Background/Purpose: It is well known that osteoporosis is paradoxically common in patients with ankylosing spondylitis (AS) in spite of their new bone formation and extraosseous calcification. However, some features of advanced disease such as syndesmophyte can cause overestimation of bone marrow density (BMD) resulting normal or high BMD values even when osteoporosis is present. The aim of this study was to evaluate lateral spine BMD and trabecular bone score (TBS) as the methods to measure actual bone loss and thus redeem the obstacles of conventional BMD in patients with advanced AS.

Methods: Fifty-four patients with AS (38 males, 16 females) who assessed BMD using dual-energy X-ray absorptiometry (DXA) were retrospectively reviewed. BMD was measured by anterior-posterior (AP) lumbar (L)1-L4, lateral (Lat) L2, L3, and proximal femur projection. Trabecular bone score (TBS) L2-L4 which provides an indirect index of trabecular microarchitecture was calculated by reanalyzing AP lumbar spine image. Radiologic variables including Bath Ankylosing Spondylitis Radiology Index for the spine (BASRI-s) score, modified Stroke Ankylosing Spondylitis Spine Score (mSASSS), and the presence of syndesmophyte were determined. AS patients were divided into mild, moderate, and advanced stages according to each radiologic variables, and the BMD measured by different methods were compared among those 3 groups using ANOVA.

Results: Radiologic progression of AS patients represented by BASRI-s, mSASSS, and the number of syndesmophyte was negatively correlated with Lat-L3 BMD (r=-0.431, r= -0.364, r=-0.464) and with TBS (r=-0.352, r=-0.355, r=-0.382). Moreover, patients in advanced stage (BASRI-s¡Ã3, mSASSS¡Ã10, syndesmophyte¡Ã1, respectively) showed significantly lower Lat-L3 BMD (P=0.047, P=0.012, P=0.006) and TBS (P=0.050, P=0.014, P=0.090) compared with those in mild stage, while they displayed no difference in conventional BMD from AP and femur DXA.

Conclusion: The Lat-L3 BMD and TBS have higher potency to mirror a bone loss in AS patients independent of their radiologic changes. Further studies evaluating TBS cutoff value for expecting vertebral fracture are needed.