What is Osteomyelitis (OMI) and what are the criteria for its diagnosis?

Osteomyelitis (OMI): Definition and Diagnostic Criteria

Osteomyelitis is an infection of bone characterized by progressive inflammatory destruction of bone, bone necrosis, and new bone formation, requiring specific imaging and laboratory criteria for diagnosis. 1

Definition and Classification

Osteomyelitis can be classified based on several key variables:

• Duration:

  • Acute: Infection present for days to weeks
  • Chronic: Infection present for months to years, often with bone necrosis (sequestrum) and sinus tract formation

• Mechanism of infection:

  • Hematogenous: Spread through bloodstream (more common in children)
  • Contiguous spread: From adjacent soft tissue infection (most common overall)
  • Direct inoculation: Following trauma or surgery
  • Vascular insufficiency: Particularly in diabetic foot infections

• Anatomical location:

  • Long bones (most common in children)
  • Vertebral (spine)
  • Foot (especially in diabetic patients)
  • Other sites

Diagnostic Criteria

Clinical Criteria

• Pain, erythema, and swelling at the affected site
• Fever (may be absent, especially in chronic cases)
• Draining sinus tract (in chronic cases)
• "Sausage toe" appearance (swollen, erythematous toe lacking normal contours) 1
• Exposure of bone in an ulcer (especially in diabetic foot)
• Positive probe-to-bone test in diabetic foot ulcers 1

Laboratory Criteria

• Elevated inflammatory markers (ESR, CRP)
  • CRP >3.2 mg/dL combined with ulcer depth >3 mm helps differentiate osteomyelitis from cellulitis 1
  • ESR >60 mm/hour combined with ulcer depth >3 mm also helps in diagnosis 1
• Leukocytosis (may be absent in chronic cases)
• Positive blood cultures (in hematogenous cases)

Definitive Diagnostic Criteria

The gold standard for diagnosis is:

• Positive bone culture with histopathological confirmation showing inflammatory cells and osteonecrosis 1
• Visualization of purulence in bone during surgery 1

Imaging Criteria

Initial Imaging

• Plain radiographs: First-line imaging test 1
  • May be normal in early disease
  • Findings in established disease: periosteal reaction, bone destruction, sequestrum
  • Sensitivity is low in early osteomyelitis but useful for anatomic evaluation

Advanced Imaging

• MRI: Modality of choice for suspected osteomyelitis 1

  • Findings: decreased T1 signal in bone marrow, increased signal on fluid-sensitive sequences
  • Nearly 100% negative predictive value (normal marrow signal reliably excludes infection) 1
  • Excellent for evaluating soft tissue involvement, abscesses, and fistulas
  • IV contrast helpful for soft tissue infection but not necessary for bone evaluation 1

• CT:

  • Less sensitive than MRI for acute osteomyelitis 1
  • Better for detecting sequestrum, foreign bodies, and gas 1
  • Useful in chronic osteomyelitis showing periosteal reaction, bone destruction, and sinus tracts
  • IV contrast helpful for assessing soft tissue involvement 1

• Nuclear Medicine Studies:

  • 3-phase bone scan: High sensitivity (83%) but low specificity (45%) 1
  • FDG-PET/CT: High sensitivity (81-100%) and specificity (87-100%) 1
  • WBC scan with sulfur colloid: Up to 90% accurate in diagnosing osteomyelitis 1
  • Useful when MRI is contraindicated or with orthopedic hardware

• Ultrasound:

  • Limited benefit in diagnosing osteomyelitis 1
  • Useful for detecting soft tissue abscesses and guiding aspiration

Special Considerations

Hardware-Associated Osteomyelitis

• MRI or FDG-PET/CT preferred when hardware is present 1
• 3-phase bone scan has low specificity in post-surgical or post-trauma settings 1

Diabetic Foot Osteomyelitis

• Probe-to-bone test is valuable in diabetic foot ulcers 1
• Ulcer area >2 cm² increases likelihood of osteomyelitis 1
• Ulcers that do not heal after 6 weeks of appropriate care should raise suspicion 1

Pitfalls and Caveats

• Distinguishing between infection and inflammation on imaging can be challenging, especially post-surgery
• Neuropathic arthropathy (Charcot foot) can mimic osteomyelitis and may coexist with it 1
• Chronic osteomyelitis requires surgical debridement in addition to antibiotics for cure 2
• Negative cultures do not exclude osteomyelitis if there was prior antibiotic therapy
• MRI findings can be altered by recent trauma or surgery, making diagnosis more difficult

By systematically applying these clinical, laboratory, and imaging criteria, clinicians can accurately diagnose osteomyelitis and initiate appropriate treatment to prevent complications such as chronic infection, bone destruction, and loss of function.