A knee replacement infection may develop after a person has had an operation to replace their knee joint. What are the symptoms of a knee replacement infection and who is at most risk of getting one?

Knee replacement surgery, also known as knee arthroplasty, is one of the most common types of elective surgery. It is estimated that by 2030 as many as 3.48 million knee replacements will take place in the United States.

Knee replacement surgery may be necessary if a person has ongoing severe knee pain or swelling that affects their ability to carry out daily activities.

Most people are free from pain and regain their mobility after surgery. However, some people who have knee replacement surgery may develop an infection.

This article explores the signs, causes, risk factors, and treatments for a knee replacement infection. It also considers how to prevent knee replacement infections from happening.

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A knee replacement infection may occur at any time after surgery.

A knee replacement infection may develop in the wound after surgery. It may also occur around the artificial implant that is used to replace the knee joint. Harmful bacteria entering the wound usually cause the infection.

A knee replacement infection can occur any time after surgery. For example:

  • during the hospital stay after surgery
  • when a person goes home after surgery
  • months or even years after surgery

According to the American Academy of Orthopaedic Surgeons, only one in every 100 people who have a hip or knee replacement will develop an infection.

The following are normal after knee replacement surgery:

  • mild swelling in knee or ankle
  • some redness around the incision or knee
  • warmth around incision or knee

These post-surgery symptoms do not indicate an infection and should not be a cause for concern. These symptoms should get better over time.

If post-surgery symptoms get worse rather than better over time, this may be a sign of infection.

A person who experiences one or more of the following symptoms may have a knee replacement infection:

  • inability to walk without pain after the point at which the doctor said walking should be pain-free
  • increasing pain and stiffness in the artificial joint
  • warmth, redness, and tenderness around the incision or the whole knee
  • grey liquid draining from the incision, especially if it smells bad
  • a fever above 100°F (37.8°C)
  • chills or night sweats
  • fatigue

After knee replacement surgery, bacteria might enter a person's body through the wound where the surgical incision was made. If bacteria reach a person's new artificial knee joint, they may multiply and cause an infection.

Some bacteria are harmless, such as those that occur naturally in the stomach, while others may harm a person and cause an infection. A person's immune system usually kills any harmful bacteria that get into the bloodstream.

When a person has a knee replacement, their knee joint is replaced with an artificial joint made of metal and plastic. Because these materials are not organic, it is harder for the body to kill bacteria on them.

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Smoking may cause a greater risk of infection after surgery.

Anyone who has a knee replacement can develop an infection after surgery, but some groups are at a greater risk of infection. These include people who:

A doctor may be able to diagnose a knee replacement infection with a visual examination.

Sometimes, the doctor may need to investigate the type of bacteria that is causing infection using one or several of the following tests:

  • Blood test: This can help measure inflammation in the body, which can indicate an infection.
  • Imaging test: This can help determine if there is an infection in the artificial joint. Examples of imaging tests include X-rays, computed tomography (CT) scans, magnetic resonance imaging (MRI), or bone scans.
  • Joint aspiration: Fluid is drawn from the knee and tested for bacteria and white blood cells. A large number of white blood cells is a sign the body is fighting an infection.

There is a range of treatments for a knee replacement infection, including both nonsurgical and surgical procedures.

Nonsurgical treatment

Some knee replacement infections are superficial, which means that the infection has reached the skin and tissue around the joint but does not affect the artificial joint itself.

A superficial knee replacement infection may be treated with oral or intravenous (IV) antibiotics.

Surgical treatment

If a knee replacement infection goes deeper than the skin and tissue around a joint, it may need to be treated surgically. Surgical treatment options include:

  • Debridement: This is a surgical washout of the joint. Any contaminated soft tissue is removed, and the artificial joint is cleaned. Plastic liners or spacers in the artificial joint may be replaced. The doctor will prescribe IV antibiotics to be taken after surgery.
  • Staged surgery: This involves a series of surgeries to remove and replace the artificial joint. This may be necessary if the infection has developed months or years after the original knee replacement.

The different stages of staged surgery typically include:

  • Removal of the artificial joint: When the infection is deep and long-lasting, the artificial joint will need to be removed.
  • Joint washout: Washing helps get rid of infected soft tissue in the joint.
  • Placement of antibiotic spacer: This helps maintain joint space and keeps the joint aligned while the infection is treated.
  • IV antibiotics: These help kill the infection. The doctor may prescribe a course that lasts up to 6 weeks.
  • New knee replacement surgery: Once the infection has been treated, another knee replacement surgery can be carried out. The doctor will remove the antibiotic spacer and give the person a new artificial knee joint.

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The risk of infection may be reduced by taking antibiotics immediately before, during, and after surgery.

Before and during knee replacement surgery, the following steps may help reduce the risk of infection:

  • Using prophylactic antibiotics: According to a 2013 review, preventive medicines may help reduce the risk of knee replacement infection.
  • Using antibiotics: These should be given immediately before, during, and after surgery for up to 24 hours.
  • Keeping operation time short: A short operation time reduces the length of time the wound is open and vulnerable to infection.
  • Reducing the number of people present: Limiting the number of people and limiting the number of times they come and go may reduce the bacteria in the room and decrease the risk of infection.
  • Using sterile equipment: The theatre, instruments, and artificial joint should all be sterilized.
  • Screening for bacteria in the nose: If a person has certain types of harmful bacteria in their nasal passage, they may increase the risk of infection. Some hospitals screen for these bacteria before operating. If harmful bacteria are found, the person will be given an antibacterial ointment to use. Some medical centers will routinely decolonize nasal passages with mupirocin several days before surgery.
  • Washing with chlorhexidine: This may help reduce the number of harmful bacteria on the skin before surgery.

After a person has had knee replacement surgery, the following measures may help reduce the risk of infection:

  • following the doctor's advice on how to treat the wound
  • clean and cover cuts, wounds, or burns as soon as they happen
  • maintain dental hygiene, as infections in the mouth may spread to the artificial joint

Although they may require surgery, knee replacement infections are treatable. Once a person has received proper treatment, the pain and swelling around the joint should improve, and they will regain movement.

Following pre- and post-surgery preventative measures will reduce the risk of further infection.