Biomedical Engineering Reference
In-Depth Information
9
Interstitial Fluid Movement in Cortical Bone Tissue
Stephen C. Cowin
9.1
Introduction
Blood and interstitial fluid have many functions in a bone. They transport nutrients
to, and carry waste from, the bone cells (osteocytes) buried in the bony matrix.
They are involved in the transport of minerals to the bone tissue for storage and the
retrieval of those minerals when the body needs them. Interstitial flow is considered
to have a role in bone's mechanosensory system. Bone deformation causes the
interstitial flow over the cell processes of the osteocyte creating a drag on the fibers
that connect the cell; the drag force created by the flowing interstitial fluid is sensed
by the cell [1-4]. A full physiological understanding of this mechanosensory system
will provide insight into the following three important clinical problems: (i) how to
maintain the long-term stability of bone implants, (ii) the physiological mechanism
underlying osteoporosis, and (iii) how to maintain bones in long-duration space
flights and long-term bed rest.
Since one purpose of this work is to describe how these fluid systems work,
consideration is limited to cortical bone in the mid-diaphysis of a long bone.
Although most of what is described is also applicable to the bone tissue at other
anatomical sites, the discussion is more concise and direct if this limitation is
stipulated.
The majority of the motive force for the blood flow is from the heart, but the
contraction of muscles attached to the bone and the mechanical loading of bone also
contribute to this motive force. The majority of the motive force for the interstitial
fluid flow is due to the mechanical loading of bone, but the contraction of muscles
attached to bone and the heart also supply some of its motive force. The influence
of the mechanical loading of a whole bone on the fluid system's maintenance of
the bone tissue is critical. The fluid flow resulting from the mechanical loading
is modeled by the theory of poroelasticity. This theory models the interaction of
deformation and fluid flow in a fluid-saturated porous medium. The theory was
proposed by Biot [5, 6] as a theoretical extension of soil consolidation models
developed to calculate the settlement of structures placed on fluid-saturated porous
soils. The theory has been widely applied to geotechnical problems beyond soil
