Biomedical Engineering Reference
In-Depth Information
Table 8.1 Typical average physical properties of HDPE and UHMWPE
a
Property
HDPE
UHMWPE
Molecular weight (10
6
g mol
-1
)
0.05-0.25
2-6
Degree of crystallinity (%)
60-80
39-75
Poisson's ratio
0.40
0.46
Tensile modulus of elasticity* (GPa)
0.4-4.0
0.8-1.6
Tensile yield strength* (MPa)
26-33
21-28
Tensile ultimate strength* (MPa)
22-31
39-48
Tensile ultimate elongation* (%)
10-1200
350-525
*Testing conducted at 23°C
a
Edidin and Kurtz.
2
8.2.1 Synthesis of UHMWPE and manufacture
of the implant
the synthesis of UHMWPe from ethylene gas in the presence of a Ziegler-
natta catalyst produces a white powder with particle size of 45-500 mm and
a molecular weight of 2.9 to 6 million g mol
-1
.
3
in a second step, the polymerized UHMWPe, in the form of resin powder,
needs to be consolidated into a sheet, rod or near-net shaped implant. Finally,
in most instances, the UHMWPe implant needs to be machined into its
final shape. A small subset of implants are consolidated into their final form
directly, in a process known as direct compression molding (dcM), without
need of additional machining.
each of these three principal steps produces a subtle alteration of the
properties of UHMWPe.
4
in some cases, such as machining, the change in
the material may occur only in the topography and appearance of the surface.
on the other hand, changes in the polymerization and conversion of the
UHMWPe can have an impact on the physical and mechanical properties
of the entire implant.
5
Another important factor in the later performance of the device is the
sterilization method employed.
6
the most common method is the gamma
irradiation of the devices, however, two main opposing effects over the material
have been detected. the gamma irradiation provokes the cross-linking of the
UHMWPe chains, which leads to an improvement in the material properties,
such as its fatigue resistance. Unfortunately, the oxidation of the polymer,
which compromises its function in the long term owing to the formation of
wear debris, is also observed.
7
8.2.2 Properties and structure of polyethylene
UHMWPe presents very interesting physical and chemical properties which
render it unique for its use in articular prostheses. UHMWPe shows the highest
