Bone mass, bone strength, muscle bone interactions, osteopenias and osteoporoses

JOSÉ LUIS FERRETTI; GUSTAVO ROBERTO COINTRY; RICARDO FRANCISCO CAPOZZA; HAROLD FROST

MECHANISMS OF AGEING AND DEVELOPMENT

ELSEVIER IRELAND LTD

Lugar: Londres; Año: 2003 vol. 124 p. 269 - 279

0047-6374

   Clasically, bone densitometrists regard the skeleton as a systemically regulated “mass” of mineralized material that borns, grows, reaches a more or less high peak, and then declines faster or slower as to develop a correspondingly high or low fracture risk. This misconception tends now to be replaced by a biomechanical description of the skeleton as a complex structure, biomechanically regulated and systemically disturbed, the strength of which depends on the intrinsic stiffness (material properties) and the spatial distribution (architectural properties) of the mineralized tissue.       The biomechanical feedback system involved (bone “mechanostat”) would not control bone “mass” to optimize bone strength; it would rather control bone architecture (through a modulation of bone modeling and remodeling) in order to optimize bone stiffness. The natural stimuli for the bone mechanostat are the customary strains of bone tissue (sensed by osteocytes) that are induced by gravitational forces and, more importantly, the contractions of regional muscles.    Thus, the development of any bone-weakening disease should be related to either  1. an intrinsic illness of the system (primary disturbs of bone cells),  2. a lack of mechanical stimulation (disuse-indiced bone losses), or  3. a systemically-induced shift of the system´s setpoint (secondary bone diseases).    The analysis of muscle-bone interactions, crucial for achieving the corresponding differential diagnosis, can be based either  1. on the densitometric determination of bone and muscle masses that would provide an anthropometric diagnosis of osteopenia (not osteoporosis because no extrapolations to bone strength can be made this way) or  2. on the cross-sectional analysis of bone structure and muscle strength provided by bone tomography, magnetic resonance or other techniques that could afford a “true”, biomechanical diagnosis of osteoporosis. .