Biomedical Engineering Reference
In-Depth Information
Application of biocompatible α-hydroxylated organic acids (glycolic,
lactic, malic, tartaric, and citric acids) and their calcium and sodium
salts for modification of both rheological and setting properties of
calcium orthophosphate cements is well described elsewhere [351,
352]. Besides, aqueous solutions of sodium orthophosphates [97,
211, 309, 353-356] and gelatinized starch [357] are also known
as setting time accelerators. An extensive list of the compounds,
which might be suitable as accelerators, retarders, additives, or
reactants in calcium orthophosphate cement formulations, might
be found in literature [99]. Interestingly, but in some cases a simple
thermal treatment of the initial reagents (in that particular case,
α-TCP powder) at ~ 500 °C could extend the initial part of the
setting reaction from a few minutes to a few hours hence providing
a potential approach to better control the setting process [358].
The subject of the reinforcing additives is discussed in detail
below in section 5.7
.
Concerning storage stability and shelf-life, the factors,
significantly influencing these properties for the initial dry powders
of calcium orthophosphate cements, were found to be temperature,
humidity, and a mixing regime of the powders. Various storage
conditions appeared to be effective in prolonging the stability of dry
brushite cements. In the order of effectiveness, they were ranged:
adding solid citric acid retardant > dry argon atmosphere ≈ gentle
mixing (minimal mechanical energy input) >> low temperature
[354]. Finally, the cement formulations must be sterilized before a
clinical use. A detailed description of the sterilization techniques for
calcium orthophosphate cements might be found elsewhere [359].
5.5
Bioresorption and Replacement of the
Cements by Bones
Due to the excellent bioresorbability of DCPD and CDHA, a newly
forming woven bone might substitute the hardened calcium
orthophosphate cements. For example, the implants made of
hardened BoneSource
TM
(an apatite cement) were partly resorbed
and replaced by natural bone, depending upon the size of the cranial
defect [103]. Replacement of BoneSource
by bone with a minimal
invasion of connective tissue was detected in another study, while
TM
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