Knee replacement is a procedure in which the surgeon removes damaged or diseased parts of the patient's knee joint and replaces them with new artificial parts. The operation itself is called knee arthroplasty . Arthroplasty comes from two Greek words, arthros or joint and plassein , "to form or shape." The artificial joint itself is called a prosthesis. Most knee prostheses have four components or parts, and are made of a combination of metal and plastic, or metal and ceramic in some newer models.
Knee arthroplasty has two primary purposes: pain relief and improved functioning of the knee joint. Because of the importance of the knee to a person's ability to stand upright, improved joint functioning includes greater stability in the knee.
Total knee replacement, or TKR, is considered major surgery. Therefore, it is usually not considered a treatment option until the patient's pain cannot be managed any longer by more conservative treatment. Alternatives to surgery are described below.
Pain in the knee may be either a sudden or gradual development, depending on the cause of the pain. Knee pain resulting from osteoarthritis and other degenerative disorders may develop gradually over a period of years. On the other hand, pain resulting from an athletic injury or other traumatic damage to the knee, or from such conditions as infectious arthritis or gout, may come on suddenly. Because the structure of the knee is complex and many different disorders or conditions can cause knee pain, the cause of the pain must be diagnosed before joint replacement surgery can be discussed as an option.
Restoration of joint function and stability is the other major purpose of knee replacement surgery. It is helpful to have a brief outline of the major structures in the knee joint in order to understand the types of disorders and injuries that can make joint replacement necessary as well as to understand the operation itself.
The knee is the largest joint in the human body, as well as one of the most vulnerable. Unlike the hip joint, which is partly protected by the bony structures of the pelvis, the knee joint is not shielded by any other parts of the skeleton. In addition, the knee joint must bear the weight of the upper body as well as the stresses and shocks carried upward through the feet when a person walks or runs. Moreover, the knee is essentially a hinge joint, designed to move primarily backwards and forwards; it is not a ball-and-socket joint like the hip, which can swivel and rotate in a variety of directions. Many knee injuries result from stresses caused by twisting or turning movements, particularly when the foot remains in one position while the upper body changes direction rapidly, as in basketball, tennis, or skiing.
The normal knee joint consists of a bone, the patella or kneecap, and a set of tendons, ligaments, and cartilage disks that connect the femur, or thighbone, to the lower leg. There are two bones in the lower leg, the tibia, which is sometimes called the shinbone; and the fibula, a smaller bone on the outside of the lower leg. There are two collateral ligaments on the outside of the knee joint that connect the femur to the tibia and fibula respectively. These ligaments help to control the stresses of side-to-side movements on the knee. The patella—a triangular bone at the front of the knee—is attached by tendons to the quadriceps muscles of the thigh. This tendon allows a person to straighten the knee. Two additional tendons inside the knee stretch between the femur and the tibia to prevent the tibia from moving out of alignment with the femur. Cartilage, which is a whitish elastic tissue that allows bones to glide smoothly against each other, covers the ends of the femur, tibia, and fibula as well as the surfaces of the patella. In addition to the cartilage that covers the bones, the knee joint also contains two crescent-shaped disks of cartilage known as menisci (singular, meniscus), which lie between the lower end of the femur and the upper end of the tibia and act as shock absorbers or cushions. The entire joint is surrounded by a thick layer of protective tissue known as the joint capsule.
Disorders and conditions that may lead to knee replacement surgery include:
There are several factors that increase a person's risk of eventually requiring knee replacement surgery. While some of these factors cannot be avoided, others can be corrected through lifestyle changes:
According to the American Academy of Orthopaedic Surgeons (AAOS), there are about 270,000 knee replacement operations performed each year in the United States. Although about 70% of these operations are performed in people over the age of 65, a growing number of knee replacements are being done in younger patients. A Canadian survey released in January 2003 stated that the number of knee replacements performed in patients younger than 55 rose 90% between 1994 and 2001. Most surgeons expect to see the proportion of knee arthroplasties performed in younger patients continue to rise. One reason for this trend is improvements in surgical technique, as well as the design and construction of knee prostheses since the first knee replacement was performed in 1968. Although most knee prostheses are still cemented in place as of 2003, cementless prostheses were introduced in the 1980s. A second reason is people's changing attitudes toward aging and their expectations of an active life after retirement. Fewer are willing to endure years of discomfort or resign themselves to a restricted level of activity.
In terms of gender and racial differences, women are slightly more likely to seek knee replacement surgery than men, and Caucasians in the United States are more likely to have the operation than African Americans. Researchers have suggested that one reason for the racial difference is a difference in social networks. People in general are influenced in their health care decisions by the experiences and opinions of friends or family members, and Caucasians are more likely than African Americans to know someone who has had knee replacement surgery.
The length and complexity of a total knee replacement operation depend in part on whether both knee joints are replaced during the operation or only one. Such disorders as osteoarthritis usually affect both knees, and some patients would rather not undergo surgery twice. Replacement of both knees is known as bilateral TKR, or bilateral knee arthroplasty. Bilateral knee replacement seems to work best for patients whose knees are equally weak or damaged. Otherwise most surgeons recommend operating on the more painful knee first so that the patient will have one strong leg to help him or her through the recovery period following surgery on the second knee. The disadvantages of bilateral knee replacement include a longer period of time under anesthesia; a longer hospital stay and recovery period at home; and a greater risk of severe blood loss and other complications during surgery.
If the operation is on only one knee, it will take two to four hours. The patient may be given a choice of general, spinal, or epidural anesthesia. An epidural anesthetic, which is injected into the space around the spinal cord to block sensation in the lower body, causes less blood loss and also lowers the risk of blood clots or breathing problems after surgery. After the patient is anesthetized, the surgeon will make an incision in the skin over the knee and cut through the joint capsule. He or she must be careful in working around the tendons and ligaments inside the joint. Knee replacement is a more complicated operation than hip replacement because the hip joint does not depend as much on ligaments for stability. The next step is cutting away the damaged cartilage and bone at the ends of the femur and tibia. The surgeon reshapes the end of the femur to receive the femoral component, or shell, which is usually made of metal and attached with bone cement.
After the femoral part of the prosthesis has been attached, the surgeon inserts a metal component into the upper end of the tibia. This part is sometimes pressed rather than cemented in place. If it is a cementless prosthesis, the metal will be coated or textured so that new bone will grow around the prosthesis and hold it in place. A plastic plate called a spacer is then attached to the metal component in the tibia. The plastic allows the femur and tibia to move smoothly against each other.
Lastly, another plastic component is glued to the rear of the patella, or kneecap. This second piece of plastic prevents friction between the kneecap and the other parts of the prosthesis. After all the parts of the prosthesis have been implanted, the surgeon will check them for proper positioning, make certain that the tendons and ligaments have not been damaged, wash out the incision with sterile saline solution, and close the incision.
The first part of a diagnostic interview for knee pain is the careful taking of the patient's history. The doctor will ask not only for a general medical history, but also about the patient's occupation, exercise habits, past injuries to the knee, and any gait-related problems. The doctor will also ask detailed questions about the patient's ability to move or flex the knee; whether specific movements or activities make the pain worse; whether the
PHYSICAL EXAMINATION OF THE KNEE. Following the history, the doctor will examine the knee itself. The knee will be checked for swelling, reddening, bruises, breaks in the skin, lumps, or other unusual features while the patient is standing. The doctor will also make note of the patient's posture, including whether the patient is bowlegged or knock-kneed. The patient may be asked to walk back and forth so that the doctor can check for gait abnormalities.
In the second part of the physical examination , the patient lies on an examining table while the doctor palpates (feels) the structures of the knee and evaluates the strength or tightness of the tendons and ligaments. The patient may be asked to flex one knee and straighten the leg or turn the knee inward and outward so that the doctor can measure the range of motion in the joint. The doctor will also ask the patient to lie still while he or she moves the knee in different directions.
IMAGING STUDIES. The doctor will order one or more imaging studies in order to narrow the diagnosis. A radiograph or x ray is the most common, but is chiefly useful in showing fractures or other damage to bony structures. X-ray studies are usually supplemented by other imaging techniques in diagnosing knee disorders. A computed tomography, or CAT scan, which is a specialized type of x ray that uses computers to generate three-dimensional images of the knee joint, is often helpful in evaluating malformations of the joint. Magnetic resonance imaging (MRI) uses a large magnet, radio waves, and a computer to generate images of the knee joint. The advantage of an MRI is that it reveals injuries to ligaments, tendons, and menisci as well as damage to bony structures.
ASPIRATION. Aspiration is a procedure in which fluid is withdrawn from the knee joint by a needle and sent to a laboratory for analysis. It is done to check for infection in the joint and to draw off fluid that is causing pain. Aspiration is most commonly done when the knee has swelled up suddenly, but may be performed at any time. Blood in the fluid usually indicates a fracture or torn ligament; the presence of bacteria indicates infection; the presence of uric acid crystals indicates gout. Clear, straw-colored fluid suggests osteoarthritis.
ARTHROSCOPY. Arthroscopy can be used to treat knee problems as well as diagnose them. An arthroscope consists of a miniature camera and light source mounted on a flexible fiberoptic tube. It allows the surgeon to look into the knee joint. To perform an arthroscopy, the surgeon will make two to four small incisions known as ports. One port is used to insert the arthroscope; the second port allows insertion of miniaturized surgical instruments ; the other ports drain fluid from the knee. Sterile saline fluid is pumped into the knee to enlarge the joint space and make it easier for the surgeon to view the knee structures and to cut, smooth, or repair damaged tissue.
Knee replacement surgery requires extensive and detailed preparation on the patient's part because it affects so many aspects of life.
LEGAL AND FINANCIAL CONSIDERATIONS. In the United States, physicians and hospitals are required to verify the patient's insurance benefits before surgery and to obtain precertification from the patient's insurer or from Medicare . Without health insurance, the total cost of a knee replacement as of early 2003 can run as high as $38,000. In addition to insurance documentation, patients are legally required to sign an informed consent form prior to surgery. Informed consent signifies that the patient is a knowledgeable participant in making health-care decisions. The doctor will discuss all of the following with the patient before he or she signs the form: the nature of the surgery; reasonable alternatives to the surgery; and the risks, benefits, and uncertainties of each option. Informed consent also requires the doctor to make sure that the patient understands the information that has been given.
MEDICAL CONSIDERATIONS. Patients are asked to do the following in preparation for knee replacement surgery:
LIFESTYLE CHANGES. Knee replacement surgery requires a long period of recovery at home after leaving the hospital. Since the patient's physical mobility will be limited, he or she should do the following before the operation:
Many hospitals and clinics now have "preop" classes for patients scheduled for knee replacement surgery. These classes answer questions about the operation and what to expect during recovery, but in addition they provide an opportunity for patients to share concerns and experiences. Studies indicate that patients who have attended preop classes are less anxious before surgery and generally recover more rapidly.
Aftercare following knee replacement surgery begins while the patient is still in the hospital. Most patients will remain there for five to 10 days after the operation. During this period the patient will be given fluids and antibiotic medications intravenously to prevent infection. Medications for pain will be given every three to four hours, or through a device known as a PCA (patient-controlled anesthesia). The PCA is a small pump that delivers a dose of medication into the IV when the patient pushes a button. To get the lungs back to normal functioning, a respiratory therapist will ask the patient to cough several times a day or breathe into blow bottles.
Aftercare during the hospital stay is also intended to lower the risk of a venous thromboembolism (VTE), or blood clot in the deep veins of the leg. Prevention of VTE involves medications to thin the blood; exercises for the feet and ankles while lying in bed; and wearing thromboembolic deterrent (TED) or deep vein thrombosis (DVT) stockings. TED stockings are made of nylon (usually white) and may be knee-length or thigh-length; they help to reduce the risk of a blood clot forming in the leg vein by putting mild pressure on the veins.
Physical therapy is also begun during the patient's hospital stay, often on the second day after the operation. The physical therapist will introduce the patient to using a cane or crutches and explain how to manage such activities as getting out of bed or showering without dislocating the new prosthesis. In most cases the patient will spend some time each day on a continuous passive motion (CPM) machine, which is a device that repeatedly bends and straightens the leg while the patient is lying in bed. In addition to increasing the patient's level of physical activity each day, the physical therapist will help the patient select special equipment for recovery at home. Commonly recommended devices include tongs or reachers for picking up objects without bending too far; a sock cone and special shoehorn; and bathing equipment.
Following discharge from the hospital , the patient may go to a skilled nursing facility, rehabilitation center, or home. Patients who have had bilateral knee replacement are unlikely to be sent directly home. Ongoing physical therapy is the most important part of recovery for the first four to five months following surgery. Most HMOs in the United States allow home visits by a home health aide, visiting nurse, and physical therapist for three to four weeks after surgery. Some hospitals allow patients to borrow a CPM machine for use at home for a few weeks. The physical therapist will monitor the patient's progress as well as suggest specific exercises to improve strength and range of motion. After the home visits, the patient is encouraged to take up other forms of low-impact physical activity in addition to the exercises; swimming, walking, and pedaling a stationary bicycle are all good ways to speed recovery. The patient may take a mild medication for pain (usually aspirin or ibuprofen) 30–45 minutes before an exercise session if needed.
The patient will be instructed to notify his or her dentist about the knee replacement so that extra precautions can be taken against infection resulting from bacteria getting into the bloodstream during dental work. Some surgeons ask patients to notify them whenever the dentist schedules a tooth extraction , root canal, or periodontal work.
Serious risks associated with TKR include the following:
Normal results include relief of chronic pain in the knee and greater range of motion in the knee joint. Realistically, however, the patient should not expect complete restoration of function in the knee, and will usually be advised to avoid contact sports, skiing, jogging, or other athletic activities that strain the knee joint.
Mild swelling of the leg may occur for as long as three to six months after surgery. It can be treated by elevating the leg, applying an ice pack, and wearing compression stockings.
One commonplace side effect of TKR is that knee prostheses sometimes set off metal detectors in airports and high-security buildings because of their large metal content. Patients who fly frequently or whose occupations require security clearance should ask their doctor for a wallet card certifying that they have a knee prosthesis.
The patient can expect a cemented knee prosthesis to last about 10–15 years, although many still function well as long as 20 years later. Cementless prostheses have not been in use long enough for reliable evaluations of their long-term durability. When the prosthesis wears out or becomes loose, it is replaced in a procedure known as knee revision surgery .
A study published in 2002 reported that the 30-day mortality rate following total knee arthroplasty was 0.5%. The overall frequency of serious complications in this time period was 2.2%. This figure included 0.4% heart attack; 0.7% pulmonary embolism; and 1.5% deep venous thrombosis. The rate of complications was highest in patients over 70, and male patients were more likely to have heart attacks than women.
A 2001 study published by the Mayo Clinic reviewed the records of 22,540 patients who had had knee replacements between 1969 and 1997. The mortality rate within 30 days of surgery was 0.21%, or 47 patients. Forty-three of the 47 patients had had preexisting cardiovascular or lung disease. Patients who had had bilateral knee operations had a higher mortality rate than those who had not.
MEDICATION. The most common conservative alternatives to knee replacement surgery are analgesics , or painkilling medications. Most patients who try medication for knee pain begin with an over-the-counter NSAID such as ibuprofen (Advil). If the pain cannot be controlled by nonprescription analgesics, the doctor may give the patient cortisone injections, which relieve the pain of arthritis by reducing inflammation. Unfortunately, the relief provided by cortisone tends to diminish with each injection; moreover, the drug can produce serious side effects.
If the knee pain is caused by rheumatoid arthritis, a group of medications known as disease-modifying antirheumatic drugs, or DMARDs, may help to slow or stop the progress of the disease. They work by suppressing or interfering with the immune system. DMARDs include such drugs as penicillamine, methotrexate, oral or injectable gold, hydroxychloroquine, leflunomide, and sulfasalazine. DMARDs are not suitable for all patients with RA, however, as they sometimes have serious side effects. In addition, some of them are slow-acting and may take several months to work before the patient feels some relief.
LIFESTYLE CHANGES. A second alternative to knee surgery is lifestyle changes. Losing weight helps to reduce stress on the knee joint. Giving up specific sports or other activities that damage the knee, such as jogging, tennis, high-impact aerobics, or stair-climbing exercise machines, may control the pain enough to make surgery unnecessary. Wearing properly fitted shoes and avoiding high heels and other extreme styles can also help to control pain and minimize further damage to the knee.
BRACES AND ORTHOTICS. Some patients with unstable knees are helped by functional braces or knee supports that are designed to keep the kneecap from slipping out of place. Orthotics, which are inserts placed inside shoes, are often helpful to patients whose knee problems are related to their gait. Orthotics are designed either to correct the position of the foot in order to keep it from turning too far outward or inward, or to correct problems in the arch of the foot. Some orthotics are made of soft material that cushions the foot and are particularly helpful for patients with osteoarthritis or diabetes.
Complementary and alternative therapies are not substitutes for arthroscopy or joint replacement surgery, but some have been shown to relieve physical pain before or after surgery, or to help patients cope more effectively with the emotional and psychological stress of a major operation. Acupuncture, chiropractic, hypnosis, and mindfulness meditation have been used successfully to relieve the pain of osteoarthritis as well as postoperative discomfort. According to Dr. Marc Darrow, author of The Knee Sourcebook , a plant extract called RA-1, which is used in Ayurvedic medicine to treat arthritis, relieved pain and leg swelling in patients participating in a randomized trial. Alternative approaches that have helped patients maintain a positive mental attitude include meditation, biofeedback, and various relaxation techniques.
Arthroscopy is the most common surgical alternative to knee replacement. It should be understood, however, as a way to postpone TKR rather than avoid it completely. The arthroscopic procedure most often used to treat knee pain from osteoarthritis is debridement, in which the surgeon cuts or scrapes away damaged structures or tissues until healthy tissue is reached. Most patients who have had arthroscopic débridement have been able to postpone TKR for three to five years.
Cartilage transplantation is a procedure in which small bone plugs with cartilage are removed from a part of the patient's knee where the cartilage is still healthy and transplanted to the area in which cartilage has been damaged. Another form of cartilage transplantation involves two operations, one to remove cartilage cells from the patient's knee for culture in a laboratory, and a second operation to place the new cells within the damaged part of the knee. The cultured cells are covered with a thin layer of tissue to hold them in place. After surgery, the cartilage cells multiply to form new cartilage inside the knee. Unfortunately, as of 2003 neither form of cartilage transplantation is usually beneficial to patients with osteoarthritis; transplantation has been most successful in treating patients whose knee cartilage was damaged by sudden trauma rather than by gradual degeneration.
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Rebecca Frey, Ph.D.
Knee replacement surgery is performed by an orthopedic surgeon, who is an MD who has received advanced training in surgical treatment of disorders of the musculoskeletal system. As of 2002, qualification for this specialty in the United States requires a minimum of five years of training after medical school. Most orthopedic surgeons who perform joint replacements have had additional specialized training in these specific procedures.
Knee replacement surgery can be performed in a general hospital with a department of orthopedic surgery, but is also performed in specialized clinics or institutes for joint disorders.