Babylon University / College of Materials Engineering
Non-Metallic Materials Engineering Department- Ceramic Branch
Subject: Bioceramics
Dr. Shaker J. Edrees
12.3.1.1. 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 micro
movements, eventually associated with the
phenomena of abrasion and with an inflammatory process. The existence
of micro movements increases the thickness of the encapsulating fibrous
layer . 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 epidictic 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
Babylon University / College of Materials Engineering
Non-Metallic Materials Engineering Department- Ceramic Branch
Subject: Bioceramics
Dr. Shaker J. Edrees
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).
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.