Yo cable_man don't get a knee replacement. Get physical therapy & learn how to do the therapy by yourself on your own; so long as you're not in pain.
I'm 59 now & back at 50 I hyperextended my knee stepping in a gopher hole. I wanted a MRI when the xray showed negative bone issues & it felt like I had bone on bone. Doc told me "MRI after physical therapy elsewise you pay 100% yourself."
3 visitas a week & 8 weeks later I figured out how to perform the exercises from physical therapy. It cost me about $1200.00 out of pocket instead 0f $2800.00 for the MRI.