Knee replacement surgery is a surgery to replace parts of your knee joint with new, artificial parts. You may need a knee replacement if you have knee damage that causes severe pain and difficulty doing daily activities, such as walking and climbing stairs. It is usually done when other treatments for knee pain haven't helped enough. The goal of a knee replacement is to relieve pain and help you move better.

People of all ages may have knee replacement surgery. But it is more common in older people. The decision whether to have surgery is based on your overall health and how much your knee bothers you.

Joint replacement surgery removes damaged or diseased parts of a joint. It replaces them with new, human-made parts. Joints may need to be replaced when they are damaged from:

• Arthritis (inflammation of joints that causes pain and stiffness)
• Years of use
• Disease

The goals of joint replacement surgery are to relieve pain, help the joint work better, and improve walking and other movements. Your provider will likely first suggest other treatments to reduce pain and help you move better, such as:

• An exercise program
• Walking aids like a cane or walker
• Physical therapy
• Medications

Complications after joint replacement surgery include infection, blood clots, and joint failure. Wearing away of the joint surface may become a problem after 15 to 20 years.

The knee joint is the largest joint of the body (Figure). It actually consists of three articulations. The femoropatellar joint is found between the patella and the distal femur. The medial tibiofemoral joint and lateral tibiofemoral joint are located between the medial and lateral condyles of the femur and the medial and lateral condyles of the tibia. All of these articulations are enclosed within a single articular capsule. The knee functions as a hinge joint, allowing flexion and extension of the leg. This action is generated by both rolling and gliding motions of the femur on the tibia. In addition, some rotation of the leg is available when the knee is flexed, but not when extended. The knee is well constructed for weight bearing in its extended position, but is vulnerable to injuries associated with hyperextension, twisting, or blows to the medial or lateral side of the joint, particularly while weight bearing.

At the femoropatellar joint, the patella slides vertically within a groove on the distal femur. The patella is a sesamoid bone incorporated into the tendon of the quadriceps femoris muscle, the large muscle of the anterior thigh. The patella serves to protect the quadriceps tendon from friction against the distal femur. Continuing from the patella to the anterior tibia just below the knee is the patellar ligament. Acting via the patella and patellar ligament, the quadriceps femoris is a powerful muscle that acts to extend the leg at the knee. It also serves as a “dynamic ligament” to provide very important support and stabilization for the knee joint.

The medial and lateral tibiofemoral joints are the articulations between the rounded condyles of the femur and the relatively flat condyles of the tibia. During flexion and extension motions, the condyles of the femur both roll and glide over the surfaces of the tibia. The rolling action produces flexion or extension, while the gliding action serves to maintain the femoral condyles centered over the tibial condyles, thus ensuring maximal bony, weight-bearing support for the femur in all knee positions. As the knee comes into full extension, the femur undergoes a slight medial rotation in relation to tibia. The rotation results because the lateral condyle of the femur is slightly smaller than the medial condyle. Thus, the lateral condyle finishes its rolling motion first, followed by the medial condyle. The resulting small medial rotation of the femur serves to “lock” the knee into its fully extended and most stable position. Flexion of the knee is initiated by a slight lateral rotation of the femur on the tibia, which “unlocks” the knee. This lateral rotation motion is produced by the popliteus muscle of the posterior leg.

Located between the articulating surfaces of the femur and tibia are two articular discs, the medial meniscus and lateral meniscus (see Figureb). Each is a C-shaped fibrocartilage structure that is thin along its inside margin and thick along the outer margin. They are attached to their tibial condyles, but do not attach to the femur. While both menisci are free to move during knee motions, the medial meniscus shows less movement because it is anchored at its outer margin to the articular capsule and tibial collateral ligament. The menisci provide padding between the bones and help to fill the gap between the round femoral condyles and flattened tibial condyles. Some areas of each meniscus lack an arterial blood supply and thus these areas heal poorly if damaged.

The knee joint has multiple ligaments that provide support, particularly in the extended position (see Figurec). Outside of the articular capsule, located at the sides of the knee, are two extrinsic ligaments. The fibular collateral ligament(lateral collateral ligament) is on the lateral side and spans from the lateral epicondyle of the femur to the head of the fibula. The tibial collateral ligament(medial collateral ligament) of the medial knee runs from the medial epicondyle of the femur to the medial tibia. As it crosses the knee, the tibial collateral ligament is firmly attached on its deep side to the articular capsule and to the medial meniscus, an important factor when considering knee injuries. In the fully extended knee position, both collateral ligaments are taut (tight), thus serving to stabilize and support the extended knee and preventing side-to-side or rotational motions between the femur and tibia.

The articular capsule of the posterior knee is thickened by intrinsic ligaments that help to resist knee hyperextension. Inside the knee are two intracapsular ligaments, the anterior cruciate ligament and posterior cruciate ligament. These ligaments are anchored inferiorly to the tibia at the intercondylar eminence, the roughened area between the tibial condyles. The cruciate ligaments are named for whether they are attached anteriorly or posteriorly to this tibial region. Each ligament runs diagonally upward to attach to the inner aspect of a femoral condyle. The cruciate ligaments are named for the X-shape formed as they pass each other (cruciate means “cross”). The posterior cruciate ligament is the stronger ligament. It serves to support the knee when it is flexed and weight bearing, as when walking downhill. In this position, the posterior cruciate ligament prevents the femur from sliding anteriorly off the top of the tibia. The anterior cruciate ligament becomes tight when the knee is extended, and thus resists hyperextension.

If you or someone you know is considering knee replacement, a new resource can help you understand how it works, how to prepare for surgery, and what to expect in recovery.

Knee replacement involves removing parts of your natural knee joint and replacing them with artificial parts. Knee replacement is the most common type of joint replacement surgery.

Several forms of arthritis can damage knees and cause so much pain and disability that knees need to be replaced. Certain knee deformities—such as bowed legs or knock knees—can wear down cartilage and create difficulties. Knee damage can also result from a problem called avascular necrosis, or osteonecrosis, in which the bones lose their blood supply, die, and eventually collapse.

If other treatments haven’t helped, your doctor may suggest knee replacement when pain and stiffness begin to interfere with your everyday activities.

If you’d like to consider knee replacement, ask your doctor to refer you to an orthopedic surgeon, a doctor specially trained to treat problems of the bones and joints.

During joint replacement surgery your doctors will:

• Give you medicine so you won’t feel pain. The medicine may block the pain only in one part of the body, or it may put your whole body to sleep.
• Replace the damaged joint with a new man-made joint.
• Move you to a recovery room until you are fully awake or the numbness goes away.

With knee or hip surgery, you will probably need to stay in the hospital for a few days. If you are an older adult or have additional disabilities, you may then need to spend several weeks in an intermediate-care facility before going home. You and your team of doctors will determine how long you stay in the hospital.

After hip or knee replacement, you will often stand or begin walking the day of surgery. At first, you will walk with a walker or crutches. You may have some temporary pain in the new joint because your muscles are weak from not being used. Also, your body is healing. The pain can be helped with medicines and should end in a few weeks or months.

Physical therapy can begin the day after surgery to help strengthen the muscles around the new joint and help you regain motion in the joint. If you have your shoulder joint replaced, you can usually begin exercising the same day of your surgery! A physical therapist will help you with gentle, range-of-motion exercises. Before you leave the hospital, your therapist will show you how to use a pulley device to help bend and extend your arm.

With total knee replacement surgery, researchers are looking beyond simply reducing pain. The hope is to return patients to the same high level of activity they had before surgery—like participating in sports or even running marathons.

"People would like to go back and be long distance runners or high performing athletes," said orthopedic surgeon Joshua J. Jacobs, M.D. NIH's National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) supports Dr. Jacobs' research.

"In the research community we want to figure out a way to make better and better implants."

- Joshua J. Jacobs, M.D.

The surgery involves replacing weight-bearing surfaces of the knee joint. Patients who commonly have this surgery have arthritis in their knees. Arthritis affects an estimated 54.4 million adults in the U.S. It causes joint inflammation, pain, swelling, and stiffness.

"Where the technology is now, the total knee replacement operations are effective for reducing pain and improving function. But we have a ways to go before we can assure our patients that this surgery can allow the extreme levels of activity that one can do on a healthy joint," said Dr. Jacobs, who works at Chicago's Rush University Medical Center.

Improving knee implants

An important step forward would be to make knee implants last longer. Current replacement joint surfaces can wear away after 15 to 20 years. Providers often delay surgery because of an implant's limited lifespan.

Despite the name, total knee replacement doesn't replace all of the knee joint. Doctors put in an artificial joint (implant or prosthesis), composed of different human-made parts.

Replacement joints are made of a mix of metal, ceramic, or plastic. They are designed so that certain materials, like metal, always border with others, like plastic. This creates smoother movement and less implant wear-and-tear.

Even though implant materials have improved since knee replacement surgery first started more than 50 years ago, Dr. Jacobs and his colleagues strive for more improvements.

They are exploring new types of material combinations. Dr. Jacobs is researching how implant surfaces or materials release different kinds of debris from a patient's natural bone or tissue. Materials with less impact on the body could make for better implants.

"That interaction actually is quite important. In many cases, it determines the outcomes of the joint reconstructions and will dictate how long the implant will function well in a particular patient," said Dr. Jacobs.

"In the research community we want to figure out a way to make better and better implants that are both longer lasting and allow the functions that our patients seek," Dr. Jacobs said.

Treatment doesn't end at surgery

Implant success also relies on patient work after surgery. Recovery and continued physical activity are key to lasting knee replacement success.

In a clinical trial, supported by NIAMS, Elena Losina, Ph.D., and her team looked at how physical activity can improve total knee replacement outcomes.

"Many researchers noticed that people who had complained about a lot of pain prior to surgery are very grateful that the pain is gone," Dr. Losina, of Brigham and Women's Hospital in Boston, explained. "But what other researchers noticed was that while the pain is gone, people were not getting more physically active."

Her clinical study used one-on-one coaching and reward money to get 200 patients to become more active after surgery. To measure progress, patients wore activity trackers and reported key information in real time.

The study helped patients understand how physical activity contributes to long-term recovery. The money offered immediate encouragement to get active.

"We found that joint intervention, which includes both coaching using motivational interviewing and financial incentives, substantially improves the number of steps that total knee replacement recipients did daily and the total minutes of physical activity," Dr. Losina said.

For patients who are advised by a physician that they may need total knee replacement surgery, Dr. Jacobs says the first step is talking to an orthopedic surgeon or specialist.

"You need to be evaluated by a health care professional who can zero in on the diagnosis of where you are with your pain," he said.

New technology and advances in surgical techniques have greatly reduced the complications involved with joint replacements. When problems do occur, most are treatable. Possible problems include:

• Infection. Areas in the wound or around the new joint may get infected. It may happen while you’re still in the hospital or after you go home. It may even occur years later. Minor infections in the wound are usually treated with drugs. Deep infections may need a second operation to treat the infection or replace the joint.
• Blood clots. If your blood moves too slowly, it may begin to form lumps of blood parts called clots. If pain and swelling develop in your legs after hip or knee surgery, blood clots may be the cause. The doctor may suggest drugs to make your blood thin or special stockings, exercises, or boots to help your blood move faster. If swelling, redness, or pain occurs in your leg after you leave the hospital, contact your doctor right away.
• Loosening. The new joint may loosen, causing pain. If the loosening is bad, you may need another operation to reattach the joint to the bone.
• Dislocation. Sometimes after hip or other joint replacement, the ball of the prosthesis can come out of its socket. In most cases, the hip can be corrected without surgery. A brace may be worn for a while if a dislocation occurs.
• Wear. Some wear can be found in all joint replacements. Too much wear may help cause loosening. The doctor may need to operate again if the prosthesis comes loose. Sometimes, the plastic can wear thin, and the doctor may just replace the plastic and not the whole joint.
• Nerve and blood vessel injury. Nerves near the replaced joint may be damaged during surgery, but this does not happen often. Over time, the damage often improves and may disappear. Blood vessels may also be injured.

As you move your new joint and let your muscles grow strong again, pain will lessen, flexibility will increase, and movement will improve.

A prosthetic joint infection (PJI) is a rare complication of joint replacement surgery, also known as arthroplasty. Arthroplasty is done to help relieve pain and restore function in a severely diseased joint, such as a knee, hip or shoulder. Approximately 0.5 to 1 percent of people with replacement joints develop a PJI. Infections can occur early in the course of recovery from joint replacement surgery (within the first two months) or much later. Signs and symptoms of PJI include fever, chills, drainage from the surgical site, and increasing redness, tenderness, swelling and pain of the affected joint.

Prosthetic joint infections are often hard to treat because of the development of a structure called a biofilm within the joint. A biofilm develops when bacteria adhere to the solid surface of the artificial joint. The biofilm can act as a kind of shield to some of the bacteria, making it difficult for the bacteria to be found and destroyed by the body's defenses or by antibiotic medications. An infected joint replacement usually requires surgery to remove the artificial parts and potent antibiotics to kill the bacteria.

Patient shares her knee pain journey

Melanie Modlin, 62, had total knee replacement last June. Her number one question after surgery was, "When am I going to feel normal again?" The answer came slowly.

For Melanie, feeling normal involved more than just becoming pain-free. She focused on rebuilding her range of motion and her strength.

"With knee surgery and replacement, it's important to get moving right away or else scar tissue can settle around that new joint," says Melanie. "If that happens, you can lose range of motion very quickly."

Exercise was key to getting better. Melanie did all the exercises her doctor and physical therapists recommended. A positive mental attitude helped, too, she says.

During her recovery, Melanie focused on "little victories." The first little victory was standing from a chair without using her arms. Another was walking with a cane instead of crutches. Later victories included walking without a cane and finally going up and down the stairs.

After four months, she felt much better. After six months, she felt fantastic. "But I know it's different for everyone," Melanie says.

The first time Melanie dislocated her kneecap, she was in eighth grade. As an adult, she experienced that same feeling a few more times. When she was 49 years old, she learned that she had osteoarthritis in her knee. Osteoarthritis causes pain, swelling, and reduced motion.

She could manage the pain on her own until one day in 2015.

On her way to work, carrying a heavy briefcase and purse, Melanie felt her right knee give way on the stairs near the parking lot. She couldn't get up. Luckily, she could call for help at NIH's National Library of Medicine, where she works.

Melanie had torn the cartilage in her knee and would need total knee replacement surgery, her doctor said.

Knowing that joint replacements last about 15 to 20 years, Melanie did everything she could to manage pain and keep active until her surgery. She did physical therapy, used crutches when necessary, and got regular cortisone shots.

But in the spring of 2017, she couldn't wait any longer. Her knee joint was bone-on-bone, meaning the cartilage had worn away, and she was in terrible pain. She needed surgery.

More than eight months after surgery, Melanie says she feels fortunate. In addition to celebrating "little victories" along the way to recovery, she also found what she calls "great gifts."

"I always thought of myself as a patient person, but now I'm even more patient," she said. "And I'm not as afraid to ask for help. People have been kind and generous. A friend stayed with me after my surgery and coworkers helped by bringing me lunch and visiting me."

One step and one victory at a time–plus the gifts of patience and gratitude–continue to help Melanie live a better life after surgery.