Disease Activity and Bone Microarchitectural Phenotype in Patients With Axial Spondyloarthritis.

Background: Axial spondyloarthritis (AxSpA) is a chronic rheumatic disease characterized by spine inflammation, abnormal bone growth, and paradoxically osteoporosis and vertebral fractures. The pathogenesis of skeletal deficits in this disease is poorly understood.

Purpose: We sought to evaluate volumetric bone mineral density (vBMD) and bone microarchitecture in patients with AxSpA and to identify disease-related factors associated with skeletal abnormalities.

Methods: We enrolled patients between 2018 and 2021 as part of a 2-year prospective study at a single institution investigating skeletal health and the skeletal effects of interleukin-17 (IL-17) treatment. Patients with AxSpA who met Assessment in SpondyloArthritis International Society (ASAS) classification criteria by X-ray or had evidence of active inflammation on magnetic resonance imaging suggestive of sacroiliitis were referred to the study by their rheumatologists. We excluded those with a history of fragility fracture, multiple myeloma, Cushing's disease, primary hyperparathyroidism, osteomalacia, untreated vitamin D deficiency, secondary osteoporosis, or other systemic rheumatic diseases, as well as use of oral steroids for 2 or more weeks in the 6 months prior or current use of hormone replacement therapy, current oral bisphosphonate, past or current intravenous bisphosphonate, teriparatide, or denosumab therapies. A total of 1606 patients were screened for eligibility. Of these, 30 participants were enrolled (mean age 43 years, 50% male). Patients with AxSpA had dual-energy X-ray absorptiometry (DXA) measurements of areal BMD (aBMD) and high-resolution peripheral quantitative computed tomography (HR-pQCT) measurements of vBMD microarchitecture and failure load by finite element analysis. Standardized disease assessment tools used included the Bath Ankylosing Spondylitis Disease Activity (BASDAI), Metrology Index (BASMI), and Functional Index (BASFI).

Results: In the 30 included patients, mean DXA and HR-pQCT Z-scores were within 1 standard deviation (SD) of normal for all indices, except for total vBMD in males (-1.2 SD below mean). Mean symptom duration was 11.7 years and mean scores for BASDAI, BASFI, and BASMI were 4.6, 3.6, and 2.7, respectively (range 1-10, 10 = severe limitation). Longer disease duration was associated with more severe skeletal deficits at the hip and tibia-specifically, lower hip aBMD, lower meta- and inner-trabecular vBMD, lower trabecular number, and higher trabecular separation and heterogeneity.

Conclusion: This study of 30 patients with AxSpA found that abnormalities in bone density and microarchitecture at weightbearing sites were associated with longer disease duration. Because of its small sample size, larger studies are needed to better characterize the pathogenic disease factors that govern skeletal damage in AxSpA.