Total knee replacement
The normal knee The knee joint has three parts: your thigh bone (the femur), shin bone (the tibia) and knee cap (the patella). This type of joint is called a hinge joint because your knee joint moves in a similar way to a hinge on a door. The surfaces of the joint are covered by articular cartilage, which is a firm slippery material about 3mm thick. A small amount of lubricating fluid is present and aids movement. This allows painless and effortless movement of the joint even under a load. The knee has four ligaments which hold the joint together and prevent unstable movement. They are tough fibrous bands attached at each end to the bone. Although the knee appears to act as a hinge, it can rotate and stretch in smaller amounts as well.
The knee takes your body weight and it must cope with walking, running, crouching, bending and lifting objects. To do this it has powerful muscles and a large range of movement. The two most important muscle groups are the quadriceps and the hamstrings. The quadriceps is a big muscle group of the front of the thigh. It straightens the knee. The hamstrings are at the back of the thigh and they bend the knee. These muscles control knee movement and are vital for the stability of the joint.
When the knee becomes arthritic
As we get older many people will develop “wear and tear” arthritis of the knee, although some will have rheumatoid arthritis which also involves other joints. Many factors may contribute to having arthritis; obesity, accidents, vigorous sport or a family history may all play a part. In osteoarthritis (wear and tear), certain changes occur in the joint. The smooth cartilage becomes flaky and develops small cracks. The bone underneath the cartilage becomes denser. The lining of the joint becomes inflamed and may thicken up. As the arthritis progresses, there may be: Severe wear of the cartilage allowing the bones to rub and grate together. Loss of the joint space. Formation of bony lumps called osteophytes. Swelling of the knee. Knock-knee or bow leg.
These changes may result in PAIN, LOSS OF MOVEMENT and LOSS OF MUSCLE POWER.
The artificial joint
The artificial knee joint closely follows the shape of the real joint. It has been designed and tested to replicate the function of the normal knee. There are many designs of artificial knee joint. Your surgeon will choose the most suitable for you. There are various types of knee replacement. A total knee replacement resurfaces the whole of your knee. This is the most common operation as more than one area of the knee is usually damaged.
A partial knee replacement resurfaces either the inner or outer half of the knee or the joint between the thigh bone and knee cap. Partial knee replacements only work for those patients with arthritis in one part of the knee. They are not an option if you have inflammatory disease such as rheumatoid arthritis, if the damaged area is more extensive, or if your knee has become very stiff. The combination of metal and plastic means the joint has low friction, wears very slowly and moves easily with your weight on it. Remember, only small slithers of bone damaged by arthritis are removed, not the whole knee.
Why do I need a knee replacement?
You should consider a knee replacement if:
- You have constant pain despite taking painkillers.
- You are unable to do everyday things and so are less independent
- You cannot walk very far now and may have to use a stick. Stairs can be very difficult.
- Your knee is getting stiff and you cannot bend it easily.
- You are unable to work because of your painful knee.
A knee replacement is a major operation. You will spend 3 – 4 hours in the operating department. The operation itself will take 1 – 2 hours. During a total knee replacement operation:
- The worn surfaces at the bottom end of the thigh bone (femur) and the top end of the shin bone (tibia) are removed.
- The surfaces are covered with metal implants fixed in place with bone cement.
- A smooth plastic insert is fixed to the top of the tibial implant. This ensures that the two surfaces glide smoothly together.
- Sometimes the underside of the knee cap (patella) is also re-surfaced with plastic.
- The layers of soft tissue, muscle and skin are repaired and stitched back together.
Partial knee replacement surgery will be slightly different depending on the part of the knee to be replaced.
You should be prepared to work hard at the exercises given to you by the therapy staff.
Most patients tell us that they are pleased with the result of their knee replacement. Some, however, are less satisfied either because a complication has arisen or their expectations are too high.