Knee replacement
Knee replacement, or knee arthroplasty ,is a common operation done to relieve the pain and disability from degenerative arthritis, most commonly osteoarthritis, but other arthritides as well.     
it 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.
Incapacitating pain from arthritis of the knee affecting everyday activities -- particularly walking -- 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.

# Technique
The surgery involves exposure of the front of the knee , with detachment of part of the quadriceps muscle (in fact the vastus medialis) from the kneecap.  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 any deformities must be  corrected,  and the ligaments balanced  so that the knee has  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.

# 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. Some also study patient satisfaction data associated with pain.
Minimally Invasive Surgery is being developed in Total Knee Replacement but has not yet found complete acceptance. The goal is to spare the patient the large cut in the quadriceps muscle which could increase post-operative pain or lengthen disability.

## Partial Knee Replacement
Unicompartmental arthroplasty (UKA), also called partial knee replacement, is an option for some patients. The knee is generally divided into three "compartments": medial (the inside part of the knee), lateral (the outside), and patellofemoral (the joint between the kneecap and the thighbone). Most patients with arthritis severe enough to consider knee replacement have significant wear in two or more of the above compartments and are best treated with total knee replacement. A minority of patients (the exact percentage is hotly debated but is probably 10-30%) have wear confined primarily to one compartment, usually the medial, and may be candidates for unicompartmental knee replacement. Advantages of UKA compared to total knee replacement (TKA) include smaller incision, easier post-op rehabilitation, shorter hospital stay, less blood loss, lower risk of infection, stiffness, and blood clots, and easier revision if necessary. While most recent data suggests that UKA in properly selected patients has survival rates comparable to TKA, most surgeons believe that TKA is the more relaible long term procedure. Persons with infectioius or inflammatory arthritis (Rheumatoid, Lupus, Psoriatic ), or marked deformity are not candidates for this procedure.

# Pre-operative work-up
Knee Arthroplasty is  major surgery.Before the surgery is attempted blood work will be obtained, usually a CBC,electrolytes, and body chemistries an APTT and Protime to measure clotting, chest Xrays, ECG,and blood crossmatching for possible transfusion. Accurate Xrays of the affected knee is needed to measure the size of components which will be needed. (templating) Medications such as warfarin and aspirin will be stopped 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
Protected weight bearing on crutches or a walker is required until the quadriceps muscle has healed and recovered its strength.  Continuous Passive Motion or CPM is commonly used, but its effectiveness is questioned.
Post operative hospitalization varies from one day to seven days on average 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). . At 6 weeks patients have usually progressed to full weight bearing with a cane. Complete recovery from the operation involving return to full normal function may take three months and some patients notice a gradual improvement lasting many months longer than that.

# Risks and complications
According to the American Academy of Orthopaedic Surgeons (AAOS), "blood clots in the leg veins are the most common complication of knee replacement surgery. Your orthopaedic surgeon will outline a prevention program, which may include periodic elevation of your legs, lower leg exercises to increase circulation, support stockings and medication to thin your blood."
Periprosthetic fractures are becoming more frequent with the aging patient population and can occur intraoperatively or postoperatively.
Also according to AAOS, "the complication rate following total knee replacement is low. Serious complications, such as a knee joint infection, occur in less than 2 percent of patients. Major medical complications such as heart attack or stroke occur even less frequently. Chronic illnesses may increase the potential for complications. Although uncommon, when these complications occur, they can prolong or limit your full recovery."
The knee at times may not recover its normal range of motion (0 - 135 degrees usually) after total knee replacement. Much of this is dependent on pre-operative function. Most patients can achieve 0 - 110 degrees, but stiffness of the joint can occur. In some situations, manipulation of the knee under anaesthetic is used to improve post operative stiffness. There are also many implants from manufacturers that are designed to be "high-flex" knees, offering a greater range of motion.
In some patients, the kneecap is unstable post-surgery and dislocates to the outer side of the knee. This is painful and usually needs to be treated by surgery to realign the kneecap. This is very rare, but possible.
In the past, there was a considerable risk of the implant components loosening over time as a result of wear. As medical technology has improved however, this risk has fallen considerably. One implant manufacturer claims to have reduced this risk of wear by 79% in fixed-bearing knees. Another implant manufacturer claims to have reduced the risk of wear by 94% in mobile-bearing, also known as rotating platform, knees. Knee replacement implants can last up to 20 years in many patients; whether or not they actually survive that long depends largely in part upon how active the patient is after surgery.

# Controversies
The minimally invasive approach to surgery is controversial. Proponents believe that the procedure allows the patient to recover more quickly. 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. They also say that the procedure is not backed by clinical results.
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 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      used. These registries may yield information about the outcomes of different designs.

# History
Following John Charnley's success with hip replacement in the 1960s 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 1970s the "Geometric" design found favour as well as John Insall'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.
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