80-year-old female patient with pain and mass in the lateral knee. Severe osteoarthritis affects the knee, particularly the lateral and patellofemoral compartments, with areas of full-thickness cartilage loss, large osteophyte formation, and subchondral degenerative marrow signal changes. There is extensive degeneration and degenerative tearing of the lateral meniscus. There is also intrasubstance degeneration of the medial meniscus without discrete tear. Enhancement within the knee joint indicate synovitis. Subchondral enhancing foci are likely degenerative in etiology, although these may conceivably represent chronic erosions. There is a small joint effusion with a large loose body in the lateral synovial recess. The cruciate and collateral ligaments as well as the extensor mechanism are intact. There is a lobulated mass of fluid signal intensity within the subcutaneous fat of the lateral aspect of the knee, which measures approximately 4.5 x 1.5 x 4.5 cm (AP x ML x CC) and demonstrates thin peripheral and septal enhancement. This may represent an extruded parameniscal cyst or a ganglion.
1.Severe osteoarthritis of the knee with extensive degeneration and tearing of the lateral meniscus.2.Fluid signal intensity mass within the lateral subcutaneous fat of the knee is compatible with either a large parameniscal cyst or a ganglion.