What causes a persistant cyst that comes out from the side of my knee replacement it was surgically removed and a poly exchange and it's back again?

Fluid collection: Cysts are typically comprised of joint fluid that escapes the joint through a communication between the joint and the subcutaneous space. The questions is whether there is still a cause for persistent fluid collections. Probably, entities such as infection, instability, loosening should be ruled out. You should talk to your surgeon.

Possible infection: I would worry about an infection in the knee replacement. Need fluid from knee analyzed as well as bloodwork.