Infection After Knee Replacement: Signs and When to Call Your Doctor
An infection after knee replacement is uncommon but serious—it happens in roughly 1 to 2 out of every 100 surgeries. The risk is highest in the first few weeks after surgery, but infections can develop months or even years later. Most infections are caught and treated successfully when you know what to watch for and act quickly.
The first sign is usually fever (over 101°F or 38.3°C) within the first few days or weeks after surgery. You may also notice increased pain, swelling, warmth, or redness around the incision that gets worse instead of better. Some people see drainage from the wound—clear, cloudy, or bloody fluid. Call your surgeon when ready if you have any of these symptoms. Do not wait for an appointment; call the office and describe what you are seeing. Most surgeons have an on-call service for post-operative emergencies.
Key Takeaways
- Fever over 101°F, increasing pain, swelling, warmth, or drainage from the incision are signs of possible infection and require a call to your surgeon the same day.
- Your surgeon will likely order blood tests and imaging to confirm infection, and may need to examine the knee directly or take a sample of fluid from inside the joint.
- Early infections caught within weeks of surgery are usually treated with antibiotics alone, while later infections or those inside the joint often require a second surgery to clean the area.
- Preventing infection starts before surgery—telling your surgeon about any infections elsewhere in your body and following wound care instructions exactly during recovery.
- Recovery from a treated infection takes longer than from the original surgery, and you may need physical therapy to regain the strength and motion you had before the infection.
How Your Surgeon Diagnoses an Infection
Your surgeon will not assume infection based on symptoms alone. The first step is usually blood work—a complete blood count and blood cultures to see if bacteria are present in your bloodstream. These tests take a few days to show results, so your surgeon may start antibiotics before the results come back if infection seems likely.
Imaging comes next. An X-ray shows whether there is unusual swelling or fluid buildup around the knee. An MRI gives a much clearer picture of what is happening inside the joint and surrounding tissues. If the surgeon suspects infection deep inside the joint, they may use a needle to draw out fluid and send it to the lab to identify the exact bacteria and which antibiotics will work against it.
In some cases, your surgeon will want to look directly inside the knee using an arthroscope—a small camera on a thin tube inserted through a tiny cut. This lets them see the joint surface, take samples, and sometimes clean out infected material at the same time.
Treatment With Antibiotics Alone
If infection is caught very early—usually within the first few weeks after surgery—and the bacteria are identified quickly, antibiotics may be enough. Your surgeon will prescribe strong antibiotics, often given through an IV at first, then switched to pills once the infection begins to clear. You will need blood tests every week or two to make sure the antibiotics are working and that the infection is not spreading.
Even with antibiotics, you will likely need to limit activity on the knee. Physical therapy may pause or slow down during treatment. The course of antibiotics usually lasts 4 to 6 weeks, and sometimes longer. You will have follow-up appointments to check that the infection is truly gone before returning to normal activity.
When Surgery Is Needed to Treat Infection
If the infection is inside the joint itself, or if it has been present for more than a few weeks, antibiotics alone usually cannot reach the bacteria. Your surgeon will need to go back into the knee to clean out the infected tissue and fluid. This is called a debridement, and it is done under general anesthesia in an operating room.
During the procedure, the surgeon removes infected material, rinses the joint thoroughly with sterile fluid, and may remove parts of the artificial knee if they are contaminated. After cleaning, the knee is closed and you begin a new course of strong antibiotics. Some surgeons place antibiotic-soaked beads or spacers inside the knee during this surgery to deliver medication directly to the problem area.
In rare cases where the infection cannot be controlled or keeps returning, the artificial knee may need to be removed entirely. This is a last resort, and your surgeon will discuss all other options first. Removal is followed by a period of antibiotics and physical therapy, and a new knee can sometimes be put in later once the infection is completely gone.
Recovery and Rehabilitation After Infection Treatment
Recovery from an infection takes longer than recovery from the original surgery. If you had only antibiotics, you may return to light activity within a few weeks, but full recovery can take 2 to 3 months. If you needed surgery to clean the knee, expect 6 to 12 weeks before you can put full weight on the leg and return to normal walking.
Physical therapy is crucial. The infection and the treatment—especially surgery—cause stiffness and weakness. A physical therapist will work with you to regain the range of motion and strength you had before the infection. This is often slower and more painful than the original post-surgery therapy, so be patient with yourself.
You will have frequent follow-up appointments with your surgeon to make sure the infection does not return. Blood tests may continue for several months. Some surgeons recommend avoiding certain activities—like running or high-impact sports—for a longer period after infection, even after you feel ready.
Reducing Your Risk of Infection Before and After Surgery
The best approach is prevention. Before surgery, tell your surgeon about any infections you have had recently—a urinary tract infection, skin infection, dental work, or any other source of bacteria. Some surgeons will delay surgery until these are treated. If you have diabetes, make sure your blood sugar is well controlled before surgery, as high blood sugar increases infection risk.
After surgery, follow wound care instructions exactly. Keep the incision clean and dry. Wash your hands before touching the area. Watch for signs of infection every day. If you notice any drainage, increased warmth, or unusual swelling, contact your surgeon right away—do not assume it will go away on its own.
Avoid activities that put stress on the knee during the early healing phase. Do not soak the knee in a bathtub or go swimming until your surgeon says the incision is fully healed. If you need dental work or any other procedure in the months after surgery, tell that doctor that you have had a knee replacement, as some surgeons recommend antibiotics before certain procedures to prevent bacteria from reaching the joint.
Frequently Asked Questions
How long after surgery can an infection develop?
Most infections show up within the first 3 months after surgery, but they can occur years later. Early infections are usually caused by bacteria introduced during surgery or in the first weeks of healing. Late infections may come from bacteria that entered the bloodstream from another part of the body—a dental infection, a urinary tract infection, or even a cut on the skin.
Will I lose my knee replacement if I get an infection?
Not in most cases. If the infection is caught early and treated with antibiotics, the knee stays in place. Even if surgery is needed to clean the joint, the artificial knee is usually saved. Removal is rare and only happens when the infection cannot be controlled any other way.
Can I go home while taking antibiotics for the infection?
Yes, if the infection is being treated with antibiotics alone and you are stable. You may receive IV antibiotics at home through a special line, or take pills by mouth. You will need regular blood tests and follow-up visits to make sure the treatment is working. If you develop new symptoms or fever returns, contact your surgeon when ready.
What if I have signs of infection but my surgeon says it is normal swelling?
Trust your instincts. If you have fever, increasing pain, or drainage that seems wrong to you, ask for blood work or imaging. A second opinion from another surgeon is reasonable if you are concerned. It is better to be checked and find out everything is fine than to wait and have a small problem become serious.
Will physical therapy be different after infection treatment?
Yes, it will likely be slower and more cautious. Your therapist will know about the infection and will start with gentler exercises. Progress may be slower than after the original surgery, and you may feel more pain during therapy. This is normal—the goal is to rebuild strength and motion safely without triggering the infection to return.