Knee replacement

Knee replacement, or arthroplastyof the knee, is a commonly performed operation done to relieve the pain and disability from rheumatoid arthritisor more often osteoarthritisof the knee.

Total knee replacement

Description

This operation is undertaken by orthopaedic surgeonsand consists of replacing the diseased and painful joint surfaces of the knee with metal and plastic components shaped to allow continued motion of the knee.

Synonyms

Knee replacement, Kneearthroplasty

Technique

The standard technique involves exposure of the front of the knee by a long incision which detaches the quadriceps muscle from the kneecap. This is a key factor in the lengthy recovery from the operation. The muscle has to heal. The kneecap is displaced to one side of the joint allowing exposure of the distal end of the thighbone (femur) and the proximal end of the shinbone (tibia). The ends of these bones are then accurately cut to shape using cutting guides oriented to the long axis of the bones. The cartilages and the anterior cruciate ligament are removed; the posterior cruciate ligament may also be removed but the collateral ligaments are preserved. Metal components are then impacted onto the bone or fixed using poly methyl methacrylate (PMMA) cement. A round ended implant is used for the femur, mimicking the natural shape of the bone. On the tibia the component is flat, although it often has a stem which goes down inside the bone for further stability. A flattened or slightly dished high density polyethylene surface is then inserted onto the tibial component so that the weight is transferred metal to plastic not metal to metal. During the operation attention must be paid to correcting any deformities and balancing the ligaments so that the knee moves through a good range of movement and is stable. In some cases the joint surface of the kneecap is also removed and replaced by a polyethylene button cemented to the kneecap. At the end of the surgery the muscle is repaired to the kneecap and the wound is closed. It is common practice to leave a drain in the knee to reduce post-operative swelling from bleeding into the knee. Blood transfusion to replace intra-operative and post-operative losses are commonly required.

Variations

There are many different implant manufacturers and all require slightly different instrumentation and technique. No consensus has emerged over which design of knee replacement is the best. Clinical studies are very difficult to perform requiring large numbers of cases followed over many years. The most significant variations are between cemented and uncemented components, between operations which spare or sacrifice the posterior cruciate ligament and between resurfacing the patella or not.

Techniques of Minimally Invasive Surgery are being developed in Total Knee Replacement but have not yet found complete acceptance. The driving force here is to spare the patient the large cut in the quadriceps muscle which increases the post-operative pain and lengthens the disability.

Unicompartmental arthroplasty is a different operation with different indications. The joint surfaces of either the inner or the outer sides of the knee are replaced.

Any dental work after this surgery requires an antibiotic before the dental work can be done.

Indications

Incapacitating pain from arthritis of the knee affecting everyday activities is the main reason to have a total knee replacement. The patient must be aware of the risks of the surgery and be prepared to take those risks rather than continue with the symptoms.

Contra-indications

An open infection in the operative area is generally regarded as an absolute contra-indication to total knee replacement. A source of infection somewhere else on the body is a relative contra-indication. Poor general medical status, mental illness or inability to cooperate with post operative restrictions are relative contra-indications.

Pre-operative work-up

Routine pre-operative work up for major surgery is required. This will often include chest Xrays, ECG, blood tests and blood crossmatching. Accurate Xrays of the affected knee is needed to measure the size of components which will be needed. (templating) It is standard practice to discontinue medications such as warfarin some days before surgery to reduce the amount of bleeding. Patients may be admitted on the day of surgery if the pre-op work-up is done in the pre-anaesthetic clinic or may come into hospital one or more days before surgery.

Post-operative rehabilitation

Patients are encouraged to move the operated knee to recover the range of motion early. Protected weight bearing on crutches or a walker is required until the quadriceps muscle has healed and recovered its strength.

Timecourse of recovery

Post operative hospitalization varies from 1 day following Minimally Invasive Surgery to an average of 7 days depending on the health status of the patient and the amount of support available outside the hospital setting. Usually full range of motion is recovered over the first two weeks (the earlier the better). Walking with protected weight bearing begins almost immediately after surgery. At 6 weeks patients have usually progressed to full weight bearing with a cane. Complete recovery from the operating involving return to full normal function may take 3 months and some patients notice a gradual improvement lasting many months longer than that.

Risks and complications

Infection of the operation site is a very severe problem which often requires further surgery and removal or revision of the joint replacement. The overall incidence of post-operative deep infection should be less than 2%

Deep vein thrombosis is common post knee replacement. Careful studies suggest about 20% of patients get a blood clot in the calf after a knee replacement. There are a number of ways to reduce this risk which are being tested. Pneumatic compression immediately after surgery and anticoagulant medication are thought to be effective.

Pulmonary embolism occurs when a blood clot moves from the leg to the lungs partially cutting off circulation to the lung. It is a dangerous complication of Deep Vein Thrombosis and may be fatal. It occurs to some degree in about 3% of patients after Total Knee Replacement but this risk can be reduced by post-operative anticoagulation medication.

The knee does not recover normal range of motion (0 - 135 degrees usually) after total knee replacement. Most patients achieve 0 - 110 but stiffness of the joint can occur. In some situations manipulation under anaesthetic is used to improve post operative stiffness.

In some patients the kneecap is unstable post-operation and dislocates to the outer side of the knee. This is painful and usually needs to be treated by surgery to re-align the kneecap.

Long term there is a significant risk of loosening of the components. The bond between the bone and the metal implant may fail or the cement may break up. This causes recurrence of pain and in severe cases a revision operation may need to be done.

Controversies

The minimally invasive approach to surgery is controversial. Proponents believe that the procedure allows the patient to recover quicker. Opponents state that the operation is made more difficult without altering the long term prognosis. They suggest that more technical errors will be made particularly during the "learning curve" when the surgical team is less familiar with the operation.

We still do not know whether cemented or uncemented components last longer in the knee. Most surgeons now cement the tibial component but opinion is divided about the femoral component. Sacrifice of the posterior cruciate is also controversial with some surgeons performing this routinely and others trying to preserve as much normalcy as possible.

Resurfacing the patella is also subject to intense scrutiny. Some studies have suggested that there is no advantage to resurfacing the patella. However, many surgeons continue to do this because resurfacing the patella at a later operation is also a very big operation.

There are many different designs of total knee replacement. All of them were devised to solve an apparent problem. Studying the outcome from one design versus another is expensive, time consuming and unrewarding because designs change frequently and may be withdrawn by the time a good long term study has been done. Many nations, led by Sweden, have set up registries of joint replacements with voluntary or mandatory reporting of the components and techniques used. These registries may yield information about the outcomes of different designs.

Prognosis

19/20 patients are very satisfied with the results of total knee replacement with pain relief and improvement in function being the expected outcome. Those that suffer one or more of the complications noted above account for a portion of the unsatisfied patients and some have undiagnosed continuing pain or stiffness.

History

Following Charnley's success with hip replacementin the 1960's numerous attempts were made to design knee replacements. Gunston and Marmor were pioneers in North America. Marmor's design allowed for unicompartmental operations but these designs did not always last well. In the 1970's the "Geometric" design found favour as well as Install's Condylar Knee design. Hinged knee replacements for salvage date back to Guepar but did not stand up to wear. The history of knee replacement is the story of continued innovation to try to limit the problems of wear, loosening and loss of range of motion.

External links

• Total Knee Replacement - Wheeless' Textbook of Orthopaedics
• Total Knee Replacement - The KNEEguru

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Knee+replacement Wikipedia article Knee replacement.