Fig a: Large joint effusion.
Fig b: Gout. Extensive soft tissue swelling and juxta-articular erosions with soft tissue mineralization, most pronounced at the popliteus hiatus on lateral femoral condyle.
Fig c: Prepatellar bursitis. Extensive prepatellar soft tissue swelling.
Fig d: Septic arthritis. Medial compartment arthroplasty with effusion, erosion of the inferior patella and subcutaneous gas. Patella alta consistent with patellar tendon avulsion.
There is a broad differential diagnosis for the atraumatic red, hot swollen knee, including crystalline arthropathy, inflammatory arthritis, and septic arthritis. Due to overlap in radiographic manifestations, diagnosis usually requires clinical history and arthrocentesis.
Look for joint effusion (fig a), soft tissue swelling and hints of underlying etiology:
- Inflammatory arthritis – diffuse periarticular osteopenia, soft tissue swelling, symmetric joint space narrowing
- Crystaline arthropathy- soft tissue tophi, juxta-articular erosions at tendinous and ligamentous insertions. (fig b)
- Prepatellar bursitis- soft tissue swelling localized to the soft tissues anterior to the patella and patellar tendon (fig c).
- Septic arthritis- rapidly progressive chondrolysis and osteolysis, joint effusion, intra-articular gas (fig d)