Using synthetic bioresorbable polymers for orthopedic tissue regeneration - Durability and Reliability of Medical Polymers - page 124

that were phagocytized by macrophages and multinucleated giant cells. 47
this adverse reaction can occur weeks and months postoperatively and might
need operative drainage. this is a major concern in orthopedic applications,
where implants of considerable size would be required, thus resulting in
release of degradation products with high local acid concentrations.
furthermore, neither PGa nor Pla possess intrinsic bioactivity. Cellular
responses to implants of these polymers are mediated mostly by surface
adsorbed proteins, surface topography, and local acidity. Crystallinity seems
to be another parameter affecting cell adhesion on Pla/PGa polymers. 48
the effects are believed to be a result of differences in protein conformation
stemming from variations in binding to amorphous versus crystalline polymer
surfaces. the chirality of crystalline regions may also play a role in controlling
cellular response, but the detailed mechanisms of these effects are not fully
understood.
Limitations of this class of materials include also insufficient mechanical
properties with regard to load-bearing applications 49 and inflammatory or
cytotoxic events owing to above-mentioned accumulation of acidic degradation
products. the decrease of pH values in the tissues adjacent to degrading
biodegradable polyesters may contribute to adverse effects. Consequently,
one of the methods for the local re-establishment of physiological conditions
could be incorporation of basic salts within the polymer. Promising results
have already been presented in some in vitro studies. 50,51

6.3 Polylactones
6.3.1 Poly( e -caprolactone) (PCL)
the most prominent and thoroughly investigated polylactone is poly( e -
caprolactone) (PCl) (fig. 6.4), an aliphatic, semicrystalline polyester with
an interestingly low glass transition temperature (-60 °C) and melting
temperature (59-64 °C). as a consequence, PCl is always in a rubbery state
at room temperature and this unusual property undoubtedly contributes to
the very high permeability of PCl for many therapeutic drugs 52 and to its
high thermal stability.
PCl is prepared by the ring-opening polymerization of the cyclic monomer
e -caprolactone, and it can be copolymerized with numerous other monomers
because of its high affinity with them. 53

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the discovery that PCl can be degraded by micro-organisms and by

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6.4 Poly( e -caprolactone) (PCL).