Background and objectives Joint inflammation and periarticular bone loss are hallmarks of rheumatoid arthritis (RA). In RA patients, systemic bone loss increases bone fracture risk. Dual-energy X-ray absorptiometry (DXA) assesses bone mineral density (BMD) in daily practice to partially estimate fracture risk. Rat adjuvant-induced arthritis (AIA) analysis already improved erosion mechanism knowledge. We explored in this model the putative correlation between systemic bone loss and arthritis severity in this model.
Materials and methods We divided 20 female Lewis rats in control (CTRL, n = 10) and AIA groups (n = 10). Rat AIA was induced by Mycobacterium butyricum injection at day 0. Arthritis was monitored by several parameters, including articular index (AI), defined from 0 for no joint inflammation to 4 for extended oedema and erythema of ankle. After rat sacrifice at day 17, BMD was assessed by DXA densitometer. For each animal, right ankle, tibia, and second lumbar vertebra (LV2) were measured. Region of interest (ROI) was reproducibly defined. BMD values were mean of two distinct analyses, including ROI reset. We performed non-parametrical statistics, including Spearman correlation “r” coefficient.
Results At day 17 post-arthritis induction, ankle BMD was lower in AIA than CTRL rats (-22%; p < 0.001). BMDtibia and BMDLV2 were also decreased in AIA group (-15% and -14% respectively; p < 0.001 for both). BMD reduction was higher in ankles than in tibia and LV2 sites (p < 0.05). BMDankle values were correlated with BMDtibia values (r = 0.79; p < 0.001) and with BMDLV2 values (r = 0.77; p < 0.001). Systemic bone loss in AIA correlated with inflammation (BMDankle and AI: r = -0.91, p < 0.001. BMDtibia and AI: r = -0.82, p < 0.001. BMDLV2 and AI: r = -0.83, p < 0.001).
Conclusions Arthritis-induced bone loss mainly affects the arthritic joint. But, in addition, a systemic bone loss is present in AIA model, including tibia and LV2 sites, like in RA. Moreover, inflammation is predictable of local but also systemic bone loss. Bone loss in AIA model mimics RA bone loss regarding as local as systemic physiopathologic effects. A better understanding of bone loss mechanisms in this animal model would improve our knowledge of systemic mechanisms and pathologic bone loss in humans.