My dad had his knee operated on, and now, 15 years later, it's starting to hurt. He hasn't done anything to hurt it again that he can think of. Otherwise he's in pretty good shape. Is the pain coming from his initial problem, or is this something new?

A previous knee injury or surgery can affect the health of the knee later on in life. A past leg or knee fracture, a ligament tear, or a meniscal injury can each cause abnormal movement and alignment in the knee joint, leading to wear and tear on the joint surfaces. Over the years, this can begin to take a toll on the knee joint, and may explain why your dad is feeling pain now.