Biomedical Engineering Reference
In-Depth Information
5.2.1 Bone Growth Factors
It has been reported that growth factors such as PDGF, IGF, bone morphoge-
netic protein (BMP), and TGF-β play an important role in bone formation and
remodeling [27,28]. These growth factors are found in considerable quantities
in bone matrix. Normally, they are retained in osteocytes. Once the osteo-
cytes are resorbed, the growth factors can be released into bone fluid and
can stimulate osteoblasts to refill resorption cavities. Unconsumed growth
factors remaining in the bone fluid can be transported to the bone surface
to deposit new bone material on it if the strain-induced fluid flow is strong
enough to overcome the resistance. Experiments have shown that a pulsed,
extremely low-frequency electromagnetic field can stimulate the multiplica-
tion of growth factors [29-31], thus indirectly accelerating the remodeling
process via growth factors.
5.2.2 Electrical Signals in Bone Remodeling
Since 1957, when some bone tissues were found to have a piezoelectric
effect [32], the electric properties of bone material have been widely inves-
tigated. It is believed that electrical signals in bone tissue play an impor-
tant role in the bone modeling and remodeling processes [2,7,8,12,33]. The
electrical signals that allow bone to adapt to its environment most likely
involve strain-mediated fluid flow through the canalicular channels. Fluids
can only be moved through bones by cyclic loading. The electrical signals
are generated in two ways: by piezoelectricity and by streaming potentials.
Streaming potentials derive from the bone fluid flow, which is generated by
bone material deformation and blood circulation and is proportional to the
strain rate.
Evidence has shown that an increase in venous pressure results in an
increase in the passage of fluid from capillary to bone matrix [34]. Increased
extravascular perfusion could be a factor in increasing periosteal bone for-
mation. This flux of fluid may increase streaming potentials in bone, act-
ing as a signal to bone cells to increase bone formation. Experiments by
Lanyon [35] showed that cyclic loading induces more bone adaptation than
static loading. Turner [36] performed experiments on cyclic loading of bone
and determined that the stimulus for bone remodeling is proportional to
the applied strain rate magnitude. Strain rate magnitude can be directly
deduced from strain magnitude and frequency of loading. These phenom-
ena can also be explained by bone electricity. It can thus be seen that both
piezoelectricity and streaming potentials have relations to strain.
As for electrical signals generated by piezoelectricity, active research in
the area of tissues such as living bone and collagen has shown that these
materials are piezoelectric and that the piezoelectric properties of bone can
also enable bone tissues to generate electrical signals that are proportional
to the strain rate [32]. All these factors predict that the magnitude of the
