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Background
The purpose of this exhibit is to review sacroiliac joint (SIJ) anatomy, describe the entity of sacroiliac dysfunction; its pathophysiology, differential diagnoses, and treatment options, and showcase pertinent pre and postoperative imaging findings.

Educational Goals / Teaching Points
Normal SIJ anatomy and function including fibrous and synovial portions, ligamentous and muscular attachments, and morphologic variations and accessory SI joints. Sacroiliac Dysfunction including etiological factors include ligamentous laxity, spinal disorders, and minor trauma, diagnosis is primarily clinical, based on motion palpation and pain provocation tests, and imaging mostly normal; primarily performed to exclude other causes of sacroiliac pain. Differential diagnosis with imaging examples including spondyloarthritis, infection, metabolic disorders, trauma, degenerative disease, referred pain, and tumor. Treatment options include physical therapy, manipulation, steroid/anesthetic injection, and SI joint fusion, including triangular titanium (iFuse) implants. Potential complications, illustrated with cases include loosening, malpositioning, peri-implant fracture or stress response, and postoperative infection.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Currently diagnostic imaging is not contributory in identifying SIJ dysfunction. However, image guided SIJ injections play an important role in the diagnosis and treatment of the disorder. Imaging also plays an important role in the postoperative setting after patients have undergone fusion to identify complications that may arise. Through our research on the topic and personal experience with these patients, the authors propose imaging findings that may suggest diagnosis including asymmetric degenerative change of the SIJ out of proportion to age or other degenerative findings in the patient, SIJ abnormalities confined to the anterior lower ilium, and asymmetric SIJ widening.

Conclusion
Anatomy of the SIJ is complex, and significant variation exists among individuals. SIJ play a crucial role in pelvic stability and their movements are governed by a strong network of ligaments and tendons. A range of pathologies can present as SIJ pain and specific physical examination maneuvers can help to localize the pain. SIJ dysfunction is defined as the inability of the SIJ to distribute biomechanical load from the upper to lower body as a result of noninflammatory, articular causes. Imaging may be normal or demonstrate features mentioned previously. Treatment includes conservative measures and/or surgical fusion. SIJ arthrodesis by insertion of triangular implants through a minimally invasive approach is gaining popularity. There are specific imaging findings and complications related to SIJ arthrodesis that the radiologist should know.