What is the recommended initial imaging approach for osteomyelitis (infection of the bone) of the coccyx (tailbone)?

Imaging for Osteomyelitis of Coccyx

MRI without and with IV contrast is the recommended initial imaging approach for suspected osteomyelitis of the coccyx, following plain radiographs, due to its superior sensitivity and specificity for detecting bone infection and associated soft tissue involvement. 1

Initial Diagnostic Algorithm

Step 1: Plain Radiographs

• Plain radiographs should be the first imaging test due to their wide availability, low cost, and ability to provide anatomic evaluation 2
• Radiographs help evaluate for radiodense foreign bodies, soft tissue gas, and exclude alternative diagnoses such as fractures or tumors 2
• Early acute osteomyelitis (<14 days) may show only normal findings or mild soft tissue swelling on radiographs, limiting their sensitivity 2
• Radiographic findings of established osteomyelitis include erosions, periosteal reaction, bone sclerosis, and mixed lucency 2

Step 2: MRI (if radiographs are normal or inconclusive)

• MRI is the modality of choice for suspected bone infection due to its:
  • Superior sensitivity for bone marrow abnormalities 2
  • Excellent soft tissue contrast and anatomic detail 2
  • Ability to detect early osteomyelitis before radiographic changes 1, 3
• MRI has a reported 100% negative predictive value for excluding osteomyelitis; a normal marrow signal reliably excludes infection 2, 4
• Positive cases show decreased T1-weighted bone marrow signal with increased signal on fluid-sensitive sequences (T2-weighted fat-saturated and STIR) 2, 4
• MRI can accurately delineate the extent of both osseous and soft tissue involvement 2

Alternative Imaging Options (When MRI is Contraindicated or Unavailable)

CT Scanning

• CT depicts cortical bone well and can detect:
  • Periosteal reaction, bone destruction, and sequestra 2
  • Soft tissue infections, abscesses, and sinus tracts 2
  • Foreign bodies and gas formation better than MRI 2
• CT is particularly useful in chronic osteomyelitis but less sensitive than MRI for early infection 2, 1

Nuclear Medicine Studies

• Nuclear medicine examinations are most appropriate when:
  • MRI is contraindicated 2
  • Infection is suspected to be multifocal 2
  • Infection is associated with orthopedic hardware or chronic bone alterations 2
• Three-phase bone scan:
  • Highly sensitive but lacks specificity 2, 5
  • Can become positive as early as 1-2 days after symptom onset 2
  • Helps distinguish cellulitis from osteomyelitis 2
• Combined approaches (bone scan with labeled leukocyte scan) enhance specificity 2, 1

Ultrasound

• Limited role in direct diagnosis of osteomyelitis 5, 6
• Useful for detecting associated soft tissue infections and fluid collections 2, 6
• Can guide aspiration for diagnostic sampling 1

Diagnostic Pitfalls to Avoid

• Relying solely on radiographs to rule out infection, as early osteomyelitis may have normal radiographic appearance 2, 1
• Misinterpreting post-traumatic changes as infection on MRI 1
• Failing to obtain contrast-enhanced MRI when evaluating soft tissue involvement 1
• Not considering multifocal involvement, which is more common in children but can occur in adults 2, 5
• Overlooking the need for tissue sampling (biopsy) in cases where imaging findings are inconclusive 2

Special Considerations for Coccyx Osteomyelitis

• The coccyx is a challenging anatomical location due to its small size and proximity to the rectum 2
• Contrast-enhanced MRI is particularly important in this region to differentiate infection from other conditions such as pilonidal disease or perirectal abscess 1
• When interpreting imaging, consider that the coccyx may be affected by pressure-related changes in bedridden patients, which can mimic infection 2

By following this algorithmic approach with plain radiographs followed by MRI when indicated, clinicians can accurately diagnose osteomyelitis of the coccyx and guide appropriate treatment to reduce morbidity and improve quality of life.