Why could knee replacement implants fail?

There are different reasons why implants can fail. Most implants nowadays, regardless of the manufacturer, are very well-made and function very well. Implants typically fail when they don't attach well to the bone. So most surgeons nowadays use bone cement to attach the implants to the bone. Some surgeons don't use bone cement and just allow ingrowth to happen with the bone and the implant. Either way you do it, sometimes they do not attach and they can loosen and that can cause some toggling on the bone and some pain. That is a reason for a revision surgery.