What Causes Painful Swelling And Tightness In The Knee Long After Total Knee Replacement?

I have a total knee replacement that was done 6 years ago. For the last year it has become very painful and after checking with the ortho physician it is not loose and is in good alignment. It is very swollen and tight. The doctor drained it and it was full of blood - - 4 vials and continued into a towel. I can t get an answer why that is happening. Any suggestions?