Biomedical Engineering Reference
In-Depth Information
O
which can experience a transformation from a liquid or pasty state to
a solid state and in which the end product of the chemical reactions
is a calcium orthophosphate.
The first self-setting calcium orthophosphate cement formulation
consists of the equimolar mixture of TTCP and dicalcium phosphate
(DCPA or DCPD) [96] which is mixed with water at a P/L ratio of 4:1;
the paste hardened in about 30 min and formed CDHA [97, 98]. This
highly viscous, non-injectable paste can be molded and, therefore,
is used mainly as a contouring material in craniofacial surgery. In
1990s, it was established that there were about 15 different binary
combinations of calcium orthophosphates, which gave pastes upon
mixing with water or aqueous solutions, so that the pastes set at
room or body temperature into a solid cement. The list of these
combinations is available in literature [99-101]. From these basic
systems, secondary formulations could be derived containing
additional or even non-reactive compounds but still setting like
cements [29, 58, 99, 102-116]. Concerning their viscosity, both
pasty cement formulations [117-120] and putties [121] of a very
high viscosity [122-125] are known as well.
According to the classical solubility data of calcium
orthophosphates (Fig. 1.6), depending upon the pH value of a
cement paste, after setting all calcium orthophosphate cements can
form only two major end-products: a precipitated poorly crystalline
HA or CDHA [126] at pH > 4.2 and DCPD (also called “brushite”
[127]) at pH < 4.2 [128]. However, the pH-border of 4.2 is shifted
to a higher value of pH in the real cement formulations. Namely,
DCPD might be formed at pH up to ~6, while CDHA normally is not
formed at pH below 6.5-7 (Table 1.1). The results of the only study
on an ACP cement demonstrated that this end product was rapidly
converted into CDHA [113]; thus, it also belongs to apatite-forming
formulations. Besides, there are some papers devoted to OCP-forming
cements [129-132]; however, contrary to the reports of late 1980s
[129] and early 1990s [130], in recent papers either simultaneous
formation of OCP and CHDA has been detected [132] or no phase
analysis has been performed [131]. Strong experimental evidences
of the existence of a transient OCP phase during cement setting were
found in still another study; however, after a few hours, the OCP
chemical formulations in the ternary system Ca(OH)
-H
PO
-H
2
3
4
2
Search WWH ::
Custom Search