composite

Cements
Bone defects are often irregular and their filling can conveniently be carried out
by hydraulic cements [DRI 95]. Plaster of Paris was one of the early cements to be
used in reconstructive surgery, but its rapid degradation has favored the emergence
of phosphocalcic cements, which have been the subject matter of considerable
development in the last few years. The applications for the filling of these bone
defects are very promising, whether in the form of a molded paste placed in the
516 Ceramic Materials
surgery site or in the form of an paste hardening in situ. These products are generally
biodegradable and are progressively being replaced by bone neo-formed tissue.
Two reactive principles can be used in phosphocalcic cements. The first consists
of achieving a reaction between one or more calcium phosphates, basic in nature
(rich in calcium) and one or more calcium phosphates, acidic in nature, (rich in
phosphate). In aqueous medium, the reaction between these phases leads to less
soluble new phases, whose crystallization brings about the setting. Various
combinations have been proposed. The second principle of cement formation
consists in using only one phase, whose hydrolysis leads to a crystallized phase and
the setting. The alpha tricalcium phosphate has been proposed; more recently, an
injectable cement with an amorphous phosphate base has been developed and
marketed [KNA 98].
The setting time of a cement is generally of the order of 15 to 45 minutes for
orthopedic or dental usage. The advantage of ionic cements is that they are easy to
use. Besides, they can easily be associated with the active principle (antibiotics,
growth factors). Nevertheless, their high porosity is harmful to their mechanical
resistance after hardening and the small size of pores does not favor cell
rehabitation.
12.4.4. Composites
Ceramic-polymer composites are generally obtained by traditional techniques of
component mixing. However, original methods have been developed, notably
injectable compounds with hydroxymethyl cellulose base capable of hardening in
vivo [WEI 97]. New avenues for research opened up, aimed at the mineralization of
organized organic matrices, rich in sites for calcium phosphate nucleation (biometric
processes). In order to help the nucleation of calcium phosphates on existing
polymer matrices, various techniques have been successfully used: phosphatation is
one of the most effective technique. It is inspired by the different methods of
calcification of bone tissues where the phosphoproteins and the phosphopolipides
will be involved in the nucleation phases and growth of apatitic phosphates. The
phosphatation of cellulose helps the mineral growth of calcium phosphate on fibers
which will be without particular affinity for calcium. Other techniques such as
silanation would also be effective.
The combinations between polymethylmethacrylate (PMMA), one of the widely
used polymer cements for prostheses stabilization and fixation, and apatites can be
achieved by using a phosphate monomer combined with apatite crystals. The
composites with compound matrices obtained did not show improvement in
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properties, biological or mechanical, justifying the additional cost involved in the
process [DEL 90].
The biomimetic processes can be used in the preparation of apatite-protein
composites at low temperature, in aqueous suspension. The proteins used generally
have the property of binding strongly with calcium phosphates (casein, albumin
etc.). Crystals of calcium phosphates involved are nanocrystalline and of apatitic
structure. The evaporation of water from the suspension results in solid materials
with generally reduced porosity and having mechanical properties close to those of
polymer cements but still relatively different from those of natural tissues [