Bone fluid flow is the major signal translating strain from mechanical stress on bone to bone cells. Realization of this mechanism was a major paradigm shift in bone physiology from totally solid mechanical model. While the translation medium has been narrowed to fluid shear stress and streaming potentials, the means by which forces driving bone fluid convection are not yet clear. An understanding of the distribution of such forces is critical for designing scaffold-type bone implants which depend upon fluid percolation to nourish passenger stem cells. Percolation driven by compression and tension generated in the sponge-like bone matrix during bone bending has been modeled. But two other sources of fluid driving forces have been inadequately addressed: 1) stresses due to weight shifts during locomotion and 2) pressures generated in circulating blood by heartbeat and skeletal muscle contraction during exercise. The latter may markedly influence capillary filtration which delivers fluid to 1) the tissues surrounding the implant and 2) the pores of the scaffold. There is now evidence that 1) resting blood pressure alone is sufficient to generate convective capillary filtration, 2) contraction of the muscle pump enhances capillary blood pressure and 3) exercise increases these effects.
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