What Causes Swelling And Pain In Knees Despite Having Total Knee Replacement?

Hi, yes I had total knee replacement and it been since May 27th of 2014. So about 10 months out and still haveing swelling and some pain. My son in law works in Ortho. P.A. looked at my x-rays and says everything looks good. I m in pool therapy now . And did 2 rounds of other PT I can t go down stairs normally yet sometimes up. Why is it takeing so long