Knee replacement 

A knee replacement operation involves replacing damaged, diseased or worn parts of the knee joint with a prosthesis made up of metal and plastic. A new knee joint can help improve mobility and reduce pain.

The knee is the largest joint and consists of the lower end of the thigh bone (femur), which rotates on the upper end of the shin bone (tibia), and the knee cap (patella), which slides in a groove on the end of the thigh bone.

Cartilage covers the ends of the thigh and shin bones, acting as a shock-absorber, while allowing the bones to move smoothly over each other. The joint is held together with ligaments, keeping the bones in the right position when bending or straightening.

If cartilage is worn away by arthritis or damaged by injury, the bones rub together, restricting movement and causing pain.

The knee joint has three compartments:

1. inner (medial)
2. outer (lateral)
3. kneecap (patellofemoral)

What parts are replaced?

Only the damaged cartilage and bone ends of the knee are replaced with metal and plastic parts (prostheses).

A single piece of curved metal replaces the end of the thigh bone, while a flat plate of metal replaces the top of the shin bone - a plastic bearing is fitted to this plate to replace cartilage.

There are two main types of knee replacement: total and partial. Your consultant will advise which is best for you, depending on the damage to your knee.

Total knee replacement

A total knee replacement means both inner and outer compartments of the knee are replaced.

Some knee prostheses require a cement to fix them in place, while others have textured surfaces to bond bone and the new joint together. A 'hybrid' knee fixation combines both.

The damaged surfaces of your thigh bone and top of your shin bone are removed and the ends shaped so the new metal parts fit.

Advantages:

• Stops or greatly reduces knee pain
• Improves quality of life
• Lasts for up to 15 years ⁽²⁾, with a second or even third replacement possible

Disadvantages:

• Hospital stay and recovery longer than partial replacement
• Blood loss may require transfusion
• Scarring may make kneeling uncomfortable
• Certain movements (bending the knee) might be difficult

Partial knee replacement

While total knee replacements are commoner, the partial (unicompartmental) procedure is a suitable alternative, if damage is limited to one compartment (usually the inner).

Osteoarthritis usually first affects the inner half of the knee and then spreads to the outer half, so only the damaged surfaces need replacing.

Advantages:

Partial knee replacement surgery is less invasive than total replacement, so hospital stay is shorter ⁽³⁾ and recovery quicker.

Disadvantages:

If arthritis in your knee worsens and a total knee replacement is required in the future, it may be more difficult to perform ⁽⁴⁾.

Kneecap replacement

For some, arthritis in the knee only affects the kneecap (patella) and the underlying groove, so it is possible to replace just those parts.

A patellofemoral replacement (patellofemoral joint arthroplasty), is less invasive than a total replacement and recovery is quicker.

What are the benefits of having the operation?

A knee replacement will reduce pain and improve mobility, so you can perform daily tasks more easily and lead a more independent life.

After six to eight weeks, you will be able to move around more easily and be able to drive, climb stairs, get up from a chair and walk up and down slopes without difficulty.

Gentle exercise, such as swimming, golf or cycling, will be possible, if you were fit before the operation. Most people are able to walk 30 minutes or more without feeling pain.

What happens during the operation?

A full knee replacement takes around two hours ⁽⁶⁾, with a hospital stay of around eight or nine days.

Surgery is normally performed under a general anaesthetic, but you can have an epidural anaesthetic, which numbs the lower half of your body and is less risky.

Infections and other complications are possible, depending on how quickly you heal. You will need to walk with crutches for between one and three months.

Refs:

1. NHS Direct: Knee Replacement/ Introduction/ Is knee replacement common?
2. NHS Direct: Knee Replacement/ Introduction/ How long does a knee replacement last?
3. Cedars-Sinai Institute for Joint Replacement
4. NHS Direct
5. Arthritis Research Campaign (ARC)
6. The Knee Society