This 3D animation from The Human Atlas describes the causes and physical effects of osteoarthritis of the knee.

In order to understand the changes that occur in the joints of a person with osteoarthritis, it is necessary to first understand how joints work. The two main bones of the knee are the femur and the tibia. In a normal knee joint, the connecting surfaces of these two bones are covered with cartilage -- a smooth and strong material that cushions the impact between the bones. The joint is also surrounded by synovial fluid, which aids in joint mobility.

In people with osteoarthritis, there is a chronic degenerative loss of cartilage resulting in bone to bone exposure, bone destruction, inflammation, and pain.

Osteoarthritis can affect any joint in the body and is commonly found in the neck, back, and hips. The cause of osteoarthritis is unknown, but aging plays a significant role in the development of this condition.