Calcium Orthophosphates as Bioceramics - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 336

In general, a characterization of calcium orthophosphate
bioceramics should be observed from many viewpoints such as the
chemical composition (stoichiometry and purity), homogeneity, phase
distribution, morphology, grain sizes and shape, grain boundaries,
crystallite size, crystallinity, pores, cracks, surface, etc. From the
chemical point of view, the vast majority of calcium orthophosphate
bioceramics is based on HA, β-TCP, α-TCP and/or biphasic calcium
phosphate (BCP, which is an intimate mixture of either β-TCP + HA
[111-126] or α-TCP + HA [8-12]) [127-136]. One should note, that
recently the concept of BCP has been extended by preparation and
characterization of biphasic TCP (BTCP), consisting of α-TCP and
β-TCP phases [137-141]. The biphasic TCP is usually prepared by
heating ACP precursors [139-141], in which the α-TCP/β-TCP ratio
can be controlled by aging time and pH value during synthesis of the
amorphous precursor [140]. Furthermore, a triphasic formulation,
consisting of HA, α-TCP and β-TCP has been prepared [142]. Further
details on this topic are available in section 1.3.14 Multiphasic and

polyphasic calcium orthophosphate compositions.
The preparation techniques of various calcium orthophosphates
have been extensively reviewed in literature [86, 143-151 and
references therein] where the interested readers are referred to.
Besides, some data are available in Chapter 1 of this topic. When
compared to both α- and β-TCP, HA is a more stable phase under
the physiological conditions, as it has a lower solubility (Table 1.1)
and, thus, a slower resorption kinetics [64, 127, 128]. Therefore, the
BCP concept is determined by the optimum balance of a more stable
phase of HA and a more soluble TCP. Due to a higher biodegradability
of the α- or β-TCP component, the reactivity of BCP increases with
the TCP/HA ratio increasing. Thus, bioresorbability of BCP

can be controlled through the phase composition [120]. Similar
conclusions are also valid for both the biphasic TCP (in which α-TCP
is a more soluble phase) and the triphasic (HA, α-TCP and β-TCP)
formulation.
As implants made of sintered HA are found in bone defects for
many years after implantation (Fig. 4.2, bottom), bioceramics made
of more soluble calcium orthophosphates [8-12, 111-126, 129-
142, 152-154] is preferable for the biomedical purposes (Fig. 4.2,
top). Furthermore, the experimental results showed that BCP had
a higher ability to adsorb fibrinogen, insulin or type I collagen than
HA [155]. Thus, according to both observed and measured bone

in vivo