Ortho Dx: A calcified mass in the back of the knee

An 82-year-old man presents with a several-year history of right knee pain. He has been seen in the past for right knee osteoarthritis and has received steroid injections, which have worked well. Another steroid injection is planned, but lateral radiograph reveals a calcified mass in the back of the knee. The radiograph shows a well-circumscribed oval ...