Usually with knee replacements the problem is arthritis in the joint, sometimes caused by overuse/abuse. A lot of AT hikers end up with bad knees from carrying too much weight up and down steep rocky slopes.
My husband had a knee replacement,obviously a different kind than Ken had. He was on a walker for a month, then began regular walking. In six months he was on the trail (PCT). Unfortunately, problems with his other knee (torn meniscus) took us off the trail a month later. About a year later, his new knee began causing serious problems. He can do very short hikes, but is in a lot of pain after 3-4 miles. The doctors have no answers as to why it went bad or how they can fix it besides tearing out the joint and starting over.
_Ginny_