Osteoarthritis (OA) is staged according to how much damage has occurred in the joint and how much joint space is left. X-rays are used most often to stage OA. In advanced stages of OA, there is a complete loss of joint space.

The femur (thigh bone) and the tibia (lower leg bone) have no cushion between them. When you use the knee, the bones slide across each other without any protection or lubrication. We call this bone-on-bone.