structural changes, including high BME occurrence in the ligamentous joint portions whereas patients with MBP had the highest occurrence of BME in the middle anterior sacrum. These findings may help differentiate axSpA patients from other back pain conditions in the early diagnostic phase.

Anatomical variation at the sacroiliac joints in young adults: estimated prevalence by CT and concomitant diagnostics by MRI

OBJECTIVE

To estimate the prevalence of atypical anatomical morphologies at the sacroiliac joints (SIJ) in young adults by CT and analyze the diagnostic ability of MRI to detect the variations in addition to concomitant MRI findings that could be misdiagnosed as inflammatory changes.

MATERIALS AND METHODS

The study sample constituted CT examinations of 155 individuals aged 18-40 years and prospectively collected comparative SIJ MRI examinations of 49, who also filled out a questionnaire on back and buttock pain. The CT and MRIs were analyzed by two musculoskeletal radiologists regarding seven SIJ variations and additional subchondral bone marrow edema (BME) by MRI.

RESULTS

CT and MRI interobserver agreements were good or very good for most variations. Mean age of the 155 individuals was 28 years, 99 (64%) were males; 88 (57%) had at least one SIJ variation, and most frequent were dysmorphic cartilaginous joint facets (n = 33, 21%), bipartite iliac bony plate (n = 27, 17%), accessory SIJ (n = 24, 16%), and iliosacral complex (n = 18, 12%), with a female predominance of all variations. The ability of MRI to detect the frequent variations was satisfying. Dysmorphic cartilaginous joint facets, accessory SIJ, and iliosacral complex were frequently observed in individuals reporting symptoms and were accompanied by BME, often located anteriorly in sacrum/inferiorly in ilium.

CONCLUSION

Atypical SIJ morphology is frequent in young adults, especially females, demanding further research into the anatomical natural variation. Most of the variations were detectable by MRI and three variations warrant further exploration as they often were accompanied by symptoms and/or BME.

Feasibility of T2 Mapping of the Sacroiliac Joints in Healthy Control Subjects and Children and Young Adults with Sacroiliitis

OBJECTIVE

To assess the feasibility of T2 mapping for evaluating pediatric SIJ cartilage at 3 Tesla (T) magnetic resonance imaging (MRI).

METHODS

Healthy control subjects and adolescents with sacroiliitis underwent a 3T MRI dedicated pelvic protocol that included a T2 mapping sequence consisting of multislice, multiecho acquisition. Healthy control subjects were prospectively recruited from our primary care practices as part of a larger imaging study, whereas adolescents with sacroiliitis were recruited specifically for this study. Regions of interest (ROIs) were hand-drawn by a senior pediatric radiologist twice and a radiology fellow twice to calibrate and test reliability using the intraclass correlation coefficient (ICC). T2 relaxation time between control subjects and cases was compared using univariate linear regression. We tested the association of T2 relaxation time in adolescents with sacroiliitis with patient-reported outcomes and the Spondyloarthritis Research Consortium of Canada sacroiliac joint (SIJ) inflammation and structural scores using Pearson correlation coefficients.

RESULTS

Fourteen subjects were evaluable (six control subjects: median age 13.7 years [interquartile range (IQR): 12.2-15.5], 67% male patients; eight cases: median age 17.4 years [IQR: 12.5-20], 88% male patients]. Acquisition time for T2 mapping sequences was approximately 6 minutes, and segmenting the ROI for each SIJ took approximately 3 minutes. The intrarater and inter-rater ICCs were 0.67 and 0.46, respectively, indicating good to fair reliability. There was a trend, albeit statistically insignificant, in longer median T2 relaxation time in cases (43.04 ms; IQR: 41.25-49.76 ms) versus healthy control subjects (40.0 ms; IQR: 38.9-48.6 ms). Although not statistically significant, cases with longer T2 relaxation time tended to occur with poorer patient-reported outcomes. Correlations with the SIJ inflammation and structural lesion scores were weak.

CONCLUSION

T2 mapping of the SIJ cartilage in children was feasible and reliable. Larger controlled and longitudinal assessments are needed to assess the validity and utility of these measurements for routine clinical practice and trials.

Main Diagnostic Pitfalls in Reading the Sacroiliac Joints on MRI

Magnetic resonance imaging of the sacroiliac joints is now frequently performed to help identify patients with early axial spondyloarthritis. However, differential diagnoses exist and should be recognized. The aim of this article is to review the most frequent differential diagnoses that may mimic inflammatory sacroiliitis in clinical practice.

Sacroiliac joint in sarcoidosis on computed tomography: a monocentric retrospective study (SISTER)

Sacroiliitis and spondyloarthritis (SpA) have been associated to sarcoidosis. Sarcoidosis bone involvement of the sacral or iliac bones has been reported to mimic SpA. We aimed to evaluate the prevalence of structural sacroiliitis and structural changes of the sacroiliac joints (SIJ) in patients with sarcoidosis by abdominal-pelvic computed tomography (AP-CT). In this monocentric retrospective study, three blinded readers evaluated AP-CT that had already been performed on patients with sarcoidosis and classified them as normal, degenerative, or inflammatory. A consensus was reached for the divergent cases. Erosion, ankylosis, and sclerosis, classically associated with sacroiliitis, were noted. SpA was defined according to the ASAS 2009 classification criteria. We identified 217 patients with proven sarcoidosis who underwent AP-CT. Only three patients had sacroiliitis by CT and four had SpA, representing 1.38% and 1.85% of the patients, respectively. Degenerative SIJs represented 28.1% of patients and were significantly associated with age, at least one pregnancy, rural lifestyle, ankylosis, diffuse idiopathic skeletal hyperostosis, sclerosis, and the presence of osteophytes. Four patients had axial bone sarcoidosis. Sacroiliitis, SpA, and degenerative changes of the SIJ have been highlighted by AP-CT in patients with sarcoidosis. Osteoarthritis of the SIJ in sarcoidosis was associated with age, pregnancy, and rural lifestyle. Further studies are needed to assess the link between SpA and sarcoidosis.

Anatomical Joint Form Variation in Sacroiliac Joint Disease: Current Concepts and New Perspectives

Purpose of Review

The aim of this article is to further the understanding of anatomical variation of the sacroiliac joint (SIJ) within the rheumatological community and point out promising fields of research in the interplay of SIJ anatomy and joint disease.

Recent Findings

Mechanical strain has long been implicated in onset and progression of axial spondyloarthritis (axSpA). Recent investigations found changes in the pattern of degenerative lesions of the SIJ in the normal population in patients with atypical joint forms. Furthermore, atypical SIJ forms are more prevalent in patients with axial spondyloarthritis and mechanical SIJ disease.

Summary

Mechanical stress from anatomical joint form variation may have an impact on development and progression of axSpA. Furthermore, mechanically induced bone marrow edema may act as an axSpA mimic on MRI and needs to be more accurately classified.

Axial Spondyloarthritis: Mimics and Pitfalls of Imaging Assessment

Axial spondyloarthritis (axSpA) is a chronic inflammatory disorder that predominantly involves the axial skeleton. Imaging findings of axSpA can be divided into active changes, which include bone marrow edema, synovitis, enthesitis, capsulitis, and intra-articular effusion, and structural changes, which include erosions, sclerosis, bone fatty infiltration, fat deposition in an erosion cavity, and bone bridging or ankylosis. The ability to distinguish between imaging lesions suggestive of axSpA and artifacts or lesions suggestive of other disorders is critical for the accurate diagnosis of axSpA. Diagnosis may be challenging, particularly in early-stage disease and magnetic resonance imaging (MRI) plays a key role in the detection of subtle or inflammatory changes. MRI also allows the detection of structural changes in the subchondral bone marrow that are not visible on conventional radiography and is of prognostic and monitoring value. However, bone structural changes are more accurately depicted using computed tomography. Conventional radiography, on the other hand, has limitations, but it is easily accessible and may provide insight on gross changes as well as rule out other pathological features of the axial skeleton. This review outlines the imaging evaluation of axSpA with a focus on imaging mimics and potential pitfalls when assessing the axial skeleton.

Joint anatomy in axial spondyloarthritis: strong associations between sacroiliac joint form variation and symptomatic disease

OBJECTIVES

The aim of this investigation was to determine the prevalence of variant sacroiliac joint forms in symptomatic patients with mechanical joint disease (MJD) and axial spondyloarthritis (axSpA) compared with control patients.

METHOD

A total of 973 patients were included in this cross-sectional study (95 axSpA; 61 MJD; 817 controls); clinical diagnosis, age and gender were noted. An established scoring system was used to classify joint forms on computed tomography scans of the pelvis. Frequencies of joint forms were compared between groups (axial spondyloarthritis; mechanical joint disease; controls).

RESULTS

Patients with MJD exhibited the highest proportion of atypical joints (80.3% in MJD vs 44.1% in axSpA and 37.5% in controls; p< 0.001). Overall, females had a significantly higher proportion of atypical joints than men (65.0% vs 17.8%; p< 0.001); proportions of atypical joints were significantly higher in males with symptomatic joint disease than in male controls: 32.2% in axSpA, 55.0% in MJD, 13.9% in controls; p≤ 0.001. Two specific joint forms were significantly more prevalent in symptomatic patients than in controls: the iliosacral complex (16.2% vs 4.2%; p< 0.001) and the crescent-shaped ilium (9.1% vs 2.8%; p< 0.05).

CONCLUSIONS

Our data demonstrate a strong association between atypical joint forms and symptomatic joint disease.

The pediatric accessory sacroiliac joint: does it exist?

OBJECTIVE

Determine the prevalence of the accessory sacroiliac joint in the pediatric population and describe variant sacroiliac joint morphology that may predispose patients to the development of an accessory sacroiliac joint.

MATERIALS AND METHODS

One hundred and seventy-eight high-resolution pelvic CT scans of patients aged 0 to 15 years were reviewed for the presence of an accessory sacroiliac joint. Patients were stratified based on age and gender. Morphology of the sacroiliac joints was detailed to assess the degree of curvature in the expected characteristic location of the accessory sacroiliac joint.

RESULTS

No accessory sacroiliac joint was identified on any of the pediatric pelvic CT scans. The sacroiliac joints demonstrated varying degrees of unilateral or bilateral curvature in the expected region of the accessory sacroiliac joint which increased in both severity and prevalence with age.

CONCLUSION

The pediatric accessory sacroiliac joint may not exist and is unlikely to be a congenital variant present at birth. However, curvature of the sacroiliac joint in the expected location of the accessory sacroiliac joint which increases in severity and prevalence with age may predispose patients to the formation of an accessory sacroiliac joint later in life.

Anatomical variations of the sacro-iliac joint: a computed tomography study

PURPOSE

To identify, describe, and compare the prevalence of sacroiliac joint anatomical variants.

METHODS

A retrospective study was performed on computed tomography scans. Joint space was measured, and variants were classified as accessory joint, ileosacral complex, bipartite bony plate, semicircular defect, iliac bony plate, and ossification centers.

RESULTS

400 scans were analyzed. Mean age was 49 years, 180 men (45%), and 220 women (55%). 209 (52.2%) patients presented an anatomical variant with higher prevalence in women (65.4% vs 36.2%), and those older than 40 years of age (60% vs 40%). Mean joint space was similar bilaterally (right 2.41 ± 0.65 mm vs. left 2.37 ± 0.65 mm). Prevalence was: 19.8% accessory joint, 6.5% ileosacral complex, 12.3% bipartite bony plate, 8% semicircular defect, 5% iliac bony plate, and 0.8% for ossification centers.

CONCLUSION

Prevalence of anatomical variants of the SIJ is higher in the Hispanic population, women, and those older than 40 years.

Comparative study of imaging and histology of sacroiliac joint in normal rats based on IVIM-DWI and DCE-MRI

Background

Currently, few studies have described the relationship between functional MRI findings and histology of normal sacroiliac joint (SIJ). Besides, due to the difficulties in access to SIJ, authentic animal models are important in providing opportunities for quantitative parameter extraction on imaging.

Aims

This study aimed at exploring the parameters of Intravoxel Incoherent Motion Diffusion-Weighted Imaging (IVIM-DWI) and Dynamic Contrast-Enhanced Magnetic Resonance Imaging (DCE-MRI) and comparing them with the histology of the SIJ in normal rats with different ages.

Methods

A total of thirty 7-week-old male Wistar rats were included in the study. The parameters of IVIM-DWI and DCE-MRI in the bone marrow and the joint space of SIJ were measured at 8, 13, 18, 23, 28, and 33 weeks. The histological analysis of the SIJ was examined using light microscopy. One-way ANOVA was used for statistical analysis.

Results

The D values in the sacral and iliac bone marrow of normal rats decreased with an increase in age. One-way ANOVA analysis indicated a significant difference in D values in different age groups (P<0.005). The normal values of D*, f, Fenh (%), Senh (%/s) in the sacral bone marrow, the iliac bone marrow, and the joint space in SIJ of normal rats were obtained. The results showed that in the six groups of rats of different ages, the histology of the SIJ surface was smooth and clear, the cartilage cells were intact, and no thickening or pannus formation was observed.

Conclusions

This study obtained the IVIM-DWI and DCE-MRI parameters of the sacral and iliac bone marrow and the synovial area of the joint space in normal rats. The parameters in normal rats can be used in future research to compare to similar parameters in animal models or patients with SIJ diseases. This study serves as a guide for future research in SIJ diseases.

Frequency of anatomical variation of the sacroiliac joint in asymptomatic young adults and its relationship with sacroiliac joint degeneration

INTRODUCTION

To evaluate the variations of the sacroiliac joint (SIJ) in asymptomatic young adults and their relationship to SIJ degeneration.

MATERIALS AND METHODS

A total of 860 SIJs were retrospectively evaluated in 430 abdominal computed tomographies (CT) of abdominal pain cases. The study population consisted of 430 patients (176 female, 254 male) aged 20-45 years with abdominal pain. The anatomical variations of the SIJ were classified as (1) accessory sacroiliac joint, (2) iliosacral complex, (3) bipartite iliac bony plate, (4) semicircular defects, (5) crescent-like iliac bony plate, and (6) ossification center. The CT findings of SIJ degeneration included joint space narrowing, sclerosis, subcortical cysts, vacuum phenomenon, and osteophytes. Chi-square and cross tabulation tests were used for statistical analysis.

RESULTS

The iliosacral complex was the most common (bilateral) anatomical variation (7.7%, n = 66). A total of 71 SIJ degenerations were detected in cases with anatomical variations. They were determined in 22 (2.6%) iliosacral complex, 19 (2.2%) bipartite iliac bony plate, 15 (1.7%) accessory sacroiliac joint, 6 (0.7%) semicircular defect, 5 (0.6%) crescent-like iliac bony plate, and 4 (0.5%) ossification center cases.

CONCLUSION

We found that iliosacral complex was the most common SIJ variation in asymptomatic young adults and also that there was significantly more SIJ degeneration among subjects with anatomical variations of the SIJ than those without. SIJ degeneration was most common in subjects with the iliosacral complex variation.

Diagnostic imaging: pitfalls in rheumatology

Although the diagnosis of arthritis and spondyloarthritis is based on clinical criteria, today the imaging methods are an indispensable aid to the rheumatologist. Imaging has not only the task of helping early diagnosis, but it has also a fundamental role in disease grading and therapeutic monitoring. In this scenario where many publications emphasize the importance of identifying synovitis and erosions at an early stage, it is essential to know the possible pitfalls which can determine both false positives and false negatives. The high variability of the musculoskeletal system anatomy makes it necessary to have a correct knowledge of all anatomical complexes, in order not to confuse them with the pathology. Moreover, the correct and standardized method of the execution and interpretation of the exams, such as ultrasound, is crucial to identifying and correctly monitoring the pathological hallmarks of the arthritis. This paper aims to provide an instrument to radiologists, highlighting the main imaging pitfalls in ultrasound and magnetic resonance which may be encountered in daily practice.