I had TKR about 8 years ago. I am 75 , play tennis and losing my bend!

I play tennis at least 3 times a week for 2 to 3 hours. The stiffness and flexibility are terrible still after all these years. I have gone back to the Dr. on 2 occasions, but x-rays show all is as it should be, what now? My current bend is LESS than 90%.