Biomedical Engineering Reference
In-Depth Information
200,000 g/mol. The high molecular weight results in a high resistance to deformation and signifi -
cantly higher abrasion and wear resistance compared with HDPE (Table 21.1). The volumetric wear
rate for HDPE might be four times higher than UHMWPE [65].
UHMWPE, after approval by FDA in the year 2000, can be replaced by a highly cross-linked
and thermally stabilized UHMWPE that has recently become the most widely used alternative to
conventional UHMWPE for hip arthroplasty. Cross-linking is a process in which polyethylene mol-
ecules are bonded together to form a stronger material (Table 21.2) [66], with substantially improved
wear resistance. Such materials are processed by irradiation with a total dose ranging from 50 to
105 kGy. Radiation cross-linking has been performed using gamma and electron beam radiations
with the polymer in the solid or molten state. Recently, a postirradiation thermal treatment of irra-
diated UHMWPE is strongly recommended to improve polymer resistance to long-term oxidative
degradation. The ionizing radiation reduces the concentration of free radicals in the polymer, which
are responsible for the polymer oxidation.
TABLE 21.1
Physical Properties of High-Density Polyethylene (HDPE) and
Ultra High-Molecular Weight Polyethylene (UHMWPE)
Property
HDPE
UHMWPE
Molecular weight (10 6 g/mol)
0.05-0.25
2-6
Melting temperature (°C)
130-137
125-138
Poisson's ratio
0.40
0.46
Specifi c gravity
0.952-0.965
0.932-0.945
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
Impact strength, Izod* (J/m of notch; 3.175 mm
thick specimen)
21-214
> 1070 (no break)
Degree of crystallinity (%)
60-80
39-75
Source :
Adapted from Kurtz, S.M., in Total Joint Replacement , Elsevier Science & Technology, San Diego,
CA, 2004. With permission.
TABLE 21.2
Physical Properties of Cross-Linked Ultra High-Molecular
Weight Polyethylene
Property
Cross-Linked UHMWPE
Melting temperature (°C)
135.8 ± 5.6
860 ± 206
Tensile modulus of elasticity* (MPa)
Tensile yield strength* (MPa)
321.1 ± 2.5
29.3 ± 7.7
Tensile ultimate strength* (MPa)
Tensile elongation at fracture (%)
212 ± 61
245.3 ± 5.3
Degree of crystallinity (%)
Source : Adapted from Lewis, G., Biomaterials , 22, 371-401, 2001. With
permission.
 
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