Septic arthritis may result from spread from a contiguous focus of osteomyelitis, hematogenous inoculation of the joint capsule and synovial membrane, or direct inoculation of bacteria after traumatic or iatrogenic injury to a joint. On radiographs, soft-tissue alterations such as swelling, joint effusion, and obliteration of the normal fat planes are detected in the early phase. When proteolytic enzymes are released from leukocytes and cartilage is destroyed, narrowing of the joint space, often associated with juxta-articular osteoporosis (which is caused by hyperemia and pain-related inactivity) may be observed. As the disease progresses, erosions of the subarticular bone may be found.
On MRI, intra-articular fluid collections can be identified with T2-weighted sequences. This is especially important in joints that are difficult to assess by physical exam and ultrasound. In T1-weighted images, joint effusion presents with low signal. On T2-weighted images, joint effusion exhibits high signal intensity, resulting in an “arthrographic effect”.
Intra-articular fluid, however, also can be identified in various other joint diseases, and the assessment of relaxation times does not allow for differentiation of pyarthrosis from other joint effusions. However, the destruction of cartilage due to bacterial arthritis is directly visible, especially in the presence of a joint effusion. In the initial phase of septic arthritis, cartilage edema may be identified on T2-weighted images. In later stages of the disease, a SI reduction is observed, which cannot be differentiated from osteoarthritis. Periarticular edema may indicate that a joint effusion is due to septic arthritis. An irregular SI reduction in the subarticular bone marrow in combination with a joint effusion is also an important clue to the presence of septic arthritis. When septic arthritis is suspected, fluid from the joint should be aspirated for microbiological analysis.