Bone Involvement in Rheumatoid Arthritis and Spondyloartritis: An Updated Review

SIMPLE SUMMARY: Chronic inflammatory arthritis, such as rheumatoid arthritis (RA) and spondyloarthritis (SpA), often have a significant impact on bone tissue, where bone is not just a passive target but actively contributes to the disease progression. This review explores the pathogenic mechanisms involving bone, highlighting the complex molecular interactions between bone cells and the immune system, a field known as osteoimmunology. It discusses the unique processes of bone erosion and systemic bone loss in RA and SpA, as well as abnormal bone formation in SpA. ABSTRACT: Several rheumatologic diseases are primarily distinguished by their involvement of bone tissue, which not only serves as a mere target of the condition but often plays a pivotal role in its pathogenesis. This scenario is particularly prominent in chronic inflammatory arthritis such as rheumatoid arthritis (RA) and spondyloarthritis (SpA). Given the immunological and systemic nature of these diseases, in this review, we report an overview of the pathogenic mechanisms underlying specific bone involvement, focusing on the complex interactions that occur between bone tissue’s own cells and the molecular and cellular actors of the immune system, a recent and fascinating field of interest defined as osteoimmunology. Specifically, we comprehensively elaborate on the distinct pathogenic mechanisms of bone erosion seen in both rheumatoid arthritis and spondyloarthritis, as well as the characteristic process of aberrant bone formation observed in spondyloarthritis. Lastly, chronic inflammatory arthritis leads to systemic bone involvement, resulting in systemic bone loss and consequent osteoporosis, along with increased skeletal fragility.