12.2.3.1. Natural materials
Different types of natural substitutes for bone tissue are available on the market,
generally of animal origin. These are subjected to physical, chemical or biochemical
processes before utilization as biomaterials. Calcium carbonates, particularly those
produced by marine organisms (coral, mother of pearl, etc.), have been used for
many years as substitutes for bone.
498 Ceramic Materials
The idea that coral can replace defective parts of bone comes from the similarity
in the structure of some coral skeletons with cancellous bone matter allowing
colonization by cells and blood vessel penetration. The exoskeleton of coral polyps
constitutes blocks of calcium carbonates with regular and interconnected porosity,
according to a structure specific for each species. After selecting, cleaning and
shaping, these materials can be implanted in bone locations, to serve as a framework
for the new bone tissue. They are degraded due to a carbonic anhydrase, thus leaving
place for newly synthesized bone tissue [GUI 95]. These materials are
biocompatible and their advantage essentially lies, on the one hand in their open
porosity which facilitates bone colonization and on the other hand, on their rapid
resorption due to the good solubility of calcium carbonates and their enzymatic
degradation. Despite good mechanical strength, this is not sufficient to allow their
use in bones subjected to high mechanical stresses (load bearing bones). Moreover,
their structure is fixed with the considered species and their chemical composition is
not well controlled, particularly with respect to trace elements.
Mother of pearl, also composed of calcium carbonate and an organic matrix, has
been proposed as a substitute for bone. Mother of pearl powder implanted in a bone
defect has shown behavior similar to that of a coral [LOP 98].
12.2.3.2. Bioglasses
Since the development of Bioglass® by L. Hench et al. [HEN 71] in the
beginning of the 1970s, several types of glasses and vitroceramics, in particular
those belonging to the family Na2O-CaO-SiO2-P2O5, have shown a certain
capability of adhering to the bone. However, this real chemical bonding with the
bone tissue occurs only for a narrow range of SiO2 content (42–52%). For SiO2
contents higher than 60% [HEN 99], the bioglass appears isolated from the bone by
a non-adhering fibrous capsule leading to failure of the implant.
Fundamental studies bearing on the bioglass-bone bonding mechanism show that
the groups Si-OH on the surface of these materials induce the formation of a layer of
apatite analogous to bone minerals, guaranteeing the durable integration of
biomaterials [HEN 91]. Moreover, the slow dissolution of silicates would improve
cell proliferation and the formation of an osteoid matrix. Furthermore, the release of
calcium and phosphorus contained in bioglasses encourages the heterogenous
nucleation of bone minerals in the osteoid matrix, thus forming the new bone very
rapidly.
Bioglasses constitute a large variety of materials depending on their composition
and structure. However, the implantation of bioglass in an area of high mechanical
stresses cannot be considered on account of their brittleness. To overcome this
Bioceramics 499
disadvantage, these glasses can be heat treated so as to obtain a vitroceramic, or
even to use this bioglass in the form of a deposit on a metallic substratum [BRI 97].
Thus, considerable research was directed towards the development of multiphase
vitroceramic materials, so as to reinforce the mechanical properties of bioglasses.
All these materials are available in different forms: massive, deposit, powder or
composite. Vitroceramics are first produced as glass and then transformed into
crystallized ceramics by heat treatment. The vitreous stage offers possibilities to
mould complex forms. The next stage of crystallization allows a fine microstructure
with little or no porosity to be obtained, which confers on the material a good
mechanical strength against impact, by virtue of the relaxation of stresses around the
pores. With the crystallization process still incomplete, the glassy part fills space
between grain boundaries, so as to create a structure without pores.
A biphasic material with excellent mechanical properties, called vitroceramic
A/W and made up of an apatitic phase (Ca10(PO4)6(OH, F)2), a wollastonite phase
(CaO SiO2) and a residual vitreous phase MgO-CaO-SiO2 is clinically used,
particularly in vertebral reconstructive surgery [KOK 85; YAM 88].
Some bioglasses contain aluminum. However, these are not well developed and
the risks of alteration of bone mineralization have been stated
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|