Causes / typical injury mechanism: Prosthetic joint infection (PJI) is a serious complication following total knee arthroplasty (TKA) and a major cause for revision surgery. The diagnosis of PJI is complex and includes the assessment of inflammatory markers, imaging analysis, and synovial fluid culture and cell count from aspiration.

Many precautions are taken to prevent a PJI after TKA, including appropriate skin preparation, administration of systemic antibiotics, and maintaining a sterile operating room environment. However, some patients are at higher risk of developing infection following the surgery and may develop an infection despite standardized protocols.

Classic history and presentation: The majority of PJIs (60%-70%) occur within 2 years from the primary TKA. Patients may present with pain, stiffness, swelling, and erythema at the site of the prior TKA. Patients may report a recent history of bacteremia, recent procedures (dental work, colonoscopy), or history of intravenous (IV) drug abuse.

Prevalence: Infection occurs in approximately 1%-2% of primary TKAs and 5%-6% of revision TKAs.

Risk factors:
Preoperative –

• Previous infection
• Previous local surgery
• Inflammatory arthropathy (eg, rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis)
• Smoking
• Morbid obesity
• Poor dental care, dental infection, and periodontal disease
• Alcohol use disorder
• IV drug abuse

Postoperative –

• Immune suppression
  • Malnutrition (serum albumin < 3.5)
  • Poorly controlled diabetes mellitus (HgA1c > 7)
  • Chronic kidney disease
  • Chronic liver disease
  • HIV infection
• Immunosuppressive drugs
  • Antitumor necrosis factor (TNF) agents (eg, infliximab, etanercept, adalimumab, certolizumab, golimumab)
  • Antimetabolites (eg, leflunomide)
  • Corticosteroids

Pathophysiology: Infection can be acquired perioperatively or hematogenously. Infection is most commonly due to growth of Staphylococcus aureus, but it can be due to a multitude of different organisms, both bacterial and fungal.

• Early perioperative infection – Caused by direct inoculation, delayed wound healing, or wound dehiscence. The most common pathogen is S aureus.
• Hematogenous infection – Associated with urinary tract infection, dental work, and other invasive procedures.

Grade / classification system:
Classification is based on timing –

• Acute PJI: Postoperative infection, occurs 2-3 weeks after surgery.
• Delayed-onset PJI: Infection occurs more than 3 months but less than 12-24 months following surgery.
• Late-onset PJI: Infection occurs more than 12-24 months following surgery.

Of note, TKA infections can also be classified as acute versus chronic and as due to hematogenous spread versus direct invasion.