When should a lumbar X-ray (X-ray of the lumbar spine) be chosen over a sacral coccyx X-ray for initial imaging?

Choosing Between Lumbar X-ray and Sacral Coccyx X-ray for Initial Imaging

A lumbar X-ray should be chosen over a sacral coccyx X-ray when symptoms are referable to the lumbar spine, particularly when evaluating for structural changes related to axial spondyloarthritis, or when there are neurological symptoms suggesting lumbar pathology rather than isolated sacral/coccygeal pain. 1, 2

Indications for Lumbar X-ray

• Symptoms referable to the lumbar spine:

  • Back pain radiating to the lower extremities (radiculopathy)
  • Neurological deficits in lower extremities
  • Progressive neurological symptoms
  • Pain that worsens with lumbar movement or positions

• Suspected axial spondyloarthritis:

  • When evaluating for structural changes such as syndesmophytes, erosions, shiny corners, vertebral body squaring, and ankylosis 1
  • The American Society for Ankylosing Spondylitis (ASAS) recommends at least radiographs of the cervical and lumbar spine should be performed in these cases 1

• After failed conservative management:

  • Persistent symptoms after 6 weeks of conservative treatment 2, 3
  • Need for evaluation of degenerative changes or alignment issues

Indications for Sacral Coccyx X-ray

• Localized sacral or coccygeal pain:

  • Pain specifically at the sacrum or coccyx
  • History of direct trauma to the sacrococcygeal region
  • Pain with sitting that improves with standing

• Suspected sacral insufficiency fractures:

  • Elderly or osteoporotic patients with lower sacral pain
  • Buttock pain, lateral hip pain, or groin pain radiating to the thigh 4
  • Especially in patients with previous lumbar vertebral compression fractures 4

Important Considerations

• Radiation exposure:

  • Lumbar spine radiographs deliver significantly higher radiation doses (2.20 mSv for AP and 1.50 mSv for lateral) compared to cervical spine or chest X-rays 5
  • Consider radiation exposure when determining appropriate imaging, especially in younger patients

• Limitations of radiographs:

  • Both sacral and lumbar radiographs have limited sensitivity for early disease processes 1
  • Wide variability in reported sensitivity (19%-72%) and specificity (47%-84.5%) for assessment of sacroiliitis on radiographs 1
  • Radiographs cannot demonstrate active inflammation 1

• When advanced imaging is preferred:

  • For suspected cauda equina syndrome, MRI lumbar spine without contrast is the preferred initial imaging modality 1, 2
  • CT or MRI may be more appropriate when there are red flags suggesting serious pathology 2, 3

Clinical Decision Algorithm

1. Assess symptom location and characteristics:

   • Lumbar spine radiographs if pain is in lumbar region with possible radiation to lower extremities
   • Sacral coccyx radiographs if pain is localized to sacrum/coccyx, especially with sitting

2. Consider clinical suspicion:

   • Lumbar spine radiographs for suspected axial spondyloarthritis with lumbar involvement
   • Sacral coccyx radiographs for suspected sacral insufficiency fractures or coccydynia

3. Evaluate for red flags requiring advanced imaging:

   • If cauda equina syndrome is suspected, bypass X-rays and proceed directly to MRI 1, 2
   • If infection, malignancy, or severe neurological deficits are present, MRI is more appropriate than any X-ray 2

4. Consider patient factors:

   • Age, osteoporosis risk, prior fractures, and radiation exposure history

Common Pitfalls to Avoid

• Automatically attributing pain to lumbar pathology without considering sacral causes, especially in elderly patients with osteoporosis 4
• Ordering inadequate imaging that doesn't capture the relevant anatomy for the patient's symptoms
• Failing to recognize that sacral insufficiency fractures are commonly missed and may coexist with lumbar pathology 4
• Relying solely on radiographs when red flags are present that warrant advanced imaging

Remember that radiographs are often the initial imaging modality, but their limitations must be acknowledged, and clinical correlation is essential for accurate diagnosis and appropriate management.