Biomedical Engineering Reference
In-Depth Information
described in literature [329, 362, 376, 391, 392]. Recent histological
and mechanical evaluation of self-setting calcium orthophosphate
cements in a sheep vertebral bone void model is available elsewhere
[393]. The interested readers are also advised to get through a paper
on the
biodegradation of brushite cements by a macrophage
cell-line [128].
To conclude this part, one should note that calcium
orthophosphate cements are able to provide short-term biologically
desirable properties and then be replaced by a new bone, which
is very important [394]. The growth rate of a newly forming bone
depends on age, sex and general metabolic health of the recipient
as well as on the anatomic site, porosity, bulk site, crystallinity,
chemical composition (brushite or apatite), particle sizes and
P/L ratio of the cements. Considering all these factors, it might
take from 3 to 36 months for different calcium orthophosphate
cements to be completely resorbed and replaced by bones [202].
However, additional sound scientific data to determine the exact
degree of biodegradability for calcium orthophosphate cements are
still needed, viz. animal studies performed in a critical-size defect
model. One must stress that the rate of cement resorption should
be balanced with the rate of new bone formation to avoid collapse
at the fracture site, which might occur if the resorption is too fast.
Interestingly, but to advance calcium orthophosphate cements as
bioabsorbable bone replaceable materials, it is essential to utilize
the patient's own blood in combination with the cements [395].
in vitro
5.6
The Mechanical Properties
As in most clinical applications self-setting calcium orthophosphate
formulations are applied in direct contact with human trabecular
bones, it may be stated as a mechanical requirement that the strength
of the formulations must be at least as high as that of trabecular bones,
which is close to 10 MPa [396]. Three-dimensional (3D) complex load
is applied during the orthopedic and dental applications because of
a combination of different forces that may include bending, torsion,
tension and compression. Unfortunately, calcium orthophosphate
cements are strong enough at compression only [195]. In theory,
after setting, they can reach the mechanical properties comparable
to those of calcium orthophosphate blocks with the same porosity.
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