Integrated Biomaterials Science

544 Dante Ronca and Giuseppe Guida
is a widespread method used to sterilize polyethylene; 2.5 Mrad is sufficient
to kill most microorganisms. However, the radiation sterilization can
significantly affect the physicochemical and toxicological properties of the
devices. The primary active species created by this irradiation are radicals
and ions. Most of the radicals formed during irradiation remain trapped in
the material and they can be converted in peroxidic radicals that further
recombine, inducing cross-links and material degradation. The introduction
of cross-links between polymer chains and chain scission which increases
crystallinity are the two main effects of the gamma radiation sterilization on
polyethylene (Rimnac et al., 1994; White et al., 1996). The presence of an
oxidized surface layer on the periphery in polyethylene components has
been reported too. Components sterilized with gamma radiation have
shown a granular structure with fusion defects and large subsurface cracks.
These findings are significant because, during gait, the rolling and sliding
motion of the knee causes high shear stresses on the polyethylene of the
tibial component and can cause wear of the articular surface. The presence
of these defects can significantly alter the resistance of the polyethylene to
high shear stresses. It seems that ethylene oxide sterilized components have
not shown evidence of a granular structure or subsurface cracking, and
show a higher ultimate tensile strength, elongation to failure, and toughness
(White et al., 1996). Moreover, when implants are inserted into living tissue,
an inflammatory response caused by the surgical trauma follows. The
experimental observations of retrieval polyethylene tibial components
showed that both chemical and physical changes take place in UHMW-PE
during the implantation time. Particularly, microscopic examination of the
articulating surfaces revealed burnishing, scratching, pitting, and delamination
as the most common modes of surface damage. The gamma radiation
produces changes in mechanical properties and a low oxidation (predamage),
but the chemical modification of the UHMWPE occurs principally
during the implantation time and it consists of an oxidation of the prosthesis
surface. A large quantity of hydroxyl and carbonyl functions is produced
during the in vivo oxidation on the exterior surface of the prostheses. The
increase of the crystallinity on the articulating surface confirms the progression
of the aging of UHMWPE due to the synergism effect of the
described phenomena (Ambrosio et al., 1996).
17.4.6. Debris
Polyethylene wear debris and associated granulomatous response are
frequently cited as the cause of bone loss and implant failure (Nolan and
Bucknill, 1992; Robinson et al., 1995). The extent of wear and consequent
production of debris particles is influenced by many factors, including