Experimental elucidation of mechanical load-induced fluid flow and its potential role in bone metabolism and functional adaptation

Several researchers have developed theories implicating some manifestation of mechanical forces such as stress, strain, and strain energy density for the initiation of cellular processes associated with functional adaptation. The mechanisms underlying dynamic bone growth and repair in response to mechanical stimuli, however, are not fully understood. Load-induced fluid flow has been postulated to provide a mechanism for the transmission of mechanical signals leg, via shear stresses, enhancement of molecular transport, or electrical effects) and the subsequent elicitation of a functional adaptation response in bone. Although indirect evidence for such fluid flow phenomena can be found in the literature pertaining to strain-generated potentials, experimental studies are inherently difficult. This motivated the authors to develop theoretical as well as ex vivo, in vitro, and in vivo experimental methods for the study of transport processes and fluid flow within bone under well-controlled mechanical loading conditions. By introducing tracer substances such as disulphine blue, procion red, and microperoxidase into the experimental system, transport and fluid flow could be visualized at tissue, cellular, and subcellular levels, respectively. Based on these studies, it could be shown that load-induced fluid flow represents a powerful mechanism to enhance molecular transport within compact bone tissue. Furthermore, the distribution of transport-elucidating tracers is a function of mechanical loading parameters as well as the location within the cross-section of the bone cortex.