In the present study, we investigated bone geometry, microstructure, and volumetric bone mineral density (vBMD) in a cohort of patients with nonradiographic axial spondyloarthritis (nr-axSpA) in order to define the early bone changes occurring in axial spondyloarthritis (axSpA) and to define potential factors for deterioration of bone microstructure. Methods: Patients with axSpA (n = 107) and healthy control subjects (n = 50) of similar age and sex were assessed for geometric, volumetric, and

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... crostructural parameters of bone using high-resolution peripheral quantitative computed tomography (HR-pQCT) at the radius. Additionally, demographic and disease-specific characteristics of patients with axSpA were recorded. Results: Patients with nr-axSpA and control subjects were comparable in age, sex, and body mass index. Geometric and microstructural analysis by HR-pQCT revealed a significantly reduced cortical area (p = 0.022) and cortical thickness (p = 0.006) in patients with nr-axSpA compared with control subjects. Total and cortical vBMD were significantly reduced in patients with nr-axSpA (p = 0.042 and p = 0.007, respectively), whereas there was no difference in trabecular vBMD. Patients with a short disease duration (< 2 years; n = 46) also showed significant reduction of cortical thickness and cortical area compared with control subjects. Patients with disease duration > 2 years (n = 55) additionally developed a decrease of cortical and total vBMD. Multiple regression models identified male sex to be associated with lower cortical vBMD and female sex to be associated with lower trabecular vBMD. Conclusions: Bone microstructure in patients with nr-axSpA is characterized primarily by deterioration of cortical bone. Cortical bone loss starts early and is evident within the first 2 years of the disease.