Mark L Morrison, PhD, and Shilesh C Jani, MS
Smith & Nephew Orthopaedics
Reconstructive Research and Innovation
1450 Brooks Road, Memphis, TN, 38116
Cross-linked Polyethylene
Engineered for TKA
Introduction
It is well known that the wear resistance of ultra-high molecular weight polyethylene (UHMWPE) improves
in proportion to the crosslinking irradiation dose. It is also well known that the mechanical properties and
oxidative stability of UHMWPE are affected by the irradiation dose, resin type, and post-irradiation heat
treatment, among others factors. Therefore, careful consideration of these variables should be made for a
given application. This balance between wear resistance and mechanical properties is particularly important
for applications in total knee arthroplasty (TKA) where the contact stresses are higher than in total hip
arthroplasty (THA).
Through extensive material and device-level testing, Smith & Nephew has developed a highly cross-linked
UHMWPE (XLPE) for TKA. This XLPE product begins with compression-molded GUR 1020 UHMWPE rods to take
advantage of the high mechanical properties of this material. These rods are then cross-linked using gamma
radiation with a dose of 7.5 Mrad (75 kGy) and are subsequently heated above the melting temperature (i.e.,
re-melted) to stabilize the material against oxidation. Tibial components machined from this stabilized XLPE
material are sterilized by ethylene oxide (EtO) gas so that no additional free radicals are created in the material.
Re-melting and EtO sterilization ensure the long-term stability of the implant against oxidation.
The purpose of this paper is to discuss the physical properties of the Smith & Nephew XLPE for knees.
Wear performance of this material is discussed in another paper.
The free-radical concentration (FRC) in UHMWPE is typically measured
through Electron Spin Resonance (ESR), where peaks in the ESR
spectrum represent the presence of free radicals. The FRC can be
calculated from the area beneath these peaks; therefore, higher and
broader peaks represent greater free-radical concentrations.
300
200
100
Intensity
No Detectable Free Radicals
Free radicals are by-products of radiation in the crosslinking process.
Free radicals are deleterious because they will react with available
oxygen (i.e., oxidation) on the shelf and/or in vivo and result in
degradation of the wear resistance and the mechanical properties of the
polyethylene. Heat treatment of UHMWPE after irradiation is commonly
utilized to remove or reduce the number of residual free radicals in
cross-linked PE components. The effectiveness of heat treatment
depends on the temperature at which it is carried out; heat treatment
above the melt temperature (referred to as ‘re-melt’) is more effective
than below the melt temperature (referred to as ‘sub-melt annealed’).
0
-100
Virgin
-200
XL Sub-melt
XL Re-melt
-200
Magnetic Field (Gauss)
Figure 1: Electron Spin Resonance (ESR) spectra showing the
effectiveness of re-melting the cross-linked UHMWPE1.
The ESR spectra plotted in Figure 1 demonstrates that the Smith & Nephew re-melted XLPE shows no discernible peaks2,
which is similar to EtO-sterilized, virgin (non-irradiated) materials1.
The Smith & Nephew XLPE therefore has a non-detectable level of free radicals. In contrast, a sub-melt annealed, cross-linked
PE shows distinct ESR peaks in Figure 11. A currently marketed, sub-melt annealed, cross-linked PE is reported to contain free
radicals in the range of 1,000 trillion (1015) free radicals per gram of PE3.
Resistant to Oxidation
As discussed above, free radicals are highly reactive and pre-disposes the material to oxidation, and oxidation will occur
when oxygen is available to react with them. Therefore, sub-melt annealed cross-linked UHMWPE will oxidize with time
and exposure to oxygen1. In contrast, re-melted cross-linked materials do not exhibit measurable oxidation on the shelf, in
vivo or during an oxidative challenge in vitro. Figure 2 shows that after a standard accelerated aging protocol the Smith &
Nephew XLPE has an oxidation index at the detection limit (0.01)2, which is comparable to EtO-sterilized virgin (non–irradiated)
materials1. However, the sub-melt annealed cross-linked PE shows measurable oxidation1 in the same oxidative challenge.
The Smith & Nephew XLPE is fully stabilized by re-melting; it contains
undetectable levels of free radicals, and is therefore resistant to
oxidation. To confirm that delamination risk has been addressed, worstcase delamination testing was conducted on XLPE2. First, XLPE disks
were subjected to accelerated aging for 14 days under 5 atmospheres
of oxygen pressure at 70°C (ASTM F2003-02). CoCr pins were used to
apply forces and linear reciprocating motion on the aged XLPE disks
at contact pressures greater than 60 MPa, which is at the upper range
of contact stresses reported in vivo5 and is greater than the tensile
strength of the material.
.45
14 days @ 70ºC and 5 atm 02
.40
Maximum Oxidation Index (OImax)
Resistant to Delamination
Delamination is a well-documented historical failure mechanism in
TKA, where, with time in vivo, the UHMWPE tibial inserts undergo
fragmentation of the articular surfaces. This behavior belies the
otherwise tough and ductile nature of UHMWPE. Oxidative degradation
was identified as a cause for this phenomenon4. Further research
showed that delamination secondary to oxidation was associated
with UHMWPE tibial inserts that were gamma sterilized in air. The act
of irradiation sterilization created free radicals, which, as discussed
above, are highly reactive. Furthermore, because the historical implant
packaging allowed oxygen to flow into the packaging, the material
underwent oxidative degradation during shelf storage. Oxidation can
occur also in-vivo, if free radicals are present in UHMWPE inserts.
.35
.30
.25
.20
.15
.10
.05
.00
Virgin
XL Re-melt
XL Sub-melt
Figure 2: Plot of the maximum oxidation indices after accelerated
aging. The XL Sub-melt material exhibited significantly greater
oxidation due to the presence of residual free radicals1,2.
In this worst-case testing, XLPE did not show any evidence of gross surface delamination (Figure 3b), while the gamma-in-air
sterilized samples (Figure 3c) exhibited extensive delamination as seen clinically4. Furthermore, no subsurface cracking was
observed under high power electron microscopy. This resistance to delamination is similar to Smith & Nephew EtO sterilized
virgin (non-irradiated) UHMWPE (Figure 3a).
(a)
(b)
(c)
Figure 3: Photographs of the (a) virgin, (b) XLPE, and (c) gamma-sterilized samples (positive control) after accelerated aging and
subsequent delamination testing.
Conclusions
Cross-linked UHMWPE has the potential for dramatic reductions in wear rates, depending on the irradiation dose used for
crosslinking. However, careful consideration in resin choice, consolidation method, crosslinking dose, and subsequent thermal
treatment parameters are required to ensure an appropriate balance between wear resistance, mechanical properties, and
oxidative stability. This balance needs to be assessed in light of the specific application; cross-linked UHMWPE for TKA and
THR will necessarily be different. A summary of currently available cross-linked polyethylene formulations for TKA is presented
in Table 1.
Smith & Nephew has struck a favorable balance between oxidative stability, mechanical properties and wear reduction in its
development of XLPE for TKA.
Table 1: Summary of the highly cross-linked UHMWPE materials for TKA on the market.
Company/Product
Smith & Nephew/XLPE
Depuy/XLKT
Zimmer/ProlongT
Stryker/X3T
GUR Resin
1020
1020
1020
1020
Radiation Dose (Mrad)
7.5
5
6.5
9 = 3x3
Thermal Treatment
Re-melt
Re-melt
Re-Melt
Sub-melt
Free Radicals
No
No
No
Yes
Oxidation*
No
No
No
Yes
*Oxidation measured according to ASTM F2102-06 after accelerated aging according to ASTM F2003-02.
References
1 M. Morrison and S. Jani, “Comparison of sequential and single-dose irradiation effects on the mechanical and physical properties of UHMWPE,” Orthop
Res Soc, San Diego, CA, Feb 11-14, 2007, 1786.
2 Data on file at Smith & Nephew.
3 S.S. Yau, A. Wang, A. Essner, M.T. Manley and J.H. Dumbleton, “Sequential irradiation and annealing of highly cross-linked polyethylenes: Resist oxidation
without sacrificing physical/mechanical properties,” Orthop Res Soc, Washington D.C., Feb 20-23, 2005, 1670.
4 J.P. Collier, D.K. Sperling, J.H. Currier, L.C. Sutula, K.A. Saum and M.B. Mayor, “Impact of gamma sterilization on clinical performance of polyethylene in the
knee,” The Journal of Arthroplasty, 1996;11(4):377-389.
5 D.L. Bartel, J.J. Rawlinson, A.H. Burstein, C.S. Ranawat and W.F. Flynn, Jr., “Stresses in polyethylene components of contemporary total knee
replacements,” Clin Orthop Relat Res, 1995;317:76-82.
Orthopaedic Reconstruction
Smith & Nephew, Inc.
1450 Brooks Road
Memphis, TN 38116
USA
www.smith-nephew.com
Telephone: 1-901-396-2121
Information: 1-800-821-5700
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40472802 02/08