Zirconia
Zirconia doped with yttrium oxide has also been proposed as a substitute for
alumina in the heads of osteoarticular prostheses. Important developmental studies
have been made [CAL 95] and these heads have been commercialized. The main
advantages of zirconia compared to alumina are a greater failure strength, mainly
bending strength, as well as a good resistance to fatigue [DRO 97]. These properties
make the use of prostheses heads of very small dimensions possible, thus reducing
the wear debris. Besides, zirconia has a better coefficient of friction and a better
wear resistance, despite the quite controversial results. One of the unknowns is the
role of zirconia in the nucleation of calcium phosphates from supersaturated body
fluids. Moreover, as we have mentioned, alumina has the advantage of a longer use
and currently gives satisfactory results.
12.2.2.4. Vitreous carbon and diamond carbon
Vitreous carbon has several interesting physicochemical and biological
properties: it is light, resistant to wear and haemocompatible. It is essentially used
for making cardiac valves and replaces natural valves taken from pigs which have a
tendency to calcify and have a more limited life. Essential problems which remain
Bioceramics 497
are the formation of thromboses and bleeding due to the degradation of the junction
between the prosthesis and the artery. Mechanical valves appear, however, to be
tolerated in the long term [PET 99].
Diamond is an interesting coating from a biological point of view: it does not
induce any cytotoxic or haemolytic effects and can be used for various vascular
applications or in cardiac surgery [DIO 92].
12.2.2.5. Other ceramics
A number of other ceramics have been subjected to biomedical tests for
implantation, without currently being developed industrially. Among these ceramics,
we can cite silicon carbide, titanium nitrides and carbides, and boron nitride. TiN
has been suggested as the friction surface in hip prostheses. While cell culture tests
show a good biocompatibility, the analysis of explants shows significant wear,
related to a delaminating of the TiN layer [HAR 97]. Silicon carbide is another
modern day ceramic which seems to provide good biocompatibility and can be used
as bone implant [SAN 98].
12.2.3. Ceramics for specific uses
Ceramics for specific uses, in addition to their traditional properties, have
biological activity. We thus refer to bioactive ceramics. These ceramics are
essentially used as bone replacements and today form part of the daily practice of
orthopedic, maxillofacial and plastic surgeons. These are used in case a loss of bone
substance (tumor, important trauma, infection, etc.) requires to be filled. According
to the type and shape of the defect to be filled, the location of the implant and
mechanical stresses, different types of bioceramics with varying biological
properties are available. Biodegradable ceramics will be resorbed and replaced by
reconstructed tissue, whereas non-biodegradable ceramics are intended for a
permanent implantation. Very often, synthetic ceramics compete with natural
materials and it seemed more interesting to start this section with a description of the
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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