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Information on knee replacement and some frequently asked questions

Knee replacements

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The knee is the largest joint in the body and it is also one of the most complex.

The knee joint is made up of four bones, which are connected by muscles, ligaments, and tendons:

  • the femur, which is the large bone in the thigh
  • the tibia, which is the large shin bone
  • the fibula, which is the smaller shin bone, located next to the tibia
  • the patella (otherwise known as the knee cap), which is the small bone in the front of the knee.

With time, the bones may wear out. When this happens the joint becomes steadily more painful and eventually a knee replacement is the only way to get rid of the pain and improve your quality of life.

What is a knee replacement?

Joint replacements are nearly always carried out because of pain that cannot be controlled by other methods such as weight loss, muscle strengthening, painkillers, physiotherapy or other surgery. The most common cause of pain is osteoarthritis.

A total knee replacement procedure consists of replacing the diseased and painful joint surfaces of the knee with metal and plastic components shaped to allow continued motion of the knee. There are many different types of knee replacement available.

The traditional type of total knee replacement involves replacing the bone at the lower end of the femur (thigh bone) and replacing the bone at the top of the tibia.

On occasion, only part (either the inner or outer bit) of the knee joint needs replacing. This type of procedure is called a unicondylar knee replacement.

You should ask your specialist about what knee replacement they recommend and what the advantages and disadvantages of it are.

Introduction to knee implants

Almost all total knee replacements use a metal femoral component that is fixed to and curves around the bottom end of the thigh bone (femur). This moves against a hard- wearing plastic (polyethylene) liner. This liner is normally attached to a metal tibial tray at the top of the shin bone (tibia) but can be fixed directly to the shin bone. The tibial component will have a stem going into the shin bone to provide additional stability.

Implants can be cemented (fixed with bone cement) or cementless (without bone cement).

Implant types

Bicondylar or total condylar knee replacement: an implant attached to both parts of the shin bone and thigh bone. A knee cap replacement (patella) is often used as well.

Patellofemoral: a two-piece knee implant that provides a joint between the knee cap (patella) and thigh bone. The patella component is made of hard-wearing plastic.

Unicondylar: an implant attached to only one part of the shin bone and one part of the thigh bone, sometimes described as a partial knee replacement.

Implant stability levels

With knee replacement procedures, different implant types offer different levels of control over stability, or constraint, within the joint. The level of stability offered by the implant and the best option for a patient will be determined by their needs in consultation with the surgeon – instability is a reason for failure of knee implants.

The following types of constraint (stability levels) are analysed by the NJR:

The artificial components making up the knee joint are not linked to each other and have no stability built-in. It relies on the patient’s soft tissue, ligaments and muscles for stability.

Some stability is built in to these designs, the cushion of the plastic tray component fixed to the shin bone has a raised surface with an internal post that fits into a special bar (called a cam) in the femoral (thigh bone) component. The pieces work together to provide additional stability to the knee as it bends and straightens.

Where the components fitted to the thigh and shin bone are attached with a hinge type mechanism. This is used when a patient’s knee is highly unstable and the soft tissue and ligaments would not be able to support other types of replacement for example, in severely damaged knees or where a very elderly patient is undergoing a re-do (revision) replacement. This type of joint will have greater limitations on the range of movement and is not expected to last as long as other types. It requires more invasive surgery to place larger implant stems into the thigh and shin bone and also puts additional stress on the bones.

Bearing surfaces

The plastic insert component fixed to the shin bone is attached firmly to the metal part beneath. The metal femoral component, attached to the thigh bone, rolls on this cushioned surface. This type of bearing may reduce the level of pain experienced following the procedure but in some cases, excessive activity or weight gain can cause a fixed bearing to wear down more quickly.

The plastic insert is less firmly fixed to the metal part beneath, it is more mobile and requires support from the soft tissues around the joint. This allows it to rotate short distances. It may allow more movement but there is a possibility of dislocation.

Thinking about your treatment options

Joint replacement is a highly successful operation that can bring relief from pain and improve mobility. Joint replacements are very hard-wearing and most patients will never need further surgery. However, going through the process can be baffling and many patients do not always understand their options or the detail of their treatment plan. It is important you feel supported by your surgeon when discussing your available options.

Working together with your practitioner is known as shared decision-making.

Shared decision-making recognises all the different factors in your life that will lead to better quality decisions, from your own research, your surgeon’s advice, through to the support from your family and friends.

Sources such as this National Joint Registry information should make you feel confident in asking questions about your surgery, your implant and your recovery. There are some commonly-asked questions below. You may wish to write these out and make some notes.

Whatever the reason for your joint replacement, there are many others going through the same process, and it may help to know you are not alone. There is additional support out there for you on shared decision-making as well as advice on looking after yourself before and after surgery.

FAQs

As you make plans for your joint replacement surgery it may be helpful for you to make a note of any questions or queries you may have either for your surgeon or clinical team at the hospital. Some commonly asked questions are: 

  • Do I need joint replacement surgery? Are there other options available to me?
  • If I do, how should I prepare for surgery?
  • What type of implant are you recommending? What are the pros and cons?..
  • What surgical technique would be used? What are the pros and cons?
  • What should I know about my aftercare?
  • What do I need to think about when I return home after my operation?

Further information about knee replacement

There are many organisations that provide additional general information about joint replacement, including specific guidance before and after surgery as well as online discussion forums. Please see links here for some of these, such as: patient groups including Versus Arthritis; and The British Hip Society (BHS) who all have further useful advice.

The National Institute for Health and Care Excellence (NICE) recently published Quality Standard Guidelines for Joint Replacement (hip, knee and shoulder). This quality standard covers care for adults before, during and after primary elective hip, knee or shoulder joint replacement and is another useful source of reference. You can find this document here

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