ADAPTATION OF BONE TO PHYSIOLOGICAL STIMULI

The ability of bone to alter its morphology in response to local physi cal stimuli is predicated upon the appropriate recruitment of bone cel l populations. In turn, the ability to initiate cellular recruitment i s influenced by numerous local and systemic factors. In this paper, we discuss data from three ongoing projects from our laboratory that exa mine how physiological processes influence adaptation and growth in th e skeleton. In the first study, we recorded in vivo strains to quantif y the locomotion-induced distribution of two parameters closely relate d to bone fluid flow-strain rate and strain gradients. We found that t he magnitude of these parameters (and thus the implied fluid flow) var ies substantially within a given cross-section, and that while strain rate magnitude increases uniformly with elevated speed, strain gradien ts increase focally as gait speed is increased. Secondly, we examined the influence of vascular alterations on bone adaptation by assessing bone brood dow and bone mechanical properties in an in vivo model of t rauma-induced joint laxity. A strong negative correlation (r(2) = 0.8) was found between increased blood flow (76%) in the primary and secon dary spongiosa and decreased stiffness (-34%) following 14 weeks of jo int laxity. These data suggest that blood flow and/or vascular adaptat ion may interact closely with bone adaptation initialed by trauma. Thi rdly, we examined the effect of a systemic influence upon skeletal hea lth. After 4 weeks old rats were fed high fat-sucrose diets for 2 yr, their bone mechanical properties were significantly reduced. These cha nges were primarily due to interference with normal calcium absorption . In the aggregate, these studies emphasize the complexity of bone's n ormal physical environment and also illustrate the potential interacti ons of local and systemic factors upon the process by which bone adapt s to physical stimuli. (C) 1997 Elsevier Science Ltd.