Expansile subchondral degenerative bone cyst secondary to osteoarthritis.

A 53-year-old man known to have osteoarthritis of the left knee was examined because of a soft-tissue swelling overlying the pes anserinus tendon. 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 open biopsy. Such presentation in the knee is rare. Awareness of the possible aggressive behaviour of such degenerative cysts should help avoid confusion with neoplasm.