How To Treat Muscle Tightness And Soreness On Knee After Having A Replacement?

Hi I had a complete knee replacement 6 yrs ago. Now all the muscles in the front of my leg ,from the top of my knee up to my groin all tightened up . I have stretched them out but now the muscles r still sore and I have pain also across the top of my knee. I have to use a cane to get around.