Why knee replacement is necessary 

Knee replacement surgery (arthroplasty) is usually necessary when the knee joint is worn or damaged to the extent that your mobility is reduced and you experience pain even while resting.

How the knee works

The knee joint acts as a hinge between the bones of the leg and is effectively two joints. The major joint is between the thigh bone of the upper leg (femur) and the shin bone of the lower leg (tibia). The smaller joint is between the kneecap (patella) and the upper leg (femur).

A smooth, tough tissue called articular cartilage covers the ends of the bones, allowing them to slide smoothly over each other. The synovial membrane covering the other surfaces of the knee joint produces synovial fluid, which lubricates the joint, reducing friction.

If the articular cartilage becomes damaged or worn, the ends of the bones rub or grind together, causing pain and difficulty moving the knee joint.

Replacing the damaged knee joint with an artificial one can help reduce pain and increase mobility.

Causes of knee pain

The most common reason for knee replacement surgery is osteoarthritis.

Osteoarthritis in the knee occurs when the articular cartilage becomes damaged through natural wear and tear. The bones have little or no protection to prevent them rubbing against each other when the knee moves.

The bones may compensate by growing thicker and producing bony outgrowths to try to repair themselves, but this can cause more friction and pain.

Other conditions that may make knee replacement necessary include:

  • rheumatoid arthritis 
  • haemophilia 
  • gout
  • disorders that cause unusual bone growth (bone dysplasias)
  • death of bone in the knee joint following blood supply problems (avascular necrosis)
  • knee injury
  • knee deformity with pain and loss of cartilage

When is surgery necessary?

You may be offered knee replacement surgery if:

  • you have severe pain, swelling and stiffness in your knee joint and your ability to move the joint is significantly reduced
  • your knee pain is so severe that it interferes with your quality of life and sleep
  • everyday tasks, such as shopping or getting out of the bath, are difficult or impossible
  • you are feeling depressed because of the pain and lack of mobility
  • you cannot work or have a normal social life

Read information about how knee replacement surgery is performed.

Surgical alternatives to knee replacement

There are alternative surgeries to knee replacement, but results are often not as good in the long term. These are described below.

Arthroscopic washout and debridement

An arthroscope (tiny telescope) is inserted through small incisions in the knee. The knee is washed out with saline and any bits of bone or cartilage are cleared away. It is not recommended if you have severe arthritis.


A keyhole operation in which small holes are made in the surface layer of bone with a small, sharp ‘pick’. This allows cells from the deeper, more blood-rich bone beneath to come to the surface and stimulate cartilage growth. It can be a good option if you have just a small area of damaged cartilage. However, the benefits are not well proven and the results are not as good as knee replacement for severe arthritis.


An open operation in which the surgeon cuts the shin bone and realigns it so that weight is no longer focused on the damaged part of the knee. It is sometimes used for younger people with limited arthritis, where it may enable a knee replacement to be postponed. However, you will usually need a knee replacement at a later date, and the operation may make knee replacement surgery more difficult if it is needed.

Autologous chondrocyte implantation (ACI)

New cartilage from your own cells is grown in a test tube and introduced into the damaged area. It is usually used for accidental injury to the knee rather than arthritis. As yet, ACI is only available as part of a clinical trial.

Mosaicplasty (cartilage replacement)

A keyhole operation that involves transferring plugs of hard cartilage, together with some underlying bone from another part of your knee, to repair the damaged surface.

Read the NICE guidance on mosaicplasty for knee cartilage defects.

Knee replacement illustration

1. Upper prosthesis
2. Lower prosthesis
3. Tibia
4. Femur

Life with osteoarthritis

Kathleen Smith describes life with osteoarthritis and how swimming helps with the pain

Page last reviewed: 16/06/2014

Next review due: 16/06/2016