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Difficulty: HardOrthopedic Surgery

A 68-year-old man is evaluated in the hospital for right knee pain and swelling that has progressively worsened over the past 3 months. He underwent a right total knee arthroplasty 2 years ago for severe osteoarthritis. His medical history includes hypertension and diet-controlled type 2 diabetes mellitus. His temperature is 37.2°C (99.0°F), blood pressure is 132/82 mmHg, and pulse is 78/min. Physical examination of the right knee reveals a moderate effusion, erythema, and warmth over the joint. Range of motion is significantly limited by pain. Radiographs of the right knee demonstrate periprosthetic radiolucency. Joint aspiration is performed, and synovial fluid analysis reveals a leukocyte count of 65,000/mm³ with 85% neutrophils. Synovial fluid culture grows methicillin-susceptible *Staphylococcus aureus* (MSSA). The patient was started on empiric intravenous vancomycin upon admission. Which of the following is the most appropriate definitive management for this patient?

  1. A
    Explantation of the prosthesis, placement of an antibiotic spacer, and continuation of intravenous vancomycin
  2. B
    Arthroscopic irrigation and debridement with retention of the prosthesis, and transition to intravenous cefazolin
  3. Explantation of the prosthesis, placement of an antibiotic spacer, and transition to intravenous cefazolinAnswer
  4. D
    Initiate long-term suppressive therapy with oral cephalexin without surgical intervention
  5. E
    Obtain a chest radiograph and urinalysis to evaluate for alternative sources of infection

Answer

The most appropriate definitive management is explantation of the prosthesis, placement of an antibiotic spacer, and transition to intravenous cefazolin.
The correct management involves both surgical source control and optimized pharmacotherapy. Because this patient is 2 years post-operative and has had symptoms for 3 months, he has a chronic prosthetic joint infection (PJI). Over this timeframe, bacteria form a mature, impenetrable biofilm on the hardware. Therefore, the standard of care in the United States is a two-stage exchange arthroplasty, which begins with the explantation of the infected prosthesis and placement of an antibiotic-impregnated spacer. Furthermore, the culture result of MSSA dictates an essential antibiotic stewardship step: empiric vancomycin must be discontinued in favor of a narrow-spectrum beta-lactam like intravenous cefazolin, which has superior bactericidal efficacy against MSSA.

Step-by-Step Solution

1
Confirm the diagnosis of Prosthetic Joint Infection (PJI).
The patient has a confirmed PJI based on symptoms, periprosthetic radiolucency, and a markedly elevated synovial leukocyte count (65,000/mm³).
Synovial fluid WBC > 1,100-3,000/mm³ with neutrophil predominance in a prosthetic joint is highly sensitive and specific for infection.
2
Determine the acuity of the infection to guide surgical management.
The infection is chronic (>4 weeks), as the patient is 2 years post-op and has had symptoms for 3 months.
Surgical management depends on the timeline. Acute infections (<3-4 weeks) can be managed with debridement and implant retention (DAIR). Chronic infections have mature biofilms and require explantation (usually via a two-stage exchange).
3
Apply antibiotic stewardship principles to the culture results.
The culture grew Methicillin-susceptible Staphylococcus aureus (MSSA).
Empiric vancomycin must be de-escalated. Beta-lactam antibiotics (e.g., cefazolin, nafcillin) provide superior bactericidal activity against MSSA compared to vancomycin.

Key Concept

Management of chronic prosthetic joint infection requires two-stage exchange arthroplasty (explantation and spacer placement), and definitive antibiotic therapy for MSSA should utilize targeted beta-lactams (e.g., cefazolin) rather than continuing empiric vancomycin.
Estimated Time:1m 30s
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Orthopedic Surgery

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