Knee pain can be caused by injuries, mechanical problems, types of arthritis, and other problems. A knee injury can affect any of the ligaments, tendons or fluid-filled sacs (bursae) that surround your knee joint as well as the bones, cartilage and ligaments that form the joint itself. Some of the more common knee injuries include ACL injury, fractures, torn meniscus, and knee bursitis. Some examples of mechanical problems that can cause knee pain include loose body, iliotibial band syndrome, a dislocated kneecap, and foot pain. More than 100 different types of arthritis exist. The varieties most likely to affect the knee include osteoarthritis, rheumatoid arthritis, gout, pseudogout, and septic arthritis.