Implant-Related Infections

Implant Infections - Bug's Unwelcome Party

• Infection of orthopaedic implant & surrounding tissue. Major complication.
• Incidence: ~1-2% primary joints; ↑ in revisions, trauma.
• Pathogenesis: Biofilm on implant = key.
  • Matrix protects bacteria from host/antibiotics.
• Common Bugs:
  • S. aureus: Acute, aggressive.
  • CoNS: Chronic, subtle.

    ⭐ Coagulase-negative Staphylococci (CoNS) are the most common cause of chronic PJI, often presenting with subtle symptoms.

  • Streptococci: Early/delayed.
  • Gram-negatives (Pseudomonas, E. coli): Hard to treat.
  • C. acnes: Shoulder, slow.
• Timing Classification:
  • Early: <3 months (e.g., S. aureus).
  • Delayed: 3-12 months (e.g., CoNS, C. acnes).
  • Late: >12 months (hematogenous).

Pathogenesis & Biofilms - Slime Shield Saga

• Initial stage: Bacterial adhesion to implant surface; a "race for the surface" against host cells.
  • Influenced by implant material (e.g., titanium, polymers) and surface characteristics (roughness, hydrophobicity).
• Biofilm development: Adherent bacteria proliferate, produce Extracellular Polymeric Substance (EPS) forming the "slime layer."
  • EPS matrix: Composed of polysaccharides, proteins, lipids, and extracellular DNA (eDNA).
  • Provides structural integrity and protection.
• Quorum sensing: Bacterial cell-to-cell communication system; regulates gene expression for biofilm maturation and virulence.
• Mature biofilm: Highly resistant to antibiotics (↓ penetration, altered bacterial metabolism) and host immune responses (e.g., phagocytosis).

⭐ Biofilms protect bacteria from host defenses and antibiotics, making eradication challenging without implant removal.

Clinical Diagnosis - Infection Detection Squad

• Presentation: Persistent joint pain (esp. rest/night), swelling, erythema, warmth, draining sinus, fever.
• Key Investigations:
  • Serum: ↑ ESR (>30 mm/hr), ↑ CRP (>10 mg/L).
  • Synovial Fluid (Arthrocentesis is crucial):
    • WBC count > 3,000/µL (knee PJI).
    • PMN% > 80% (knee PJI).
    • Culture (aerobic & anaerobic) - Gold Standard.
    • Leukocyte Esterase (++), Alpha-defensin.
• Imaging:
  • X-ray: May show loosening, osteolysis (often late).
  • Nuclear scans (WBC scan, FDG-PET): For complex cases.
• MSIS Criteria for PJI Diagnosis:
  • 1 Major (e.g., sinus tract, ≥2 positive cultures) OR
  • ≥3 Minor (e.g., ↑ESR/CRP, ↑Synovial WBC/PMN%, +Histology, +LE, +Alpha-defensin).

⭐ The Musculoskeletal Infection Society (MSIS) criteria are pivotal for diagnosing Periprosthetic Joint Infection (PJI), combining clinical, lab, and histological findings.

Management & Prevention - Battle & Blockade

Prevention (Blockade):

• Pre-op: Optimize patient (glycemia, nutrition), S. aureus decolonization, screen infections.
• Intra-op: Strict asepsis, prophylactic antibiotics (Cefazolin <60 min pre-incision, for 24h), antibiotic cement if high-risk.
• Post-op: Wound care, early mobilization.

Management (Battle):

• Principles: Multidisciplinary, microbial Dx, surgical debridement, prolonged targeted Abx.
• Surgical Options:
  • DAIR: Acute PJI (<4 wks post-op or <3 wks symptoms hematogenous), stable implant, susceptible pathogen.
  • One-Stage Revision: Single surgery; healthy patient, sensitive pathogen, good soft tissue.
  • Two-Stage Revision: Chronic PJI. Stage 1: Removal, debridement, spacer, Abx (4-6 wks). Stage 2: Reimplant.
  • Salvage: Resection, arthrodesis, amputation (rare).
• Antimicrobials: Biofilm agents (Rifampicin for Staph), culture-guided, 4-6 wks IV/oral, then possible suppression.

⭐ Two-stage revision arthroplasty is often considered the gold standard for treating chronic PJI, especially with resistant organisms.

High‑Yield Points - ⚡ Biggest Takeaways

• Biofilm formation is crucial in pathogenesis and antibiotic resistance.
• Staphylococcus aureus and S. epidermidis are the most common causative organisms.
• Early infections (<3 months) are often by high-virulence pathogens; delayed infections (3-24 months) by low-virulence organisms.
• Late infections (>24 months) usually result from hematogenous spread.
• Diagnosis relies on synovial fluid cell counts, multiple cultures, and inflammatory markers like ESR/CRP.
• Management involves surgical debridement, possible implant revision, and long-term antibiotics.