Biomedical Engineering Reference
In-Depth Information
chain scission is not a prevalent phenomenon in UHMWPE. On the other hand,
hydrogens have higher mobility than the macroalkyl radicals of the polymer and
C±H bonds are not readily recombined. In the amorphous phase, crosslinks
between polymer chains are formed by the combination of two free radicals on
different chains (H crosslinks) or by the reaction of a macroalkyl radical with a
terminal vinyl group (Y crosslinks). On the other hand, the free radicals in the
crystalline phase do not undergo recombination but are trapped until they can
migrate to the crystalline/amorphous interface. This migration takes a very long
time, which is proportional to the thickness of the crystalline lamellae. When the
free radicals have escaped the crystalline phase, they can react with diffused
oxygen and start a cascade of reactions (Fig. 3.3), which result in the formation
of oxidation products and chain scission. Molecular weight degradation due to
chain scission and recrystallization of degraded chains results in the
deterioration of mechanical properties of the material.
Gamma sterilized UHMWPE is slightly more wear resistant than UHMWPE
sterilized without radiation sterilization because even the low dose of irradiation
causes some crosslinking in the material, reducing its plasticity (Affatato et al.,
2002). However, wear of UHMWPE coupled with the detrimental mechanical
degradation resulting from oxidation caused was linked to severe osteolysis and
implant loosening (Wroblewski, 1988; Amstutz et al., 1992; Jasty and Smith,
1992; Buechel et al., 1994). Therefore, in the next decade, the focus was on
finding methods to minimize wear and oxidation. One method to combat
oxidation was gamma sterilization in inert atmosphere. While this eliminates
most oxidation on the shelf, the implants oxidized in vivo. Also, radiation
crosslinking was introduced to decrease wear.
3.4 The introduction of high dose crosslinking
The first highly crosslinked UHMWPEs were used in the 1970s and 1980s in
small cohorts of patients (DuPlessis et al., 1977; Oonishi et al., 1992;
Wroblewski et al., 1996). The industry-wide acceptance of highly crosslinked
UHMWPE came in the late 1990s, when large orthopedic manufacturers started
producing highly crosslinked UHMWPEs (Table 3.2).
3.4.1
First generation highly crosslinked UHMWPEs
Radiation crosslinking of UHMWPE was introduced to decrease the wear rate
observed with gamma sterilized conventional UHMWPE (Muratoglu et al.,
1999), which was associated with the occurrence of osteolysis and implant
loosening. Radiation doses ranging from 50 to 100 kGy were used (Table 3.2).
Since radiation-induced free radicals were determined to be the cause of oxida-
tion, post-irradiation thermal treatments were used in first generation highly
crosslinked UHMWPEs to reduce or eliminate the presence of free radicals. One
