Surgical Site Infection Is the Most Common Post-Replacement Problem

Surgical site infection (SSI) is the most common infection after knee replacement. It occurs when bacteria enter the incision during or after surgery and multiply in the wound. Most SSIs develop within the first two to three weeks after surgery, though some can appear months later. The infection may stay in the skin and soft tissue around the incision, or it can spread deeper into the joint itself—a much more serious complication that may require additional surgery.

Not every infection looks the same. Some are mild and respond to antibiotics taken by mouth. Others are severe enough to require hospitalization and intravenous antibiotics. A small number become chronic infections that persist even after treatment, which is why recognizing the early signs matters.

Key Takeaways

  • Surgical site infections develop most often in the first two to three weeks after knee replacement, though they can appear later.
  • Redness, warmth, swelling, drainage, and fever are the main warning signs—contact your surgeon when ready if you notice any of these.
  • Deeper infections that reach the joint space are less common but more serious and may require a second surgery to clean out the infection.
  • Your surgeon will prescribe preventive antibiotics before surgery and give you wound care instructions to reduce infection risk.
  • Certain factors like diabetes, obesity, and a weakened immune system raise your infection risk after knee replacement.

Signs of Infection You Should Not Ignore

The first sign of infection is usually increased warmth and redness around the incision. You may also notice the area becomes more swollen than it was a few days after surgery, or the swelling that was improving suddenly gets worse. These changes happen because your body is fighting bacteria.

Drainage from the incision is another red flag. A small amount of clear or slightly bloody fluid in the first week is normal, but drainage that is thick, yellow, green, or foul-smelling suggests infection. Increased pain at the incision site—especially pain that gets worse rather than better as days pass—also warrants a call to your surgeon. Fever of 101°F or higher is a serious sign that infection has entered your bloodstream.

Some people also notice the knee becomes stiff, hot, and painful to move, separate from the normal stiffness that comes with healing. If you have any combination of these symptoms, do not wait for your next scheduled appointment. Call your surgeon's office when ready, even if it is after hours or on a weekend.

Deeper Infections That Reach the Joint

A periprosthetic joint infection (PJI) occurs when bacteria reach the artificial knee joint itself rather than staying in the skin and soft tissue. This is less common than surface infections but far more serious. It may develop weeks or even months after surgery, sometimes triggered by bacteria entering the bloodstream from an unrelated source—a dental procedure, a urinary tract infection, or a skin wound elsewhere on your body.

Symptoms of a deep joint infection include persistent pain and swelling in the knee, difficulty bearing weight, and sometimes fever. The knee may feel unstable or give way when you try to walk. Because the infection is inside the joint, antibiotics alone usually cannot reach it in high enough concentrations to clear it. Most people with PJI need surgery to remove the infected tissue and sometimes the artificial joint itself, followed by a period of antibiotics and then replacement with a new prosthesis.

This is why your surgeon may prescribe antibiotics before any future dental work or other procedures if you have had a knee replacement. The goal is to prevent bacteria from reaching your bloodstream in the first place.

Risk Factors That Increase Your Infection Chances

Some people are at higher risk for infection after knee replacement. Diabetes is one of the strongest risk factors because high blood sugar impairs your immune system's ability to fight bacteria. Obesity increases risk because fatty tissue has a poorer blood supply, making it harder for antibiotics and white blood cells to reach the surgical site. Rheumatoid arthritis and other autoimmune conditions that require immunosuppressant medications also raise your risk.

Age over 75, previous surgery on the same knee, and a history of infection anywhere in your body all increase your chances of developing an SSI. If you smoke, quitting before surgery—ideally at least four weeks before—significantly lowers your infection risk. Poor nutrition before surgery also matters; your body needs adequate protein and vitamins to heal properly and fight infection.

If you fall into any of these categories, tell your surgeon before your operation. They may take extra precautions, such as giving you antibiotics for a longer period after surgery or being more aggressive about treating any signs of infection early.

What Your Surgeon Does to Prevent Infection

Modern knee replacement surgery includes multiple infection-prevention steps. You will receive intravenous antibiotics before the surgery begins—usually within 60 minutes of the first incision. The surgical team uses sterile technique throughout the operation, and many surgeons use antibiotic-coated implants or add antibiotics to the cement that holds the prosthesis in place.

After surgery, you will receive oral antibiotics for a set period, usually one to two weeks. Your surgeon will give you detailed instructions for keeping the incision clean and dry. You will be told when you can shower, how to cover the incision if needed, and what signs mean you should call when ready. Follow these instructions exactly—they are not suggestions but proven steps to lower your infection risk.

You may also be asked to avoid certain activities or environments in the weeks after surgery. For example, you should avoid soaking the incision in a bathtub or swimming pool until the wound is fully closed and healed. Avoid dirty or dusty environments where bacteria are more likely to be present.

What Happens If Infection Is Diagnosed

If your surgeon suspects an infection, they will take a sample from the incision or joint—either with a needle or by opening the wound—and send it to a lab to identify the bacteria. This takes a few days, but your surgeon will usually start antibiotics right away rather than waiting for results. If the infection is caught early and stays in the soft tissue, oral antibiotics taken for two to four weeks often clear it completely.

If the infection is deeper or does not respond to antibiotics, your surgeon may need to clean out the infected tissue surgically. In some cases, this means removing the artificial joint temporarily, cleaning the area thoroughly, and then replacing it with a new prosthesis. This is a longer recovery than the original surgery, but it is often necessary to save your knee.

Throughout treatment, you will have follow-up appointments and possibly repeat lab tests to confirm the infection has cleared. Do not stop taking antibiotics early just because you feel better—finish the full course your surgeon prescribes, even if symptoms disappear.

Frequently Asked Questions

How common is infection after knee replacement?

Surgical site infections occur in roughly 1 to 2 percent of knee replacements. Deep joint infections are much rarer, happening in fewer than 1 percent of cases. These numbers have improved over the past decade because of better surgical techniques and preventive antibiotics.

Can I get an infection months after my knee replacement?

Yes. While most infections appear in the first few weeks, bacteria can reach the joint months or years later through the bloodstream during an unrelated infection or procedure. This is why your surgeon may recommend antibiotics before dental work or other surgeries.

What if I have a fever but no other symptoms?

Fever alone after knee replacement is unusual and should be reported to your surgeon, even if you feel otherwise well. It may signal infection, or it could be from an unrelated cause. Your surgeon needs to know to rule out infection.

Will I need another surgery if I get an infection?

Not always. If the infection is caught early and stays in the skin and soft tissue, antibiotics alone may clear it. If the infection reaches the joint or does not respond to antibiotics, surgery to clean out the infected material is usually necessary.

How long does it take to recover from treating an infection?

Recovery depends on how serious the infection is. A surface infection treated with oral antibiotics may resolve in two to four weeks with no lasting impact on your recovery. A deep infection requiring surgery adds several weeks to your overall healing time and may delay your return to normal activities.