Wear processes in polymer implants - Durability and Reliability of Medical Polymers - page 150

fibre-reinforced (CFR) UHMWPE in both pelvic and tibial implants. They
describe wide failure particularly in the tibial implants. This was attributed
to poor bonding between the carbon fibres and UHMWPE matrix that was
exacerbated by the poor creep resistance of UhmWpE. however, the high
creep resistance of pEEK minimises the risk of de-bonding at the interface
between carbon filler and matrix polymer.
The wear performance of pEEK and its composites was extensively reported
in general tribological science terms. however, this is not the case when
only biomedical applications are considered, with only a limited number of
studies published relating to the biotribology of pEEK. Scholes and Unsworth
(2009) reported wear rates of between 0.1 and 0.2 ¥ 10 -6 mm 3 n -1 m -1 for
both polyacrylonitrile (pan) and pitch pEEK composites against CoCr alloys.
These wear rates were shown to be much lower than for unreinforced pEEK
rubbing against a similar surface (wear rate approximately 7.5 ¥ 10 -6 mm 3
n -1 m -1 ). Similar experiments were conducted using ceramic counterfaces,
again demonstrating low rates of wear, trends and magnitudes that were
similar to the results for metallic bearing surfaces (Scholes and Unsworth,
2007).
all polymer-bearing couplings were also examined. Both austin et al.
(2009) and Scholes and Unsworth (2010) tested unreinforced pEEK/pEEK
and CFr pEEK (both pan and pitch) rubbing against similar counterfaces.
Wear rates reported agree that pan CFr pEEK produced the lowest wear.
The average steady-state wear rates for CFr pEEK, of various compositions,
rubbing against metallic, ceramic and polymer counterfaces are summarised in
Table 7.2. Finally, it should be noted that the results reported were generated
using pin-on-plate apparatus, highly simplified from the loads and stress
states that are found in the actual implants.

7.4.4 Silicone
Silicone was used to manufacture single-piece finger and wrist replacement
implants. Because the number of finger and wrist implants is much less than
that for lower limb joints, there has been little research investigating the wear
of silicone for this use. pylios and Shepherd (2008) investigated the wear of
medical-grade silicone rubber against titanium and UhmWpE using a pin-
on-disc apparatus. The lubricants used were ringer's solution and bovine
serum diluted with distilled water. The wear factors of the silicone rubber
against titanium were 67 ¥ 10 -6 mm 3 n -1 m -1 (ringer's solutions) and 40 ¥
10 -6 mm 3 n -1 m -1 (diluted bovine serum). against UhmWpE the wear factors
were 88 ¥ 10 -6 mm 3 n -1 m -1 and 84 ¥ 10 -6 mm 3 n -1 m -1 , when lubricated
with ringer's solution and diluted bovine serum, respectively.
Finger joint simulators were used to test single-piece silicone implants,
but there were no attempts to quantify the amount of wear debris generated

￿ ￿ ￿ ￿ ￿ ￿