For practical purposes, the Prepatellar Bursa is a "Subcutaneous" structure, so any code for this is going to be for a technically simple and uncomplicated procedure. "Loose Body" of the Prepatellar Bursa is usually a mass of fibrous tissue that may be relatively loose and mobile within the bursa, sometimes totally free. It can slide around in the bursa and be a nuisance. Although pathologically not a "tumor" in the sense that a "tumor" is an abnormal growing mass of tissue, it is a mass of fibrous tissue, and could be equated to a "tumor." As such, you could try 27327: Excision of "tumor" (loose body mass) of the knee area, subcutaneous, less that 3 cm. (which they usually are). If you don't like that code, then you could go with 27599: Unlisted procedure, knee, and pair it with 27327.
Hopefully this helps you.
Respectfully submitted, Alan Pechacek, M.D.