skip to main content

DXA Artefact in AS — SCE Rheumatology MCQ

Instant feedback + full explanation. One question, done properly.

ModerateSpondyloarthropathyDXA Artefact in ASSCE Rheumatology

A 58-year-old man with longstanding ankylosing spondylitis undergoes posteroanterior DXA scanning. His lumbar spine T-score is −0.5, whereas his femoral neck T-score is −2.8. Which mechanism best explains the apparently preserved lumbar spine bone mineral density?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: A — Syndesmophytes and ligamentous calcification enter the DXA projection, increasing measured lumbar areal density

Explanation lettering: D = shown as A · E = shown as B · A = shown as C · C = shown as D · B = shown as E

The correct answer is D. Posteroanterior lumbar DXA is a two-dimensional measurement of projected areal bone mineral density. In established ankylosing spondylitis, syndesmophytes and ligamentous calcification are included within that projection and can falsely increase the measured lumbar value despite loss of vertebral trabecular bone. NICE therefore cautions that spinal DXA values may be elevated in axial spondyloarthritis and that hip measurements may be more reliable. Inflammation does not normally cause protective diffuse trabecular osteosclerosis (A), and DXA does not overestimate density by excluding cortical bone (B). Hip enthesophytes would not explain a spuriously low femoral-neck value (C). Reduced movement artefact from ankylosis is not the principal cause of this characteristic discrepancy (E).

Reference: National Institute for Health and Care Excellence. Spondyloarthritis in over 16s: diagnosis and management (NG65), recommendation 1.8.5. 2017; reviewed March 2025. https://www.nice.org.uk/guidance/ng65/chapter/Recommendations