An accessory sacroiliac joint

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).

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.

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.

Progressive sacroiliitis due to accessory sacroiliac joint mimicking ankylosing spondylitis: A case report

RATIONALE

An accessory sacroiliac (SI) joint usually has little clinical significance. However, severe arthritic changes can cause chronic buttock or low back pain and can be misdiagnosed as another disease presenting with sacroiliitis such as ankylosing spondylitis (AS).

PATIENT CONCERNS

A 33-year-old woman was diagnosed with AS due to chronic buttock pain and progressive sacroiliitis on plain X-ray and magnetic resonance imaging (MRI). Her buttock and low back pain gradually worsened despite proper treatment for AS.

DIAGNOSIS

Computed tomography revealed an accessory SI joint with arthritic changes.

INTERVENTIONS

Nonsteroidal anti-inflammatory drugs (NSAIDs) and restricted movement were prescribed.

OUTCOMES

The symptoms were controlled with NSAIDs, rest, and restriction of excessive movement. The medication could be stopped after the pain subsided.

LESSONS

An accessory SI joint can be a cause of chronic back pain and can be misdiagnosed as AS with sacroiliitis when progressive arthritic changes are observed. Therefore, additional imaging studies other than conventional X-ray or MRI may be required for accurate diagnosis.

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.

Sacroiliac joints: anatomical variations on MR images

OBJECTIVE

To describe the appearance of the anatomical variations of the sacroiliac joints (SIJ) on MR images and to highlight the potential MR features that might be misleading.

METHODS

Consecutive MRI examinations of SIJs performed at our department from January 2015 to December 2016 were retrospectively analyzed. As one of our goals was to describe potential misleading edematous or structural changes associated with SIJ anatomical variations, patients fulfilling the ASAS criteria were excluded from this study to ensure that signal intensity changes would not be related to sacroiliitis. Five anatomical variations and a dysmorphic appearance of the SIJ were detected.

RESULTS

The final group consisted of 157 patients. Unilateral or bilateral anatomical variations of the SIJ were found in 50 patients (accessory SIJ, iliosacral complex and sacral defect in 17, 18 and 21 patients, respectively, and synostosis in one patient). A dysmorphic appearance of the SIJ was found in 26 patients. No case of an unfused ossification centers was depicted. Structural and/or edematous changes of the facing bones were quite frequently observed in accessory and dysmorphic SIJ. Iliosacral complex and sacral defects could be associated with prominent vessels running along their bony surfaces.

CONCLUSION

Several anatomical variations of the SIJs are relatively commonly seen on MR images, particularly in females. These variations may be associated with signal intensity changes, which may be mechanical and not necessarily inflammatory in nature.

KEY POINTS

• Anatomical variations of SIJ may involve the cartilaginous or ligamentous part of the joint • Anatomical variations of SIJ are sometimes associated with edematous and/or structural changes of the adjacent bone • Anatomical variations of the SIJ can be misleading on MR imaging.