Biomedical Engineering Reference
In-Depth Information
superplasticizers, i.e. 50 vol. %, allowed for a significant increasing
of the P/L ratio from 3.13 to 3.91 g/ml, without affecting the
maximum strength and/or the workability of the cement [407]. This
effect was explained by an inhibiting effect of the aforementioned
additives on the crystal growth kinetics of newly forming crystals of
calcium orthophosphates, which resulted in smaller crystallites and,
hence, a denser and more interdigitated microstructure. However,
the increased strength was attributed mainly to the polymer's
capacity to bridge between multiple crystallites (thus forming a
more cohesive composite) and to absorb energy through a plastic
flow [406]. Other factors affecting strength are the materials used
in the solid phase, particle sizes, incorporation of fillers (see section
5.7. Reinforced formulations and concretes
for details), the P/L ratio
and various additives to the liquid phase [106].
As presence of pores simplifies for cracks to run throughout the
ceramic mass, the mechanical properties of the hardened cements
were found to decrease exponentially with the porosity increase
[408]. In theory, calcium orthophosphate cements can be made with
almost any porosity. However, for most commercial formulations,
the pores are of 8-12 μm in diameter and, after the cement is set,
porosity occupies about 40-50% of its volume [409]. Pressure
can be applied to reduce the porosity of hardened cements [144,
410, 411]. Usually, the pore dimensions in hardened cements are
too small to allow a fast bone ingrowth. Thus, there is a lack of
macroporosity. Besides, unless special efforts have been performed,
the available macroscopic pores are not interconnected. Due to these
reasons, after injection, osteoclastic cells are able to degrade the
hardened cements layer-by-layer only, starting at the bone-cement
interface throughout its inner part (in other words, from the outside
to the inside). This is the main drawback of the classical cement
formulations when compared to calcium orthophosphate ceramic
scaffolds with an open macroporosity [197, 198].
Since compression strength is reciprocally proportional to
porosity [371], the former might be adjusted by varying the P/L
ratio in the hardening mixture. Elevated compression strength
would be applicable in cranioplasty for regions requiring significant
soft-tissue support. For smaller bone defects, such as root canal
fillings, low-compression cements might be used [137]. Concerning
the tensile strength of calcium orthophosphate cements, as a rule of
thumb, it appears to increase two-fold with each 10 vol. % decrease
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