Biomedical Engineering Reference
In-Depth Information
OCP was first proposed by W. E. Brown to participate as the initial
phase in enamel mineral formation and bone formation through
subsequent precipitation and stepwise hydrolysis of OCP [184,
185, 191]. It plays an important role in
formation of apatitic
biominerals. A “central OCP inclusion” (also known as “central
dark line”) is seen by transmission electron microscopy in many
biological apatites and in some synthetically precipitated HA [192-
196]. Although OCP has not been observed in vascular calcifications,
it has been strongly suggested as a precursor phase to biological
apatite found in natural and prosthetic heart valves [197, 198]. In
surgery, OCP is used for implantation into bone defects [199-205].
For the comprehensive information on OCP, the readers are referred
to other reviews [179, 189].
in vivo
1.3.6 β-TCP
β-tricalcium phosphate (β-Ca
; the chemically correct
names are calcium orthophosphate tribasic beta or tricalcium
bis(orthophosphate) beta) cannot be precipitated from aqueous
solutions. It is a high temperature phase, which only can be prepared
at temperatures above 800°C by thermal decomposition of CDHA or
by solid-state interaction of acidic calcium orthophosphates, e.g.,
DCPA, with a base, e.g., CaO. Apart from the chemical preparation
routes, ion-substituted β-TCP can be prepared by calcining of bones
[206]: such type of β-TCP is occasionally called “bone ash”. In β-TCP,
there are three types of crystallographically nonequivalent PO
(PO
)
3
4
2
4
3−
groups located at general points of the crystal, each type with
different intratetrahedral bond lengths and angles. At temperatures
above ~1125°C, β-TCP is transformed into a high-temperature
phase α-TCP. Being the stable phase at room temperature, β-TCP is
less soluble in water than α-TCP (Table 1.1). Furthermore, the ideal
β-TCP structure contains calcium ion vacancies that are too small to
accommodate calcium ions but allow for the inclusion of magnesium
ions, which thereby stabilize the structures [207, 208]. Both ion-
substituted [209-212] and organically modified [213-215] forms of
β-TCP can be synthesized as well. The maximum substitution of mg
2+
in β-TCP was found to correspond to the Ca
(Mg(1)
,Mg(2)
)
2.61
0.28
0.11
stoichiometric formula [212]. The modern structural data
on β-TCP are available in refs. [216-218], those on Vicker's and
Knoop microhardness studies might be found if Ref. [219], while
(PO
)
4
2
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