Knee replacement

Surgery to replace a worn-out knee joint is very common. The outcomes are usually very good and much better than they were when the surgery was first introduced.

When a knee replacement is needed

The usual reason for a knee replacement is because of very painful arthritis in the knee.

A knee replacement is a major operation and is usually only considered when all other options have been exhausted. This is usually due to severe pain in the knee, which is not resolved by regular painkillers. It may affect walking, working and disturb sleep. Before considering surgery, regular painkillers, weight loss, exercise and physiotherapy will usually be recommended as "conservative" treatment options. When these are no longer working, a knee replacement may be a good option.

Most people who decide to have a knee replacement are already taking painkillers every day but are still not able to walk far and often need to use a stick. The research on knee replacements suggests that the people who do best after a knee replacement are those with severe arthritis but not so bad that the joint is completely destroyed.

One of the reasons for this is likely to be because it is important to have strong muscles around the knee in order to make the best recovery, and people who have the most advanced disease tend to have weaker leg muscles.

The main reason for needing to have a knee replacement is arthritis in the knee:

Osteoarthritis

Osteoarthritis (OA) of the knee is the most common reason for a knee replacement. This is often known as "wear and tear arthritis" and is a wearing away of the cartilage which sits on the ends of the bones in a joint and protects them from damage. It can be primary (by far the most common) or secondary:

• Primary osteoarthritis:

  • Is more common in people who have a close relative with osteoarthritis.

  • Is more common as people get older and in people who are obese.

• Secondary osteoarthritis:

  • Happens after some other damage to the knee, such as a cartilage injury or infection (septic arthritis).

Rheumatoid arthritis

Rheumatoid arthritis (RA) is a less common cause and knee replacements for this reason are reducing. Rheumatoid arthritis is an auto-immune inflammatory condition and usually affects other joints before the knees. Due to significant advances in treatments for rheumatoid arthritis, most people no longer need joint replacements.

Other

Any condition that can cause damage to the cartilage of the knee might result in needing a knee replacement, such as:

• Haemophilia.

• Sero-negative arthritis (other inflammatory conditions of the joints).

• Avascular necrosis (death of the bone in a joint due to blood supply problems).

• Gout.
  

Preventing knee replacements

The most effective treatment for the symptoms of osteoarthritis of the knee is weight loss. Losing weight also helps following a knee replacement, as people who are obese or overweight tend to heal less well after having the surgery.

Other treatments that are recommended for all patients with osteoarthritis of the knee include:

• General exercise - walking, etc.

• Strength training - to increase the strength of the muscles in the legs.

• Water-based exercise - swimming and water aerobics.

• Painkillers.

• Using a walking stick or cane. Use the walking aid on the side opposite to the affected (or worst) leg. For example, if you have a bad right knee, hold the walking aid in your left hand. Then move the bad leg and the aid at the same time, so that the load is shared.

Tests before knee replacement surgery

Usually about six weeks or so before the operation, there will be a 'pre-admission' or 'pre-assessment' clinic. At this clinic a nurse will assess fitness for knee surgery, including fitness for an anaesthetic.

There are several tests that may be needed and they include:

• Blood tests - to check that there is no anaemia and that the kidneys are working well enough for the operation to be performed.

• Specific blood tests - people with diabetes will often need to have a blood test to confirm that their diabetes is well controlled. Poorly controlled diabetes can cause problems with healing and also with undergoing surgery itself - so a knee replacement is likely to be cancelled if the diabetes is not well enough managed.

• Urine test - to make sure there is no urine infection and that there isn't any glucose in the urine.

• Blood pressure.

• Infection screen - this includes looking for meticillin-resistant Staphylococcus aureus (MRSA). MRSA is a bacteria that can be difficult to treat and can cause complications of a knee replacement.

• A heart tracing (electrocardiogram, or ECG).

There may be the opportunity to speak to an anaesthetist, physiotherapist or occupational therapist at this clinic but this isn't always possible.

Risks and benefits

Before any operation, you have the opportunity to discuss all the potential risks of the surgery for you. This should be clear and in plain language that you understand fully. If you have other medical problems, such as heart disease, diabetes or a tendency to deep vein thrombosis, or if you are obese, you should also have explained to you how these things may increase the risks of the operation for you.

Potential risks include, but are not limited to, bleeding, infection, blood clots, nerve or ligament damage and ongoing pain. Benefits are reduction of pain and improved mobility.

The National Institute for Health and Care Excellence (NICE) issued new guidance on knee replacements in 2022. If a specialist recommends knee replacement, they advise that:

• They should also advise on 'prehab' - how people can get into the best shape to recover well from the operation - when they put someone on the waiting list for surgery.

• People with osteoarthritis only in one half of the knee joint (the inner side) should be given the choice of partial or total replacement.

• Before going home after surgery, people should be given advice on rehabilitation to improve their recovery.

What type of anaesthetic will I need?

There are two different types of anaesthetic for this operation:

• A spinal anaesthetic

• A general anaesthetic

At the pre-assessment clinic you can talk about the type of anaesthetic for your knee replacement.

Care after the operation

It is important to consider options for support and care after a knee replacement. Most people like to be independent, but, following a knee replacement, some support with day-to-day activities is likely to be necessary for a few days or even weeks.

Physiotherapy is important after a knee replacement in order to build up the leg muscles and restore mobility.

Knee replacement operation

The operation usually takes between 1 and 3 hours. The surgeon makes a cut down the front of the knee, moves the knee cap (patella) to one side and then cuts away the damaged surfaces of the ends of the thigh bone (femur) and shin bone (tibia) along with a little bit of the underlying bone.

The two surfaces that have been removed are then replaced with specially shaped artificial surfaces. The new surface that covers the top of the shin bone (tibia) is usually made of metal and plastic.

Sometimes it is only made of metal and a separate piece of plastic is inserted; this is called a mobile-bearing knee replacement. The plastic, whether separate or part of the covering of the shin bone (tibia), allows the two ends of the bones to glide over each other smoothly. Your knee cap (patella) may also be given a new surface, although sometimes it's left alone.

Some surgeons are using minimally invasive techniques using specially designed surgical instruments and telescopes - these reduce the size of the incisions that are made. Traditionally the incision is around 20 - 25 cm long but with a minimally invasive technique this can be reduced to 10 - 15cm. This may be an option for people of normal weight who have no underlying medical conditions.

Discharge from hospital is normal when people are mobile enough to be safe at home.

Types of knee replacement surgery

Knee replacements can be divided into two types:

Total knee replacement (total knee arthroplasty)

• Most knee replacement operations involve replacing the surface of the bottom end of the thigh bone (femur) and the upper surface of the shin bone (tibia)

• A total knee replacement may also involve replacing the knee cap (patella) with a dome-shaped plastic one.

Uni-compartmental (partial) knee replacement

• If arthritis only affects one side of the knee (usually the inner side) a partial knee replacement may be suggested.

• A partial knee replacement is less invasive and the recovery is usually quicker.

Whether total or partial, the replacement parts are made of a combination of metal and plastic; the metal parts replace the surfaces of the thigh bone (femur) and shin bone (tibia) and the plastic replaces the meniscus or menisci. (See 'causes' section for more information about the anatomy of the knee joint).

The metal parts may be fixed in place using special cement (cemented) or they may not be fixed (uncemented) but designed so that the bone grows over them and fixes them in place that way. All knee replacements used to be cemented but, over time, the cement can break down and the surgery may need to be repeated. This is usually over 10-20 years. Cementless knee replacements do not have this risk but they are not suitable for people with osteoporosis; they are also newer so there is less evidence about their long-term success. Currently short-term success seems to be as good as with cemented knee replacements.

Complex or revision knee replacement

This may be needed if arthritis has damaged more than the usual amount of bone or when a previous knee replacement has to be re-done (revised). Sometimes, in very complex situations such as following surgery for bone cancer, the components will be designed specifically to fit in the knee.

Which type should I have?

The surgeon will discuss this on an individual basis. It will depend on how much of the knee is affected by arthritis.

A study of over 500 patients with osteoarthritis of the inner (medial) part of their knee has compared the effectiveness of total and partial (uni-compartmental) knee replacement. The two groups were followed up five years after surgery, and asked to complete questionnaires about pain, activity and day-to-day living.

The results showed that outcomes from partial and total knee replacement were similar in terms of pain and well-being, as well as in the risk of complications and the likelihood of needing further surgery.

The researchers suggest that partial knee replacement should be the first choice for surgery in people with osteoarthritis affecting only one half of the knee.

Recovering from knee replacement surgery

For the majority of people, knee replacements are very successful. There is a lot of evidence from research showing that patients have less pain and are much more mobile after surgery and this often greatly improves their quality of life. Outcomes are getting better too.

About 5 people out of 100 are unhappy with their knee replacement following surgery.

Will I need to be seen again after my operation?

Most people are seen again by their surgeon about 8 weeks after surgery. Some people continue to be offered follow-up after this.

How long will my new joint last?

In recent years, improvements in medical equipment and surgical techniques have meant that many knee replacements last longer than they did in the past.

A new study looking at over 6,000 people who have had knee replacement shows:

• More than 4 in 5 people who have total knee replacements can expect them to last for at least 25 years.

• 7 in 10 people who have a uni-compartmental knee replacement can expect it to last for at least 25 years.

Complications of knee replacement surgery

Bleeding

• Blood transfusion may be needed.

• However, tranexamic acid is now advised by NICE in all knee replacement surgery to reduce the risks of bleeding.

Pain and stiffness

• Pain can be reduced by different anaesthetic techniques used at the time of the operation.

It is important to ensure adequate pain relief by taking painkillers after the operation. It is necessary to be able to move about and then start to walk as soon as possible after the operation.

• It is extremely important to follow the advice from your physiotherapist regarding exercises to do following your knee replacement:

  • In particular, not moving the knee enough can cause the scar and the tissues around the knee to 'glue' up.

  • Occasionally this has to be treated by forcefully moving the knee under anaesthetic, followed by intensive physiotherapy.

Venous thromboembolism

• Venous thromboembolism occurs when a clot of blood forms inside a vein.

• All patients are given thromboprophylaxis (medication, foot pumps, below knee stockings) unless it would be dangerous to do so. (Thromboprophylaxis is the name for anything that reduces the chance of getting a venous thromboembolism).

• This reduces the chance of suffering from the most severe but rare form of thromboembolism, which is a pulmonary embolism (PE). It reduces the risk of dying from a PE by 70%.

• Someone who has had a venous thromboembolism before is more likely to have another during surgery. Cancer and chemotherapy, as well as being obese, also increase the risk of this complication.

Nerve damage

• It is common to have a numb area of skin to the outer side of the operation scar. This may improve over two years but doesn't always recover completely.

• Occasionally a particular nerve, called the common peroneal nerve, is damaged during a knee replacement:

  • This can cause foot drop.

  • Foot drop weakens the foot so that the front of the foot does not lift properly during walking.

  • Peroneal nerve damage is more common when the arthritis in the knee is very severe.

  • Half of the people who develop foot drop recover completely without any treatment.

Ligament damage

• There are four ligaments that cross the knee and sometimes they can be damaged during a knee replacement.

• If one of the knee ligaments is damaged it may be possible to mend it during the operation or a brace around your knee may need to be worn for a while to allow it to heal.

Blood vessel damage

• Damage to the blood vessels is rare.

Other complications include:

• Urinary tract infection - related to having a tube (catheter) put into the bladder during the operation.

• Constipation - due to painkillers and immobility.

• Chest infection - more likely following a general anaesthetic and in people who already have a lung condition, such as chronic obstructive pulmonary disease (COPD).

• Wound infection and wound breakdown (also knee joint infection - see below).

• Painful scar - this may make it difficult or uncomfortable to kneel and some people avoid kneeling after a knee replacement for this reason.

• Dislocation of the knee - this is rare but can occur with certain types of knee replacements.

• Fracture or breakage of a prosthesis (or femur or tibia) is very rare.

What are the possible later complications?

Long-term complications include the knee replacement 'failing' and infection of the knee joint.

Failure

• Knee replacements can wear out; they can become loose or break - this is often referred to as knee replacement failure. They then need to be re-done (revised) which is a much more complex operation.

• Pain, instability and stiffness after surgery are other reasons for knee replacement revision.

• Needing to have the knee replacement done again is more common if the first knee replacement was done when young.

• Overall about 4-5 out of every 100 people who have a knee replacement will need to have it revised within 20 years.

• A knee replacement is likely to last longer if a healthy weight and if not doing a heavy manual job.

Infection

• Infection of a knee replacement can be extremely problematic. An infected knee prosthesis may need to be removed and it may not be safe or possible to replace it.

• Between 1 in every 100-200 people who have a knee replacement develop a knee joint infection.

• The risk of infection is greater in men but it is not known why this is. The risk is also higher in people who have both knees operated on at the same time, people who smoke, people who were younger at the time of surgery or people being discharged to a nursing home. The longer the surgery took, the more likely infection is to occur. People with diabetes, kidney disease, obesity, lung or heart disease are also more at risk of infection.

• 8 out of every 10 people who get a joint infection, do so within the first year of their operation. The highest risk is in the first 3 months

• In one study, 1 in 4 of the people who got a knee joint infection, never completely recovered. This causes significant long-term disability.