Ankylosing spondylitis (AS) patients seems to be at risk of osteoporosis but bone screening is not often performed. The objective was to evaluate the effect of vertebral ankylosis on scanographic bone attenuation coefficient (SBAC) on lumbar vertebrae in AS patients.
This study included AS patients fulfilling New York criteria who underwent both thoraco-abdomino-pelvic computed tomography and X-rays during routine follow-up. The modified stoke ankylosing spondylitis spinal score (mSASSS) was scored on X-rays, and the presence of at least one syndesmophyte (mSASSS≥2) defined mSASSS+ patients. Ankylosis of a lumbar vertebra was defined by the presence of bone bridges to its two adjacent vertebrae. The SBAC was measured from L1 to L5, and the fracture threshold was set at SBAC≤145 HU.
A total of 73 AS patients were included (mean age: 60.3 (±10.7) years, 65 men (89%)). Sixty patients (82.2%) were mSASSS+; 13 patients (17.8%) presented ankylosis of at least one lumbar vertebra. The SBAC of each lumbar vertebra was not significantly different between mSASSS- and mSASSS+ patients. The SBAC was lower for patients with at least one bone bridge than for patients without (p<0.05). Patients with lumbar vertebral ankylosis had a higher risk of presenting an SBAC≤145 HU (OR: 4.95 (95% CI: 1.1-17.4)).
The presence of a bone bridge and complete ankylosis of lumbar vertebra were associated with a higher risk of SBAC under the fracture threshold, suggesting structural deterioration of trabecular bone in ankylosed vertebrae in AS patients.

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