CT-based hounsfield unit as an alternative osteoporosis assessment in ankylosing spondylitis patients with bamboo spine.

This retrospective study compared bone mineral density (BMD) measurements assessed by unenhanced computed tomography (CT)-based HU at L1 vertebrae to those by plain spine radiographs in ankylosing spondylitis (AS) patients with or without bamboo spine (BS). We also assessed whether BS affects dual-energy X-ray absorptiometry (DXA)-based osteoporosis diagnosis using unenhanced CT scans at the L1 vertebrae. All patients underwent DXA, unenhanced CT scans, and plain spine radiographs. The cohort of 524 AS patients was classified into BS (n = 189) and non-BS (n = 335) groups (Hu threshold for osteoporosis, 135). The BS group had significantly higher spine (P < 0.001) and hip (P < 0.001) T-scores and significantly lower L1 HU values (P < 0.001) than the non-BS group. Among non-BS patients, lowest T-scores correlated positively with L1 HU values (R-value, 0.437; P < 0.001); in BS patients a weak but significant correlation was observed between L1 HU values and hip T-scores (R-value, 0.341; P < 0.001). Using the Hu threshold, 77.8% of BS and 55.2% of non-BS patients were reclassified as osteoporotic despite DXA indicating no osteoporosis. Whether CT-based HU improves the accuracy of BMD evaluation in the AS population requires confirmation in larger cohort studies.