Biomechanical background for a noninvasive assessment of bone strength and muscle-bone interactions.

New concepts and methods of study in bone biomechanics defy the prevailing idea that bone strength is determined by a systemically-controlled "mineralized mass" which grows until reaching a peak and then is lost at individually-specific rates. In case of bones, "mass" represents actually the substratum of a structure, the stiffness of which does not depend on the mass, but on the intrinsic stiffness and the spatial distribution of the mineralized material. A feed-back system called "bone mechanostat" seems to orient the osteoblastic and osteoclastic processes of bone, modeling and remodeling, according to the sensing by osteocytes of strains caused in the structure by mechanical usage of the skeleton, in specific directions as determined principally by the customary contractions of regional muscles and impact forces. The endocrine-metabolic systems, crucial for the normal skeletal development, modulate the work of osteocytes, blasts and clasts in a systemic way (i.e., not related to a specific direction of the stimuli). Therefore, they tend actually to interact with, rather than contribute to, the biomechanical control of bone structure. Furthermore, no feed-back loop enabling a cybernetic relationship of those systems with bone is known. Instead of passively letting hormones regulate their "mass" in order to optimize their strength, bones would actively self-regulate their architecture following an anisotropic pattern in order to optimize their stiffness (the only known variable to be ever controlled in the skeleton) and strength "despite of" the endocrine systems. Three practical questions derive from those ideas: 1. Osteoporoses are not "intense osteopenias" but "osteopenic fragilities". 2. The diagnosis of osteopenia could be solved densitometrically; but that of bone fragility is a biomechanical problem which requires auxiliary resources for evaluating the stiffness and the spatial distribution of the mineralized material. 3. Osteopenias and osteoporoses should be on time evaluated as related to the mass or strength of the regional muscles, respectively, in order to differentiate between the "primary" (intrinsic lesion of the mechanostat) or "secondary" (systemic) etiologies and the biomechanical origin (disuse) in each case, with important therapeutic implications.