What Causes Severe Knee Pain After A Knee Replacement Surgery?

I had total left knee replacement June 2007 - osteoarthritis. The whole knee area has always ached since the replacement surgery. I have had physical therapy several times, and done strengthening exercises, but no let-up of the pain/ache. Orthopedic drs, as well as general practice drs, have no answers. I ve had xrays several time by a couple different ortho drs, but they have no answers. I ve got OA in most joints, my back, hips and knees being the worst. Probably going to have back surgery next fall. New problem: my left hip has gone totally south and I am going to get it replaced in the next 45 days. It started bothering me last June, and the pain has gradually gotten worse. As my hip has gotten worse, my knee pain has been a much greater challenge. Been on Norco for 2-3 years, and moved to Percocet two months ago. I take 3/daily, but really one more for overnight coverage. I am 72 years old. I understand, kind of, whats going on, the knee aches and pains have really baffled me. Any idea or suggestions?