This 3D animation from The Human Atlas explains Osteoarthritis of the hip.

The skeletal system protects the internal organs and provides the framework for the body. The hip joints are located in the pelvis. They connect the torso to the legs and support most of the body's weight.

The bones of the pelvis--the pubis, the ischium, and the ilium--form a ball-and-socket joint together with the head of the femur, the long thigh bone. The head of the femur and the socket (acetabulum) are covered with a layer of smooth cartilage. The cartilage cushions the joint and allows the femur to move with very little friction.

Osteoarthritis is a degenerative joint disease that is caused by the chronic breakdown and eventual loss of cartilage within the joints. As the cartilage wears away, the bones that meet at the joint begin to rub against each other. This can cause extreme pain and can severely reduce movement and flexibility of the joint.

Growths of bone, called bone spurs, can also form around the edges of the joint and cause pain. Joint swelling can also occur if the synovial membrane lining the joint becomes irritated, producing excess fluid that collects inside the joint.

More than half of the population age 65 or older have osteoarthritis in at least one joint. Osteoarthritis usually results from injury to a joint or from wear and tear over time. Heredity, lack of use, and being overweight also contribute to the development of osteoarthritis.

Treatment can include weight loss, physiotherapy, and medication. If the condition becomes severe and mobility is greatly reduced, hip replacement surgery may be necessary.