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If the prosthesis is not sitting properly in the knee, it can cause wear and tear and bleeding. This can cause a hematoma. Other than this it can be a slipped prosthesis, a Baker’s cyst (cyst in popliteal fossa at the back of the knee caused by fluid built up under pressure within the knee cavity in response to an inflammatory problem within the knee), an abscess, a lipoma or a muscle tumor. Try and consult a doctor and get the basic investigations done like X-ray of the joint and if this does not reveal anything then a MRI of the joint. For all you know, the problem may not be related to the prosthesis.
Hope this helps. Take care!
my mom had one of her knees replace & she eventually got fluid build-up which was very painful & made her knee swell up