MRI Findings of Septic Arthritis and Associated Osteomyelitis in Adults

The purpose of this study was to describe the soft-tissue, synovial, and osseous MRI findings of septic arthritis. At 1.5 T (T1-weighted, T2-weighted or STIR, and contrast-enhanced images), 50 consecutive cases of septic arthritis were evaluated by two observers for synovial enhancement, perisynovial edema, joint effusion, fluid outpouching, fluid enhancement, and synovial thickening. The marrow was assessed for abnormal signal on T1- and T2-weighted images or after contrast enhancement. We noted whether the marrow signal was diffuse or abnormal in bare areas. MRI findings were compared with microbiologic, clinical, and surgical data and diagnoses. The frequency of MRI findings in septic joints was as follows: synovial enhancement (98%), perisynovial edema (84%), joint effusions (70%), fluid outpouching (53%), fluid enhancement (30%), and synovial thickening (22%). The marrow showed bare area changes (86%), abnormal T2 signal (84%), abnormal gadolinium enhancement (81%), and abnormal T1 signal (66%). Associated osteomyelitis more often showed T1 signal abnormalities and was diffuse. Synovial enhancement, perisynovial edema, and joint effusion had the highest correlation with the clinical diagnosis of a septic joint. However, almost a third of patients with septic arthritis lacked an effusion. Abnormal marrow signal-particularly if it was diffuse and seen on T1-weighted images-had the highest association with concomitant osteomyelitis.