Biomedical Engineering Reference
In-Depth Information
3.2 Orientation on the articular surface of a UHMWPE acetabular liner after
retrieval from the patient (scale bar represents 10m) (a) and UHMWPE wear
particles after being isolated from bovine serum used in hip simulator wear
testing of gamma sterilized UHMWPE (scale bar represents 1m) (b).
3.3.2
Processing
When temperature is increased above the melting temperature, UHMWPE
simply turns from opaque to translucent, but its melt flow rate (MFR) is
extremely low, which means that for all practical purposes it behaves as a solid
in the melt state. For this reason, traditional polymer processing techniques such
as injection molding or screw extrusion are not feasible for processing
UHMWPE resin into consolidated form for implant manufacturing. Most
commonly, UHMWPE is processed through slab compression molding of
powder or using extrusion by impaction with a ram (Kurtz, 2009a). In both
cases, high temperatures and high pressures are used to enhance the diffusion of
the long polymer chains and the effective fusion of the resin powder. The
processing time as well as the cooling (recrystallization) rate can make
significant differences to the resultant properties of the material. There are
specifications for the properties of consolidated UHMWPE for use in surgical
implants (ASTM, 2009).
Implants can be manufactured by machining from UHMWPE stock material
consolidated by compression molding or ram extrusion or they can be direct
compression molded to the desired implant shape. While the latter offers a
single-step process, the effects of any additional processing such as sterilization
or crosslinking on the manufacturing tolerances may be significant.
A recent development in the manufacture of polyethylene implant com-
ponents is the use of antioxidants. Despite the pervasive use of antioxidants in
other applications using polyethylene such as food packaging, this was carefully
avoided in medical-grade polyethylene mainly due to the fusion defects and
sporadic oxidation attributed to the presence of calcium stearate in the past
