Biomedical Engineering Reference
In-Depth Information
Setting of calcium orthophosphate cements is a continuous
process that always starts with dissolution of the initial compounds
in an aqueous system. This process supplies ions of calcium and
orthophosphate into the solution, where they chemically interact
and precipitate in the form of either the end products or precursor
phases, which causes the cement setting [138-140]. This was
confirmed by Ishikawa and Asaoka, who showed that when TTCP
and DCPA powders were mixed in double-distilled water, both
powders were dissolved. The dissolved calcium and orthophosphate
ions in the solution were then precipitated in the form of CDHA on
the surface of the powders [141]. The precipitate can be either a gel
or a conglomerate of crystals. Therefore, the hardening mechanism
is either a sol-gel transition of ACP [113] or entanglement of the
precipitated crystals of other calcium orthophosphates (Fig. 4.9) [29].
For example, for the classical Brown-Chow cement formulation, after
the initial setting, petal- or needle-like crystals enlarge epitaxially and
are responsible for the adherence and interlocking of the crystalline
grains, which result in hardening. After ~ 2 h, the newly formed
crystals become rod-like, resulting from higher crystallinity with the
observation of more material at the inter-particle spaces. During this
period, the cement setting reaction proceeded at a near-constant
rate, suggesting that the reaction rate was limited by factors that are
unrelated to the amounts of the starting materials and the reaction
products present in the system. Such factors could be related to the
surface area of DCPA or TTCP or to the diffusion distances over which
the calcium and orthophosphate ions migrate in order to form CDHA
[142-144]. At ~24 h, the crystals are completely formed, being very
compacted in some areas of high density, and well separated in areas
with more porosity [106, 111, 112].
The chemical reactions occurring during setting of calcium
orthophosphate cements depend on their chemical composition.
However, it can be stated that only two major chemical types of the
setting reaction are possible. The first type occurs according to the
classical rules of the acid-base interaction, i.e. a relatively acidic
calcium orthophosphate reacts with a relatively basic one to produce
a relatively neutral compound. The first cement by Brown and Chow
is a typical example of this type because TTCP (basic) reacts with
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