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Knee Replacement

Improvements in surgical techniques, technology and materials have made knee replacement surgery one of the most successful orthopedic procedures, with over 700,000 performed successfully in the US annually. After a knee replacement, most people experience reduced or eliminated knee pain, increased ability to move and an overall improvement in quality of life.


What is knee replacement surgery?

Knee replacement is where portions of the bones that form the knee joint are removed and replaced with artificial implants. It is performed primarily to relieve knee pain and stiffness caused by osteoarthritis.

Most people who get this surgery have advanced knee arthritis, in which the knee cartilage is worn away and the surface of the knee becomes pitted, eroded, and uneven. This causes pain, stiffness, instability and a change in body alignment. Knee replacement surgery can also help some people who have a weakened knee joint caused by an injury or other condition.

What are the different types of knee replacement surgery?

There are two main types:

  • total knee replacement, where the entire joint is replaced with artificial surfaces
  • partial knee replacement, where only one damaged compartment of the knee is replaced

Total knee replacement is the more common of these two procedures.

Total knee replacement overview

In a total knee replacement, damaged bone and cartilage from the knee joint are cut away and replaced with artificial implants to restore the natural motion and function of the knee.

Prior to surgery, you will undergo either general anesthesia (in which you will be unconscious) or regional anesthesia (in which strategic nerve blocks temporarily disable your ability to feel pain in the surgical area).

Prior to surgery, you will undergo one of two types of anesthesia:

  • general anesthesia, in which you will be unconscious during surgery
  • regional anesthesia, in which strategic nerve blocks temporarily disable your ability to feel pain in the surgical area

An orthopedic surgeon shaves down the damaged bone areas and affixes implants (prostheses) over the ends of the bone so that they glide smoothly against one another. These implants are generally made of metal and plastic, and each implant is customized for the individual to provide maximum compatibility.

To understand a total knee replacement, also known as total condylar knee arthroplasty, you must be familiar with the structure of the knee, a complex joint that consists of three bones:

  • the femur (thighbone)
  • the tibia (shinbone)
  • the patella (kneecap)

Tap the below image for an interactive diagram of anatomy and conditions of the knee. (For viewing on mobile devices, hold your tablet or phone in the landscape position.)

Knee joint diagram with labels clockwise from top indicating the femur, femoral condyle, medial collateral ligament (MCL), medial meniscus, tibia, anterior cruciate ligament (ACL) and the patella (kneecap)

Strong ligaments connect the powerful muscles of the thigh and calf to the bones around the knee to control knee motion and function. Cartilage (such as the meniscus) and other soft tissues cover and cushion the bones to help them glide together smoothly. When you bend or straighten your knee, the end of the femur rolls against the end of the tibia, and the patella glides in front of the femur.

When the cartilage that cushions the joint degrades or is worn away completely, the bones rub together and become scraped and rough. This causes inflammation known as osteoarthritis, which results in pain and stiffness that make walking and other movement difficult. The implants used in knee replacement are smooth like the surfaces of a healthy knee.

Graphic showing elements of the healthy knee, arthritic knee, and total knee replacement.

How is a total knee replacement done?

First, the orthopedic surgeon makes an incision (cut) in the knee and moves the patella (kneecap) to the side. If are any bone spurs (small bony growths) are present, as sometimes occurs in osteoarthritis, they will be removed.

Next, the two menisci between the femur and tibia are removed, as are the anterior cruciate ligament (ACL) and, in some cases, the posterior cruciate ligament (PCL). In some types of knee replacement, the PCL is retained.

During the main phase of the operation, the surgeon cuts and remove cartilage and some bone from the upper part of the tibia and lower sections of the femur. The femoral sections removed are two knobby protuberances called the femoral condyles. The tibia and femur are then be capped with metal implants to create new surfaces for the joint. The surface of the femoral component mimics the shape of the original femoral condyles. If the kneecap has also degraded, the surface on its underside may also be cut away and replaced with a polyethylene implant.

Finally, the various layers of tissue are repaired with dissolvable sutures and the skin incision is closed with sutures or surgical staples. A bandage will be wrapped around the knee and the patient is be taken to recovery.

An image of a knee replacement implant from an article about total knee replacement from Hospital for Special Surgery
Fixed-bearing knee implant with a polyethylene articulating surface (plastic tray) sandwiched between the metal tibial implant and metal femoral implant.

An image of a knee replacement implant within the knee joint from an article about total knee replacement from Hospital for Special Surgery
Side-view illustration of a knee with a fixed-bearing knee implant in place.

X-ray of a total knee arthroplasty from an article about Arthritis of the Knee from Hospital for Special Surgery
X-ray of a knee after total knee replacement, showing the implanted prosthesis)


Knee replacement prosthetic implants overview

The selection of implant design and materials depends on each individual patient. The main implant components are made of metal – usually titanium or chrome-cobalt alloys. The implants are fixated in place either with a cement bonding agent or by osseointegration, in which a porous metal stem extends into the tibia and the patient's natural bone grows into it. A plastic platform or spacer will be inserted between the tibial and femoral implant surfaces. The spacer is made of polyethylene.

Most femoral components are made of metal alloys (cobalt chromium) or metal-ceramic alloys (oxidized zirconium). The patellar component is plastic (polyethylene). The tibial insert component is also plastic (polyethylene). The tibial tray component can be made of the following materials:

  • Cobalt chromium (metal alloy)
  • Titanium (metal alloy)
  • Polyethylene (plastic)

Video: Animations of total and partial knee replacement

Animation of a total knee replacement

Partial knee replacement overview

A partial knee replacement is also known as unicompartmental knee arthroplasty. In this surgery, damaged cartilage and bone are removed and replaced only in one diseased compartment of the knee. This differs from a total knee replacement, in which bone and cartilage from the entire joint are replaced.

Partial knee replacement (also known as unicompartmental knee arthroplasty or unicondylar knee arthroplasty) is suitable for people who experience arthritis only in one compartment (section) of the knee joint, rather than throughout the joint. It can also provide relief from pain and stiffness in some people who have medical conditions that make them poor candidates for total knee replacement surgery.

Animation of a partial knee replacement

Can I avoid or postpone a knee replacement?

The choice on whether to have surgery to address arthritis of the knee joint depends on multiple factors, including:

  • the condition of the knee joint
  • the patient’s age and activity level

In cases where arthritic damage is minimal and/or if the patient is not very active, nonsurgical treatments by be tried, including:

  • physical therapy
  • NSAIDs (nonsteroidal anti-inflammatory drugs), such as ibuprofen
  • weight loss to reduce pressure on the knee

How do I know if I need knee replacement surgery?

You may need surgery if:

  • Your knees are stiff and swollen.
  • There is pain throughout the day, even at rest.
  • Walking, getting up or climbing stairs is difficult and painful.
  • Medication and therapy do not offer enough relief.
  • Knee cartilage is so damaged and worn away that you are walking "bone on bone," in which the bones of the joint are scraping together.

How long does a knee replacement last?

Knee replacement implants are expected to function for at least 15 to 20 years in 85% to 90% of patients. However, the implants do not last forever.

After a period of 15 to 20 years, general wear and tear may loosen the implant. Depending on the patient, this may cause no symptoms, or it may cause any of the following:

  • pain
  • loose implant particles
  • knee instability
  • infection

When these symptoms arise, orthopedic surgeons recommend having knee revision surgery to replace the original implant. Infection, especially, requires a prompt revision surgery. Infection after knee replacement surgery is rare, but a knee replacement implant cannot defend itself from infection if bacteria are introduced to the body. Learn more about infection prevention in joint replacement.

Can I go home after knee replacement surgery?

Most patients stay in the hospital one or two nights after surgery. Some patients are able to return home the same day after an outpatient procedure.

Contact one of our treating physicians for knee replacement to learn whether you are an appropriate candidate for outpatient (als known as "ambulatory") knee replacement, and learn more by reading Outpatient Knee Replacement Surgery: Frequently Asked Questions.

Knee replacement recovery time and recuperation

Total knee replacement surgery generally takes about 60 to 90 minutes, but you should expect to be in the operating room for over two hours. Rehabilitation (physical therapy) will begin within 24 hours of surgery.

After your surgery, the nursing staff will position you in bed and help you turn until you are able to move on your own. You may have a pillow between your legs if ordered by your surgeon.

Very soon after surgery, a physical therapist will come to your room to teach you appropriate exercises and review your progress. Gentle exercises to improve your range of motion can help prevent circulation problems as well as strengthen your muscles.

Your rehabilitation program will begin as soon as you are medically stable and there are orders from your doctor to begin postoperative mobility. All patients begin rehabilitation within 24 hours of their surgery. Your motivation and participation in your physical therapy program is key to the success of your surgery and recovery. The physical therapist will assist you in the following activities:

  • sitting at bedside with your feet on the floor
  • transferring in and out of bed safely
  • walking with the aid of a device (walker, cane, or crutches)
  • climbing stairs with aid of a device

How long does it take to walk after a knee replacement?

Most patients progress to a straight cane, walker or crutches within two or three days after surgery. As the days progress, the distance and frequency of walking will increase.

Patients are usually able to drive a car within three to six weeks after surgery and resume all other normal activities by or before six weeks. Complete recuperation and return to full strength and mobility may take up to four months. However, in many cases, patients are significantly more mobile one month after surgery than they were before they had their knee replacement

What can I expect at HSS?

Hospital for Special Surgery has been at the forefront of modern knee replacement since the operation was first performed in the late 1960s. HSS surgeons and engineers invented the modern total knee implant (called the total condylar knee) in the early 1970s. The Hospital has since led the field ever since in a number ways:

How to prepare for knee replacement

There are certain steps that can improve your recovery time and results. It is important to follow your knee replacement surgeon’s instructions both before and after surgery, as well as that of your rehabilitation therapist’s recommendations.

Here is additional material on preparing for knee replacement:

Articles related to knee replacement

Read these articles to learn more about specific issues related to joint replacement surgery.

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