Will an X-ray (X-radiation) of the knee show infection?

X-Ray of the Knee for Detecting Infection

X-rays of the knee are not sensitive or specific for diagnosing infection and should not be relied upon as the sole diagnostic tool for knee infections. 1 While radiographs are recommended as the initial imaging study when infection is suspected, they often appear normal in early infection or show only nonspecific findings.

Role of X-rays in Knee Infection Diagnosis

Initial Evaluation

• Radiographs are appropriate as the first imaging study for suspected knee infection, but have significant limitations 1
• In early acute osteomyelitis (less than 14 days), radiographs may be normal or show only mild soft tissue swelling 1
• X-rays can demonstrate indirect signs of infection such as:
  • Joint effusion
  • Soft tissue swelling
  • Periosteal reaction
  • Gas in soft tissues
  • Erosions (in more advanced cases) 1

Limitations of X-rays for Infection Detection

• Radiographs cannot directly image post-knee arthroplasty periprosthetic soft-tissue abnormalities 1
• X-rays are neither sensitive nor specific for diagnosing infection 1
• The radiographic appearance of an infected knee can range from "normal" to subtle periprosthetic lucency to advanced bone destruction 1
• It is often not possible to distinguish infection from other conditions like mechanical loosening or particle disease based solely on radiographs 1

Diagnostic Algorithm for Suspected Knee Infection

Step 1: Initial Radiographs

• Begin with standard radiographs to provide anatomic overview and exclude other pathologies like fractures or tumors 1, 2
• Look for indirect signs: effusion, soft tissue swelling, periosteal reaction, bone destruction 1

Step 2: Laboratory Testing

• Order ESR and CRP as these are more sensitive than radiographs for detecting infection 1
• When both ESR and CRP are negative, infection is unlikely 1
• Positive results for either ESR or CRP warrant further evaluation 1

Step 3: Joint Aspiration

• Joint aspiration is extremely useful in diagnosing joint infection and should be performed if laboratory markers are elevated or clinical suspicion remains high 1
• Image-guided aspiration (using ultrasound, fluoroscopy, or CT) is recommended for accurate needle placement 1

Step 4: Advanced Imaging

• If infection is still suspected after initial evaluation, MRI is the test of choice due to its superior soft tissue contrast and high sensitivity for pathologic edema 3, 2
• In patients with prosthetic joints, specialized protocols may be needed to reduce metal artifacts 1

Common Pitfalls and Caveats

• Relying solely on radiographs to rule out infection can lead to missed diagnoses, as radiographic changes often appear late in the disease process 1
• In chronic infections, radiographs may show bone sclerosis but can still miss the full extent of infection 1
• For patients with prosthetic joints, radiographic findings like loosening, periostitis, focal osteolysis, and radiolucent lines can be seen in both infected and non-infected knees, making differentiation difficult 1
• Minor differences in positioning can greatly alter the appearance of periprosthetic lucencies, potentially leading to false interpretations 1
• Low-grade or chronic infections may be particularly difficult to diagnose with any single imaging modality 1