Biomedical Engineering Reference
In-Depth Information
bone and metals, a stress protection effect usually may occur causing the bone
to atrophy over the long-term period. as consequence, the stress-shielded
bone does not heal completely and is susceptible to re-fracture after removal
of the metallic implant (Codran, 1969; tormala
et al
., 2002). Moreover, the
metal chemical composition may evoke an allergic reaction by electrical
potential difference which may promote an escalation of uncontrolled
corrosion phenomena (tonino
et al
., 1976). More specifically, ion release
may cause local adverse tissue reactions as well as allogenic responses with
negative effects on the bone mineralization and adverse systemic responses
like local tumour formation (Lamovec
et al
., 1988; martin
et al
., 1988). To
provide a less rigid system with minor problems of corrosion and a reduced
need for removal, biodegradable polymeric composites and polymer/ceramic
composites have been investigated for potential use in intramedullary rods,
bone plates, fixation pins and screws, and bone regenerative scaffolds.
in the case of biodegradable composite, the stress-shielding phenomena
associated with the use of rigid metallic implants may also be drastically
reduced. indeed, the continuous degradation of the implant causes a gradual
load transfer to the healing tissue, preventing stress-shielding atrophy by
stimulation of healing and bone remodelling.
9.5.1 Partially and totally degradable fibre
reinforced composites
Some requirements must be fulfilled by ideal prosthetic biodegradable
materials, such as biocompatibility, adequate initial strength and stiffness,
retention of mechanical properties throughout a sufficient time to assure
its biofunctionality and non-toxicity of degradation by-products. From the
mechanical point of view, degradable polymers and composites have to possess
a modulus of elasticity much closer to bone, one which decreases over time
as the healing bone becomes stronger and stiffer (Flahiff
et al
., 1996). It
has been demonstrated that synthetic resorbable polymers like polylactide
and polyglycolide fulfil several demands of ideal ostheosynthesis materials
in term of biocompatibility and suitable stiffness, ensuring a progressive
transfer of stresses to healing bone, obviating the need for a removal
operation. However, they are too weak and flexible for safe clinical use in
bone surgical applications. to overcome the limited mechanical response
of non-reinforced materials based on aliphatic polyesters like PLa (i.e. low
bending stiffness up to 4-6 GPa in the dry state at room temperature), the
addition of thermoplastic fibres may promote the adjustment of the bending
modulus up to 50 GPa as a function of the fibre content and their orientation
(Kulkarni
et al
., 1971).
in the last 30 years, partially resorbable composites have been obtained
by combining a degradable polymeric matrix with high modulus fibres that
