Longevitỷ
Crosslinked Polyethylene Liners
Dale Swarts; Michel Laurent, Ph.D.; Roy
Crowninshield, PhD Research and Development Zimmer, Inc Warsaw, Indiana Crosslinking of the molecular structure in polyethylene
offers an opportunity to improve the material’s resistance to wear and to
reduce the number of wear particles produced. The clinical history with crosslinked polyethylene began with
Dr. Oonishi (1) who used gamma crosslinked acetabular cups more than
twenty years ago. Others (2,3) have
used both radiation and chemically crosslinked polyethylene components
clinically. More recently, advanced
techniques of improving the wear resistant of polyethylene through
crosslinking have been developed. However,
since crosslinking can affect other mechanical properties of polyethylene,
care must be taken to understand not only the wear performance but also to
fully characterize other properties of crosslinked polyethylene. Thorough analysis of various processes and the
resulting material properties has led to the development of a highly
crosslinked polyethylene. This
material shows promise in addressing clinical wear issues and has mechanical
properties which meet ASTM, ISO, and FDA guidelines. Since the material has no measurable level
of free radicals, it can be expected to maintain its mechanical properties
with aging. An electron beam irradiation process and melt
annealing techniques are used to produce Longevity
Crosslinked Polyethylene. Acetabular
components fabricated from this material have been tested to 20-million
cycles in hip simulator studies. The
graph below presents the average wear (weight loss) of 28-mm and 32-mm head
size cups of both crosslinked and normal gamma-sterilized components. Testing of the components in a third-body abrasion
environment or in the presence of scratched femoral heads produces a higher
rate of wear. However, third body
abrasion and scratched femoral head tests demonstrate that the
crosslinked material maintains a wear advantage over normal polyethylene in
adverse situations. Elimination of free radicals and thus the
potential for oxidation was determined by testing material that had been
artificially aged at elevated temperature in a pressurized oxygen environment. Reduction of free radicals reduces the
potential for oxidation and the subsequent alteration of mechanical
properties. Longevity Polyethylene retains 100 percent of its ultimate
tensile strength after accelerated aging. The crosslinking of polyethylene offers promise to
improve the longer-term clinical outcome of total hip replacement through the
reduction in wear particle generation.
This material demonstrates wear improvement in hip simulation testing
compared to conventional processed polyethylene during both ideal and adverse
wear situations. References: 1.
Oonishi H, Saito M,
Kadoya Y. Wear of high dose gamma
irradiated polyethylene in total joint replacement – long-term radiological
evaluation. Orthopaedic Research
Society 44th Annual Meeting; 1998. 2.
Wroblewski BM, Siney PD,
Fleming PA. Low friction arthroplasty
of the hip using alumina ceramic and crosslinked polyethylene – a ten-year
follow-up report. J Bone Joint Surg. 1999;81-B;1:54-55. 3.
Grobbelaar CJ, Du
Plessis TA, Marais F. The radiation
improvement of polyethylene prostheses.
J Bone Joint Surg. 1978;60-B(3):370-374. |
