I had a custom partial knee replacement about 6 months ago and recently developed a golf ball size lump where the the knee joint is is quiet painful. I am 40 years old and have been diagnose with arthritis through out my knee but not bad enough for a total knee replacement it was the joint under the knee cap that was replaced.
Hello, I have studied your case and I think you might be having infection in your knee that can cause such lump. These things should not be taken lightly, and you need to see doctor as soon as possible. If there is any infection then this fluid needs aspiration, and then it should be sent to culture, and then you should receive antibiotics accordingly.  I hope this answer will be useful for you. Let me know if there is any other followup questions. Thanks