Biomedical Engineering Reference
In-Depth Information
A number of other non-degradable polymers applied in orthopedic
surgery include PE in its different modifications such as low density
PE, HDPE, and UHMWPE (used as the articular surface of total hip
replacement implants [139, 140]), polyethylene terepthalate, PP, and
PTFE which are applied to repair knee ligaments [141]. Polyactive™, a
block copolymer of PEG and PBT, was also considered for biomedical
application [142-147]. Cellulose [148, 149] and its esters [150, 151]
are also popular. Finally yet importantly, polyethylene oxide, PHB
and blends thereof have also been tested for biomedical applications
[50].
Nonetheless, the most popular synthetic polymers used in
medicine are the linear aliphatic poly(α-hydroxyesters) such as PLA,
PGA and their copolymers—PLGA (Table 6.3). These materials have
been extensively studied; they appear to be the only synthetic and
biodegradable polymers with an extensive FDA approval history [50,
132, 152-156]. They are biocompatible, mostly non-inflammatory,
as well as degrade
through hydrolysis and possible enzymatic
action into products that are removed from the body by regular
metabolic pathways [49, 127, 132, 156-161]. Besides, they might be
used for drug delivery purposes [162]. Poly(α-hydroxyesters) have
been investigated as scaffolds for replacement and regeneration
of a variety of tissues, cell carriers, controlled delivery devices for
drugs or proteins (e.g., growth factors), membranes or films, screws,
pins, and plates for orthopedic applications [127, 132, 153, 154,
156, 163-165]. Additionally, the degradation rate of PLGA can be
adjusted by varying the amounts of the two component monomers
(Table 6.3), which in orthopedic applications can be exploited to
create materials that degrade in concert with bone ingrowth [160,
166]. Furthermore, PLGA is known to support osteoblast migration
and proliferation [59, 132, 157, 167], which is a necessity for bone
tissue regeneration. Unfortunately, such polymers on their own,
though they reduce the effect of stress-shielding, are too weak to
be used in load bearing situations and are only recommended in
certain clinical indications, such as ankle and elbow fractures [156,
161]. In addition, they exhibit bulk degradation, leading to both a
loss in mechanical properties and lowering of the local solution pH
that accelerates further degradation in an autocatalytic manner.
As the body is unable to cope with the vast amounts of implant
degradation products, this might lead to an inflammatory foreign
body response [132, 156, 163]. Finally, poly(α-hydroxyesters) do
in vivo
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