Fluid pressure relaxation depends upon osteonal microstructure: modeling an oscillatory bending experiment

When bone is mechanically loaded, bone fluid flow induces shear stresses on bone cells that have been proposed to be involved in bone's mechanosensory system. To investigate bone fluid flow and strain-generated potentials, se veral theoretical models have been proposed to mimic oscillatory four-point bending experiments performed on thin bone specimens. While these previous models assume that the bone fluid relaxes across the specimen. thickness, we hypothesize that the bone fluid relaxes primarily through the vascular p orosity (osteonal canals) instead and develop a new poroelastic model that integrates the microstructural details of the lacunar-canalicular porosity, osteonal canals, and the osteonal cement lines. Local fluid pressure profi les are obtained from the model, and we find two different fluid relaxation behaviors in the bone specimen, depending on its microstructure: one assoc iated with the connected osteonal canal system, through which bone fluid re laxes to the nearby osteonal canals; and one associated with the thickness of a homogeneous porous bone specimen ( similar to 1 mm in our model), thro ugh which bone fluid relaxes between the external surfaces of the bone spec imen at relatively lower loading frequencies. Our results suggest that in o steonal bone specimens the fluid pressure response to cyclic loading is not sensitive to the permeability of the osteonal cement lines, while it is se nsitive to the applied loading frequency. Our results also reveal that the fluid pressure gradients near the osteonal canals (and thus the fluid shear stresses acting on the nearby osteocytes) are significantly amplified at h igher loading frequencies. (C) 1999 Elsevier Science Ltd. All rights reserv ed.