Biomedical Engineering Reference
In-Depth Information
parameters that controlled the mechanical properties of calcium
orthophosphate concretes [424, 445]. Fiber reinforcement of porous
formulations (mannitol was used as a porogen) was discovered
as well [446]. Namely, reinforcement by aramid fibers (volume
fraction of 6%) was found to improve the properties of a calcium
orthophosphate concrete with the strength increasing threefold at
0% mannitol, sevenfold at 30% mannitol and nearly fourfold at 40%
mannitol. Simultaneously, the work of fracture increased by nearly
200 times, however the modulus was not changed as a result of fiber
reinforcement [446]. Addition of 20 wt. % of acrylamide and 1 wt.
% ammonium polyacrylate to the liquid increased the compressive
and tensile strength of α-TCP bone cement by 149 and 69% (55
and 21 MPa), respectively [447]. A positive influence of polyamide
fibers [448] and bioactive glass [449] is also known. Interestingly,
but reinforcement of calcium orthophosphate cements might be
performed by infiltration of a preset cement formulation by a
reactive polymer and then cross-linking the polymer
[450].
In the cases, when bioresorbable reinforcement fibers are used,
strength augmentation is attained at the initial stages [319-326]. For
example, the initial strength of a concrete was threefold higher than
that of the unreinforced cement control [319]. The work of fracture
(toughness) was found to increase by two orders of magnitude for
other biocomposites of calcium orthophosphate with resorbable
fibers (namely, Vicryl polyglactin 910 (Ethicon, Somerville, NJ) [320]
and a mesh of copolymer of polyglycolic and polylactic acids [324]).
When implanted
in situ
, bioresorbable fibers would provide initial
strength and then dissolve to form interconnecting macroscopic
channels, which could facilitate bone ingrowth into implants [143,
144, 319, 324]. For example, interconnected macropores were
formed in a hardened formulation at 84 days' immersion in a
physiological solution [324]. One should note that, apart from the
mechanical properties of the reinforcing materials, the structure
of the incorporated fibers, regular or random, appears to be
crucial for the resulting flexural strength and modulus of elasticity
[322]. A higher strength might help extending the use of calcium
orthophosphate cements to larger stress-bearing repairs, while the
macropores might facilitate tissue ingrowth and integration of the
hardened cement with an adjacent bone. To extend this idea further,
several types of fibers with different rates of bioresorbability might
be simultaneously incorporated into a cement formulation.
in vivo
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