What is the difference between axial and appendicular Dual-Energy X-ray Absorptiometry (DEXA)?

Difference Between Axial and Appendicular DEXA Scans

Axial DEXA scans measure bone mineral density (BMD) in the central skeleton (spine and hip), while appendicular DEXA scans measure BMD in the peripheral skeleton (forearm, wrist, heel). The distinction is important because these measurements serve different clinical purposes and have varying diagnostic value 1.

Anatomical Differences

• Axial DEXA:

  • Measures the lumbar spine (L1-L4 vertebrae) and proximal femur (total hip and femoral neck)
  • Evaluates trabecular bone-rich areas that are common sites of osteoporotic fractures
  • Considered the gold standard for osteoporosis diagnosis according to WHO criteria

• Appendicular DEXA:

  • Measures peripheral skeletal sites such as the distal forearm, wrist, or heel
  • Evaluates predominantly cortical bone
  • Used as an alternative when axial measurements are not possible or as a screening tool

Clinical Utility and Diagnostic Value

Axial DEXA is strongly preferred for primary diagnosis of osteoporosis for several reasons:

• The WHO definition of osteoporosis (T-score ≤ -2.5) was established based on axial measurements 1
• Axial sites are more metabolically active and show earlier changes in bone loss
• Axial sites are common locations for osteoporotic fractures, making measurements more clinically relevant

According to the American College of Radiology appropriateness criteria, axial DEXA (lumbar spine and hip) receives the highest rating (9/9) for BMD screening, while appendicular measurements like distal forearm DEXA receive a lower rating (5/9) 1.

Technical Considerations

• Precision and Accuracy:

  • Axial DEXA typically provides better precision for monitoring changes in BMD over time
  • Appendicular measurements may have poorer correlation with fracture risk compared to axial measurements 2

• Radiation Exposure:

  • Both have minimal radiation exposure, but axial DEXA has slightly higher radiation levels
  • Axial DEXA: Relative Radiation Level (RRL) of ☢ (<0.1 mSv)
  • Appendicular DEXA: Similar or slightly lower radiation exposure 1

Clinical Applications

• When to use axial DEXA:

  • Primary screening and diagnosis of osteoporosis
  • Monitoring response to treatment
  • Fracture risk assessment
  • Follow-up evaluations

• When to use appendicular DEXA:

  • When axial DEXA is unavailable
  • In patients with severe obesity (BMI >35 kg/m²) where axial measurements may be limited
  • In patients with extensive spinal degenerative changes or prior spinal surgery
  • As a secondary tool when axial measurements are compromised by artifacts 1

Correlation Between Measurements

Research shows that appendicular BMD measurements correlate poorly with axial measurements, with correlation coefficients ranging from 0.24 to 0.34 3. This means that peripheral measurements may not accurately reflect the bone status at axial sites where osteoporotic fractures commonly occur.

Common Pitfalls

1. Discordance between sites: A study of prostate cancer patients found that while many were osteoporotic at the forearm (appendicular), only some showed osteoporosis at the lumbar spine (axial) 3

2. Overreliance on appendicular measurements: Appendicular DEXA has poorer ability to discriminate persons with fractures and appears to be less sensitive than axial measurements 2

3. Confounding factors in axial measurements:

   • Lumbar spine measurements can be affected by degenerative changes, aortic calcification, and vertebral fractures
   • Hip measurements can be affected by osteoarthritis 1

In summary, while both axial and appendicular DEXA scans measure bone mineral density, axial measurements are the preferred method for diagnosing osteoporosis and assessing fracture risk due to their stronger correlation with clinically significant outcomes.