Biomedical Engineering Reference
In-Depth Information
Mechanical properties
In addition to tensile mechanical testing, there are a few mechanical testing
methods that are commonly used in assessing the properties of UHMWPEs
designed for joint implants. One of these is the so-called small punch test (Kurtz,
1999; ASTM, 2008b) using deformation of small discs of UHMWPE resulting
in a load±displacement curve, an ultimate load at failure and a work-to-failure
which is calculated as the area under this curve. These can be used to compare
materials with different processing history and it has the advantage of using
small enough samples to allow mechanical analysis of explanted components
(Kurtz et al., 2005), which is not possible by other characterization methods.
There are compressive and shear forces on UHMWPE during articulation, and
there may be residual tensile stresses that arise from accumulated stress during
repeated load cycles (Sathasivam and Walker, 1998). The reversal of stress
direction causes fatigue cracks to initiate causing delamination, especially in total
knee implants (Bartel et al., 1986, 1995). Fatigue crack initiation and propagation
resistance of UHMWPE is a crucial property for predicting long-term performance
of joint implants especially under adverse loading conditions. Fatigue testing is
performed in cyclic tension on a compact tension specimen (ASTM, 2008a), which
is designed to concentrate applied stress at a notch tip with an existing crack. A
stress factor at crack inception (K
i
) is calculated as a measure of fatigue crack
propagation resistance. This is the stress factor range (dependent on the load ratio,
sample geometry and crack length), at which the crack growth rate exceeds 10
ÿ6
mm/cycle. The K
i
of unirradiated UHMWPE is reported as 1.6±2.0MPam
1/2
(Baker et al., 2000; Oral et al., 2009). The temperature at which the testing is
performed (25 vs. 40 ëC), the frequency used in the test, testing environment (air vs.
water) and the crack initiation method are some factors that account for the
variation in the results obtained from different laboratories.
Impact testing is performed by fracturing a double notched Izod impact
specimen by contacting with a swinging pendulum and calculating the work-to-
fracture (ASTM, 2009).
Oxidative properties
Oxidation in UHMWPE takes place through a cascade of reactions (Fig. 3.3).
The macroalkyl/macroallyl radicals in UHMWPE react with oxygen to form
peroxy free radicals, which then attack unreacted polymer chains to form
hydroperoxides, generating new free radicals which can fuel the oxidative
reactions. The thermal decomposition of the hydroperoxides into carbonyl-
containing species is accompanied by chain scission and molecular degradation.
What is termed `oxidation' in UHMWPE is the amount of detectable carbonyl
containing species by vibrational spectroscopy, commonly Fourier transform
infrared spectroscopy (FTIR) (ASTM, 2006c). It has been shown that there can
be absorbed lipid species in UHMWPE in vivo (Costa et al., 2001), some of
