Toxicity of constituting elements
Several elements present in ceramics, either as impurity or as constitutive
elements, may cause toxicity. It is customary to distinguish essential trace elements,
oligoelements necessary for the life, from toxic trace elements. It appears, however,
that all these elements have different toxic limits. The latter are difficult to
determine in an absolute manner and in the case of implanted substances, the local
effect on tissues should be distinguished from the remote effect of the implant on
other tissues or organs (Table 12.2).
The toxicity of alumina, lead, arsenic, mercury and cadmium are well
documented. These studies however often relate to the elements ingested in
nutriments. Very few studies have been made on the toxicity of elements associated
with implants and in their immediate neighborhood and the standards in vigor are
not often based on any experimental data.
The solubility of phases and their reactivity in a biological environment is
evidently a determining factor. Thus, for example, alpha alumina implants have
never shown toxicity associated with aluminum, despite the large quantities of
aluminum introduced into the organism, while degradable glasses containing smaller
proportions of aluminum may considerably impair the formation of bone tissue [NIZ
99]. Similarly, fluorapatite, which is very insoluble, does not induce any negative
reaction despite the toxicity of fluorine. As far as dental amalgams are concerned,
the poisoning effect of mercury appears very controversial. The toxicity of elements
depends equally on their degree of oxidation and on their state of complexation.
Thus, the arsenates, eventually present as trace elements in calcium phosphates, are
much less toxic than arsenites; likewise, certain organic derivatives of phosphorus
are highly toxic, while orthophosphates are harmless.
Lastly, high temperature heat treatments (sintering, plasma spraying) tend to
reduce the strength of volatile impurities in ceramics, particularly mercury. It can
also help the segregation of certain impurities at grain boundaries or on the surface
and increase their bioavailability.
Bioceramics 511
Elements Biological effect
Mg Essential element, well regulated in healthy substance, may affect locally the
cell functioning; inhibits the crystal growth of bone mineral.
Al Neurological effects of the element are not well established. At low doses,
it helps cell multiplication. At high doses, it inhibits cell development, alters
mineralization and inhibits crystal growth of the bone mineral.
Si Essential element, may induce a cell toxicity in the form of isolated silicate
ions, seems to help the formation of the neo-formed layer of apatite.
As
Pb,
Hg,
Cd
These trace elements are relatively spread out and can be found in most
ceramics. They can have important neurological consequences and seriously
disturb cell function.
Maximum acceptable concentrations are usually standardized.
Ti Does not seem to have any important toxic effect.
Metal and oxide particles can be toxic.
Zr Zirconium has a higher toxicity, at equal dose, than aluminum (test on mice).
It will set essentially in the ovaries and to a lesser degree
in the bones and the lungs.
Table 12.2. Toxicity of elements encountered in inorganic biomaterials
12.3.3.5. Inflammatory reaction connected with particles
Insoluble particles, irrespective of their nature, can provoke serious
inflammatory reactions. These phenomena can be compared to those provoked by
crystal coating disease: arthritis (whitlockite, apatite, calcium pyrophosphates), gout
(uric acid), calcification of tendons (calcium phosphates). The inflammatory reaction
is generally a periodic phenomenon which can however seriously damage the tissues
and the biomaterials and eventually lead to the loosening of the prosthetic device.
The predominant factor in the inflammatory phenomena connected with particles is
the size of the latter. Particles smaller than 50 micrometers are phagocyted by the
macrophages for possible degradation in the lysosomes. Phagocytose activate
macrophages which in turn synthesize various cytokines, growth factors which are
active on the effector cells. Among these are immunizing cells with remote action or
cells with local action like the osteoclasts which, once activated, are responsible for
massive bone destruction. The inflammatory phenomenon is all the more
accentuated the finer the size of the particles. Other factors are, however, involved,
such as chemical composition, crystallographic nature, crystal morphology,
composition of surface, the adsorption and desorption of proteins. Particles with
prominent angles, for example, appear more phlogogenic than those with a globular
morphology. However, the majority of studies on inflammatory processes have been
carried out on soft tissues and generally the inflammatory responses of bone tissues
appear more limited.
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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