Calcium Orthophosphates as Bioceramics - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 378

precipitating at the bone/biomaterial interface. Strange enough but
a careful seeking in the literature resulted in just a few publications
[459, 538, 635, 636], where the bioactivity mechanism of calcium
orthophosphates was briefly described. For example, the chemical
changes occurring after exposure of a synthetic HA bioceramics
to both (cell culture)

conditions were studied. A small amount of HA was phagocytozed
but the major remaining part behaved as a secondary nucleator as
evidenced by the appearance of a newly formed mineral [635].

in vivo (implantation in human) and in vitro

In vivo ,

a cellular activity (e.g , of macrophages or osteoclasts) associated

with an acidic environment were found to result in partial dissolution
of calcium orthophosphates, causing liberation of calcium and
orthophosphate ions to the microenvironment. The liberated ions
increased a local supersaturation degree of the surrounding biologic
fluids, causing precipitation of nano-sized crystals of biological
apatite with simultaneous incorporating of various ions presented
in the fluids. Infrared spectroscopic analyses demonstrated that
these nano-sized crystals were intimately associated with bioorganic
components (probably proteins), which might also have originated
from the biologic fluids, such as serum [459].
Therefore, one should better rely on the bioactivity mechanism
of other biomaterials, particularly of bioactive glasses—the concept
introduced by Prof. Larry L. Hench [49-52]. The bonding mechanism
of bioactive glasses to living tissues involves a sequence of 11
successive reaction steps. The initial 5 steps occurred on the surface
of bioactive glasses are “chemistry” only, while the remaining 6
steps belong to “biology” because the latter include colonization
by osteoblasts, followed by proliferation and differentiation of the
cells to form a new bone that had a mechanically strong bond to the
implant surface (Fig. 4.13). Therefore, in the case of bioactive glasses
the border between “dead” and “alive” is postulated between stages
5 and 6. According to Hench, all bioactive materials “form a bone-like
apatite layer on their surfaces in the living body and bond to bone
through this apatite layer. The formation of bone-like apatite on
artificial material is induced by functional groups, such as Si-OH (in
the case of biological glasses), Ti-OH, Zr-OH, Nb-OH, Ta-OH, -COOH
and -H

.

(in the case of other materials). These groups have
specific structures revealing negatively charge and induce apatite
formation via formations of an amorphous calcium compound, e.g

PO

2 4

. ,

calcium silicate, calcium titanate and ACP” [49-52].