Introduction
This booklet aims to help people who have osteoarthritis of the knee, and their families and friends. We first explain how osteoarthritis of the knee develops, how you can recognize the symptoms, and how doctors diagnose and treat it. We then offer hints and advice on living with it more easily, including answers to common questions.
Near the end of the booklet you will find addresses of organizations that can offer further help, including information on how to contact the Arthritis Research Campaign (arc). There is also a brief glossary of medical words (like cartilage). We have put these in italics when they are first used in the booklet.
What is osteoarthritis?
Osteoarthritis is a disease which affects the joints in the body. The surface of the joint is damaged and the surrounding bone grows thicker. 'Osteo' means bone and 'arthritis' means joint damage and swelling (inflammation). When joints are swollen and damaged they can be painful. They can also be difficult to move. Some other words are used to describe osteoarthritis, including 'osteoarthrosis', 'arthrosis' and 'degenerative joint disease'.
Osteoarthritis of the knee is a very common form of osteoarthritis. Other joints which are often affected include joints in the hands, the spine, the hip joint and the big toe joint (see arc booklet 'Osteoarthritis').
How does osteoarthritis of the knee develop?
To understand how osteoarthritis develops you need to know how a normal joint works. A joint is where two bones meet. Most of our joints are designed to allow the bones to move in certain directions. The knee is the largest joint in the body, and also one of the most complicated because it has many important jobs to do. It must be strong enough to take our weight and must lock into position so we can stand upright. But it has to act as a hinge, too, so we can walk. It must also withstand extreme stresses, twists and turns, such as when we run or play sports.
The knee joint is where the thigh bone (femur) and shin bone (tibia) meet. The end of each bone is covered with cartilage which has a very smooth, slippery surface. The cartilage allows the ends of the bones to move against each other almost without friction. The knee joint has two extra pieces of cartilage (called meniscal cartilages or menisci) which help to distribute the load evenly within the knee. A normal knee joint is shown in Figure 1.
The joint is surrounded by a membrane (the synovium) which produces a small amount of thick fluid (synovial fluid). This fluid helps to nourish the cartilage and keep it slippery. The synovium has a tough outer layer called the capsule which helps hold the joint in place. The knee cap (patella) is another important part of the knee joint. The underneath of the patella is also covered with cartilage. The patella is attached to the thigh muscles by a very large tendon. The patella is fixed to the bone just below the knee joint at the front of the tibia.
The tendons are strong connecting tissues which attach the muscles to the bones on either side of the joint. They also help to keep the joint in place. When a muscle contracts it shortens, and this pulls on the tendon attached to the bone and makes the joint move. Figure 2 shows how the muscles are attached to the bones above and below the joint.
The knee joint is held in place by four large ligaments. These are thick, strong bands which run within or just outside the joint capsule. Together with the capsule, the ligaments prevent the bones moving in the wrong directions or dislocating. The thigh muscles (quadriceps) also help to hold the knee joint in place.
When a joint develops osteoarthritis, the cartilage gradually roughens and becomes thin. This happens over the main surface of the knee joint or at the cartilage underneath the patella. The surrounding bone reacts by growing thicker. The bone at the edge of the joint grows outwards (this forms osteophytes or bony spurs) (see Figure 3). This bone growth can affect both the femur and the tibia, as well as the patella.
The synovium swells slightly and may produce extra fluid, which then makes the joint swell. This extra fluid causes what some people call 'water on the knee'.
The capsule and ligaments slowly thicken and shrink, as if they were trying to push the joint back into shape. The muscles that move the joint gradually weaken and become thin or wasted. This can make the knee joint unstable so that it 'gives way' when you put weight on it.
When we look at an osteoarthritic joint under a microscope, we see that the joint is trying to repair itself. All the tissues are more active than usual. New tissues, such as the bony spurs (osteophytes), are produced to try to repair the damage. In some types of osteoarthritis, especially in the small finger joints, the repair is successful. This explains why many people have osteoarthritis but experience very few problems. Unfortunately, in osteoarthritis of the knee the repair does not usually work. Osteoarthritis may then seriously affect the joint, making it painful and difficult to move.
Osteoarthritis is a slow process that develops over many years. In most cases there are only small changes which affect only part of the joint. Sometimes, though, osteoarthritis can be more severe and extensive.
In severe osteoarthritis the cartilage can become so thin that it no longer covers the thickened bone ends. The bone ends touch, rub against each other, and start to wear away. The loss of cartilage, the wearing of bone, and the bony overgrowth at the edges all combine to change the shape of the joint. This forces the bones out of their normal positions and causes deformity.
What causes osteoarthritis of the knee?
Many factors seem to increase the risk of osteoarthritis developing in the knee joint. The risk does increase as we get older, but osteoarthritis of the knee joint is not a problem in all elderly people. It often runs in families. Genetic factors are very important. Genes may affect collagen, one of the main building blocks of cartilage, or the way the bone reacts and repairs itself, or even the inflammatory process.
Osteoarthritis of the knee is twice as common in women as in men. It mainly occurs in women who are over the age of 50, but there is no strong evidence that it is directly linked to the menopause. It is often associated with mild arthritis of the joints at the end of the fingers (causing bony swellings called Heberden's nodes).
Osteoarthritis of the knee is also more common in some racial groups than others. For example, it is more common in Afro-Caribbean people than in white people.
Osteoarthritis of the knee is common in people who are overweight, especially middle-aged women. Being overweight also increases the chances of osteoarthritis getting worse once it has developed.
Normal use does not normally lead to osteoarthritis, and neither does exercise (including running) unless it is excessive. However, injuries to the knee joint often lead to osteoarthritis in later life. A common cause is a tear of the meniscal cartilage or ligaments after a twisting injury. This is a common injury in footballers, who can face extra risks. The damaged cartilage can lead to osteoarthritis in later life, and we now know that the operation to remove the torn cartilage (meniscectomy) substantially increases the risk of osteoarthritis developing after a number of years.
Does osteoarthritis of the knee vary for different people?
Osteoarthritis of the knee affects different people in different ways. Some people have a problem with only one knee, others with both knees. Pain is the main problem for some people, while others find their main problem is difficulty in walking. Some people may notice little change in their condition over the years, while in other people the osteoarthritis keeps getting worse. As a result, it is not very helpful to compare the experience of one person with another, and we cannot predict the eventual outcome for any one individual with osteoarthritis.
How can I tell if I have osteoarthritis of the knee?
People with osteoarthritis of the knee joint usually complain that the knee is painful or aching. Your knee joint may feel stiff at certain times, often in the mornings or after rest. Walking for a few minutes usually eases the stiffness. You may have pain all around the joint or just in one particular place, and the pain may be worse after a certain activity, such as using stairs. The pain is usually better when you rest. It is unusual to have pain in the knee joint which wakes you up at night, except in severe osteoarthritis.
You will probably find that your pain will vary. There may be good days and bad days, or even good and bad months, for no apparent reason. Changes in the weather may make a difference in some people. All joints have nerve endings which are sensitive to pressure. The nerve endings may respond to the drop in atmospheric pressure which occurs before it rains.
If you develop more severe osteoarthritis, your movement will be restricted. Walking any distance or climbing stairs can be a problem. Sometimes your knee joint may give way because of weak thigh muscles or damaged ligaments.
How do doctors diagnose osteoarthritis of the knee?
Your doctor will be looking out for the problems mentioned above. When your joints are examined, your doctor can feel the bony swelling and creaking of the joint and see any restricted movement. Your doctor will also be looking for tenderness over the joint, and any extra fluid.
The thigh muscles are usually thinner and weaker than normal. With very severe osteoarthritis in the knee, the knee joint will tend to give way because of the damaged ligaments.
