A serious complication of total knee replacement (TKA) is a periprosthetic joint infection (PJI). This occurs when bacteria colonize the artificial joint and surrounding tissues, often forming a protective layer called a biofilm. Although the overall incidence is low (affecting 1% to 2% of patients), PJI is a leading reason for needing revision surgery. Understanding this infection and the necessary medical response is important for anyone considering or recovering from knee replacement.
Recognizing the Signs of Infection
Infections are categorized as acute or chronic, depending on how long they have been present. An acute infection typically appears soon after surgery, often within the first few weeks, and presents with clear, pronounced symptoms. These signs include a high fever, persistent pain, and noticeable warmth or spreading redness around the surgical site. Cloudy or purulent fluid drainage from the incision is also a significant indicator of an acute issue.
A chronic infection develops months or years after the knee replacement, and its symptoms are often more subtle. Patients may experience new or worsening joint pain, along with unexplained swelling. The presence of a sinus tract—a small opening in the skin that drains fluid directly from the joint space—is a definitive sign of chronic PJI. In long-standing cases, the infection may cause the implant to loosen, leading to mechanical instability and declining joint function.
Confirming the Diagnosis
If a doctor suspects an infection based on symptoms, a medical investigation is initiated to confirm PJI. The process begins with blood tests measuring inflammatory markers, such as C-reactive protein (CRP) and Erythrocyte Sedimentation Rate (ESR). While these markers are typically elevated during infection, their levels alone are not sufficient for a definitive diagnosis.
Plain X-rays are used for initial screening, though they are limited because early infection and simple implant loosening can appear similar. The definitive step is joint aspiration, where fluid is drawn from the knee joint using a needle. This synovial fluid is analyzed for its white blood cell count, providing strong evidence of infection. The fluid is also cultured to identify the specific bacteria and determine its antibiotic susceptibility, which guides the treatment plan.
Treatment Pathways for Periprosthetic Infection
Treatment selection is individualized based on the timing of the infection, implant stability, and the patient’s overall health. For patients who cannot tolerate surgery, long-term antibiotic suppression may be considered. This non-surgical approach manages the infection as a chronic disease to control symptoms, but it rarely results in a cure.
Acute Infection: DAIR Procedure
For acute infections (symptomatic for less than four weeks) where the knee replacement remains firmly fixed, the preferred option is Debridement, Antibiotics, and Implant Retention (DAIR). This procedure involves surgically washing out the joint, removing infected tissue, and exchanging the plastic liner while keeping the main metal components in place. After surgery, the patient receives an extended course of specific intravenous antibiotics to eradicate the remaining bacteria.
Chronic Infection: Two-Stage Revision
If the infection is chronic or if the DAIR procedure fails, the standard of care is the two-stage revision arthroplasty. The first stage involves the complete surgical removal of all prosthetic components, cement, and infected tissue. An antibiotic-loaded cement spacer is then placed into the knee joint. This spacer delivers high concentrations of antibiotics directly to the area while maintaining joint space.
The patient receives several weeks of intravenous antibiotics to clear the infection before the second surgery. Once blood tests confirm that infection markers have normalized, the patient returns for the second stage. Here, the antibiotic spacer is removed, and a new, sterile knee replacement implant is inserted.
A less common option is the one-stage revision, which removes the old components and replaces them with a new implant in a single procedure. This is typically reserved for select patients with low-virulence organisms and healthy soft tissues.
Steps to Reduce Future Risk
Risk management for PJI begins by optimizing the patient’s health before the initial surgery. Pre-operative steps focus on improving the body’s ability to fight infection:
- Patients with diabetes should achieve excellent blood sugar control (hemoglobin A1c below 7%), as elevated levels impair immune response and wound healing.
- Improving overall nutritional status and reducing body weight are important, as obesity and poor nutrition are associated with higher infection rates.
- Patients who smoke should quit, as tobacco use restricts blood flow and compromises the body’s ability to fight bacteria.
After the knee replacement, maintaining meticulous oral hygiene is a lifelong preventative measure, since dental infections can release bacteria into the bloodstream that may travel to the implant. For future invasive procedures, such as extensive dental work or cystoscopies, patients are often instructed to take a short course of prophylactic antibiotics. This prevents transient bacteria in the bloodstream from initiating a new infection in the artificial joint.