Periprosthetic Joint Infection

MSIS Criteria

The Musculoskeletal Infection Society (MSIS) proposes a new definition for prosthetic joint infections:

Major Criteria (Diagnosis can be made when [1] major criterion exists)

  1. Sinus tract communicating with prosthesis
  2. Pathogen isolated by culture from 2 separate tissue/fluid samples from the affected joint

Minor Criteria (Diagnosis can be made when [4/6] of the following minor criteria exist)

  1. Elevated ESR (>30mm/h) or CRP (>10mg/L)
  2. Elevated synovial WBC (>1,100 cells/μL for knees, >3,000 cells/μL for hips)
  3. Elevated synovial PMN (>64% for knees, >80% for hips)
  4. Purulence in affected joint (Note: This finding alone is insufficient as fluid from metal-metal articulation, gout, etc., can resemble pus)
  5. Pathogen isolation in 1 culture
  6. 5 PMN per HPF in 5 HPF at ×400 magnification (intraoperative frozen section of periprosthetic tissue)

Epidemiology

  • Occurs in ~1% of primary joints and ~5% of revisions
  • Knees > Hips

Risk Factors

  • Superficial infection
  • Hematoma and wound ooze
  • Inflammatory arthropathy
  • Diabetes
  • Immunosuppressive drugs
  • Poor nutrition
  • Revision surgery
  • Peri-operative infection at another site
  • Malignancy or prior radiotherapy

Prevention

Pre-Operative

  • Screening for MRSA, ulcers, and UTI
  • Admission on the day of surgery to a ring-fenced ward
  • Urine dipstick test on admission day
  • Optimization of comorbidities and nutrition

Peri-Operative

  • Laminar flow – Ex Flow (well-maintained and regularly tested)
  • Low traffic
  • Antibiotics within 1 hour of incision
  • Antibiotic-loaded cement
  • Shaving at the time of surgery
  • Good handwashing technique
  • Draping with disposable drapes and ioband
  • Opening sets within laminar flow
  • Efficient surgery, good hemostasis, and sound wound closure
  • No hypothermia or hypotension

Post-Operative

  • Interactive dressing
  • Minimize dressing changes
  • Cultivate infection control – use of alcoholic gel; bare below elbows, etc.
  • Minimize unnecessary transfusions
  • Post-operative antibiotics for 24 hours
  • Early mobilization and physiotherapy
  • Optimal medical management
  • Timely but safe discharge

Classification

  1. Acute – Within 1 month
  2. Late Chronic – >1 month, indolent chronic infection
  3. Acute Hematogenous – Many years later in a previously sterile joint
  4. Subclinical – Found only on intra-operative cultures

History & Examination

  • Hints at the type of infection and differentiates it from other causes
  • Pain – Cardinal feature (95% of infected joints have pain)
    • Rest pain, night pain, worsening pain (differentiates from loosening)
  • Systemic upset
  • Recent infection elsewhere – acute hematogenous

Radiology

X-Ray

  • Periosteal reaction
  • Foci of osteolysis (rather than lucent lines seen in loosening)
  • Cortical destruction
  • Bone loss without significant prosthetic wear

Bone Scans

  • Standard bone scan: 99% sensitivity, but only 40% specificity
  • Triple-phase scans: Improve specificity to 95%
    • Technetium-99 – Detects inflammation
    • Indium-111 – Detects leukocytes (may also be present in loosening)

PET Scan

  • Uses fluorinated glucose – migrates to areas of high metabolic activity
  • 98% sensitivity and specificity reported

Blood Tests

Combined CRP and ESR

  • 99% sensitivity, 90% specificity
  • ESR elevated up to 90 days post-op; CRP up to 21 days
  • Rising trend is worrisome

Interleukin-6

  • Useful for diagnosis and monitoring
  • Expensive

Joint Aspiration

  • Sensitivity: 50-90%, Specificity: 95%
  • Sensitivity increases with repeat aspirations and avoiding antibiotics
  • Specificity improves with good technique (reduces false positives)

Synovial Fluid WCC

  • 1,100 WCCs & 70% neutrophil differential diagnostic in knees (Parvizi)

  • 3,500 WCCs and 75% neutrophil differential diagnostic in all joints (Dalle Valle)

Microbiology

Gram Stain

  • Poor sensitivity (25%)
  • Negative Gram stain is meaningless
  • Positive Gram stain almost certainly indicates infection

Frozen Section

  • 5 PMNs/HPF = positive

  • Specificity: 85%, Sensitivity: 95%

  • Useful for equivocal pre-op evaluation or worrisome intra-op appearance

Molecular Techniques

  • PCR amplifies bacterial DNA (risk of false positives due to contamination)
  • Bacterial gene and protein targeting (not widely available)

Bacteriology of Periprosthetic Infection

Staphylococcus

  • Staph aureus: Main coagulase-positive species
    • Secretes coagulase, converting fibrinogen to fibrin → Clotting protects against phagocytosis
  • Staph epidermidis: Main coagulase-negative species
    • Less virulent, but highly pathogenic within a biofilm

Glycocalyx and Biofilm

  • Glycocalyx: Protective layer secreted by bacteria; increases resistance to phagocytosis and adhesion to metal
  • Biofilm: Bacterial community within a glycocalyx
    • Takes ~4 weeks to form
    • Makes bacteria even more resistant to phagocytosis, better at adhering, and self-sufficient

Management Options

  1. Non-Operative – Suppression with long-term antibiotics
  2. Washout and Debridement with Prosthetic Retention
  3. Single- or Two-Stage Joint Revision
  4. Resection Arthroplasty/Arthrodesis
  5. Amputation

Washout and Prosthetic Retention

  • Large volume irrigation
  • Exchange easily removable implants (e.g., femoral heads, polyethylene)
  • Long-term antibiotics (6 weeks)
  • Only viable <4 weeks due to biofilm formation
  • Suitable for acute or acute hematogenous infections

Two-Stage Joint Revision

Prerequisites

  • Fit patient with adequate bone and soft tissue
  • Serologic and tissue-proven eradication of infection

First Stage

  • Thorough debridement, removal of all prosthetic material (including cement)
  • Multiple cultures (≥5 samples)
  • Spacer implantation

Interim Stage

  • IV antibiotics for 6 weeks
  • Antibiotic-free period for 2 weeks → Aspiration and ESR/CRP testing

Second Stage

  • Frozen section (>5 PMNs in 3/5 samples = ongoing infection)
  • Negative: Proceed to revision; Positive: Re-debride and re-insert spacers

Results: 90-95% success

Single-Stage Revision

Prerequisites

  • Identified sensitive organism
  • Adequate bone and soft tissue coverage

Surgical Principles

  • Aggressive debridement, irrigation, and removal of prosthesis
  • New prosthesis implantation with antibiotic cement
  • Long post-operative antibiotics

Results:
- Not as successful as two-stage revisions
- Better in hips than knees

Excision Arthroplasty

  • For non-ambulatory or frail patients

Arthrodesis

  • For ambulatory patients

Amputation

  • For cases with inadequate bone or soft tissue

Antibiotic Cement Spacers

Benefits

  • Maintains tissue planes
  • Prevents instability
  • Preserves mobility
  • Allows local antibiotic delivery
  • Eases the second stage

Types of Spacers

  1. Static: Minimal motion, risk of soft tissue issues
  2. Dynamic: Allows mobility, better soft tissue maintenance

Antibiotic Choice

  • Heat-stable, water-soluble antibiotics (e.g., vancomycin, tobramycin)

Factors Affecting Elution

  1. Antibiotic type
  2. Cement type
  3. Porosity
  4. Surface area
  5. Mixing technique

Antibiotic Dosing

  • Limit: 8g per 40g of cement
  • Common: 3.2g tobramycin + 3g vancomycin per 40g
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