The enduring success of the low-friction arthroplasty first advanced
by Sir John Charnley as a solution for severe hip arthritic problems
may be appreciated from the fact that in 1999 more than 500,000
hip and knee arthroplasties were performed in the United States.
The prevalence of aseptic loosening attributed to polyethylene debris-induced
osteolysis has been in the single digits in most contemporary series,
with some reports describing prostheses surviving for twenty to thirty
years (Figs. 1-A and 1-B).
Until recently, gamma irradiation in air has been the predominant
method of sterilization of ultra-high molecular weight polyethylene
components and, despite current concerns, it represents the only
gold standard against which contemporary material improvements will
be measured over time.
The ultra-high molecular weight polyethylene used in hip and knee
arthroplasty components results from polymerization of ethylene
gas into a fine resin powder of submicrometer and micrometer size
distribution. It is consolidated with use of ram-extrusion or compression-molding
techniques. Structurally, the polymer is made up of repeating carbon-hydrogen
(-CH2-) units that are arranged in ordered (crystalline) and disordered
(amorphous) regions. Irradiation to sterilize components breaks
the polymer chains, creating free radicals, which in an air environment
combine with oxygen, facilitating ongoing oxidative degradation
of the polymer (Fig. 2).
Continued exposure of the component to oxygen through prolonged
shelf storage in air before clinical use results in a progressive
stiffening and embrittlement of the polymer, reducing wear resistance
and fatigue strength. This is thought to represent a major contributing
factor influencing in vivo polymer failure (Figs. 3 and 4).
Besides reacting with oxygen, however, free radicals can also combine,
creating cross-links between adjacent molecules. Bench-testing has
suggested that these cross-links improve wear performance. Eliminating
oxygen from the sterilization process by employing inert gas or
a vacuum environment contributes to this improvement.
Alternative sterilization methods employing ethylene oxide or gas
plasma without ionizing radiation avoid oxidation but do not realize
potential wear performance benefits resulting from increased cross-linking
(Fig. 5).
Previous attempts to improve the performance of ultra-high molecular
weight polyethylene have included carbon-fiber reinforcement (Poly-2)
and, more recently, polymer reprocessing to enhance mechanical properties
by hot isostatic pressing (Hylamer). The former was withdrawn from
the market because of an unexpectedly high wear rate (Fig. 6), while the
latter has been linked to debris-induced osteolytic response, especially
when sterilized by radiation in air, in early reports (Fig. 7).
Heat pressing was yet another attempt to improve the finish of the
articular surface, but it was associated with polyethylene fatigue
and early delamination (Fig. 8).
These findings suggest that the preclinical evaluations of the above
polyethylenes did not fully predict in vivo performance.
The new generation of cross-linked polyethylenes represents a class
of emerging ultra-high molecular weight polyethylene alternatives
whose common denominator is an appreciation of the importance of
increased cross-linking and minimization of oxidative degradation
to reduce wear. Both chemical and thermal/radiation processing
solutions have been advocated, with a number of the latter being
recently cleared by the Food and Drug Administration for commercial
product distribution.
Process differences include: (1) heating above or below the melt
temperature of the polyethylene, (2) the radiation source, (3) dose
level, and (4) end-point sterilization (Fig. 9).
In general, increasing the radiation dose dramatically reduces polymer
wear in laboratory hip joint simulation (Fig. 10). Free-radical
suppression through kinetic recombination at increased temperature
in an oxygen-free environment or quenching through remelting represent
efforts to stem the oxidation process.
The above processes also change either the amorphous or both
the amorphous and the crystalline regions of the resulting polymers,
affecting mechanical properties and potentially reducing fatigue
characteristics. Fatigue damage is a factor associated with high
cyclic stresses in nonconforming ultra-high molecular weight polyethylene
knee components. It is not known how these polyethylenes will perform
in knees. Current market application is primarily limited to acetabular components,
with limited Food and Drug Administration clearance for use in the
knee.
There is some laboratory evidence suggesting that increased cross-linking
may decrease resistance to fatigue crack propagation (Figs. 11-A and 11-B), a finding that
could have implications for some modular acetabular cup designs
(Fig. 12).
Currently, there is no direct clinical evidence of the promised long-term in
vivo integrity, suggested by laboratory evaluation, of
the cross-linked polymer being marketed today.
The absence of information on the long-term clinical performance
of highly cross-linked ultra-high molecular weight polyethylene
components prompted the Medical Devices Agency of the United Kingdom
in June 1999 to issue a Safety Notice (MDA SN1999[23])
advising caution in their use and careful patient monitoring.
For the past three decades, ultra-high molecular weight polyethylene
hip and knee components have been predominantly sterilized by gamma
irradiation in air and have shown remarkable overall resilience
in terms of clinical function.
Nevertheless, aseptic loosening attributed to polyethylene debris-induced
osteolysis is of contemporary concern. As the indications for total
joint replacement expand to younger patients and life expectancy
increases, the interest in alternative bearing materials has accelerated.
It is now known that irradiation in an environment in which oxygen
is present encourages oxidation of ultra-high molecular weight polyethylene
components, resulting in embrittlement and a decrease in wear performance.
This process continues when components are shelf-stored in air or
in permeable packaging for prolonged periods before use.
Sterilization in oxygen-free environments with barrier packaging
and shelf-dating reduce the risk of material compromise.
The use of ethylene oxide and gas plasma as alternative sterilization
methods avoids oxidative degradation, but there is loss of the potential
benefits with respect to polymer wear reduction derived from cross-linking.
Recently a number of "improved" polymers have
emerged whose common benefit resides in increased cross-linking
concurrent with minimization of oxidation. Pin-on-disk testing has
not been shown to predict in vivo performance,
but hip simulator models suggest a significant reduction in wear
with these new polymers.
These processes, however, change the chemical structure of the
polymer, affecting both static mechanical properties and fatigue
characteristics.
A number of these polyethylenes have received clearance by the
Food and Drug Administration and are commercially available despite
the absence of clinical reports.
Corporate responsibility to assess short-term performance by means
of evidence-based studies is necessary and should be a consideration
in the surgeon’s selection of highly cross-linked polymer
components.