Biomedical ceramics and their field of use
12.2.1. Usage properties of biomedical ceramics
Ceramics have numerous uses in the field of biomaterials, mainly because of
their physicochemical properties. Their chemical inertness helps to minimize
organic reactions of the host organism and their hardness and resistance to abrasion
makes them suitable for substitution of hard tissues (bones and teeth). Some
ceramics also have excellent tribological properties and are utilized in friction
couples intended to replace malfunctioning joints. Finally, other properties
(appearance, electrical insulation) also determine certain biomedical applications.
12.2.2. Multipurpose ceramics
A number of implanted ceramics have not actually been designed for specific
biomedical applications and are used in different implantable systems because of
their properties and their good biocompatibility.
12.2.2.1. Alumina
Alumina is one of the most widely used multipurpose ceramics. It is essentially
used in orthopedics for its good tribological properties and its outstanding chemical
inertia. One of the advantages of alumina is that it is a very bad substrate for the
crystalline growth of calcium phosphates, which can alter other friction couples
[ROY 93]. It constitutes the heads of femoral prostheses and is used also in the
development of the acetabulum. Early applications raised some problems of
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mechanical strength, now very rare, and led to the creation of wear debris. These
problems were attributed to different causes: too large grain size of sintered piece,
loosening at grain boundaries, insufficient density and shaping flaws. Today, the
alumina used have evolved and most of these problems have been eliminated.
Properties of alumina used have been stringently standardized. In all cases, it refers
exclusively to rhombohedral alpha phase, and high purity. The lifespan of alumina
heads is now very often longer than the patient’s. The main cause for failure is the
wear of high density polyethylene used in the acatebulum. Alumina acatebulum may
also be used. Their characteristics are identical to those of the heads, the essential
problem here being the alumina-bone contact. Alumina is in fact considered to be a
bioinert ceramic and it does not directly bind with the bone. There is always a thin
layer of fibrous tissue between the bone tissue and alumina which can cause
osteolysis, pain and loosening. Hip prosthesis stem made totally in alumina were
also developed, but these initiatives seem unlikely to lead to development on an
industrial scale, due to inadequate mechanical properties such as a very high
brittleness and a Young’s modulus which is very different from that of bone tissue.
Alumina has also been proposed as an adhesive underlayer for bioactive coating,
generally biodegradable, on the stem of metallic prostheses [DEM 98]. The
undercoat deposition is generally obtained by plasma spraying. However, this
process leads to several phases, whose biological properties are still little known.
Nevertheless, these phases are relatively more soluble than alpha alumina and the
release of aluminum in vivo can induce bone lesions (osteomalacia) [FRA 94]. Other
uses of alumina ceramics to be mentioned are: inner ear ossicles, ocular prostheses,
electrical insulation for pacemakers, catheter orifices. Finally, alumina is also used
in numerous prototypes of implantable systems (cardiac pumps for example).
12.2.2.2. Alumino-silicates and glasses
Alumino-silicates are essentially used in dental prostheses, either as massive
ceramic, or cermet or in ceramic-polymer composites. Polymers, usually associated
with alumino-silicates, are also increasingly used in the filling of cavities replacing
amalgams suspected to have toxic effects [MJO 97]. The alumino-silicates used are
characterized by a glassy structure, sometimes incorporating crystalline phases
(vitroceramic). Particular care is taken by manufacturers in the coloring of materials
to ensure a perfect visual integration with natural teeth. Contrary to artificial teeth in
resin, the color of tooth ceramic remains stable. However, the enamel of natural
teeth has often a tendency to turn yellow with age and differences may then appear.
More important problems like bonding with biologic tissue (bone, enamel, dentine,
oral epithelium) and mechanical strength arise in the case of crowns and implants.
From an esthetic point of view, the ceramic crown is superior to a metallic one or to
those made of cermets [BAS 98], but their making entails a high level of precision,
on the part of both practitioner and prosthesist. Their brittleness also leads to
breakages, notably on molars [FUZ 98]. Regarding mechanical properties,
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microstructure plays an important role in the resistance of ceramics, especially the
size and nature of crystalline phases which are associated with alumino-silicates
(mica, alumina, zircon, etc.). Moreover, they are relatively sensitive to fatigue.
Cermets always pose problems of metal-ceramic bonding and also appear less
resistant to abrasion than ceramics. These different materials introduced in the oral
cavity, do not seem to have any incidence on the development of dental plaque and
the appearance of dental caries. However, these can cause abrasion on the dental
enamel of the opposite teeth during mastication, which can result in losses much
more important than those caused by crowns in metal alloys. The reverse can also be
observed and some ceramics could be damaged prematurely by friction and pressure
on the opposite teeth. Progress is yet to be made in this respect.
Different types of fixation of crowns or implants have been studied. Implants,
more often in titanium, are directly fixed in the jawbone and are crowned by the
artificial tooth. An osteo-conductive bioceramic coating is sometimes done, as for
artificial hips, in order to favor the integration of the implant in bone (see section
12.2.3). Dental crowns are simply attached with organic resins, the positioning is
particularly crucial during the setting period and could determine the longevity of
the substitute.
Alumino-silicate glasses were also proposed as substitutes for bones; their
chemical composition is then adapted for making these materials bioactive and this
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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