Passive degradation by biological fluids
Modes of passive degradation are not different from what we can observe in
other aqueous media. We may, for example, observe a dissolution at grain
boundaries associated with the release of particles and hydrolysis phenomena,
sometimes limited to the surface of the material. In the case of multiphase ceramics,
phenomena connected to the transfer of material between phases of different
solubility may appear. The release of particles in tissues or biological fluids may
sometimes produce inflammatory reactions which we will analyze later. Certain
plasma sprayed coatings of calcium phosphate can be associated with small
proportions of calcium oxide, whose hydration is accompanied by a large increase in
molar volume, leading to the formation of cracks and to a detachment from the
metallic substrate.
12.3.3.2. Mechanical degradation
Mechanical degradation is frequently observed in ceramics used in friction
couples. The wear debris causes an acceleration of the degradation process,
deterioration of the friction couples and above all induces a local inflammation
which could be responsible for some loosening of implants. These modes of
degradation can be observed in vitro and the results obtained confirm the very good
performance of the alumina-alumina couple. We must however note that solutions
imitating biological fluids which are used during mechanical tests (Ringer solution)
are not supersaturated with respect to apatitic calcium phosphates, like biological
fluids, and the aspects connected with the nucleation of this phase are often
neglected. It is not rare, however, to find calcium phosphate particles, on the
explanted friction surfaces of prostheses [FRA 97].
Other phenomena of degradation can be due to micromovements of implants
subjected to periodical mechanical forces (stems of hip prostheses for example).
These phenomena generally affect the ceramic coatings of some metallic prostheses.
Like in the previous case, the production of wear debris can produce inflammations
and have devastating effects on the performance of the prostheses, thus accelerating
the process.
12.3.3.3. Cell degradation
In the first few days after surgery, all implants are subjected to attacks from cells
in charge of defending the integrity of the organism (multinucleate giant cells,
510 Ceramic Materials
macrophages, monocytes, etc.). The main cell involved in this degradation is the
macrophage. They are cells which can, initially, create detrimental conditions of pH
and oxidation for materials. It has thus been shown that ceramics in bioactive
calcium phosphates deposited on metallic implants suffered a significant dissolution
in the first few days after the implantation. These phenomena however stop naturally
when these implants are not toxic in nature and do not generate particle
inflammation. However, in the opposite case, the local inflammatory reaction can
take place and deteriorate the implant and neighboring biological tissues.
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 .
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