My OH had a partial knee replacement last year; his knee previously kept collapsing after he had injured it. The operation has been successful in that at least he now can walk without a stick - but he has had a lot of discomfort and pain; x-rays show nothing but his consultant believes his body is trying to reject the 'foreign body'. Apparently this is very very rare. A year on it has settled down a lot.

Full knee replacements are much more established and I know a number of people who have had very successful results; it has literally given them a new lease of life.

Good luck!