EXPANSILE SUBCHONDRAL DEGENERATIVE BONE-CYST SECONDARY TO OSTEOARTHRITIS

A 53-year-old man known to have osteoarthritis of the left knee was ex amined because of a soft-tissue swelling overlying the pes anserinus t endon. Plain radiography and computed tomography revealed an expansile subchondral cyst of the proximal tibia. The cyst was multiloculated, had a sclerotic margin and an extraosseous soft-tissue component, and interrupted the cortex. Proof of a degenerative cyst was obtained by o pen biopsy. Such presentation in the knee is rare. Awareness of the po ssible aggressive behaviour of such degenerative cysts should help avo id confusion with neoplasm.