Babylon University / College of Materials Engineering
Non-Metallic Materials Engineering Department- Ceramic Branch
Subject: Bioceramics
Dr. Shaker J. Edrees
12.2.3.3. Calcium phosphates
Calcium phosphate-based ceramics constitute, at present, the preferred
bone substitute in orthopedic and maxillofacial surgery. They are very
similar to the mineral phase of the bone, by their structure and/or their
chemical composition. Calcium phosphates usually found in ceramics
are:
– hydroxyapatite (HAP): Ca10(PO4)6(OH)2;
– tricalcium phosphate β (β TCP
These ceramics are bioactive and can be degraded to various degrees.
The stoichiometric HAP, characterized by an atomic ratio Ca/P = 1.67
and a hexagonal structure, is the nearest relative of biologic apatite
crystals. Moreover, the HAP is the least soluble and the least resorbable
calcium phosphate. When an HAP ceramic is implanted in a bone site, the
bone tissue formation is observed on its contact (osteoconduction) .
Besides, in certain conditions, calcium phosphate ceramics can induce the
formation of bone tissue in ectopic sites. HAP implants appear in the
form of dense ceramics or with variable porosity or again, in the case of
prostheses, as thin coatings deposited by plasma sprayed on a metal The
β TCP, characterized by an atomic ratio Ca/P = 1.5, is perfectly
biocompatible and bioresorbable. Like HAP, it is capable of developing a
chemical bond with the bone and to stimulate its growth, but its
resorption is more rapid. It is difficult to make pure HAP or β TCP and
biphasic materials HAP-β TCP have been developed initially by
accident and later deliberately; they combine the physicochemical
properties of each of the compounds. These can be advantageously used
to prepare materials with controlled resorption and bone substitution .
The presence of pores in materials provides anchor points for the bone
and improves the mechanical quality of the bone/implant interface; the
increase in the specific surface further encourages cell colonization and
the revascularization. While calcium phosphate-based bioceramics are
excellent materials for bone
reconstruction, they have a low mechanical strength (less than that of the
bone), not lending themselves to machining. This resistance diminishes
while porosity increases, making the utilization of very porous implants
very delicate. Even if the HAP remains the most important calcium
phosphate, from an industrial point of view, the development of low
Babylon University / College of Materials Engineering
Non-Metallic Materials Engineering Department- Ceramic Branch
Subject: Bioceramics
Dr. Shaker J. Edrees
temperature processes, particularly those concerning mineral cements and
coating on metal, have led to the utilization and emergence of other
calcium phosphates which are more reactive. lists the different calcium
phosphates used as biomaterials.
The development of calcium phosphate-based ceramics at high
temperature requires taking into account the thermal stability of these
compounds. We can distinguish two schemes of decomposition according
to the temperature: irreversible
decompositions (condensation of hydrogen phosphate ions,
decomposition of carbonate ions, of hydroxide ions, etc.) at low
temperature
(150–1,000°C)
and
reversible
decomposition
(decomposition of the apatite into TCP, TTCP and lime) at
high temperatures (T > 1,000°C).
12.2.3.4. Oxides and hydroxides
As discussed in the previous section, it would seem that the layer of
hydrated silica, formed at the surface of implanted bioglasses and
vitroceramics, plays a very important role in the formation of the neoformed apatitic crystals, hence the idea of using pure silica or titanium
hydrogels as bioactive compounds. Thus, gels of silica,
titanium or zirconia, after being subjected to heat treatment, can induce
the formation of the apatite when these are immersed in a metastable
solution analogous to biological fluids, while alumina gels are not
bioactive. The development through sol-gel processes is an interesting
method for the preparation of bioactive materials,
since it helps to obtain a product favoring the heterogeneous nucleation of
the apatite. While the silica gel is heated to a temperature greater than
900°C, the formation of the apatite is delayed [LI 93a]. It therefore
appears that the speed of rehydroxilation and the rate of hydroxyl group
on the surface of the silica gel control the formation of apatite. In the
same way, sufficiently hydrated titanium dioxide, that is with Ti- OH
groups on the surface, can join with the bone while it is placed in a bone
site.
This property offers the possibility to render titanium bioactive by a
treatment before implantation, in such a way as to form a gel or a very
hydrated layer at the surface of the implant