What are the pitfalls of Dual-Energy X-ray Absorptiometry (DEXA) scans for osteoporosis screening?

Pitfalls of DEXA Scans for Osteoporosis Screening

DEXA scans can falsely elevate bone mineral density measurements in patients with spinal degenerative changes, scoliosis, or vascular calcifications, leading to underdiagnosis of osteoporosis in precisely those patients who need treatment most. 1

Major Technical and Anatomical Pitfalls

Degenerative Spine Disease

• Lumbar spine BMD measured by DEXA can be falsely elevated due to degenerative changes such as osteophytes, facet joint arthritis, and vertebral sclerosis. 1
• Studies demonstrate that DEXA imaging is less accurate than CT Hounsfield units in calculating BMD in patients with spinal degeneration. 1
• Degenerative changes are particularly problematic because they are most common in elderly patients—the exact population being screened for osteoporosis. 1, 2

Structural Abnormalities

• DEXA assessment is inaccurate in patients with scoliosis, as spinal curvature distorts the two-dimensional projectional measurements. 1
• Vertebral compression fractures can paradoxically increase apparent BMD due to collapsed vertebral height and increased bone density per unit area. 2
• Surgical hardware, calcifications, or external objects (such as buttons or jewelry) can falsely elevate BMD readings. 2

Body Habitus Limitations

• Extreme body sizes pose significant challenges: very tall, very small patients, and those with BMI >35 kg/m² may require alternative imaging modalities like quantitative CT. 1, 2
• The two-dimensional nature of DEXA means larger bones may appear to have the same density as smaller bones despite having superior structural strength. 3

Measurement and Interpretation Pitfalls

Projectional Limitations

• DEXA generates two-dimensional images of complex three-dimensional structures, reporting bone density as bone mineral content divided by bone area rather than true volumetric density. 3
• The projectional properties of DEXA summate cortically predominant end plates and posterior elements with cancellous vertebral body measurements, decreasing sensitivity to detect trabecular bone loss over time. 1
• DEXA cannot distinguish between cortical and trabecular bone, yet these compartments have vastly different rates of bone loss with aging. 1

Vascular Calcification Interference

• Abdominal aortic calcifications can lead to overestimation of lumbar spine BMD, particularly in elderly patients. 4
• One study found that 9.2% of patients classified as having normal BMD by DEXA were subsequently diagnosed with osteoporosis when quantitative CT was used. 1

Underdiagnosis in Fracture Patients

• DEXA significantly underdiagnoses osteoporosis in patients who already have vertebral fractures: one study found 44% of fracture patients were classified as osteoporotic by DEXA versus 81% by quantitative CT. 1
• In patients with osteoporotic spine or hip fractures, 80% had low Hounsfield units on CT despite having normal DEXA spine/hip BMD. 1

Clinical Decision-Making Pitfalls

Imperfect Fracture Prediction

• Although DEXA is the gold standard, most individuals who sustain fragility fractures have T-scores above the -2.5 diagnostic threshold for osteoporosis. 3
• The arbitrary T-score cutoff of -2.5 means many at-risk patients are missed by screening. 3

Positioning and Technical Errors

• Incorrect patient positioning, with body and limbs not aligned to midline or with excessive rotation, can produce inaccurate results. 2, 5
• Inconsistent positioning between baseline and follow-up scans invalidates serial comparisons, making it impossible to accurately assess treatment response or disease progression. 2
• Errors in demographic information, improper scan analysis, and mistakes in interpretation can all lead to wrong clinical decisions. 5

Monitoring Limitations

• Changes in BMD must exceed the "Least Significant Change" threshold to be considered real versus measurement variability, but this is facility-specific and often not calculated. 2
• Evidence is lacking about optimal intervals for repeated screening, creating uncertainty about follow-up timing. 1

Alternative Modalities to Consider

When DEXA Should Be Avoided

• Quantitative CT should be considered as the primary imaging modality in patients with severe degenerative spine disease, extremes in height, obesity (BMI >35), or when monitoring treatments affecting primarily trabecular bone. 1
• CT Hounsfield unit values >160 demonstrate significant reduction in osteoporosis risk, while values <110 are significantly correlated with osteoporosis. 1

Quantitative Ultrasound Limitations

• Quantitative ultrasound cannot be used to diagnose osteoporosis because it does not measure BMD and WHO classification criteria do not apply. 1
• Discordance between quantitative ultrasound and central DEXA is not infrequent, and there is insufficient evidence to support its use as a screening tool. 1

Common Clinical Scenarios to Avoid Errors

Pre-Surgical Assessment

• For patients undergoing spinal fusion surgery, DEXA may be inadequate because degenerative changes falsely elevate BMD precisely in the surgical population. 1
• CT-based assessment is more accurate for predicting implant fixation and screw loosening risk in spine surgery patients. 1

Vertebral Fracture Assessment

• Vertebral Fracture Assessment (VFA) should be performed during DEXA if T-score is <-1.0, historical height loss ≥4 cm, or prospective height loss ≥2 cm. 6, 2
• An osteoporotic fracture supersedes any DEXA measurement and establishes the diagnosis regardless of T-score. 2

Artifact Recognition

• Clinicians must actively identify and document artifacts that may falsely elevate BMD, including surgical hardware, vascular calcifications, and vertebral compression fractures. 2
• Vertebrae with structural abnormalities should be excluded from analysis, but this requires careful review of the scan images, not just the numerical report. 2