Total Knee Replacement (TKR) became popular during the 1970’s and 1980’s as a treatment for end-stage degeneration of the knee joint. Since then, many advances have been made. Today, almost 60,000 TKRs are performed in the UK each year.
A Total Knee Replacement operation replaces the bottom end of the femur (thighbone) and the top end of the tibia (shinbone) with a prosthesis (man-made joint). Often, the patella (kneecap) must also have its back surface replaced.
Knee with Replacement (Click to Enlarge)
1. Quadriceps Tendon
2. Patella
3. Patellar Tendon
4. Tibia
5. Fibula
6. Femoral Prosthesis
7. Polyethylene Insert
8. Tibial Prosthesis
9. Lateral Collateral Ligament
10. Femur
There are many different knee replacements available. The majority consist of metal components that are fixed to the bottom end of the femur (femoral component) and top end of the tibia (tibial component); a plastic liner is inserted between the two. In some designs this plastic liner is mobile.
Most knee replacements are designed to be used with orthopaedic cement, which acts as a grout. Others are designed to encourage bone growth onto the prosthesis to give long term stability (uncemented).
Oxinium™ Knee Replacement
Standard Knee Replacement
It is the plastic liner that wears out with time, probably related to activity levels. Technological advances, however, have produced bearing surfaces which can withstand higher activity levels and will probably last much longer. For example the Genesis II™ knee replacement made by Smith & Nephew has a femoral component made of Oxinium™ which has been shown in laboratory studies to reduce the wear on the plastic liner by 90%.
Patients who are young and physically active may therefore benefit from technologies such as Oxinium™ since the risk of implant failure is significantly reduced. The surgeon should choose the most appropriate type of knee replacement for each patient.
Because of its anatomy, most of the weight is borne through the medial (inner) compartment of the knee. As a result, the articular cartilage on the inner side and the medial meniscus tend to wear more quickly than on the lateral side. Consequently, the knee tends to become ‘bowed’ and even more weight is borne through the medial compartment.
In time it is likely that most patients will have progression of their arthritis to the other compartments – the lateral and patello-femoral (kneecap joint). However patients who have arthritis confined to the medial compartment, and whose symptoms are uncontrolled with non-operative treatment, may be suitable for an operation to replace only this part of the knee. Replacement of only one compartment of the knee is termed a unicompartmental knee replacement.
In some cases, arthritis is confined to either the patello-femoral (kneecap) or lateral part of the knee. Unicompartmental replacement of these parts of the knee is also possible but is performed far less frequently than replacement of the medial compartment. The results are less predictable.
About 2000 unicompartmental knee replacements are performed in the UK each year. The most commonly used is the ‘Oxford Uni’ made by Biomet. It is designed to be used with cement, and consists of metal femoral and tibial components with a mobile plastic bearing in between
(see illustration).
Medial unicompartmental knee replacement
A unicompartmental knee replacement is NOT a suitable treatment option for:
Postoperative X-rays of a Unicompartmental Knee Replacement (Click to Enlarge)
Postoperative X-rays of a Total Knee Replacement (Click to Enlarge)
The benefits of a unicompartmental knee replacement compared to a total knee replacement are:
Note: The long term survival of a unicompartmental knee replacement is as good as that of a total knee replacement.
The majority of patients experience the following benefits following knee replacement:
Patients are usually required to attend a Pre-admission Clinic a couple of weeks before the proposed operation date; investigations will be undertaken and the operation discussed.
Knee replacement surgery involves an inpatient stay of 4-5 days; the patient will generally be admitted the day before the operation. The consultant and anaesthetist will see the patient prior to surgery and the knee undergoing the replacement will be marked.
Most knee replacements are performed under a spinal anaesthetic (where the patient is awake but normally sedated), but occasionally a general anaesthetic is used. The damaged bottom end of the femur is shaped so that a metal femoral component can be attached; the top end of the tibia is removed and replaced with a metal tibial component; a plastic liner is inserted between the two. If necessary the back surface of the patella is also replaced.
At the end of the operation a drain may be inserted into the knee joint to draw off excess blood. A dressing is applied to the wound and the knee will be wrapped in a crepe bandage. Foot pumps, elastic stockings and occasionally an injection to thin the blood are used to lower the risk of blood clots forming in the legs.
Post-operative pain is normal after a knee replacement. It can be controlled by many methods, from pain pumps (Patient Controlled Analgesia, PCA) to simple tablets, and usually lessens dramatically after the first 2-3 days. The pain of arthritis usually disappears within a day or two of surgery.
Occasionally, the bladder stops working after a spinal anaesthetic; a catheter may need to be inserted into the bladder for a day or two. Once removed, most people have normal return of bladder function.
Blood tests and X-rays will be taken in the early post-operative period to ensure that blood levels have not substantially changed and that the knee replacement is in a satisfactory position.
The physiotherapists will assist patients in mobilising after the operation and will supervise an exercise programme. It is extremely important that patients follow this exercise programme and take the necessary precautions with their new knee.
Patients can usually return to work after 8-12 weeks although this period may be longer for heavy manual work. By three months most patients can participate in low impact sports such as golf, bowls, cycling and swimming.
The majority of patients undergoing knee replacement do not experience any complications. In fact 95-98% of patients are extremely happy with their knee replacement and report that it has given them back their life. However no operation can guarantee success.
Complications can occur as a result of the anaesthetic, the knee replacement itself or as a general result of having major surgery:
