Hip osteoarthritis (degenerative arthritis of the hip)

What is osteoarthritis?

Arthritis means "joint inflammation". It causes pain and swelling in the body's joints such as the knees or hips. There are many types of arthritis, but osteoarthritis is the most common type. Also known as degenerative joint disease, osteoarthritis is more likely to develop as people get older.

Osteoarthritis usually occurs when microtrauma (injury at a microscopic level) to the joint or some other unknown factor triggers an immune reaction, which - in an ill-fated attempt to repair the damage - causes inflammation that leads to a breaking down of cartilage tissue that causes pain, swelling and deformity.

Cartilage is a firm rubbery material that covers the ends of bones in normal joints. It is primarily made up of water and proteins. The primary function of cartilage is to reduce friction in the joints and serve as a "shock absorber". The shock-absorbing quality of normal cartilage comes from its ability to change shape when compressed. It can do this because of its high water content.

When the cartilage is damaged - for example in a joint stressed by abnormal weight bearing loads - attempts by the immune system to repair the damaged cartilage cause it to swell. The cartilage becomes thin, soft and cracked, exposing the bone beneath and leading to the formation of small cysts and new outgrowths of bone called osteophytes, which further disrupt the way the joint works and aggravate the problem. The joint space becomes narrowed, further altering the mechanics and adding to the stresses within the joint. Other tissues in the joint such as the surrounding membrane (the synovium) ligaments and tendons may also be affected. This degeneration is a gradual process that may go on over many years although there are occasional exceptions.

The two main types of osteoarthritis are:

• Primary: More generalised osteoarthritis that affects the fingers, thumbs, spine, hips and knees
• Secondary: Osteoarthritis that occurs after significant or macroscopic injury or inflammation in a joint

How does osteoarthritis affect the hip joint?

Patients who have osteoarthritis of the hip sometimes have problems walking. Diagnosis can be difficult at first. That's because pain can appear in different locations including the groin, thigh, buttocks or knee. The pain can be stabbing and sharp or it can be a dull ache, and the hip is often stiff.

What causes osteoarthritis of the hip joint?

The causes of osteoarthritis of the hip are not known. Factors that may contribute include joint injury, increasing age and being overweight.

In addition osteoarthritis can sometimes be caused by other factors:

• The joints may not have formed properly. This is increasingly thought to be to blame when osteoarthritis develops when a person is young or in midlife - there are often problems with the shape of the bones in the hip joint, which may be inherited. These abnormalities may be very small but enough to mean that it doesn’t work effectively as a ball-and-socket joint, leading to mechanical stresses in the joint.
• There may be genetic (inherited) defects in the cartilage or possibly in the immune system, making it react abnormally to damage in the joint.
• The person may be putting extra stress on his or her joints, either by being overweight or through activities that involve the hip such as running or other intensive weight-bearing sports.