Biomechanical effects of inflammatory diseases on bone-rheumatoid arthritis as a paradigm

Inflammatory diseases, such as rheumatoid arthritis (RA), influence the bone remodelling process and increase the risk of fracture. Bone can be viewed as a composite material comprising of two phases: the organic phase, constituted predominantly by collagen type I, and the mineral phase, composed primarily by calcium phosphate, in the form of mineral crystals. The mineral component confers bone with strength and stiffness while the organic phase is responsible for bone toughness and ductility and acts as a scaffold for the mineralisation process. The efficacy of bone as a structural material depends on the balance between these different bone components and their biomechanical properties. The main determinants of mechanical properties of bone are the amount of mineral, the collagen content, the orientation of the collagen fibers and minerals and the accumulation of microcracks in the bone matrix. In a mice model of arthritis mechanical testing has shown that arthritic femurs have a significantly lower Young's modulus, yield stress and work until ultimate stress. This evidence suggests that one of the major explanations for the increased fracture risk in RA is related to the changes on bone components induced by inflammation that result in compromised biomechanical properties.