Courtesy: Prof Alfonso Manzotti, Luigi Sacco Hospital, Milan, Italy

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Patient Profile Commonly Associated with PJI

Periprosthetic joint infection is more frequently encountered in patients with:

• Highly complex surgical cases

• Multiple comorbidities (pluripatological patients)

• History of multiple surgical procedures

• Poor treatment compliance

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The Clinical Problem

• A painful total knee arthroplasty (TKA) without an identifiable mechanical cause should be considered infected until proven otherwise.

• Patient-reported outcomes are often poor:

  • Only 23% report satisfaction

  • Approximately 18% report complete dissatisfaction

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Epidemiology and Microbiology

Common Causative Organisms

• Coagulase-negative staphylococci: 30–43%

• Staphylococcus aureus: 12–23%

• Streptococci: 9–10%

• Enterococci: 3–7%

Infection Rates

• Primary joint replacement (first 2 years):

  • Hip and shoulder: <1%

  • Knee: <2%

  • Elbow: <9%

• Revision arthroplasty:

  • Infection rates significantly higher, reported up to 40%

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Classification of PJI

Early PJI (< 6 months)

• Usually caused by high-virulence organisms

• Common pathogens:

  • Staphylococcus aureus

  • Gram-negative bacteria

• Often related to:

  • Perioperative contamination

  • Early postoperative hematogenous or lymphatic spread

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Routes of Infection

• Perioperative:
  Inoculation during surgery or immediately postoperatively

• Hematogenous:
  Spread via blood or lymphatics from a distant infectious focus

• Contiguous:
  Direct extension from adjacent infection
  (e.g., penetrating trauma, pre-existing osteomyelitis, skin or soft-tissue infection)

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Evolution of PJI Diagnostic Criteria

• 2004 – Zimmerli criteria

• 2010 – AAOS guidelines

• 2013 – IDSA guidelines

• 2018 – International Consensus Meeting

• 2021 – European Bone and Joint Infection Society (EBJIS)

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Diagnostic Criteria for PJI

Major Criteria

• Sinus tract communicating with the prosthesis

• Isolation of the same pathogen from two separate tissue or fluid samples

Minor Criteria

PJI is diagnosed if three of the following five are present:

1. Elevated ESR and CRP

2. Synovial WBC count >3,000 cells/µL or positive leukocyte esterase

3. Synovial PMN percentage >80%

4. Isolation of a microorganism in a single culture

5. 5 WBCs per high-power field in 5 high-power fields on histology

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Diagnostic Protocol for Suspected PJI

1. Clinical history and physical examination

2. Blood tests and joint aspiration

3. Standard radiographs

   • Wedge-shaped bone resorption

   • Periosteal scalloping

4. Advanced imaging (when indicated):

   • Nuclear medicine studies

   • CT or MRI

5. Further specialized investigations in atypical cases

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Blood Tests and Joint Aspiration

Serum Biomarkers (Minor Criteria)

• C-reactive protein (CRP)

• D-dimer

• Plasma fibrinogen (excellent biomarker)

• ESR (note: normal in ~4% of PJIs)

Serum-negative PJI:
Approximately 4% of patients have normal ESR and CRP.

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Synovial Fluid (SF) Analysis

Used by ~97% of surgeons, but culture-negative infections occur in 20–40%.

Components of SF Analysis (5-in-1)

1. Aerobic and anaerobic culture

2. Leukocyte esterase

3. Synovial WBC count

4. Synovial PMN percentage

5. Alpha-defensin (routine use not mandatory)

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Diagnostic Thresholds for Synovial WBC and PMN%

• Synovial calprotectin is emerging as a useful marker for chronic PJI.

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Tips and Tricks for Synovial Fluid Analysis

• Use pre-prepared aspiration sets

• Always perform optical microscopy

• Aspirate at least 15 mL of fluid:

  • 5 mL for anaerobic culture

  • 5 mL for aerobic culture

  • 2 mL for cell count

  • 2 mL for leukocyte esterase

  • 1 mL for alpha-defensin

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Second-Level Investigations

Ultrasound

• Pros:
  Low cost, widely available, useful for soft-tissue monitoring and biopsy guidance

• Cons:
  Low sensitivity and specificity

CT Scan

• Pros:
  Useful for bone biopsy guidance

• Cons:
  Metal artefacts, radiation exposure, contrast risks, lower accuracy than MRI

MRI

• Pros:
  High diagnostic accuracy with modern metal-artifact-reduction sequences
  Radiation-free

• Cons:
  Peri-implant edema may cause false positives

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Nuclear Medicine Imaging

Tc-MDP / HDP Bone Scan

• High sensitivity

• Low specificity

• Useful as a screening tool in chronic infections

99mTc-HMPAO / In-Oxine WBC Scan

• High sensitivity and specificity

• Particularly useful in acute infection

• Can be combined with bone marrow scan

• Limitations: availability, radiation, blood handling

FDG-PET/CT

• High sensitivity

• Low specificity

• High radiation dose

• Difficult interpretation and high cost

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Intraoperative Investigations

• Obtain 3–5 sterile tissue samples

• Avoid swabs

• Use blood culture bottles (e.g., BACTEC)

• Sonication of removed implants:

  • More sensitive than standard periprosthetic tissue cultures

  • Similar specificity

• DTT and sonication have similar sensitivity and outperform standard cultures

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Advanced Microbiological Techniques

• Microbial DNA PCR (panels of 8–20 organisms)

• Next-generation sequencing (NGS)

• Particularly helpful in culture-negative infections

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Take-Home Messages

• Diagnosis of PJI is never presumptive

• Always start with clinical history and examination

• Apply standardized PJI diagnostic criteria

• Follow a stepwise, protocol-based approach

• Multidisciplinary collaboration improves accuracy

• Proceed to advanced diagnostics only when results are inconclusive

• Emerging technologies will enhance diagnostic precision in the future