Biomedical Engineering Reference
In-Depth Information
at the microstructural level and in the anatomical structures that determine the
flow. In [1], the resistance to fluid flow and the source of the SGP reside in the
PLC, that is, to say, in the fluid annulus that surrounds the osteocytic processes,
that is, the space between the cell membrane of the osteocytic process and the
walls of the canaliculi- the space containing the glycocalyx or fiber matrix. In [2],
the presence of the glycocalyx increases the SGPs and the hydraulic resistance to
the strain-driven flow. The increased SGP matches the phase and amplitude of the
measured SGPs. In the model [83], this fluid resistance and SGP are explained by
assuming that an open, continuous small pore structure (
≈
16 nm radius) exists
in the mineralized matrix.
Experimental evidence indicating that the collagen-hydroxyapatite porosity of
the bone mineral is unlikely to serve as the primary source of the SGP is
obtained from several sources, including the estimates of the pore size in the
collagen-hydroxyapatite porosity [58] and the impermeability of the lacunar-
canalicular/collagen- hydroxyapatite porosity interface described in Section 9.7. It
is thought that this impermeability is inconsistent with the suggestion of Mak
et al
.
[90] that both the PLC and the collagen- hydroxyapatite porosity are sources of the
experimentally observed SGPs. It was noted by Mak
et al
. [90] that, since there were
many assumptions associated with the physical constants in their model, their
study should be considered as a parametric study of their model. For example,
the authors assume a value for the interface permeability between the PLC and
the collagen-hydroxyapatite porosity that appears quite high in view of the tracer
studies summarized in Section 9.7.
9.8.4
The Poroelastic Model for the Cortical Bone
Poroelasticity is a well-developed theory for the interaction of fluid and solid
phases of a fluid-saturated porous medium. It is widely used in geomechanics,
and it has been applied to bones by many authors in the last 40 years. A review
of the literature related to the application of poroelasticity to the bone fluid is
presented in [7]. This work also describes the specific physical and modeling
considerations that establish poroelasticity as an effective and useful model for
deformation-driven bone fluid movement in the bone tissue. The application of
poroelasticity to bone differs from its application to soft tissues in two important
ways. First, the deformations of the bone are small while those of soft tissues are
generally large. Second, the bulk compressibility of the mineralized bone matrix
is about seven times stiffer than that of the fluid in the pores, while the bulk
compressibilities of the soft tissue matrix and the pore water are almost the same.
Poroelasticity and electrokinetics can be used to explain SGPs in a wet bone. It is
noted that SGPs can be used as an effective tool in the experimental study of local
bone fluid flow, and that the knowledge of this technique will contribute to the
answers for a number of questions concerning bone mineralization and the bone
mechanosensory system.
