Biomedical Engineering Reference
In-Depth Information
The lacunar- canalicular/collagen-hydroxyapatite porosity interface is consid-
ered next. Evidence suggests that this interface is generally impermeable. Again,
the evidence is from tracer studies. This conclusion is supported by the studies
on the alveolar bone of five-day-old rats using the small tracer microperoxidase
(MP) (2 nm) [68]. These studies clearly showed that the MP only penetrated the
unmineralized matrix surrounding the lacunae and the borders of the canaliculi
(see figure 9.13 of that study) and was absent from the mineralized matrix. Using
more mature rats, another study confirmed the failure of the small (2 nm) MP
tracer to penetrate the mineralized matrix tissue from the bone fluid compartments
[69]. Further confirmation comes from studies that observed that the tracers of
ruthenium red (MW 860, 1.13 nm in the largest dimension) and procion red (MW
300-400) did not penetrate the bone mineral porosity, but were present in the PLC
[70] (S.D. Doty, 1997, Private communication).
An important physiological consideration arises from the fact that the bone serves
as a reservoir for calcium and phosphorus, and these mineral reserves should be
connected to the circulation. Clearly, these minerals must cross the PV/PLC
interface, but should they cross the lacunar-canalicular/collagen- hydroxyapatite
porosity interface? That is, to ask, can the necessary minerals be supplied by
the bone lining cells from the bone matrix they are situated upon, or should
the osteocytes be involved in this process? Estimates show that sufficient min-
eral can be supplied by the bone lining cells, consistent with the suggestions
above that the lacunar- canalicular/collagen-hydroxyapatite porosity interface is
generally impermeable and that the permeability of the PV/PLC interface is
equal to the permeability of the PLC. However, the possibility that the interface
permeability between the PLC and the collagen- hydroxyapatite porosity might
be changed by physiological demands is worthy of consideration. The exact
method of mineral retrieval and redeposition lies at the root of many studies
[62, 70-73].
9.8
Interstitial Fluid Flow
9.8.1
The Different Fluid Pressures in Long Bones (Blood Pressure, Interstitial Fluid
Pressure, and Intramedullary Pressure)
Since the blood is encased in very thin-walled blood vessels that are contained
within the PV, the interstitial fluid pressure is less than the blood pressure. The
difference between the blood pressure and the interstitial fluid pressure is the
transmural pressure. The PV is a vast low-pressure reservoir for interstitial fluid
that can interchange that fluid with the PLC. This is the case because the lineal
dimension associated with the bone fluid PV is 2 orders of magnitude larger than
the lineal dimension associated with the PLC, and the interstitial fluid pressure in
the PV is typically lower than the blood pressure within the blood vessels.
