Biomedical Engineering Reference
In-Depth Information
orthophosphate cements appears to be of paramount importance
to achieve the excellent bioresorbability, other experimental
approaches have to be developed [596].
Recently, a layered structure was designed by combining a
macroporous layer of calcium orthophosphate cement with a
strong fiber-reinforced calcium orthophosphate cement layer. The
rationale for such construction was for the macroporous layer to
accept tissue ingrowth, while the fiber-reinforced strong layer
would provide the needed early strength [597].
In the case of calcium orthophosphate concretes, future studies
could combine in one formulation porogens and biodegradable
fibers of different shapes and dissolution rates to form after
in vivo
hardening calcium orthophosphate scaffolds with sustained strength.
In such a system, one porogen is quickly dissolved, which creates
macropores to start a bone ingrowth process, while the second
type of fibers provides the required strength to the implant. After
significant bone ingrowth into the initial pores increased the implant
strength, the second set of fibers would then be dissolved to create
additional macropores for bone ingrowth [319]. Such complicated
formulations have already been developed. For example, chitosan,
sodium orthophosphate and hydroxypropylmethylcellulose were
used to render calcium orthophosphate cements fast setting and
resistant to washout, while absorbable fibers and mannitol porogen
were incorporated for strength and macropores, respectively. Both
strength and fracture resistance of this concrete were substantially
increased and approached those values for sintered porous HA
implants [598]. Turning on a bit of imagination, one might predict
development of polymeric forms of drugs (already available [599,
600]), hormones, growth factors, etc. (e.g.,
prepared by either
incorporation into or cross-linking with either water-soluble or
bioresorbable polymers). Coupled with reinforcing biodegradable
fibers and porogens, such types of “healing fibers” might be added
to calcium orthophosphate concretes, which not only will accelerate
the remedial process, but also will allow simultaneous improvement
in both their strength and injectability.
Stability (insolubility) in normal physiological fluid environment
and resorbability under acidic conditions produced by osteoclasts
appears to be among the most important
in vivo
characteristics
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