What is Osteoarthritis?

Osteoarthritis

What is osteoarthritis?

Osteoarthritis is a condition that affects your joints. It's the most common joint problem. When you hear people, particularly older people, saying their arthritis is bothering them, they usually mean osteoarthritis.

Osteoarthritis can make your joints stiff and painful. It's most common in the joints in your knees, hips, hands and spine. Some people find that osteoarthritis makes it hard for them to do everyday things, such as getting out of chairs, reaching high shelves or combing their hair. Unlike other types of arthritis, osteoarthritis only affects your joints. It does not have an effect on any other part of your body.

Osteoarthritis can't be cured, but there are treatments that can ease the symptoms.

Key points for people with osteoarthritis

It's most common in older people.
Osteoarthritis is the most common joint problem. As many as 2 in 10 people over the age of 60 have osteoarthritis.
The parts of your body most commonly affected by osteoarthritis are the joints in your knees, hips, hands and spine.
Most people don't have a severe form of the condition. Only a few will need surgery to replace an affected joint. Surgery can work very well, but it doesn't work for everyone.

Asking questions and gathering information will help you cope with your disease. You may already know more than you think you do about osteoarthritis. To test your knowledge, see How much do you know about osteoarthritis?

What is a joint?

To understand osteoarthritis, it helps to know how your joints work.

This is a normal hip joint.

A joint is where two bones meet. Some joints don't move.

Osteoarthritis only affects joints that move, like your hip and knee joints.

Your hip joint

You can't see most of your hip joint from the outside of your body. It's where your thigh bone (doctors call it the femur) meets your hip bone (part of the pelvis).

The top part of the thigh bone is shaped like a ball, and the lower part of the hip joint is shaped like a socket. The way these bones fit together allows you to move quite a lot at your hip joint. It allows you to sit, kick a ball and climb stairs.

Your knee joint

This joint is formed by your thigh bone (femur), shin bone (tibia) and knee cap (the piece at the front of your knee that doctors call the patella).

This type of joint is called a hinge joint because it allows your knee to move in the same way a hinge on a door allows the door to move.

How does a joint work?

Joints are designed to cope with walking and jumping and repetitive movements. They are made of the following things.

Cartilage: This is a hard, slippery material that coats the end of a bone where it meets another bone. Cartilage allows bones to move smoothly without rubbing together. It also helps to stop too much stress being put on any one part of the bone. This stops bones from shattering when pressure is put on the joint: when you stand on one leg, for example.
Capsule: The parts of bones that join to make a joint are held together in a capsule. This is fibrous tissue that is made stronger in places by thicker bands of tough tissue called ligaments.
Muscles: Muscles around the joint help keep it in position and control its movement.
Synovial membrane: The joint capsule has a lining called the synovial membrane, which makes a fluid called synovial fluid. This fluid lubricates the joint and helps it to move smoothly. It also feeds the outer area of the cartilage and keeps it healthy.

What happens in osteoarthritis?

Osteoarthritis is a disease of the joints. It happens when the cartilage at the ends of bones becomes damaged. The bone in your joints then tries to repair the damage. (Bone can grow and mend itself.) But instead of making things better, in osteoarthritis the bone grows abnormally and makes things worse. For example, the bone can grow into a knobbly shape and make the joint painful and unstable.

No one really understands why bone does this. And it's not clear if the cartilage damage happens before the bone grows abnormally.

What happens in the joint?

Osteoarthritis is a complicated disease. All the parts of a joint depend on each other, and when something goes wrong in one part, it affects the others. No one is sure whether the cartilage breaks down first or whether the bone does. But changes are seen in both. The cartilage and bone are both destroyed and then repaired abnormally.

Cartilage becomes thinner and softer, and shows signs of pitting (having little holes in it) and roughening. It may split in places.
The bone cells under the cartilage grow in an abnormal way. They break down and repair themselves more quickly than usual. The new bone that grows under the cartilage is thicker than usual in some places but thinner in others. It may develop tiny cracks in the thinner areas. It may also get stiffer and be less able to act as a "shock absorber." Meanwhile, the surface of the joint loses bone in some places while it forms too much new bone in other places. This bone growth happens faster than it normally would. And, in places, the bone forms outgrowths called spurs that have a "cap" of cartilage. These bone spurs can change the shape of joints, which means they can't move so well.
Bits of bone or cartilage can break off into the joint.
The synovial membrane, the thin layer lining the capsule of the joint, becomes red and inflamed because of all the change in the bone and cartilage. It produces more fluid than usual, and this fluid has cells in it that normally fight infection in the body.
Muscles and ligaments, the fibers around the joint, become weaker and smaller. This makes the joint less stable. No one knows why the muscles become weaker. They may become smaller because they get used less.
The bones in the joint can get out of line with each other because of all the changes in the bone. The surfaces of the joint may no longer fit snugly. Instead, they rub against each other in a way that causes pain. The ends of the bones can get out of shape.

Why me?

Osteoarthritis is not due simply to aging or wear and tear of the cartilage at the end of the bones. It's a disease that affects all of the joint: the cartilage, the bone, the membrane, the fluid within the joint, the ligaments and the muscles. Exactly what triggers the condition in one person and not in another is unknown. And it may be that what causes osteoarthritis in one joint is different from what causes it in another.

However, there are certain things that make some people more likely than others to get osteoarthritis. These are called risk factors. Being older and being overweight are two common risk factors for the disease.

To learn more, see Risk factors for osteoarthritis.

This information was last updated on Jan 07, 2009

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment.
© BMJ Publishing Group Limited 2009. All rights reserved.

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