Knee Replacement Overview

boy bouncing a soccer ball on his knee
Knees: far more fun than elbows.

It's the biggest joint in the body, it's more complex than it first appears, and it's fun to bounce soccer balls off of -- it's the knee. Your knee allows your lower leg to be extended outward or pulled back toward your body. It's a crucial part of our ability to move around (lock your legs in a straight position for an hour if you have any doubts). All good things must come to an end, though, and so it is with this meeting of the femur (your thighbone), the tibia (your shin) and the patella (the kneecap).

Most of the time, arthritis (an inflammation of a joint) is the reason why a knee joint must be replaced. However, sports injuries, excessive wear-and-tear and genetics also lead to badly damaged knees. A condition caused by lack of blood flow to the knee, avascular necrosis, also kills bone tissue. Fortunately, advancements in science and medicine enable us to replace this important joint, extending and increasing not only mobility but also the quality of life for people with damaged knees.


Early implant devices were little more than simple hinges, but modern artificial knees come much closer to replicating the knee's unique gliding and rolling motions while providing strength and stability. Hundreds of thousands of people undergo total knee replacement surgery, also known as total knee arthroplasty, all over the world each year -- there are more than 400,000 knee replacements each year in the United States alone [source: Sharpe]. With the population growing and techniques improving, that number will only increase.

­Generally, people over 65 make up the largest candidate pool for total knee replacements [source: Arthritis Research Campaign]. The procedure can also performed on younger patients suffering from pain and loss of mobility, though it's best to put off this surgery as long as possible. The life span of an artificial knee is around 10 years. Middle-aged recipients will likely need a second replacement down the road, and the second procedure is often not as successful as the first (we'll talk about why a little later).

In this article we'll discuss why knees need to be replaced, how total knee replacements are performed and what to expect from your new knee.


Anatomy of the Knee

anatomy of the knee
2009 HowStuffWorks

The knee joint is the intersection of your femur (your thigh bone), your tibia (your shin bone) and your patella (your kneecap).

The quadriceps is the big group of muscles that make up your thigh. These muscles run down the length of your femur and taper off as they approach the patella. They taper down into a tough piece of connective tissue called the quadriceps tendon. This tendon attaches to the top of your kneecap. On the other side of the kneecap, this connective tissue (now called the patellar tendon) continues downward and connects the bottom of the kneecap to the top of the tibia.


The kneecap rests in a groove (the trochlea) at the bottom of the femur, where it's able to slide forward and backward. The kneecap is as much a part of the tendon as it is a bone with two tendons attached. Without the patella, your tendon would directly hug the joint, grinding back and forth in the groove of the trochlea and wearing away with use. As it is, the patella holds the tendon slightly away from the joint, protecting it and maximizing the tendon's function, which is an important one: When the quadriceps contracts, the quadriceps tendon contracts, pulling your kneecap back. This pulls the patellar tendon back, which (being attached as it is to the top of your shin) straightens out your leg. Relax the quadriceps, and your leg bends.

There's more to the picture, though. The shape of the femur and tibia have subtle variations in different people, as do the form of the ligaments. In addition to the quadriceps tendon and the patellar tendon above and below the kneecap, there are four tendons that directly attach the femur to either the tibia or the fibula (the smaller lower leg bone that runs beside your shin). These tendons keep your leg perfectly aligned, adjusting every moment in relation to each other and the movement of the joint. If they didn't exist, not only would walking normally be impossible, your lower leg could very well spin like a top below your knee! The ligaments, tendons, bone and cartilage must all be correctly aligned and remain healthy for the knee to work properly.

So, it's the biggest joint in the body, it supports almost our entire body weight and it has an intricate design -- what could possibly go wrong?


Arthritis of the Knee: Enemy No. 1

The knee joint is often damaged by arthritis. Arthritis of the knee is painful and gets worse with time. Though there are many different forms of arthritis, three kinds are the common culprits of knee pain:

  • Osteoarthritis (also sometimes called degenerative arthritis) is the most common form of arthritis that affects the knee. Obesity, diabetes and frequent physical battering of the joint itself (think: baseball catcher's knees) are common causes, although we don't always know why osteoarthritis occurs. Osteoarthritis usually catches up to us in middle age or beyond -- among 40-year-olds, about two out of five show signs of osteoarthritis, but only half experience symptoms at this stage. Osteoarthritis can be very painful. Surviving tissue may be inflamed, bone can grind against bone when a joint is moved, and all of the aggravating stimulation to the end of the bone can cause it to form a bone spur, a newly formed bony protrusion, near the joint, leading to further discomfort.
  • Rheumatoid arthritis occurs when the body's immune system attacks healthy tissue in reaction to bacteria, toxins and parasites. It can take just a year for some malfunctioning white blood cells around your knee joint to cause permanent damage to your joint [source: Abbott Laboratories]. This usually affects both knees simultaneously. Over time, rheumatoid arthritis changes the way you walk and move, and hobbling around leads to further bone and tissue damage. Rheumatoid arthritis can show up at any point in life.
  • Post-traumatic arthritis occurs as a result of external damage to the knee. It may take months or years for signs of arthritis to appear after an injury.

As knees become arthritic, they stiffen, swell and grow weaker. When cartilage wears away, it can create too much space between the bones, and it can alter the tracking of the patella. Likewise, if bone spurs (outgrowths caused by bone stimulation) develop, there will be too little space between the joints.


As arthritis advances, the pain happens not only when your knee is in motion, but even when you're resting. The knee may start "going out" or "giving way," or, alternately, it may stiffen up and refuse to bend.


Who is a Good Candidate for Total Knee Replacement?

health care provider checking a patient's knee
Total knee replacement should be a last resort -- go through all your other options first.

Although the surgery is becoming somewhat routine, total knee replacement isn't right for everyone. For one thing, the replacement doesn't last forever. Unlike a mechanical heart stent that lasts longer than the patient, mechanical knees wear down just like real knees. So if you get a knee replacement in your early 50s, you're probably going to need another one sometime in your mid- to late 60s. The problem is that the second is often less successful than the first one, because the bones have already been drilled into, leaving less of a "bracket" for the replacement joint the second time around.

For this reason (and to avoid major surgery in the first place), people who have pain in their knees should explore and exhaust all other options before turning to knee replacement. About 75 percent of people affected by osteoarthritis in their knees won't need knee surgery [source: American Academy of Orthopaedic Surgeons]. If you can alleviate the pain and regain range of motion in the knee, you have fulfilled the functions of a knee replacement.


Everyday swelling and pain related to arthritis can sometimes be controlled with over-the-counter medications such as ibuprofen, aspirin or another nonsteroidal anti-inflammatory drug (NSAID). Other drugs with more long-term effects -- such as hydroxychloroquine, penicillamine or methotrexate -- can be taken to slow the advance of arthritis as well as the onset of symptoms.

The more body weight you have, the harder your knee has to work. Instead of activities that have a high impact on your knee joint, like jumping and running, take up swimming, walking or biking. Getting in shape will also strengthen your leg muscles and help keep your ligaments flexible. Often, some type of knee support -- an elastic bandage, a cane or just well-fitted shoes -- will increase the utility and the lifespan of the joint.

Regular injections of hyaluronic acid in the joint (a process called viscosupplementation) can temporarily replace the lost fluid that once kept the joint nicely lubricated. This treatment requires three to five injections a week and works best with mild or moderate arthritis. Magnetic pulse therapy and acupuncture may also provide relief, though these methods haven't received much in the way of scientific testing.


Surgical Alternatives to Total Knee Replacement

There are surgical treatments other than a total knee replacement to alleviate pain and regain mobility in a damaged knee. Torn cartilage and other matter can be removed from the knee during arthroscopic surgery. Another procedure, an osteotomy, realigns the knee by cutting bone off either the femur or tibia.

A less common procedure, the unicompartmental knee arthroplasty, has shown promise in treating knee joints with arthritic damage on only one side of the knee. For instance, if you had arthritis damage on the left side of your knee, only the damaged portions on the left side would be removed, reshaped and replaced by metal and polyethylene replicas. About 6 percent of patients with arthritic knees are candidates for unidepartmental knee arthroplasty [source: Port]. If the untreated side eventually becomes arthritic, the patient can still have a total knee replacement.


One relatively new technique that requires less tissue damage during the procedure is called a minimally invasive knee replacement. Although the same type of implant is inserted, the surgeon works with a smaller incision. There is less scarring with this method and less overall tissue damage, resulting in a shorter hospital stay. The procedure is difficult to perform and requires different tools and instruments to operate on the joint. Depending on the joint damage and other factors such as obesity, a minimally invasive knee replacement may not be the ideal choice for some patients.

If you have hip troubles that may require a hip replacement, you'll want to get your hip taken care of before your knee. Why can't you do both at once? Because in order to properly exercise and rehab your new knee, you will need mobility in your hip.

Not all knees are built alike, nor is all knee damage similar. An orthopedic surgeon can help steer you toward the right treatment for your specific knee situation.


The Knee Prosthesis

­Attempts at removing portions of bone to alleviate pain in knee joints were made as early as the 1860s, but the first artificial replacements were implanted in the 1940s. These early attempts focused only on the femur, but by the 1950s replacements that affixed to both the femur and tibia were being used.

These early models wrongly assumed that the knee worked basically like a hinge. With all the rolling, gliding, sliding and recentering that occurs with each knee movement, these models weren't successful. More recent designs have made greater strides toward replicating the knee's complicated design while also maintaining the structural integrity and efficiency of the artificial implant.


Implants share three common components: an attachment to the femur, an attachment to the tibia and a replacement for the back of the patella. They're usually made of metal like cobalt chrome or titanium and they cup the femur where damaged tissue has been removed from the end, front and back. A specialized type of acrylic cement can seal the device to the bone end, or (depending on the type of prosthesis) pegs or a long stem may be inserted into holes drilled into the bone. Other methods of attachment include using implants that have a coating designed to promote the bone tissue to grow into it. Sometimes both methods -- cement and bone-growth attachment -- are used.

For the tibia, a metal plate may be screwed to the flattened bone end, to which is attached the tibial insert, usually a polyethylene plastic piece that will act as the articular surface (the part of the bone end that is spongy) and prevent the tibia's metal plate and the femur's attachment from rubbing together.

A polyethylene plastic implant is used to replace tissue from the back of the patella. The patellar implant will move against the femoral implant, so a precise fit is crucial.

Another type of implant is known as a rotating platform or mobile bearing knee. As opposed to a fixed bearing knee in which the tibial insert is in a fixed position, the mobile bearing knee has a tibial insert that moves between the two fixed points on either bone end. This allows greater rotation, but can also lead to an increased likelihood of dislocation.


Preparing for Knee Surgery


­Total knee replacements are performed by orthopedic surgeons. Many specialize in only this procedure. After it's been determined the damage to your knee necessitates a replacement, you'll receive a physical checkup and blood analysis, as well as X-rays that will pinpoint the damaged areas. The patient may donate a unit or two of blood for use during surgery should it be needed.


Before the procedure takes place, the patient should make arrangements to have some help in the weeks following the surgery. Patients should be prepared for an altered life immediately after surgery: stairs should be avoided, as should carrying any significant weight. The living quarters that a patient plans to return to post-surgery should be free of clutter and have handrails in the bathroom and shower. A ride home from the hospital should also be arranged.

You will have a choice when it comes to anesthesia. You can either go "all in" and receive general anesthesia, rendering you unconscious for the procedure, or you can receive a spinal anesthesia or epidural anesthesia. Both involve inserting a needle into your lower back and guiding it between the spine's vertebrae, reaching either the cerebrospinal fluid or the epidural space, at which point local anesthetics and narcotics are delivered. The patient that receives the spinal or epidural anesthesia will remain awake for the procedure, but will be extremely relaxed and may not remember the procedure. The benefits of choosing this method include greater pain control once the surgery is finished and less risk of falling into shock during the procedure. On the other hand, many people desire a complete and total removal of consciousness long before the scalpels come out.

One drawback of general anesthesia is that it takes longer for the patient to fully return to his or her senses, which adds time between the end of the procedure and the start of simple physical therapy techniques needed to help the leg heal.


Knee Replacement Procedure

When the big day arrives, the patient will be connected to heart leads and monitors that register heart rate and the body's level of oxygen.

The knee will remain bent during the surgery so that the surgeon can see as much of the joint as possible. The area around the knee will be cleaned with antiseptic liquid, a tourniquet may be applied above the knee to limit bleeding and then an incision will be made. The incision is usually 6 to 12 inches (15 to 30 centimeters) long.


Once the kneecap has been carefully moved aside, the surgeon concentrates on removing damaged bone tissue, being careful to leave not only enough bone to attach the implant, but enough bone to attach a second replacement later if the first one wears out over time.

Generally, bone is removed from the end of the femur, as well as from the front and back sides of the bone end. This allows the prosthesis to fit correctly. Bone is also removed from the top of the tibia, so that the end of the tibia is flattened. Any damaged tissue on the back of the kneecap will also be removed. The bones will be measured and the surgeon will insert the prosthesis once he or she is sure it will fit correctly.

Once the implant is secured, the surgeon may realign the ligaments to guarantee optimal function. Any tissue that has been cut in the operation to provide access to the joint will be sewed back in place. A drainage tube may be inserted to allow fluid to exit the wound. Then the incision is closed, the knee is bandaged and the procedure is over. The entire thing lasts about two hours.

All done. Now what happens? And what can go wrong?


Recovering from Knee Replacement Surgery

Patients suffering from bone diseases walk with the help of nurses and U.S. volunteers after undergoing joint replacement surgeries
Patients suffering from bone diseases walk with the help of nurses and U.S. volunteers after undergoing joint replacement surgeries at the West China Hospital of Sichuan University.
China Photos/Getty Images

­After the surgery is complete, the patient is taken to a recovery room until the anesthesia begins wearing off. Once awake, the patient will be transferred to a hospital room. As soon as the patient has shaken off the effects of anesthesia, a physical therapist will begin working with the patient to strengthen and heal the leg. It's important to begin moving the foot and ankle as soon as possible to keep swelling and clotting at bay. The patient may have their leg attached to a device known as a continuous passive motion machine that moves the leg on its own to keep it from growing stiff.

Within a day, the patient will try standing (under supervision), and by the next day may be taking steps with the aid of a walker. Blood clotting is a risk after surgery, so the patient may be prescribed blood thinners or be fitted with special support hose or compression boots, which inflate around the leg. Most total knee replacements require the patient to remain under hospital care from three days to a week, but this may be adjusted depending on individual rates of recovery. By the time the patient leaves, the bandages and sutures will likely be removed.


Over the next several weeks, the patient should be careful, get lots of rest and faithfully perform all exercises as instructed for joint rehabilitation. About a month and a half after surgery, the patient can expect to be walking around with a cane, and in another couple of weeks, driving will be possible again.

Patients can resume sexual activities whenever they feel able, but are advised to limit positions for a while to those covered on the copyright page of the Kama Sutra.


Potential Knee Replacement Complications

There are both short- and long-term complications that can arise from a total knee replacement. All surgeries carry the risk of anesthetic mishaps and excessive bleeding. There is also a small (less than 3 percent) risk of infection immediately after the surgery. Antibiotics are routinely given to patients in the first day after the surgery to lessen the risk. However, infection remains a risk long after the surgery has been performed. If the infection can't be controlled, the implant will have to be removed and replaced.

Of course, one serious long-term complication is that the implant itself will deteriorate with time, especially if its bearer is physically active. Although there are lifestyle choices that can extend the life of the implant, such as weight loss and avoidance of high-impact activities, the implant is still going to wear out eventually, especially if the recipient is middle-aged or even younger.


Since so many operations are performed each year, most facilities that offer the surgery aren't only skilled at their craft, but well practiced. However, it's not rare for a replacement knee to be inserted with an alignment that leads to increased wear and strain on the implant, leading to discomfort or a dislocation of the artificial joint.

The patella, which is moved aside during the procedure, may suffer from instability or tissue unevenness, or come into contact with the device, causing discomfort.

Most joint failures will be caused by the implant loosening from the bones. This is caused by degeneration of the bone tissue. The joint is lined by a thin layer of tissue called a synovial membrane. The synovial membrane breaks down foreign matter in the joint and provides lubrication. The synovial membrane can become inflamed, the joint fills with excess fluid, and the extra force this generates during joint movement begins to wear away at the bone tissue. If you have synovitis, your knee will feel warm and look puffy.

Take heart, though -- only about one patient in 50 will have significant problems as a result of a total knee replacement [source: Mayo Clinic].


The Long-term Outlook for a Knee Replacement


­Life for most recipients of a total knee replacement is vastly improved after healing from the procedure. Pain that was once always present is often eradicated, allowing normal activity levels during the day and sound, healthy sleep at night. About nine out of 10 patients report having significantly less pain in their joint after it has been replaced [source: Mayo Clinic]. Post-operative satisfaction, though, is related to how unsatisfied the patient was before the surgery -- another reason why it's preferable to exhaust all other options before getting a total knee transplant.

The new knee isn't a perfect knee. Most people won't be able to fully extend their leg (but they'll come close) or kneel without discomfort. It may be painful to climb steep stairs or take a seat in a chair that's low to the ground.

Compared to how the old knee was feeling right before the surgery, the replacement will feel much better. Exercise is possible -- it just won't involve running, jumping or juking and jiving. Basketball, racquetball and jogging are out, but swimming, walking and biking are encouraged. The recipient should take care to avoid falls, since damage to the implant may require further surgery and another replacement.

The orthopedic specialist will want to touch base yearly with the recipient to make sure no functional or mechanical problems are developing.

Artificial-knee recipients should inform dentists and doctors about the implant before undergoing any procedure. If bacteria is introduced into the body during dental or medical work, it could lead to a serious infection around the joint. Patients are usually advised to take prescribed antibiotics before any medical or surgical procedure following a knee replacement.

Lots More Information

Related HowStuffWorks Articles

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  • American Academy of Orthopaedic Surgeons. "Arthritis of the Knee." Oct. 2007. (March 20, 2009)
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