I had a right knee replacement in early 2012. It appears it's worn out. The doctor doesn't know how much will have to be replaced, from only the plastic between the metal parts to a complete replacement. I have lowered my saddle and use lower gears so I can keep riding through the summer - I had already scheduled (and paid for) the Erie Canal ride - 400 miles in 8 days (but mostly flat). I made a rough estimate that in the 13 years since the replacement, my right leg has gone up and down on the pedals about 10 million times - my goal is to average 10 miles a day and I beat it almost every year. I think the 30 year life of a replacement knee may not apply to active cyclists.