Knee replacement surgery (total knee arthroplasty) is a common and successful orthopedic procedure. The most serious complication following this surgery is an infection around the artificial joint, known as a Periprosthetic Joint Infection (PJI). Although relatively rare, occurring in about 1% to 2% of primary surgeries, PJI can severely impact a patient’s health and the longevity of the implant.
Identifying Periprosthetic Joint Infection
PJI involves a microbial infection in the joint space and the surrounding tissue where the implant is located. PJI is distinct from a simple wound infection because the bacteria adhere directly to the non-living surface of the metal and plastic components.
The most frequent bacteria behind PJI are Gram-positive cocci, often part of the normal skin flora. Specifically, Staphylococcus aureus and coagulase-negative staphylococci (such as Staphylococcus epidermidis) cause a majority of these infections. S. aureus is associated with acute, aggressive infections, while coagulase-negative staphylococci are often found in chronic cases.
A key factor making PJI difficult to treat is the formation of a biofilm on the implant surface. This biofilm is a complex matrix of bacteria embedded in a self-produced slime, which acts as a protective shield. The biofilm defends the bacteria from the immune system and renders standard antibiotics ineffective, necessitating specialized treatment.
Recognizing the Signs and Timing of Infection
The signs of an infected knee replacement can range from subtle to severe, but persistent pain is the most frequently reported symptom. Patients may also observe classic inflammatory signs around the knee, including increased warmth, redness (erythema), and swelling that does not resolve over time. Drainage from the surgical wound, particularly if it persists beyond initial healing, is a significant indicator. Systemic symptoms, such as fever or chills, can also occur, especially in acute infections.
The timing of symptom onset is a crucial factor in classifying the infection, which guides the appropriate treatment strategy. Infections are categorized based on when they appear relative to the surgery.
Acute infections typically present within the first three months following the operation, often with pronounced local and systemic symptoms. Chronic infections, which are more common, develop after three months and present with less dramatic signs, such as pain and gradual deterioration of joint function. This distinction is important because chronic infections have established a mature biofilm, making them harder to manage.
Understanding the Factors that Increase Risk
Several patient-related and surgical-related factors can increase an individual’s susceptibility to developing PJI after knee replacement.
Patient-Specific Risks
One of the strongest patient-specific risk factors is diabetes mellitus, particularly when blood sugar levels are poorly controlled, as this impairs the immune response. Obesity is another significant factor, with the risk of infection increasing as the patient’s Body Mass Index (BMI) rises.
Other systemic health issues also contribute to a heightened risk, including poor nutritional status, preoperative anemia, inflammatory conditions like rheumatoid arthritis, and current tobacco use. These conditions compromise the body’s ability to heal and fight off pathogens.
Surgical Risks
Factors related to the surgical procedure itself also play a role in infection risk. A prolonged operative time (surgery lasting longer than 90 minutes) increases the potential for wound contamination. Patients who require blood transfusions during or immediately after the surgery are also at a higher risk of infection. A prior history of infection in the joint or having had multiple previous surgeries on the knee elevates the likelihood of PJI.
Medical Management and Treatment Options
Diagnosing a PJI begins with clinical evaluation and laboratory tests to confirm the presence of infection. Blood tests measuring inflammatory markers, such as the Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP), are routinely used to screen for elevated inflammation. If these markers are high, the definitive diagnostic step is joint fluid aspiration, where a sample is withdrawn from the knee and sent for culture and cell count analysis.
Treatment depends heavily on the timing of the infection and the stability of the implant. For acute infections, a procedure called Debridement, Antibiotics, and Implant Retention (DAIR) may be attempted. This involves aggressively cleaning the joint without removing the knee replacement components, followed by a prolonged course of targeted antibiotics.
If the infection is chronic, or if the DAIR procedure fails, the standard of care is typically a two-stage revision arthroplasty. The first stage involves removing all infected implant components, thoroughly debriding the area, and placing an antibiotic-loaded cement spacer. The patient then receives several weeks of systemic antibiotics. The second stage, performed months later after confirmation that the infection is eradicated, involves removing the cement spacer and implanting a new, sterile knee replacement.