Infectious sacroiliitis: insights from long-term follow-up and the diagnostic contribution of computed tomography and magnetic resonance imaging

Purpose Infectious sacroiliitis (ISI) is a rare condition with non-specific symptoms, often leading to delayed diagnosis We aim to describe the clinical, microbiological, and radiological characteristics of infectious sacroiliitis (ISI), and to highlight the diagnostic and follow-up value of imaging modalities. Materials and methods We conducted a retrospective review of 35 ISI cases diagnosed over 27 years. Clinical, microbiological, and imaging data were collected. CT and MRI findings were compared, and follow-up imaging was analyzed when available. Results The mean age was 40.9 years, with a female predominance (60%). Pyogenic organisms were identified in 45.7% of cases, Mycobacterium tuberculosis in 34.3%, and Brucella spp. in 20%. The average diagnostic delay was 102 days. The most frequent symptoms were fever (80%) and buttock pain (77.1%), and inflammatory markers were elevated in approximately 90% of cases. Blood cultures were positive in 6.4%, while biopsy and abscess aspiration confirmed the etiological diagnosis in 28.6% and 14.3%, respectively. Imaging played a central role: CT revealed bony erosions (84.6%) and periarticular involvement (69.2%), while MRI, performed in 13 cases, showed signal abnormalities (85.7%), contrast enhancement (91.7%), and detected soft tissue abscesses (30.8%) and extension to adjacent bone or soft tissues (38.5%). Among the 10 patients who underwent both CT and MRI, MRI detected joint space widening more frequently than CT (70% vs. 50%, p = 0.038) and revealed periarticular infiltration in all cases compared to 60% on CT (p = 0.045). Conversely, CT identified bone sequestra in 50% of patients, whereas none were visualized on MRI (p = 0.041). Radiological follow-up showed improvement in 83.3% of cases, although residual lesions persisted. Conclusion ISI is difficult to diagnose due to its non-specific presentation. Imaging is essential for early diagnosis, guiding treatment, and monitoring response.