What imaging modalities are recommended for diagnosing prosthetic joint infections?

Imaging for Prosthetic Joint Infection

Plain radiography should be performed as the first-line imaging modality in all patients with suspected prosthetic joint infection (PJI), but advanced imaging studies such as bone scans, leukocyte scans, MRI, CT, and PET scans should not be routinely used for PJI diagnosis. 1

Diagnostic Algorithm for PJI

First-Line Diagnostic Tests

1. Plain radiography (mandatory)

   • Although neither sensitive nor specific for diagnosing infection
   • Radiographic appearance can range from normal to subtle periprosthetic lucency to advanced bone destruction
   • Cannot reliably distinguish between infection, aseptic loosening, or particle disease 1
   • Serial radiographs may help identify subtle signs of loosening

2. Laboratory tests (to be performed concurrently)

   • Erythrocyte sedimentation rate (ESR)
   • C-reactive protein (CRP)
   • The combination of abnormal ESR and CRP provides the best sensitivity and specificity 1
   • CRP has sensitivity of 73-91% and specificity of 81-86% for PJI diagnosis when using a cutoff of ≥13.5 mg/L 1

Second-Line Diagnostic Tests

1. Joint aspiration (when infection is suspected)

   • Extremely useful in diagnosing PJI with high sensitivity and specificity 1
   • Should include:
     • Total cell count and differential leukocyte count
     • Aerobic and anaerobic cultures
     • Crystal analysis if clinically indicated
   • Important: Withhold antimicrobial therapy for at least 2 weeks prior to aspiration to increase organism recovery 1
   • Newer biomarkers like α-defensin may improve diagnostic accuracy 1

2. Blood cultures

   • Only indicated if:
     • Fever is present
     • Acute onset of symptoms
     • Conditions that increase risk of bloodstream infection (e.g., Staphylococcus aureus infection elsewhere) 1

Advanced Imaging Modalities

Despite their availability, advanced imaging modalities should not be routinely used for PJI diagnosis 1. This includes:

• Bone scans
• Leukocyte scans
• Magnetic resonance imaging (MRI)
• Computed tomography (CT)
• Positron emission tomography (PET) scans

Intraoperative Diagnosis

If surgery is performed, the following should be obtained:

• At least 3 (optimally 5-6) periprosthetic tissue samples for aerobic and anaerobic cultures 1
• Histopathological examination of periprosthetic tissue samples (when infection is in doubt) 1
• Examination of the explanted prosthesis itself 1

Definitive Evidence of PJI

The following findings provide definitive evidence of PJI:

1. Presence of a sinus tract communicating with the prosthesis 1
2. Presence of purulence around the prosthesis without another known etiology 1
3. Two or more intraoperative cultures yielding the same organism 1
4. Growth of a virulent microorganism (e.g., S. aureus) in a single specimen of tissue biopsy or synovial fluid 1

Common Pitfalls to Avoid

1. Relying solely on radiography: While necessary, radiographs alone are insufficient for PJI diagnosis
2. Performing aspiration while on antibiotics: This significantly reduces culture yield; antibiotics should be withheld for at least 2 weeks prior
3. Ordering advanced imaging prematurely: These should not be first-line tests and add unnecessary cost and time
4. Insufficient sampling during surgery: At least 3 (optimally 5-6) tissue samples should be collected
5. Misinterpreting a single positive culture: A single culture of a common contaminant (e.g., coagulase-negative staphylococci) should be interpreted cautiously

Special Considerations

• Fluoroscopically positioned radiographs may provide optimal visualization of the prosthesis-bone interface, especially for uncemented prostheses 1
• A "dry tap" during aspiration does not rule out infection 1
• Weekly repeat aspirations are recommended if the first aspiration is negative but clinical suspicion remains high 1
• Recent studies suggest that combining synovial fluid α-defensin and CRP tests may achieve sensitivity of 97% and specificity of 100% for PJI diagnosis 1

By following this systematic approach to imaging and diagnostic testing, clinicians can optimize the diagnosis of prosthetic joint infections while avoiding unnecessary tests, reducing costs, and improving patient outcomes.