Implant stabilization
The introduction of a chemically inert foreign body in a living organism gives
rise to a series of reactions leading to the formation of a fibrous encapsulating tissue.
This fibrous tissue isolates the foreign body which could not be destroyed and
ensures at the same time the joining between the healthy tissue and the implant. This
primary form of biointegration of the implant is considered to be of relatively poor
quality, notably for the bone tissue where this layer allows a relative mobility of the
implant and the existence of micromovements, eventually associated with the
phenomena of abrasion and with an inflammatory process. The existence of
micromovements increases the thickness of the encapsulating fibrous layer [HOL
92; PIL 95]. Movements of low amplitude (less than 50 micrometers) do not appear
to have any effect. Above 200 micrometers, on the contrary, we observe the
formation of a thick fibrous tissue. Other than the surgical skill, various
morphological factors especially determine the mechanical anchoring of the implant:
a porous or rough surface favors mechanical stabilization of the implant. Likewise,
partially biodegradable surfaces resorbed in an irregular manner may be considered
as favorable to mechanical integration.
12.3.1.2. Ceramic-tissue bonding
The integration of the implant can also be ensured by the interactions of a
physicochemical nature with the tissue. These phenomena have been more
particularly studied for ceramics used in orthopedics. Interactions with the mineral
part have been more particularly described; those with the collagenic part of the
bone are not known, despite the fact that they may play an equally important role. In
the case of implants of apatitic structure, it has been shown by high resolution
electron microscopy that there exists a continuity between apatite crystals of
ceramics and those of bone tissues [BON 91]. These interactions of epitactic type
justify in some measure the use by apatites as orthopedic biomaterials. With other
types of non-apatitic materials, even other amorphous materials such as bioglasses,
the formation of a carbonated apatite, analogous to bone mineral, is also implied,
even though epitaxy relations do not necessarily exist. Phosphate ions appear to play
an important role at the interface between certain materials and the bone mineral.
Different authors have clearly shown that the bonding between apatitic type
ceramics and the bone is among the strongest, and is generally the line of fracture is
situated in the bone tissue and not at the bone-implant interface. An exception that
remains generally unexplained is the bioglass-bone bonding. The interactions with
organic matrix, as well as the possibility of having locally weaker but greater
number of bonds, have been suggested. The type of interaction involved is in fact
highly dependent on the number of crystals in direct contact with the biomaterial.
But the methods of nucleation of these crystals can be affected by a number of
factors associated with the implanted biomaterials (nucleation sites) and its
environment (protein adsorption for example).
Bioceramics 505
12.3.1.3. Mechanical stresses
Cell activity, of the bone tissue in particular, is closely connected to mechanical
stimuli. This effect is now well established, based on observations that in the
absence of mechanical stress the remodeling of bones tends to slow down. The
implant integration in the bone tissue depends also on biomechanical factors.
Besides, since the Young’s modulus of ceramics is generally much higher than that
of the bone tissue, the implant can cause mechanical stresses at the bone interface.
Moreover, it produces a modification of the lines of force (stress shielding) which
can result in bone defects close to the implant. This phenomenon is sometimes
visible in x-rays and manifests itself by a diminution of the bone density near the
implant related to the lack of mechanical stimulation of the tissue.
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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