My Legs Feel Like They Can't Hold Me Up. I

My legs feel like they can't hold me up. I started complaining about difficulty walking when I had two total knee replacements in 1999 and my knees felt unusually heavy. As the years have gone by, my legs feel heavier and heavier and I have a burning sensation in them all the time. I've had MRIs of my spine which do not give a clue. My legs hurt so much I have to sit most of the time. Could it be just plain old arthritis, which I know I've had for many years?