A knee replacement is an operation to replace damaged parts of the knee joint. It can be either a total knee replacement (TKR) or a partial (unicompartmental) knee replacement. The new part is called a prosthesis.
Surgery to replace a worn-out knee joint is very common. It is increasingly popular, as the outcomes have become better and better over the last decade or so.
When a knee replacement is needed
The usual reason that someone has a knee replacement is because they have very painful arthritis in their knee.
You should always bear in mind that a knee replacement is a major operation and you should really only be considering it when you have run out of other options. A doctor can tell you that you have arthritis in your knee and they can tell you that you could have a knee replacement but only you can decide if the time is right for you. Most people who decide to have a knee replacement are already taking painkillers every day but are still not able to walk far and need to use a stick.
Looking at all of the research on knee replacements (and it's good to know that there is lots), it would seem that the people who do best after a knee replacement are the ones with severe arthritis but not so bad that the joint is completely destroyed. This could be because it's really 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 very weak leg muscles.
The main reason for needing a knee replacement operation is pain in the knee - a knee replacement operation is essentially a painkilling operation. The pain can affect how far you can walk and may affect your ability to work. It is usually at its worst when you stand on the affected leg and is often really bad at night. You won't necessarily need a knee replacement if you have been told you have arthritis in your knee, as there are lots of other treatments that will help if the symptoms aren't severe. However, if the pain is severe despite taking painkillers, losing weight and physiotherapy, and you are finding that you are increasingly disabled by it, a knee replacement operation may be a sensible option.
Symptoms will often vary from day to day for no apparent reason. This is really common. Some people think their symptoms vary according to the weather or according to how much they have been doing - but it can be completely random.
Sometimes you will be aware of a grating or grinding feeling coming from your knee. This is called crepitus. On its own this does not necessarily indicate a serious problem with your knee.
The main reason for needing to have a knee replacement is arthritis in your knee:
Osteoarthritis (OA) of the knee is the most common reason for a knee replacement. It can be primary or secondary:
- Primary osteoarthritis:
- Is not caused by previous damage or injury to the joint.
- It is more common in people who have a close relative, particularly a brother or sister, with osteoarthritis.
- It is more common as you get older and in people who are obese.
- Secondary osteoarthritis:
Rheumatoid arthritis (RA) is a less common cause and knee replacements for this reason are reducing. This is because the treatments for rheumatoid arthritis have improved.
Any condition that can cause damage to the cartilage of the knee might result in needing a knee replacement, such as:
- Sero-negative arthritis.
- Avascular necrosis.
Preventing knee replacements
The most effective treatment for the symptoms of osteoarthritis of the knee is weight loss.
Losing weight also helps if you do end up needing a knee replacement, as people who are obese or overweight just don't do as well as people of normal weight after having a knee replacement.
Other treatments that are recommended for all patients with osteoarthritis of the knee include:
- General exercise - walking, t'ai chi, etc.
- Strength training - to increase the strength of the muscles in the legs.
- Water-based exercise - swimming and water aerobics.
- Using a walking stick or cane - and in the UK an umbrella! Use the walking aid on the side opposite to your 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
About six weeks or so before your operation you will have an appointment for a 'pre-admission' or 'pre-assessment' clinic. At this clinic a nurse will assess your fitness for your knee surgery.
There are several tests that may be needed and they include:
- Blood tests - to check that you aren't anaemic and that your kidneys and liver are working well enough for you to undergo the operation.
- Urine test - to make sure you haven't got a urine infection and that there isn't any glucose in your urine.
- Blood pressure.
- Infection screen - this includes looking for meticillin-resistant Staphylococcus aureus (MRSA). MRSA is a germ (bacterium) that is difficult to treat and can cause complications of a knee replacement.
- A heart tracing (electrocardiogram, or ECG).
You may have the chance to speak with an anaesthetist, physiotherapist, occupational therapist or social worker at this clinic but this isn't always possible.
Risks and benefits
Make sure that at some point before your 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.
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. An anaesthetist will explain to you which type of anaesthetic is most suitable for you but your preference will always be taken into account. Most people have a spinal anaesthetic.
Care after the operation
Please give some thought as to how you will be looked after once you have had the operation, well in advance. Most people like to be independent, but you are going to need support with day-to-day activities for a while. If you have an able-bodied partner, this might fall to them, but otherwise you may need a friend or relative to come to stay with you for a while. Some people may arrange to stay in a care home until they have their mobility and independence back.
Knee replacement operation
The operation usually takes between 1 and 2 hours. The surgeon will make a cut down the front of your knee and then cut 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 - sometimes called keyhole surgery. This means that they make just one or two very small cuts instead of one long cut and use specially designed surgical instruments and telescopes. Your surgeon will discuss with you if this is available.
You will be able to go home once you are eating and drinking normally and are mobile enough to be safe where you are going after you leave hospital.
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 your thigh bone (femur) and the upper surface of your shin bone (tibia)
- A total knee replacement may also involve replacing your knee cap (patella) with a dome-shaped plastic one.
Unicompartmental (partial) knee replacement
- If your arthritis only affects one side of your knee (usually the inner side) you may be offered a partial knee replacement.
- A partial knee replacement involves less of your knee being operated on and the recovery is usually quicker.
- It is more likely that a partial knee replacement can be done using minimally invasive techniques.
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 your bone grows over them and fixes them in place that way.
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 your knee.
Which type should I have?
Your surgeon will discuss this with you. It will depend on how much of your knee is affected by arthritis - it may not be possible to know this until your surgeon has started your operation.
If you have a partial knee replacement it is more likely that you will need to have it done again, than if you have a total knee replacement (TKR). Sometimes the reason for choosing to have a partial knee replacement is that it leaves the option to have a TKR at a later date. However it's also more likely that you will need to have your total knee replacement re-done, if you had a partial knee replacement done before having your total knee replacement.
There are over 150 different designs of knee replacement and some of the differences between all the different types and makes of knee replacement parts aren't known, particularly how they perform in the long term. In many countries, registries have been set up so that anyone who has had a knee replacement is entered into the register. The information collected is used to monitor how their replacement is performing. In the UK, patients also enter information about their health and quality of life before and after their operation.
A study of over 500 patients with osteoarthritis of the inner (medial) part of their knee has compared the effectiveness of total and partial (unicompartmental) 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.
Because partial knee replacement is a less major operation, this study may influence specialists - 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.
You can find out more about the study in the further reading section below.
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, as more research is carried out on what the best operation is and how to reduce the risk of complications.
However about 8 people out of 100 are unhappy with their knee replacement 2-17 years later. If they have had to have their knee replaced a second time (revised), they are twice as likely to be unhappy with the outcome.
Will I need to be seen again after my operation?
Within about eight weeks of your operation, you will be followed up by the hospital where you had your surgery. You will usually be offered further follow-up appointments.
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 unicompartmental knee replacement can expect it to last for at least 25 years.
Clearly this will vary between different people, and you still need to take care of your new knee.
Possible complications of knee replacement surgery
Blood transfusion may be needed.
Pain and stiffness
- Pain can be reduced by different anaesthetic techniques used at the time of your operation.
- It is important to make sure that you get adequate pain relief. You need to be able to move about and then start to walk as soon as you are able after your 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 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%.
- If you have already had a venous thromboembolism before or are closely related to someone who has, this makes it more likely that you will suffer from one when you have your knee surgery. Cancer and chemotherapy, as well as being obese, also increase your risk of this complication.
- 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 your foot so that you don't lift the front of your foot properly as you walk.
- 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.
- There are four ligaments that cross the knee and sometimes they can be damaged during a knee replacement.
- If one of your knee ligaments is damaged it may be possible to mend it during the operation or you may have to wear a brace around your knee for a while to allow it to heal.
Blood vessel damage
- Damage to the blood vessels is rare.
- If it occurs it would usually need further surgery to repair it.
Other complications include:
- Urinary tract infection - related to having a tube (catheter) put in your 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 - of the femur or tibia - or breakage of a prosthesis is rare.
What are the possible later complications?
Long-term complications include the knee replacement 'failing' and infection of the knee joint.
- 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 after surgery, instability and stiffness are other reasons for knee replacement revision.
- Needing to have your knee replacement done again is more likely the younger you are when you have it done in the first place:
- 3½ years after your operation you are five times more likely to need to have it revised if you were under 55 when you first had it done than if you were over 75.
- Overall about 4-6 out of every 100 people who have a knee replacement will need to have it revised within 10 years.
- On the other hand, for between 80 and 90 out of every 100 people, their knee replacement lasts more than 20 years.
- Your knee replacement is likely to last longer if you are not obese or overweight and if you don't do a heavy manual job.
- Infection of a knee replacement can be catastrophic. 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 get a knee joint infection.
- The risk of infection is greater in men but it is not known why this is.
- 8 out of every 10 people who get a joint infection, do so within the first year of their operation.
- In one study, 1 in 4 of the people who got a knee joint infection, never got rid of it completely. This causes significant long-term disability.
Further reading and references
Joint replacement (primary): hip, knee and shoulder; NICE Clinical Guidance (June 2020)
Mini-incision surgery for total knee replacement; NICE Interventional Procedures Guidance, May 2010
Carr AJ, Robertsson O, Graves S, et al; Knee replacement. Lancet. 2012 Apr 7379(9823):1331-40. doi: 10.1016/S0140-6736(11)60752-6. Epub 2012 Mar 6.
Hofstede SN, Nouta KA, Jacobs W, et al; Mobile bearing vs fixed bearing prostheses for posterior cruciate retaining total knee arthroplasty for postoperative functional status in patients with osteoarthritis and rheumatoid arthritis. Cochrane Database Syst Rev. 2015 Feb 4(2):CD003130. doi: 10.1002/14651858.CD003130.pub3.
Ferket BS, Feldman Z, Zhou J, et al; Impact of total knee replacement practice: cost effectiveness analysis of data from the Osteoarthritis Initiative. BMJ. 2017 Mar 28356:j1131. doi: 10.1136/bmj.j1131.