Biomedical Engineering Reference
In-Depth Information
cement and at the cement-bone interface which leads to long-term stability
of the total replacement hip joint.
Finally, fatigue is the effect of repeated load cycles below the level needed
to fail the material in a single load application.
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Polymers do not have an
endurance limit (level of stress below which the material will not fail) and
consequently they will always fail after sufficient load cycles have been
applied. Fatigue failure normally originates at a point of high tensile stress
concentration.
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nevertheless, tensile stresses in bone cements can be relaxed
very rapidly. it is this stress relaxation that provides a form of self-protection
for the entire bone cement.
8.3.4 PMMA-based antibiotic loaded acrylic
cements (ALAC)
the addition of antimicrobial agents to acrylic bone cement was begun as
early as 1969.
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it is very useful because it reduces the risk of infection.
PMMA bone cement is a meshwork of PMMA chains and consequently
antibiotics enclosed in these meshes are released by elution from the bone
cement. the elution properties of acrylic bone cements correlate directly
with the ability to absorb water during bone cement preparation.
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For this
application, the antibiotics need to present a certain physicochemical profile
that allows them to be eluted from the bone cement. Among these properties
are high solubility in water, heat stability during the polymerization reaction,
no chemical interaction with PMMA or mediators of the polymerization,
low effect on the mechanical strength of the bone cement and finally good
release from the bone cement.
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there are two main uses of antibiotic loaded acrylic cements (ALAc), both
prophylaxis and therapy. Prophylactic use is determined by the pathogens
expected at the site of the prosthesis. Gentamicin, a well-known antibiotic,
turned out to be a suitable agent for prophylactic use in ALAc.
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on the
other hand, for the therapy of periprosthetic infection, the pathogen must
be identified prior to revision surgery so that appropriate antibiotics can be
selected in advance according to the susceptibility pattern of the individual
bacterial strain for application in ALAc.
8.4 Biodegradable polymers
the biodegradable polymers are a class of polymers that will degrade and
therefore resorb once they are in contact with tissues and fluids of the body.
the main advantages of these polymers are: (1) once their function has been
accomplished they will disappear and (2) the tissue will grow after their
disappearance. For these same reasons, they must fullfil stringent requirements
in terms of their biocompatibility. in addition to the potential problem of
