[Imaging of bacterial infections of the sacroiliac joint]

Infection of the sacroiliac joint can be pyogenic or granulomatous and is usually unilateral. There are a number of predisposing conditions including drug abuse and intra articular steroid injection, but in 44% of cases, no definite predisposing factors can be identified. Considerable delay between presentation and diagnosis is recognized. The clinical picture may be non-specific and variable, and clinical suspicion may be low due to the relatively low incidence of the condition. This is compounded by difficulties in clinical examination of the SUs. The diagnosis is based on a history suggestive of infection, clinical or radiographic localization to the SUs, and a positive blood culture or joint aspirate. The pathology of pyogenic sacroiliitis is reviewed with respect to the anatomy of the SU, and the differential diagnoses considered. The imaging findings, and relative merits of all the modalities are discussed with particular consideration given to changes over the course of the disease. Imaging strategies are evaluated and proposed. As the commonest presenting symptom is low back pain, consideration should be given to the addition of a STIR sequence covering the SUs on all routine lumbar spine MR examinations. MR imaging is the most sensitive and specific imaging modality, while CT-guided arthrocentesis improves diagnostic confidence. Tc99MDP blood pool imaging mirrors the clinical features of resolution, and scintigraphy may be the best method to monitor response to treatment. Targeted antibiotic therapy usually leads to a full recovery. A high incidence of clinical suspicion, with MR imaging at an early stage are the essential prerequisites to an accurate diagnosis of bacterial sacroiliitis.