PURPOSE OF REVIEW: We review the pathways, cytokines, and concepts important to the pathogenesis of bone resorption and formation in rheumatoid arthritis (RA) and spondyloarthritis (SpA).

RECENT FINDINGS: Research in bone biology has shed light on the pathogenesis of the joint destruction that occurs in RA and in peripheral SpA. However, understanding the mechanisms behind the bone formation seen in peripheral and axial SpA has been challenging. Mouse models have been used to gain an understanding of key signaling pathways, cytokines and cells regulating inflammation in these diseases. Biologic therapies directed against these targets have been developed to control both inflammation and effects on bone. Although biologic therapies improve joint inflammation in both RA and SpA, leading to a decrease in pain and improving quality of life for patients, the long-term effects of such therapies must also be evaluated by assessing their impact on structural progression. Inhibition of radiographic progression in both RA and peripheral SpA has been easier to demonstrate than in axial SpA. Here, we discuss the similarities and differences among biologic therapies as they pertain to radiographic progression.