You felt a lump behind your knee and went straight to the worst-case scenario. In this episode, Dr. Schultz — a hip and knee surgeon who has had a Baker’s cyst himself — explains what a popliteal (Baker’s) cyst actually is, why an irritated or arthritic knee produces the extra fluid behind it, how to tell a benign cyst from something that needs imaging, and when a lump behind the knee truly warrants a call to your doctor.