Prosthetic joint infections

Question 1

Causative agents in prosthetic joint infections

Answer 1

• Coagulase negative staph most common
• P acnes common in shoulder
• Time dependent infections:
  
  • Early (< 3 months) - S aureus and gram negative rods
  • Delayed (3-24 months) - coagulase negative staph and P acnes
  • Delayed longer - coagulase negative staph

Question 2

Laboratory approach to prosthetic joint infections

Answer 2

• CRP: cut-off 13.5 mg/L gives relatively high S and S
• Synovial fluid leukocyte count and differential (higher S and S)
  
  • Knee: 1700/ml and 65% neuts
  • Hip: 4200/ml and 80% neuts
• Culture synovial fluid
• Tissue culture, optimally from 5-6 sites
• Intraoperative frozen
  
  • > 5 neutrophils/ HPF in 5 fields ( not very sensitive, but is specific)
  • Intraoperative gram stain is also not sensitive, but is specific