The Journal of Bone and Joint Surgery 82:1708 (2000)
© 2000 The Journal of Bone and Joint Surgery, Inc.
Effect of Sterilization Method and Other Modifications on the Wear Resistance of Acetabular Cups Made of Ultra-High Molecular Weight Polyethylene
A Hip-Simulator Study*
Harry McKellop, Ph.D. ,
Fu-wen Shen, Ph.D.,
Bin Lu, M.S.,
Patricia Campbell, Ph.D. and
Ronald Salovey, Ph.D.
Investigation performed at the J. Vernon Luck Orthopaedic
Research Center and the Joint Replacement Institute, Los Angeles
Orthopaedic Hospital, and the Departments of Orthopaedics, Biomedical Engineering,
and Materials Science, University of Southern California, Los Angeles,
California
*No benefits in any form have been received or will be received
from a commercial party related directly or indirectly to the subject
of this article. Funds were received in total or partial support
of the research or clinical study presented in this article. The
funding sources were the National Institutes of Health Grant 40996
and the Los Angeles Orthopaedic Hospital Foundation. Materials were donated
by DePuy-DuPont Orthopaedics; Howmedica, Incorporated; Intermedics
Orthopedics, Incorporated; Poly Hi Solidur, Incorporated; Spire Corporation;
and Zimmer, Incorporated.
The J. Vernon Luck Orthopaedic Research Center (H. McK., F.-W.
S., and B. L.) and the Joint Replacement Institute (P. C.), Orthopaedic
Hospital, 2400 South Flower Street, Los Angeles, California 90007.
E-mail address for H. McKellop: hmckellop{at}laoh.ucla.edu
Department of Materials Science, University of Southern California,
University Park Campus, Los Angeles, California 90089.
Background: Wear of ultra-high molecular
weight polyethylene acetabular cups in hip prostheses produces billions of
submicrometer wear particles annually that can cause osteolysis
and loosening of the components. Thus, substantial improvement of
the wear resistance of ultra-high molecular weight polyethylene could
extend the clinical life span of total hip prostheses. It has become
apparent that the conditions under which ultra-high molecular weight
polyethylene cups have been sterilized can markedly affect their
long-term wear properties, and new sterilization methods and other
modifications have been developed to minimize the negative effects.
Methods: In the present study, a hip-joint simulator
was used to assess whether it is preferable to sterilize ultra-high
molecular weight polyethylene cups without gamma irraSdiation, to
avoid radiation-induced oxidative degradation, or to sterilize with gamma
irradiation while the cups are packaged in a suitable low-oxygen
atmosphere to minimize oxidation while retaining the increased wear
resistance conferred by the radiation-induced cross-linking. Ion-implanted
cups and cups made of a highly crystalline polyethylene (Hylamer)
also were investigated. Cups made of each material were subjected
to wear-testing prior to and after artificial thermal aging to accelerate
oxidative degradation.
Results: The results of the present study demonstrated
that the cross-linking induced by gamma irradiation improves the
wear resistance of ultra-high molecular weight polyethylene, while
oxidation reduces it. Without thermal aging, the two types of cups that
were sterilized with gamma irradiation while in low-oxygen packaging
exhibited about a 50 percent lower rate of wear than did either
the nonsterilized cups or the nonirradiated cups sterilized with
gas plasma. There was a comparable advantage in the rate of wear
after fourteen days of thermal aging. However, after thirty days
of aging, the cups sterilized with gamma irradiation in low-oxygen
packaging wore several times faster than did the nonirradiated cups.
Ion-implanting improved the wear resistance without thermal aging,
but after extensive thermal aging the oxidation and wear were greater
than those of the controls. Hylamer cups (that is, those that were
sterilized with gas plasma) exhibited wear properties very close
to those of the nonsterilized ultra-high molecular weight polyethylene
cups (the controls) with or without aging.
Conclusions: Sterilizing an ultra-high molecular
weight polyethylene acetabular cup without radiation (for example,
with ethylene oxide or gas plasma) avoids immediate and long-term
oxidative degradation of the implant but does not improve the inherent
wear resistance of the polyethylene. Sterilizing with use of gamma
irradiation with the implant packaged in a low-oxygen atmosphere
avoids immediate oxidation and cross-links the polyethylene, thereby increasing
its wear resistance, but long-term oxidation of the residual free
radicals may markedly reduce the wear resistance. Ideally, cross-linking with
gamma irradiation to reduce wear should be done in a manner that
avoids both immediate and long-term oxidation.
Clinical Relevance: The present study demonstrated
how the fabrication and sterilization processes influence the resistance
to oxidation and wear of the various types of ultra-high molecular
weight polyethylene that are currently available. As an exact quantitative
relationship between days of thermal aging and years of real-time
aging (on the shelf and/or in vivo) has not yet
been established, it is not possible to predict precisely when,
if ever, the in vivo wear rate of cups sterilized
with gamma irradiation while in low-oxygen packaging would exceed that
of nonirradiated cups. Nevertheless, the results of these wear tests
with use of a hip simulator suggest that, for at least ten years
of clinical use, the in vivo wear rate of cups sterilized with gamma irradiation
while in low-oxygen packaging will be substantially lower than that
of cups sterilized without irradiation. The fundamental interactions among
radiation, cross-linking, and oxidation exhibited by the specific
materials included in the present study may also apply to acetabular
cups of other types of polyethylene. Understanding these fundamental
interactions will assist the surgeon in making an informed choice
among the materials examined in the present study and among other types
of modified polyethylene already in clinical use, including those
sterilized with ethylene oxide, those sterilized with gamma irradiation
in other forms of low-oxygen packaging, and the various new cross-linked
and thermally stabilized polyethylenes.
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