Knee Infection After Replacement: Causes, Risk Factors, and Prevention

What causes knee infection after knee replacement?

Knee infection after knee replacement, known as periprosthetic joint infection (PJI), primarily results from bacteria entering the joint during surgery, spreading through the bloodstream from another site of infection, or contaminating the surgical wound post-operatively.

Understanding Periprosthetic Joint Infection (PJI)

Total knee replacement (TKR), or total knee arthroplasty (TKA), is a highly successful procedure for alleviating severe knee pain and improving mobility. However, a significant complication, though relatively rare, is periprosthetic joint infection (PJI). PJI occurs when bacteria colonize the prosthetic joint components and surrounding tissues, leading to inflammation, pain, and potentially requiring further surgery to eradicate the infection. The consequences of PJI can be severe, impacting a patient's quality of life and the longevity of the implant.

Primary Pathways of Infection

Infection can arise through several distinct mechanisms, each presenting unique challenges for prevention and management.

• Direct Contamination (Intraoperative): This is the most common pathway, occurring when bacteria are introduced into the surgical site during the operation itself. Despite stringent sterile techniques, the operating room environment is not entirely sterile, and microscopic airborne particles or direct contact from surgical instruments, drapes, or the surgical team can carry pathogens. The longer the surgery, the greater the exposure time and thus the theoretical risk.
• Hematogenous Spread (Postoperative): Bacteria from an infection elsewhere in the body can travel through the bloodstream and seed the prosthetic joint. This can occur from seemingly minor infections such as:
  • Dental procedures (e.g., tooth extraction, deep cleaning)
  • Urinary tract infections (UTIs)
  • Skin infections (e.g., cellulitis, boils)
  • Respiratory tract infections (e.g., pneumonia)
  • Gastrointestinal infections The prosthetic joint, being a foreign body, can be more susceptible to bacterial colonization once bacteria arrive via the bloodstream.
• Local Wound Contamination (Postoperative): After surgery, the incision site remains a potential entry point for bacteria until it is fully healed. Poor wound care, inadequate dressing changes, or the presence of a wound complication (e.g., persistent drainage, dehiscence) can allow bacteria from the skin or environment to enter the joint space.

Key Risk Factors for Infection

While any patient undergoing knee replacement is at some risk, certain factors significantly increase susceptibility to PJI. These can be broadly categorized:

• Patient-Related Factors:
  • Obesity: Higher body mass index (BMI) is associated with increased surgical time, larger incision size, and often more complex wound healing, increasing infection risk.
  • Diabetes Mellitus: Poorly controlled blood sugar levels impair immune function and wound healing.
  • Immunosuppression: Conditions like rheumatoid arthritis, HIV, or the use of immunosuppressant medications (e.g., corticosteroids, biologics) weaken the body's ability to fight infection.
  • Peripheral Vascular Disease: Reduced blood flow can compromise tissue healing and immune response.
  • Chronic Kidney Disease: Impairs immune function and overall health.
  • Malnutrition: Compromises immune system integrity and wound healing.
  • Smoking: Impairs circulation, oxygen delivery to tissues, and wound healing.
  • Previous Surgical Site Infection: A history of infection in the same joint increases the risk of recurrence.
  • Dental Issues: Untreated dental infections can serve as a source for hematogenous spread.
  • Skin Conditions: Psoriasis or other skin conditions near the surgical site can increase local bacterial load.
• Surgical Factors:
  • Prolonged Operative Time: Longer surgeries increase exposure to potential contaminants.
  • Excessive Blood Loss: May necessitate transfusions, which can be associated with increased infection risk.
  • Poor Surgical Technique: Inadequate aseptic technique or excessive tissue trauma can increase risk.
  • Operating Room Environment: Air quality, traffic flow, and adherence to sterile protocols.
• Postoperative Factors:
  • Wound Complications: Hematoma, seroma, or persistent drainage from the surgical wound provide a medium for bacterial growth and an entry point.
  • Poor Postoperative Hygiene: Inadequate patient adherence to wound care instructions.
  • Presence of Other Infections: Any concurrent infection (e.g., UTI, pneumonia) can lead to hematogenous spread to the new joint.

Common Pathogens Involved

The majority of PJIs are caused by common skin bacteria.

• Staphylococcus aureus: This is the most prevalent pathogen, including Methicillin-resistant Staphylococcus aureus (MRSA).
• Coagulase-negative Staphylococci (e.g., Staphylococcus epidermidis): Often associated with infections that develop more slowly (low-virulence).
• Streptococcus species: Another common group of bacteria.
• Gram-negative bacteria (e.g., E. coli, Pseudomonas aeruginosa): Less common but can cause severe infections, often originating from urinary or gastrointestinal sources.
• Polymicrobial Infections: Sometimes, multiple types of bacteria are involved.

Symptoms and Diagnosis

While this article focuses on causes, it's important to recognize that PJI typically presents with persistent pain, swelling, warmth, redness, and sometimes fever or drainage from the wound. Diagnosis involves a combination of clinical assessment, blood tests (e.g., ESR, CRP), joint aspiration for fluid analysis (cell count, culture), and sometimes imaging studies.

Prevention Strategies

Minimizing the risk of PJI requires a multi-faceted approach involving the patient, surgical team, and healthcare environment.

• Preoperative Optimization:
  • Control Chronic Conditions: Meticulous management of diabetes, obesity, and other co-morbidities.
  • Smoking Cessation: Patients are often advised to stop smoking well in advance of surgery.
  • Dental Clearance: Addressing any active dental infections before surgery.
  • Skin Preparation: Using antiseptic washes before surgery.
  • Nutritional Optimization: Ensuring adequate nutritional status.
• Intraoperative Measures:
  • Strict Aseptic Technique: Meticulous adherence to sterile protocols by the entire surgical team.
  • Antibiotic Prophylaxis: Administration of intravenous antibiotics shortly before incision and sometimes post-operatively.
  • Laminar Air Flow Operating Rooms: Specialized ventilation systems to reduce airborne contaminants.
  • Minimizing Operative Time: Efficient surgical execution.
• Postoperative Care:
  • Proper Wound Care: Patient education and adherence to instructions for dressing changes and hygiene.
  • Antibiotic Use: Appropriate use of antibiotics for any new infections elsewhere in the body (e.g., UTIs, dental procedures), often requiring consultation with the orthopedic surgeon.
  • Vigilance for Symptoms: Prompt reporting of any signs of infection to the healthcare team.

Conclusion

Periprosthetic joint infection following knee replacement is a serious complication, but understanding its causes and risk factors is the first step in prevention. By addressing patient-specific risks, adhering to strict surgical protocols, and maintaining diligent postoperative care, the likelihood of this challenging complication can be significantly reduced, ensuring the best possible long-term outcomes for patients.