Background/Purpose: Axial Spondyloarthritis (axSpA) is characterized by abnormal bone formation that produces syndesmophytes, while paradoxically, inflammation causes increased bone resorption and skeletal fragility. Detection of osteoporosis using DXA can be challenging as syndesmophytes falsely elevate spine areal bone mineral density (aBMD). In contrast, high-resolution peripheral QCT (HRpQCT) measures volumetric BMD (vBMD) and microarchitecture and is not subject to this artifact. This study investigated skeletal health in a cohort with axSpA using aBMD, trabecular bone score (TBS), a method for estimation of spine trabecular bone quality, and HRpQCT. We hypothesized that TBS and HRpQCT would reveal skeletal abnormalities in axSpA, while aBMD by DXA would appear normal.

Methods: Patients fulfilling the imaging arm of the ASAS criteria who were enrolled as part of an ongoing longitudinal study had DXA measurements of aBMD of spine, hip and1/3 radius, and TBS. Trabecular (Tb) and cortical (Ct) vBMD and microarchitecture were measured by HRpQCT (Xtreme CT2, voxel size ~61 µm) at the distal radius and tibia. DXA in axSpA were compared with normative populations of age and sex matched individuals (Z-Scores). HRpQCT values were compared to a healthy reference population of young sex-matched adults. T-tests compared skeletal parameters in subjects and normative populations. Spearman correlations related skeletal parameters to disease features.

Results: 18 subjects were enrolled; 56% male, age 42+13 yrs. On average, subjects had symptoms for 14 yrs and had received a diagnosis of axSpA 6 yrs prior to study enrollment. BASDAI and other criteria reflected high disease activity (Table 1). The majority of subjects had normal aBMD by DXA. TBS was normal in the majority (67%). By HRpQCT at the radius, axSpA had lower Tb vBMD (-0.9 SD) and Tb thickness (-1.7 SD) than the reference population. At the tibia, axSpA had lower total (-0.9 SD), Ct (-1.5 SD) and Tb vBMD (-1.2 SD) and Tb thickness (-1.4 SD; p< 0.01 for all). Longer duration of symptoms was related to worse vBMD and microarchitecture, tibia: total vBMD (r=-0.83; p< 0.0001), Tb density (r=-0.76; p< 0.0005), Tb number (r=-0.51; p< 0.04) and Tb separation (r=0.61; p< 0.01) and radius: Tb density (r=-0.64; o< 0.1), Tb number (-0.50; p< 0.04) and Tb separation (r= 0.57; p< 0.02). Current disease activity did not relate to HRpQCT abnormalities.

Conclusion: Subjects with active axSpA had low vBMD and microarchitectural abnormalities by HRpQCT despite normal DXA and TBS. Abnormalities were most pronounced at the tibia and related to duration of symptoms. Larger studies are needed to confirm our findings, which suggest that long-term impairment in mobility may contribute to skeletal abnormalities in patients with axSpA.

Table 1 Novarts

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/patients-with-axial-spondyloarthritis-have-abnormal-microarchitecture-despite-normal-areal-bone-mineral-density-and-trabecular-bone-score-by-dxa/