Knee Replacement Surgery

Robin Allum, FRCS
Original article: 25/02/2003
Reviewed: 27/10/2011
Next review due: 27/10/2014

Introduction

The development of knee replacement has been slower than hip replacement. Whereas the clinical results of total hip replacement have been satisfactory from the early 1960s, it is fair to say that total knee replacement did not reach the same level of success until the late 1970s and early 1980s. The knee is a more complex joint to replace. The original designs were simple hinges. There is a rotational stress on the knee joint and this caused the hinges to loosen. Initially also the prostheses were relatively large and a significant amount of bone had to be removed for their insertion. This presented a very difficult situation if they failed, as there was very little in the way of stability left in the knee joint. The modern designs are really resurfacing replacements in which relatively small quantities of bone are removed which leads to less in the way of problems if the operation fails. The results of knee replacement are now as good as hip replacement and it would seem that the incidence of loosening in the long term is in fact less in the knee than in the hip. It is hoped therefore that the current generation of knee replacements will actually be longer lasting than hip replacements. According to the National Joint Registry, over 80,000 knee replacements are now carried out annually in the UK.

Indications

The primary indication for knee replacement is pain. Typically pain significantly limits activities, particularly walking. There is often pain at night and pain at rest. There may also be deformity, stiffness and swelling. These are relative rather than absolute indications for surgery. Increasing deformity does clearly cause a problem and surgery should be carried out before deformity is severe otherwise the result may be compromised. If the knee is significantly stiff then the range of movement should be improved by knee replacement: a range of approximately 120 degrees is the maximum that could be expected with surgery.

The Technique of the Operation

Essentially the operation involves shaving off the ends of the bones - the femur (thigh bone), the tibia (shin bone) and the patella (knee cap). The patella is not always replaced, opinion amongst surgeons varies. The femur and tibia are then resurfaced with metal. A plastic spacer is inserted between the two metal components and this is attached to the tibial component. The patella, if replaced, is resurfaced with plastic. The implants are usually anchored to the bone by acrylic cement although some surgeons favour fixation without cement which involves additional fixation such as screws or some form of specialised surface to the implant that will integrate with the bone. When cutting the ends of the bone it is important to correct any deformity to ensure satisfactory alignment of the knee joint. The ligaments and other soft tissues need to be carefully balanced and correctly tensioned. If they are too loose then the joint will be unstable and if they are too tight there will be restricted movement.

The surgical wound is normally repaired in three layers, the capsule or covering of the joint, the subcutaneous fat layer under the skin and the skin itself. Instead of conventional interrupted sutures the skin closure now is normally with a subcuticular suture which lies immediately below the skin as this method gives a more cosmetic scar. In a small proportion of cases when the arthritis is localised to one area of the joint, a partial replacement is carried out but this is not appropriate in rheumatoid arthritis.

Recovery

A drainage tube is normally placed inside the knee for the first 24 hours so that if bleeding occurs the blood will be sucked out of the knee and will not cause pain and swelling. The blood can then be transfused back as it is better for the patient to have their own blood.There are a number of methods for effective pain relief. Strong pain killing drugs such as morphine and pethidine can be given on a regular basis by injection but it is preferable to use these with a Patient Controlled Analgesia (PCA system) where the patient presses a button to control the administration of the drugs. A limit is set on the amount of painkiller that can be given so that over-doseage does not occur. It is possible to numb the main nerves in the leg and this can be a very useful technique in the first 24-48 hours as it provides very effective pain relief. It inevitably also paralyses the muscles and therefore would hinder mobilisation if it were continued for a longer period. Similarly a spinal or epidural anaesthetic can be continued in the early post-operative period. A cryocuff or ice jacket can be placed around the knee to reduce pain and swelling. Anti-inflammatory drugs, particularly Voltarol are also useful in the post-operative period and are often given as suppositories. After the drain has been removed the patient is then mobilised.The haemoglobin level is normally checked after 48-72 hours. This is the blood count which will drop if there is significant bleeding during surgery or in the post-operative period. A transfusion may be necessary if the level is low. An X-ray is taken during the first few days. It is difficult to make rules with respect to mobilisation as every patient is different but the majority of patients will be fit enough for discharge home 5 - 7 days following the operation at which time they will be walking with support and able to negotiate stairs. After approximately 6 weeks most patients will be back to normal day to day activities including driving ( less for driving if it is the left knee and an automatic car) although it can take up to 12 months for a full recovery to be made. The knee may well be sore, tender, warm and irritable for several months. The scar takes a long time to settle down as the front of the knee is somewhat vulnerable. Kneeling is initially quite painful, this does become easier but the ability to kneel is rarely normal following a knee replacement.

Problems

Patients now need to be able to give an informed consent for surgery and this means having an understanding of problems that may occur.

A metal and plastic knee will never be as good as the original and will rarely be entirely pain free. A survey from the National Joint Registry, of 10,000 patients more than one year after surgery has shown that 81.2% of patients were satisfied but the remainder (almost one in five) were in some way disappointed, mainly because of pain. In a multi-national study, patients were asked at one year after operation whether they would undergo surgery again. In Australia 25% said that they would not, in the UK 17% and in the USA 12%. In a small percentage of patients persistent pain is a problem due to no obvious reason and this can be difficult to bring under control. These issues highlight the importance of discussing and managing your expectations before the surgery.

In any major surgery to the lower limbs there is always a risk of deep vein thrombosis (blood clots) which can lead to pulmonary embolism. This occurs when a piece of clot breaks away from the vein in the leg and travels to the lungs, blocking part of the circulation to a lung. Various measures can be taken to reduce the risk of thrombosis and at the present time there is still considerable debate as to the most effective method. The current regime uses three techniques. Firstly Heparin anticoagulant tablets which are usually continued for 2 weeks following the operation, secondly calf pumps until the patient is mobile, and lastly compression (TED) stockings worn for 6 weeks after surgery. There is an increased risk of blood clots with a long haul flight in the first 6 weeks.

Just as fillings work loose in teeth the implant and cement can work loose in the bone in time. There is no such thing as a mechanical device which is 100% reliable but as stated earlier this appears to be less of a problem in knee replacement than hip replacement. Well over 90% of knee replacements remain solidly fixed in the bone for at least 10 - 15 years.

Artificial joints are vulnerable to infection because clearly they have no biological means of fighting bacteria. Infection can cause the artificial joint to loosen by damaging the bonds between the implant, cement and the bone. It may not be possible to control infection simply with antibiotics and the artificial joint may have to be removed. A new joint can be inserted at a later date but the results are less reliable than with the primary procedure and there is a significant incidence of continuing infection under these circumstances. Superficial infection in the wound itself is quite common and this will normally respond to an appropriate course of antibiotics.

Prevention is better than cure. Patients are screened for MRSA prior to admission, the operation is carried out in a laminar flow (clean air) operating theatre, powerful antibiotics are given at the time of surgery and the cement that anchors the implant to the bone contains antibiotics.

The patella is a very important part of the knee joint. If the alignment of the knee is incorrect then the patella may be unstable and this can cause a problem. Numbness alongside the scar is normal as the nerves in the skin are inevitably damaged by the incision. Occasionally the main nerve on the outer side of the knee (the lateral popliteal nerve) can be stretched during the surgery. This tends to occur when there has been severe deformity with angulation and the lower leg is pointing outwards (a valgus deformity) and can lead to temporary or permanent numbness and weakness in the foot with a foot drop. The foot cannot be lifted from the ground and this makes walking difficult. Rarely the main blood vessel in the leg (the popliteal artery) can be damaged and this is particularly likely to occur if there is pre-existing disease in the artery. A blockage may occur which could cut off the circulation to the leg. Urgent surgery is required to remedy this.

After a partial replacement arthritis may occur in other areas of the joint and the partial replacement will need to be converted to a total replacement. There are small risks associated with the anaesthetic and nerve blocks or spinal/epidural blocks which your anaesthetist will explain.

Important Points

• Over 77,000 knee replacements are now carried out annually in the UK.

• The primary indication for surgery is pain.

• Most patients are in hospital for 5 - 7 days.

• Return to normal day to day activities including driving takes about six weeks (less for driving if it is the left knee and an automatic car).

• ·Full recovery can take up to 12 months.

• A metal and plastic knee will never be as good as the original. Up to one in five patients may be disappointed.

• The main risks are residual pain, blood clots, loosening, infection, kneecap problems and nerve and blood vessel damage. These have to be balanced against the benefits.

References available on request