Biomechanics

Therapeutic agents used to treat osteoporosis reduce the incidence of vertebral and nonvertebral fractures in osteoporotic women. The antiremodeling agents, such as the bisphosphonates, prevent bone loss by suppressing the remodeling rate, perhaps increasing bone volume slightly, and increasing mineralization of the tissue. The anabolic agents, of which rhPTH(1–34) is the only one approved, accomplish this in a manner that is almost completely the opposite in terms of biological process. rhPTH(1–34) causes net bone gain by stimulating both modeling and remodeling, by increasing bone volume significantly through direct bone apposition to trabecular and endocortical surfaces, and by reducing the mean degree of tissue mineralization (a natural consequence of enhanced remodeling). Each of these treatments maintains or increases bone strength and is similarly effective at preventing fractures. However, because of their different mode of action, each has different consequences for bone matrix quality (defined here by microdamage accumulation and by the properties of mineral and collagen) and the mechanical properties of the tissue. Although bone's composite nature makes it a relatively tough material—more like fiberglass than glass—the accumulation of damage will nevertheless reduce its residual mechanical properties until the damage is repaired through remodeling. Agents that suppress remodeling are associated with both microdamage accumulation and increased mineralization. The biological importance of damage and mineralization to bone's mechanical properties is still a source of debate.