Total Knee Replacement

 

 

Background to total knee replacement

 

The operation of total knee replacement is designed to relieve the symptoms of arthritis in the knee.  Although it is a very successful operation it is a major procedure and it should be undertaken only for severe symptoms.  It is only the patient who knows how severe their pain is but reasonable guidelines are as follows.

 

The surgery is indicated if your knee is preventing you walking for normal day to day purposes due to either pain in the knee, stiffness or instability.  If you cannot walk down the road to the post box or the newsagent or you cannot walk when you are on holiday or you want to go shopping, that is clearly a significant problem.  Some people find that surgery is required if the knee is so painful that they cannot sleep at night.  In a younger patient it is reasonable to undertake a joint replacement if you cannot go to work because of knee pain.

 

What is the knee joint like?

 

The knee joint is the joint between the lower end of the femur or thigh bone and the upper end of the tibia or shin bone.  The knee is a complex hinge which allows both bending and a small amount of rotation. The surfaces of the bones are covered with cartilage which is a shiny white lubricated surface.  Also in the middle of the knee joint there are meniscal cartilages which are pieces of fibrous gristle-like cartilage which normally aid the distribution of force in the joint.  These are the cartilages which are commonly torn by footballers.  The joint is stabilised by two large ligaments in the middle of the joint called the cruciate ligaments and a ligament either side of the joint called the medial and lateral collateral ligaments.  At the front of the joint the knee-cap runs over the front of the femur. The knee-cap is a special bone in the tendon of the quadriceps muscle which is the muscle at the front of the thigh which straightens out the knee.  

 

The process of arthritis is the destruction of the articular cartilage surface of the joint.  There are two main types of arthritis which affect the knee.

 

Osteoarthritis is a condition in which the articular cartilage wears out.  It can be caused by major injuries to the joint.  It is more common in people who have had sporting injuries at a young age, but the main cause is genetic.  The condition runs in families and it is a common part of the ageing process.  Osteoarthritis is nothing to do with osteoporosis which is a thinning of the bones which causes fractures but does not cause arthritis. 

 

Rheumatoid arthritis is a disease rather than being a simple wearing out process.  In rheumatoid arthritis the lining of the joint is inflamed and it is this inflammation which damages the joint surface.  

 

How is the joint replaced?

 

The knee joint is opened from the front of the knee using an incision which runs on average 15 to 20cm (6 to 8 inches) down the front of the knee.  The knee-cap is moved to one side to allow access to the joint and the patella tendon is carefully preserved.  The top of the tibia is cut at right angles to the shaft of the tibia and the femur is cut so that it will end up parallel to the tibial cut.  The end of the femur is shaped using a number of cutting blocks to fit the component.  The balance between the ligaments on the inside and the outside of the knee is then checked and the overall line of the joint is checked.  If the knee is fixed in flexion prior to the surgery the tight capsule at the back of the knee is released.  This is a slightly hazardous process because of the blood vessels just behind the capsule, but with care it is normally possible to get the knee to fully extend.  The patella is not normally resurfaced unless the patient has been suffering from patellar pain prior to the surgery.  

 

There are trial implants which are fitted to the bones to check the knee for tension and alignment.  Assuming all is well the two components are fitted to the bone using bone cement.  Bone cement is plastic cement which is mixed from two components.  It grouts into the bone and it sets solid in about ten minutes.  Over the few months after the surgery the cut surface of the bone regrows in close adhesion to the bone cement.  

 

The component fitted to the lower end of the femur is made from chrome cobalt. The component fitted to the tibia consists of a titanium metal tray with a polyethylene insert.  All the components are available in a variety of sizes and the polyethylene inserts are available in different thicknesses so that the tension of the knee can be adjusted.   

 

The knee replacement that I routinely use is a Zimmer Nexgen Knee which was developed by Dr John Insall of New York, initially working with Mr Mike Freeman of the London Hospital.

 

The risks and benefits of total knee replacement

 

Overall total knee replacement is a very successful operation.  It works well in around 90% of cases in the long term.  The main benefit of the operation is relief of arthritic pain.  Most people who have a total knee replacement still know that they have an artificial knee, it does not feel completely like a normal knee, but they get rid of the great majority of their arthritic pain, if not every ache and pain around the joint.  

 

The surgery is however a major procedure which has occasional very serious complications.  The complication that we worry about most is infection.  If the artificial knee becomes infected it certainly has to be operated on again and the problem can leave the patient every bit as bad as they were to start off with, particularly if the infection cannot be fully eliminated.  The incidence of infection in routine knee replacements at the Royal Orthopaedic Hospital is around 1%.  The risk is higher in patients with rheumatoid arthritis, diabetes, obesity and other medical problems.  Even so the risk is only around 2%.  There are risks of damage to nerves and blood vessels around the knee.   Damage to blood vessels at the back of the knee is a rare but serious complication.  Damage to nerves around the knee is a little more common.  There is a key nerve which wraps around the head of the fibula which is the nerve responsible for upward movement of the foot.  This nerve is particularly prone to damage if a valgus or outward bending deformity of the knee is corrected.  

 

The risk to the patient’s life from a total knee replacement is very small but it cannot be totally ignored.  For a fit individual the risk is probably around 0.3%.  The risk is obviously higher in people with medical problems and it is certainly a bit higher in patients over the age of 80.  The main medical complications are heart attacks during or after the surgery and the formation of blood clots in the legs which may float off to the chest with dire consequences.  The risk of this happening is reduced by giving the patient regular doses of Aspirin around and after the time of surgery. 

 

Overall therefore the risk of a major complication from the surgery is probably around 2-3%.  

 

Between the 90% of operations which are very successful and the 2-3% where there is some major problem, there is a group of 5-10% of patients who have a slightly suboptimal result, the procedure is not a disaster but it is spoiled by some problem, particularly excessive stiffness, knee-cap pain, etc.  The operation is not a disaster by any means but it is not as good as some.

 

In the long term around 90% of total knee replacements are still working ten years after the surgery and I have a lot of patients with well functioning knees between ten and fifteen years post-surgery.  Overall in the long term between five and fifteen years post-surgery total knee replacements actually seem to cause less trouble than total hip replacements.

 

The care of the patient before and after total knee replacement

 

Prior to your surgery it is important that you tell the treating team and the anaesthetist about any medical problems you have and you will be seen at a pre-op assessment clinic for this purpose and to have a full medical check-up.  

 

All patients undergoing total knee replacement surgery have a blood count and their biochemistry checked together with the blood inflammatory markers which are used to monitor the activity of rheumatoid arthritis and check up on infection.  Some patients need their chest x-rays and ECGs done.  Patients with rheumatoid arthritis need to have a neck x-ray prior to surgery as rheumatoid arthritis can cause instability in the cervical spine.  Patients can prepare themselves for surgery by understanding what is going to happen and by keeping as physically fit as possible prior to the surgery, you should therefore do your muscle exercises as much as you can within the limits of the fact that your knee is obviously painful. If you have rheumatoid arthritis your medical management should be optimised by the rheumatologist and the rheumatologist may also treat you with medications such as calcium and bisphosphonates to improve your bone density.  

 

If you are a smoker you should stop.  Smoking certainly increases the risk of a surgical complication following any procedure and total joint replacement is more likely to be problematic in a smoker.  

 

If you are overweight you must reduce your weight.  Weight is calculated using the Body Mass Index which gives a reliable measurement of how much you weigh in relation to your height.  Your Body Mass Index should be 20 to 25.  If it is 25 to 30 you are overweight. If it is 30 to 35 you are obese and you are better to lose weight.  Once your Body Mass Index is over 35 you are at more serious risk of complications following knee replacement and above a BMI of 40 things are really unacceptably dangerous in my view.  The point is that not only is there a higher early risk from surgery, but also there is a higher risk of the joint replacement becoming loose.  Additionally if you are very overweight and you reduce weight the knee pain may become better and you may avoid surgery altogether.  

 

The situation with total knee replacement is different to the situation with total hip replacement where for some reason the patient’s weight does not have such an important influence on the outcome.

 

Progress after total knee replacement

 

Following the surgery the anaesthetist will have placed a drip in your arm.  You may have an epidural catheter or a nerve block catheter both of which are used to numb the pain in your leg so that you do not have to have such a strong general anaesthetic.  Knee replacement surgery can be done either under a general anaesthetic or an epidural or spinal anaesthetic.  These days it is most commonly done with a combination of a light general anaesthetic and some type of nerve block.  The anaesthetist will usually be happy to comply with the patient’s wishes.  Some people cannot stand the idea of being put completely to sleep but other people cannot stand the idea of a needle in their spine.  There are times when an epidural anaesthetic is better, for example, a patient who has severe chest disease.

 

Following the surgery your skin will be closed with metal clips.  You will have a dressing on the skin which is simply placed on the skin and wrapped up with cotton wool. The idea of this is to reduce the risk of blistering, particularly in patients with severe rheumatoid arthritis.  This dressing is removed at 24 to 48 hours and a conventional stick-on dressing applied.  The patient is then mobilised initially usually by a physiotherapist with an assistant.  

 

It is very hard work after a total knee replacement.  The physiotherapist will get you doing thigh muscle exercises and straight leg raising exercises almost as soon as you wake up and they will be urging you to bend the knee.  In the vast majority of patients the more exercise you can do the better and the more you can push yourself to bend the knee, the better.   

 

One thing that can be a problem is getting full extension in the knee, ie. straightening it out completely.  This is particularly a problem if the knee had a flexion or bending deformity prior to the surgery.  Under these circumstances you can keep the knee stretched out by supporting under the lower calf with a well padded pillow and then exercising by pushing the knee down towards the bed.  

 

Most patients will leave hospital after three to four days if they are local patients who can be supported by the community scheme and possibly a little longer if they come from a long distance or they live alone.

 

Patients with rheumatoid arthritis may have difficulties caused by upper limb problems such as painful shoulders.  If you have painful shoulders and elbows prior to the surgery, if you let us know we can inject these joints whilst you are under the anaesthetic which often helps with post-operative progress.  

 

The clips are removed from the skin at about twelve days, maybe fourteen days in patients with particularly frail skin, on steroids, etc.  By this time you should be mobile around your house with the aid of two elbow crutches and able to get up and down stairs without excessive difficulty.  By six weeks post-operation you should be mobile with one stick and you should be able to drive a car, particularly if it was the left knee that was replaced.  

 

        Swelling in the leg is common after hip and knee surgery. It is important to keep the leg muscles moving and not to leave your foot sitting still on the ground excessively after the surgery. 

 

         For more details on swelling click here. 

To get back to driving your car a useful guide is that if you can walk up and down stairs relatively briskly you should be able to drive a car.  The best thing is to take your car to a quiet cul-de-sac and make sure you can do an emergency stop before you set off down the M6.

 

Conclusion

 

Total knee replacement is a major operation.  Most of the time it is dramatically successful and the chances of success are increased by careful attention to detail both from the surgeon, other members of the team and the patient.