Septic knee: multimodality approach.

Septic arthritis of the knee is an acute infection that can rapidly destroy articular cartilage if not promptly recognized and treated. Early diagnosis is often difficult because symptoms overlap with crystal or inflammatory arthropathies and post-traumatic effusions, while fever and laboratory markers may be absent or nonspecific. Synovial fluid analysis remains the diagnostic cornerstone, but imaging is essential to confirm effusion, define the extent of synovitis and periarticular involvement, and detect complications such as osteomyelitis or abscess. Radiography provides a rapid baseline, ultrasound is highly sensitive for effusion and guides aspiration, and CT is valuable for cortical destruction and deep soft-tissue mapping. MRI offers the most comprehensive early assessment, revealing synovial enhancement, bone-marrow edema, and occult abscesses. Nuclear medicine studies, including labeled leukocyte scintigraphy and FDG PET/CT, are useful in complex or periprosthetic settings and for monitoring therapy. Staphylococcus aureus remains the leading pathogen, followed by streptococci and Gram-negative bacilli, with atypical and fungal infections mainly in immunocompromised hosts. A stepwise, multimodality imaging strategy enhances diagnostic confidence and guides timely intervention, while advances such as metal-artifact reduction, radiomics, and hybrid imaging hold promise for improving accuracy and standardization in native knee infections.