The most common reason for knee replacement surgery is knee pain, usually from osteoarthritis (“wear and tear”). However, what you may not have realized, is that when you tore you ACL in your 20s, it may have initiated a slow process of joint deterioration of your knee joint which caused it to “wear out” or develop arthritis. Another reason for “wear and tear” or osteoarthritis developing in your knee joint, could be simply because of your posture. For instance if you are “bow-legged” or “pigeon-toed” and have been all your life, then it will speed up that wear and tear on your knees. (Consider the tires on a car – if they are not aligned properly, then your tires wear out quicker and need to be replaced). Also included in posture, would be if you are overweight. The additional weight increases pressure on the knee joint, thus causing wear and tear. Every pound of excess weight exerts about 4 lbs of extra pressure on the knees. So a person who is 10 lbs overweigth has 40 lbs of extra pressure on their knees. Or, on a lighter note, losing 10 lbs is equal to 40 fewer pounds per step that your knees must support.