It can be caused by overuse of the knee, injuries to the joint, or having a kneecap that's not properly in line. The damage to the cartilage can be mild or severe. Patellofemoral pain syndrome: Also called runner's knee, this is a common cause of knee pain. It can be caused by overdoing physical activities, tissues around your knee that are too tight or too weak, and misalignment of your kneecap.