Bones ability to tolerate loading is determined by its material composition and structural design. During growth, growth plate trabeculae condense by appositional growth forming the metaphyseal cortices (corticalization). Rapid distal radial longitudinal growth in early puberty outpaces trabecular condensation producing transitory intracortical porosity predisposing to fractures.
In adulthood, remodeling removes and replaces damaged bone. Around midlife in women, the volume of bone formed decreases and the volume resorbed increases producing a negative bone balance which produces structural decay, especially after menopause when remodeling intensity increases. Remodeling is always initiated upon an internal surface of bone. Remodeling upon haversian canal surfaces enlarges the canal focally producing intracortical porosity (in cross section) which fragments (trabecularizes) the cortex thinning it from within. Pores coalesce increasing the surface area; remodeling becomes self-perpetuating; more bone is lost from a decreasing cortical bone volume so bone loss accelerates. Remodeling upon trabeculae removes them so trabecular remodeling is self-limiting. Resorption upon the endocortical surface adjacent to marrow also thins the cortex. The porous structure loses its ability to resist cracking predisposing to fractures. Most bone loss is cortical, most occurs after 65 years and most fractures are appendicular, not vertebral.
Antiresorptives reduce remodeling intensity slowing structural decay. Structure is partly restored as refilling of resorptive cavities present at the start of treatment proceeds with the concurrent appearance of fewer new remodeling cavities. Osteons formed months earlier mineralize more completely instead of being replaced with younger less fully mineralized bone matrix. More homogenous and fully mineralized bone is more brittle it cannot absorb energy by deforming. Thus, protracted remodeling suppression reduces structural decay but may compromise bones material composition. Understanding why and how bones break requires the study of bones material composition and structure its qualities. Bone densitometry was a good beginning.
Declaration of interest: The author declares that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.
Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector
05 - 09 May 2012
European Society of Endocrinology