Other than injuries, knee complaints are a common topic of conversation amongst those who have been on this earth for some time. Basically you have got two bones stacked on top of each other, strapped together by ligaments, with a couple of cartilage washes in the middle taking up the slack. It's biomechanically difficult for the knee joint to be sound. So the strength in the knee comes from the ligaments and from muscles in the thigh that pass over and support the joint.
You may be one of those people that have inherited more elastic ligaments which consequently makes the joint unstable and prone to injury. Years of weight bearing from up above will also gradually wear the joint out which is when you might end up with a replacement.
Good knee health relies on stability and this is achieved by increasing the strength of the thigh muscles which helps hold the knee together. I have had a number of patients who have averted exploratory knee surgery by strengthening the thigh muscles. In fact I have cured my own knee pain by following these exercises and strengthened my thighs. If you are worried about your knees then see an osteopath then start on the SFL knee program and you will be amazed.