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:
- Osteoarthritis (OA). Osteoarthritis is a disorder in which the cartilage in the knee joint gradually breaks down, allowing the surfaces of the bones to rub directly against each other. The patient experiences swelling, pain, inflammation, and increasing loss of mobility. OA most often affects adults over age 45, and is thought to result from a combination of wear and tear on the joint, lifestyle, and genetic factors. As of 2003, OA is the most common cause of joint damage requiring knee replacement.
- Rheumatoid arthritis (RA). Rheumatoid arthritis is a disease that begins earlier in life than OA and affects the whole body. Women are three times as likely as men to develop RA. Its symptoms are caused by the immune system's attacks on the body's own cells and tissues. Patients with RA often suffer intense pain even when they are not putting weight on the affected joints.
Trauma. Damage to the knee from a fall, automobile accident, or workplace or athletic injury may trigger the process of cartilage breakdown inside the joint. Trauma is a common cause of damage to the knee joint. Some traumatic injuries are caused by repetitive motion or overuse of the knee joint; these types of injury include bursitis, or housemaid's knee, and so-called runner's knee. Other traumatic injuries are caused by sudden twisting of the knee, a direct blow to a bent knee
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.
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.
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. 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.