Cơ xương khớp pdf

Highly Cross-linked
Polyethylene in Total
Hip Arthroplasty
Abstract
Although total hip arthroplasty is a common and highly successful
procedure, its long-term durability has been undermined by the
cellular response to polyethylene wear debris and the subsequent
effects on periprosthetic bone. Research elucidating the effects of
sterilization on polyethylene wear has facilitated the development
of a more wear-resistant material—highly cross-linked
polyethylene. Laboratory testing has demonstrated that highly
cross-linked polyethylene has markedly improved wear resistance
compared with conventional polyethylene under a variety of
conditions. Early clinical data have supported these results. To
make informed decisions about this already widely available and
frequently used product, the practicing orthopaedic surgeon should
have a basic understanding of the production process as well as
knowledge of the most current laboratory and clinical data.
T
otal hip arthroplasty (THA) is
one of the most successful sur-
gical procedures ever developed. Ce-
mented and cementless component
fixation provide excellent pain relief,
return of function, and intermediate
longevity in patients with degenera-
tive conditions of the hip. Ultra-
high–molecular-weight polyethylene
(UHMWPE) articulating with a metal
head has been the p redominant bear-
ing surface since the inception of

modern THA. Although the success
of this bearing couple is well docu-
mented, clinical studies and retrieval
analyses have shown that polyethyl-
ene wear and osteolysis are the ma-
jor factors limiting the longevity of
THA.
1-5
Extensive research undertaken to
elucidate the physical and biologic
mechanisms behind polyethylene
wear and osteolysis
6,7
has led to the
development of highly cross-linked
UHMWPE. The term highly cross-
linked polyethylene is commonly
used to describe this new generation
of polymers. We use this term to de-
scribe intentionally cross-linked ma-
terial; however, different manufac-
turers use proprietary methods to
produce various levels of cross-
linking in their components. Labora-
tory work and early clinical trials
have demonstrated that highly
cross-linked polyethylene is signifi-
cantly more wear-resistant than con-
ventional polyethylene. A synthesis
of the manufacturing processes, in

addition to laboratory and clinical
data regarding highly cross-linked
UHMWPE, may aid the orthopaedic
surgeon in making an informed deci-
sion regarding THA.
Alexander C. Gordon, MD
Darryl D. D’Lima, MD
Clifford W. Colwell, Jr, MD
Dr. Gordon is Orthopaedic Surgeon,
Illinois Bone and Joint Institute, Morton
Grove, IL. Dr. D’Lima is Director,
Orthopaedic Research Laboratories,
Scripps Center for Orthopaedic
Research and Education, La Jolla, CA.
Dr. Colwell is Director, Musculoskeletal
Center; Director, Scripps Center for
Orthopaedic Research and Education;
and Shiley Chair, Orthopaedic
Research, Scripps Center for
Orthopaedic Research and Education.
None of the following authors or the
departments with which they are
affiliated has received anything of value
from or owns stock in a commercial
company or institution related directly or
indirectly to the subject of this article:
Dr. Gordon, Dr. D’Lima, and Dr. Colwell.
Reprint requests: Dr. Gordon, Illinois
Bone and Joint Institute, 9000
Waukegan Road, Morton Grove, IL

60053.
J Am Acad Orthop Surg 2006;14:
511-523
Copyright 2006 by the American
Academy of Orthopaedic Surgeons.
Perspectives on Modern Orthopaedics
Volume 14, Number 9, September 2006 511
Polyethylene Resins
and Manufacturing
Polyethylene Resin
Polyethylene is simply a repeat-
ing chain of ethylene monomer mol-
ecules; the modifiers low-density,
high-density, and ultra-high–molec-
ular weight refer to the molecular
weight, chain length, and arrange-
ment of the polymer chains. The
condensed polymers have crystalline
and amorphous regions, the percent-
age and arrangement of which affect
the properties of the material (Figure
1). In general, polymers with higher
percentages of crystalline regions
have higher elastic moduli and dem-
onstrate better resistance to crack
propagation, but they may be more
susceptible to the effects of oxida-
tion.
8
Ruhrchemie AG, a predecessor

company of Ticona, began commer-
cial manufacture of UHMWPE resin
in the 1950s. Ticona is currently the
leading manufacturer of medical-
grade UHMWPE resin, with plants
in Bishop, Texas and Oberhausen,
Germany . A ll Ticona resins have the
designation “GUR,” followed by a
numeric modifier. Their medical-
grade resins are named GUR 1020,
1120, 1050, and 1150. The first of the
four numerals (1) indicates that the
polymer is designated for ortho-
paedic implantation. The second nu-
meral notes the presence (1) or ab-
sence (0) of calcium stearate. The
third digit is an indicator of molecu-
lar weight, and the fourth is an inter-
nal corporate code.
Hercules Powder manufactured
another UHMWPE resin known as
the 1900 series. Most recently pro-
duced by Basell Polyolefins, the 1900
series line (1900 and 1900H) has re-
mained the same as when Hercules
produced it. In 2002, Basell sold the
1900 resin technology and ceased
production of this product. Although
new 1900 resin is not being pro-
duced, some orthopaedic device

manufacturers have stockpiled the
material and continue to use it in
their implants. The 1900 series res-
ins have a lower mean molecular
weight and larger mean particle size
than do the GUR 1050 resins, which
may affect their clinical perfor-
mance.
Edidin et al
9
andWonetal
10
stud-
ied the effects of resin type and man-
ufacturing method on the wear and
degradation of the 1900 and GUR
resins. Won et al
10
analyzed retrieved
tibial bearings from the Miller-
Galante (MG) I and II (Zimmer, War-
saw, IN) knee arthroplasty designs.
The tibial bearings had the same ge-
ometry; both were gamma-sterilized
in air. T he MG-I bearings were made
from direct compression-molded
1900 resin, and the MG-II compo-
nents were manufactured from ex-
truded GUR 415 stock. The re-
searchers found notably higher rates

of delamination and subsurface
damage consistent with oxidation in
the MG-II components. They con-
cluded that compression-molded
1900 resin was more resistant to ox-
idation than GUR material.
Edidin et al
9
studied differences
between 1900 and GUR resins in the
laboratory setting using an accelerat-
ed aging technique to determine sus-
ceptibility to oxidation. They found
that after accelerated aging and gam-
ma sterilization in air, compression-
molded GUR 1050 and 1900 resins
as well as extruded 1050 stock de-
graded similarly in mechanical test-
ing. The authors noted more rapid
degradation of the 1900 resins than
of the GUR resins under similar test-
ing conditions, but only minor dif-
ferences in the post-aging mechani-
cal properties of the three resins.
Implant Fabrication
Implant manufacturers obtain
UHMWPE as powdered resin or
stock material from converting com-
panies, such as Poly Hi Solidur (Fort
Figure 1

A, Molecular structure of ethylene and of ultra-high–molecular-weight polyethylene (UHMWPE). n = the degree of
polymerization. B, Crystalline and amorphous regions of UHMWPE. (Reproduced with permission from Kurtz SM: The
UHMWPE Handbook: Ultra-High Molecular Weight Polyethylene in Total Joint Replacement. San Diego, CA: Elsevier
Academic Press, 2004, pp 4, 6.)
Highly Cross-linked Polyethylene in Total Hip Arthroplasty
512 Journal of the American Academy of Orthopaedic Surgeons
Wayne, IN) and Perplas Medical
(Bacup, Lancashire, UK). Compo-
nents are fabricated from the resin
by direct compression molding or
machined from converted stock sup-
plied by ram-extruded bars or mold-
ed sheets. The mechanical proper-
ties of the final product are affected
by the specific temperature, pres-
sure, and cooling rate used in the
processes.
8,11
Direct compression-
molded components made from
1900 resin have demonstrated excel-
lent clinical performance despite be-
ing sterilized by gamma radiation in
air. In his study of direct compres-
sion-molded components in the hip
and knee, Ritter
12
found osteolysis in
2.5% of hips at a mean of 21 years
and in 0% of knees at a mean of 8

years. He concluded that the clinical
performance of direct compression-
molded 1900 resin is superior to that
of other polyethylene components.
Because there is no clear consensus
on the best resin or fabrication
method for UHMWPE bearings in
THA, orthopaedic implant manufac-
turers decide which resin and fabri-
cation method best suits their im-
plants.
Sterilization and Aging
of Conventional
Polyethylene
Polyethylene THA bearings are ster-
ilized by one of two general meth-
ods—surface treatment and irradia-
tion. These methods, as well as
numerous other variables in the ster-
ilization process, have specific ef-
fects on the in vitro and in vivo per-
formance of UHMWPE acetabular
liners.
Surface Treatment
The two commonly used surface
sterilization treatments are ethylene
oxide (EtO) gas and gas plasma. Al-
though highly toxic, EtO is well-
suited for polyethylene because the
gas does not chemically react with

the component. Safe and effective
EtO sterilization requires special en-
vironmental conditions during ster-
ilization and appropriate timing to
allow the gas to diffuse in and out of
the component. Gas plasma treat-
ment is performed at a lower tem-
perature and in a shorter time frame
than EtO surface sterilization. In gas
plasma treatment, less toxic sub-
stances (eg, peracetic acid, hydrogen
peroxide gas plasma) are used to
eliminate potential contamination.
This method is newer than EtO, and
data regarding its use are limited.
11
Irradiation
Gamma radiation and its effects
on the mechanical properties of
polyethylene have been well docu-
mented, with a resultant large-scale
overhaul of polyethylene production
for THA. Irradiation of polyethylene
causes cleavage of the polymer
chains, leading to the production of
free radicals. After radiation, the
chains may bond at their original
scission point or cross-link with one
another. When neither occurs, the
cleaved end of the polymer chain re-

mains a free radical. When steriliza-
tion and packaging of the compo-
nent take place in the presence of
oxygen, the free radicals generated
by the radiation are able to combine
with oxygen molecules during stor-
age and after implantation (Figure 2).
This leaves the component suscepti-
ble to the effects of oxidation, which
are now known to adversely affect
its mechanical properties.
In a retrieval analysis of compo-
nents from multiple manufacturers,
Sutula et al
13
investigated the sub-
surface white band found in their re-
trievals. Infrared spectroscopy dem-
onstrated that this subsurface white
band corresponded to an area of high
oxidation and was present only in
components sterilized by gamma ir-
radiation in air. The appearance of
this band was time-dependent, and
all components in which the white
band was observed had been steril-
ized more than 3 years before the ob-
servation. The authors found that
the presence of the subsurface white
band corresponded with decreased

tensile strength, severe embrittle-
ment of the subsurface zone, and an
increased incidence of rim cracking
and delamination in retrieved liners
(Figure 3).
McKellop and coauthors
14,15
ex-
amined the effects of sterilization
method, calcium stearate addition,
and thermal aging on the wear per-
formance of UHMWPE in two hip
simulator studies. Before initiating
the studies, all irradiated samples re-
ceived a mean dose of 2.7 Mrad.
Despite differences in molecular
weight and the presence or absence
of calcium stearate, gas plasma–ster-
ilized components demonstrated
wear rates comparable with each
other. Among components not sub-
jected to accelerated aging, the EtO-
sterilized samples had significantly
higher wear rates than those steril-
ized with gamma radiation in air (P
= 0.0001) or in a vacuum (P = 0.0001).
Additionally, components that were
Figure 2
Effects of irradiation in an oxygen
environment on UHMWPE.

(Reproduced with permission from
Greenwald AS, Bauer TW, Ries MD,
Committee on Biomedical Engineering,
Committee on Hip and Knee Arthritis:
New polys for old: Contribution or
caveat? J B one Joint Surg Am 2001;
83(suppl 2):27-31.)
Alexander C. Gordon, MD, et al
Volume 14, Number 9, September 2006 513
gamma radiated in air had signifi-
cantly higher wear rates than did
those irradiated in a vacuum (P =
0.01) (Figure 4).
After thermal aging, all gamma-
irradiated cups, including those ster-
ilized with methods to decrease
oxidation (eg, ion implantation, ni-
trogen packaging, oxygen scavenger)
demonstrated oxidative degradation
and a subsequent increase in wear.
The unsterilized and gas plasma–
treated cups wore at the same rates
before and after thermal aging, and
both types of cups showed no oxida-
tion. After accelerated aging, cups
that were gamma-irradiated in air
had the highest wear rates of all test
specimens (Figure 5). The investiga-
tors concluded that prior to aging,
the cross-linking effect of radia-

tion—even in an oxygen environ-
ment—provided improvements in
wear compared with components
that were never sterilized or were
EtO-sterilized. After oxidation and
Figure 5
Wear rates of six types of polyethylene for two cycle intervals after artificial aging
for 14 days at 80°C. All gamma-sterilized cups had a mean radiation dose of 2.7
Mrad. (Reproduced with permission from McKellop H, Shen FW, Lu B, Campbell P,
Salovey R: 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. J Bone Joint Surg Am 2000;82:170 8-1725.)
Figure 3
Photographs of Charnley components
that were never implanted. The section
without the band (top) was never
sterilized. The section with the
pronounced white band (bottom) was
sterilized by gamma radiation in air
14 years earlier. (Reproduced with
permission from Sutula LC, Collier JP,
Saum KA, et al: The Otto Aufranc
Award: Impact of gamma sterilization
on clinical performance of polyethylene
in the hip. Clin Orthop Relat Res
1995;319:28-40.)
Figure 4
Wear rates of six types of polyethylene for two cycle intervals without artificial aging.
All gamma-sterilized cups had a mean radiation dose of 2.7 Mrad. (Reproduced
with permission from McKellop H, Shen FW, Lu B, Campbell P, Salovey R: 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. J
Bone Joint Surg Am 2000;82:1708-1725.)
Highly Cross-linked Polyethylene in Total Hip Arthroplasty
514 Journal of the American Academy of Orthopaedic Surgeons
embrittlement of the polymer, how-
ever, the advantage of irradiation is
lost.
Sychterz et al
16
studied steriliza-
tion variables in a clinical setting.
They reviewed radiographs of pa-
tients who had undergone cement-
less THA fixation whose conven-
tional acetabular liners had been
sterilized with (1) gamma radiation
in air, (2) gamma radiation in a vac-
uum and barrier-packaged, or (3) gas
plasma. The liners that were
gamma-irradiated in a vacuum and
those irradiated in air wore at signif-
icantly lower rates than did those
sterilized by gamma radiation in air
or gas plasma (P < 0.01). The authors
also concluded that the c ross-linking
provided by gamma sterilization,
even in air, provided better wear re-
sistance than did gas plasma in con-
ventional polyethylene.

Before the introduction of highly
cross-linked polyethylene, two prod-
ucts meant to be improvements on
conventional polyethylene were mar-
keted but subsequently discontin-
ued—highly crystalline UHMWPE
(Hylamer, DePuy, Warsaw, IN) and
carbon fiber–reinforced polyethylene
(Poly II, Zimmer). Hylamer has been
more extensively studied than Poly
II in THA, with reports of Hylamer
wearing at rates comparable to those
of conventional polyethylene.
15,16
De-
spite this finding, some studies
17,18
indicate high wear rates and severe
osteolysis in patients implanted with
Hylamer liners sterilized by gamma
radiation in air. In a retrieval analy-
sis, Collier et al
19
noted that for a
given level of oxidation, Hylamer lin-
ers that were gamma-sterilized in air
sustained more wear and damage
than did conventional polyethylene
sterilized in the same manner. They
suggested that the increased crystal-

linity of Hylamer makes it more sus-
ceptible to oxidation than conven-
tional polyethylene. Most reports of
Poly II are from the knee arthroplasty
literature, but one report o f carbon fi-
ber polyethylene in THA discussed
two instances of severe tissue reac-
tion and prosthetic loosening associ-
ated with this material.
20
Highly Cross-linked
Polyethylene
Manufacturing
The highly cross-linked compo-
nents available for implantation are
machined from ram-extruded bar
stock of GUR 1050 resin. Although
the exact methods are proprietary
and differ among manufacturers, the
steps to produce cross-linked poly-
ethylene follow the same general
sequence: radiation cross-linking,
thermal treatment, and terminal
sterilization
21
(Figure 6).
The first step is a cross-link–induc-
ing radiation dose of 2.5 to 10 Mrad
provided by cobalt 60 (gamma) or an
electron beam source. This is fol-

lowed by thermal treatment, in
which the polyethylene is heated be-
low, at, or above its melting temper-
ature, depending on the manufac-
turer. This step is meant to quench
free radicals, allowing the polyethyl-
ene chains to preferentially cross-link,
thus diminishing the chances for ox-
idative degradation. The heating
methods, which are proprietary, may
be combined with electron beam ir-
radiation because this process mea-
surably heats the polymer. The final
step is terminal sterilization and bar-
rier packaging. Te rminal sterilization
of these components is usually a sur-
face treatment, but some manufactur-
ers use a sterilizing dose of gamma ra-
diation in an inert atmosphere.
In a study attempting to deter-
mine the effects of these specific
steps, Muratoglu et al
22
found high-
er levels of free radicals and more
post-aging oxidation in polymers
Figure 6
1050 Extruded
rod
Machine cup Radiation

(1) 5 Mrad
(2) 10 Mrad
7.5 Mrad
radiation
125 C
Warming oven
Warming oven
3 Mrad
Sterilize, N
2
Heat above melt
(>135 C)
Heat anneal
9.5 Mrad
Electron beam
10 Mrad
Electron beam
Heat anneal in
package
Machine cup Machine cup Heat above melt
(>135 C)
Sterilize
(1) Gas plasma
(2)
2.5 Mrad
St
erilize,
N
2
/Vacuum

Machine cup Machine cup
Ethylene oxide
sterilize
Gas plasma
sterilize
Process
Heat
stabilized
CISM
(cold irradiated
subsequent melt)
CIAN
(cold irradiated
adiabatic non-melt)
WISM
(warm irradiated
subsequent melt)
Longevity
Durasul
Zimmer
Stryker
Howmedica
Osteonics
Crossfire
(1) Marathon
(2) XLPE
(1) DePuy/
Johnson & Johnson
(2) Smith+Nephew
Duration

Product
Company
Stryker
Howmedica
Osteonics
°
Heat above melt
C)
WIAM
(warm irradiated
adiabatic melting)
(>135
Ethylene oxide
Zimmer
°
°
°
Processing steps for highly cross-linked polyethylene, by manufacturer.
(Reproduced with permission from Greenwald AS, Bauer TW, Ries MD, Committee
on Biomedical Engineering, Committee on Hip and Knee Arthritis: New polys for
old: Contribution or caveat? J Bone Joint Surg Am 2001;83(suppl 2):27-31.)
Alexander C. Gordon, MD, et al
Volume 14, Number 9, September 2006 515
treated with sub-melt temperature
annealing and terminal gamma ster-
ilization (Crossfire; Stryker How-
medica Osteonics, Mahwah, NJ)
than in those that were melted and
gas sterilized after cross-linking radi-
ation (Longevity; Zimmer). In a

retrieval analysis of explanted cross-
linked liners from different manu-
facturers, Bhattacharyya et al
23
hy-
pothesized that Crossfire would
show more in vivo oxidation than
melt-stabilized polyethylene, such
as Longevity or Durasul (Zimmer).
Within 3 years of implantation, the
authors found elevated oxidation
levels and one component with a
subsurface white band among the
Crossfire liners; they did not detect
any oxidation in the other two types.
Bhattacharyya et al
23
concluded that
the free radicals formed by sub-melt
temperature annealing and gamma
sterilization can lead to in vivo oxi-
dation.
Laboratory Studies
Laboratory studies have demon-
strated that higher degrees of cross-
linking improve wear resistance and
decrease particulate volume in a hip
simulator.
24
Although increased

femoral head size causes increased
volumetric wear rates in hips im-
planted with conventional polyeth-
ylene,
25
wear-simulator studies of
highly cross-linked polyethylene
have demonstrated greatly dimin-
ished wear compared with conven-
tional polyethylene in liners articu-
lating with 22-, 28-, 32-, and 46-mm
heads.
Hermida et al
26
compared highly
cross-linked liners with nominally
cross-linked liners articulating with
28- and 32-mm femoral heads. The
highly cross-linked liners had been
sub-melt temperature annealed and
sterilized with gamma radiation; the
nominally cross-linked liners were
polyethylene that was conventional-
ly sterilized by gamma radiation in
nitrogen. The 28- and 32-mm highly
cross-linked liners had significantly
(P < 0.001) less wear than did their
conventional counterparts, but the
wear of 28- and 32-mm highly cross-
linked cups did not differ significant-

ly (Figure 7). The authors concluded
that larger femoral head size may
not be predisposed to increased wear
in highly cross-linked liners.
Muratoglu and colleagues
27,28
have
extensively studied electron beam
cross-linked, melt-annealed, and
EtO-sterilized (Durasul) UHMWPE.
They studied the mechanical proper-
ties, oxidation levels, effect of femo-
ral head size, and wear rates com-
pared with those of conventional
polyethylene. The authors found
markedly less wear of the highly
cross-linked liners compared with
gamma-sterilized/inert implants for
femoral head sizes ranging from 22 to
46 mm. After weighing the compo-
nents, they determined that there
was no detectable wear from the
highly cross-linked specimens and
that the head penetration noted was
solely the result of plastic deforma-
tion. This was corroborated by the
presence of machining marks on the
cross-linked liners after 20 million
cycles; these marks had been worn
away on the conventional polyethyl-

ene specimens. The mechanical and
molecular analysis of Durasul
showed no oxidation after acceler-
ated aging and no evidence of free
radicals, but it did demonstrate a de-
crease in ultimate tensile strength
(UTS) and yield strength compared
with gamma-sterilized/inert polyeth-
ylene. Despite the inferior mechan-
ical properties of the highly cross-
linked polyethylene, the testing
results fell well within American So-
ciety for Testing and Materials
(ASTM) standards for medical-grade
UHMWPE. However, ASTM stan-
dards do not imply that a polyethyl-
ene component is suitable for clini-
cal use and do not include a
specification for fracture toughness.
The diminished crack propagation re-
sistance of cross-linked polyethylene
may have clinical implications.
Other researchers have tested
highly cross-linked polyethylene un-
der more adverse conditions, such as
wear in the presence of a third body
or a rough countersurface. One study
comparing gamma/nitrogen–cross-
linked, barrier-packaged polyethyl-
Figure 7

Cumulative wear rates of highly cross-linked (X) and nominally cross-linked (O)
acetabular liners articulating with 28- and 32-mm heads. (Reproduced with
permission from Hermida JC, Bergula A, Chen P, Colwell CW Jr, D’Lima DD:
Comparison of the wear rates of twenty-eight and thirty-two-millimeter femoral
heads on cross-linked polyethylene acetabular cups in a wear simulator. J Bone
Joint Surg Am 2003;85:2325-2331.)
Highly Cross-linked Polyethylene in Total Hip Arthroplasty
516 Journal of the American Academy of Orthopaedic Surgeons
ene articulating with femoral heads
of differing surface roughness report-
ed significantly (P = 0.004) less wear
of the cross-linked liners.
29
These in-
vestigators found that severely
roughened balls (surface roughness,
0.9 µm) during and after the initial
wear-in period had wear rates higher
than that of the conventional poly-
ethylene articulating with a smooth
surface, thus negating the effects of
cross-linking on wear.
Although it is not known wheth-
er this degree of roughening occurs
in vivo, Minakawa et al
30
attempted
to quantify the third-body damage of
retrieved femoral heads. They deter-
mined that cobalt-chrome heads

could suffer varying degrees of dam-
age; cobalt-chrome heads had a
mean surface roughness of 0.4 µm
on their most damaged areas. The
authors found a single component
with damage >2.0 µm, which sug-
gests that the conditions in the
study by McKellop et al
29
could oc-
cur in vivo.
Bragdon et al
31
compared the wear
resistance of gamma/nitrogen and
cross-linked polyethylene in an en-
vironment of polymethylmethacry-
late (PMMA) or alumina third-body
particles. As expected, the speci-
mens with the alumina particles
wore much more than did those
with PMMA or without third-body
particles. Although the authors
found that the presence of a very
hard third body (eg, alumina) affect-
ed the wear of cross-linked polyeth-
ylene, PMMA particles had a very
small effect. The cross-linked liners
in this study demonstrated signifi-
cantly (P < 0 .0001) l ess wear than did

conventional polyethylene in all
testing conditions. Taylor et al
32
also
studied the effects of PMMA parti-
cles on wear of cross-linked polyeth-
ylene. They found lower wear in the
cross-linked specimens than in con-
trols; however, they did note signif-
icant surface damage and wear rates
that were much higher than those
reported by Bragdon et al.
31
Although much research has fo-
cused on the wear rates of cross-
linked polyethylene, other reports
have focused on the characterization
of the wear particles generated dur-
ing these tests. Ingram et al
33
mea-
sured the size of wear particles pro-
duced by wearing 5- and 10-Mrad
cross-linked polyethylene against
smooth and rough surfaces, then
tested their biologic activity by de-
termining the levels of tumor necro-
sis factor-α production by macro-
phages cultured with the wear
debris. They found that increased

levels of cross-linking, associated
with wear from the rougher surface,
led to a higher percentage of debris
in the submicron range and in-
creased biologic activity. Wear
against a smooth surface resulted in
nanometer-sized particles in non-
and cross-linked specimens, thus de-
creasing their biologic activity.
These data suggest that although ab-
solute wear is decreased with cross-
linking, the particles generated are
biologically active and have the po-
tential to induce osteolysis.
Collier et al
34
obtained cross-
linked acetabular liners from six US
orthopaedic implant manufacturers
to determine the effects of the differ-
ing manufacturing techniques on
the mechanical properties, crystal-
linity, and pre- and post-aging oxida-
tion levels of the various compo-
nents (Table 1). Their goal was to
determine the properties of clinical-
ly available polyethylene liners and
relate those properties to the wear
rates published by the manufactur-
ers. The authors did not do a head-

to-head comparison of wear rates.
These cups were subjected to an ac-
celerated aging protocol. Before ag-
ing, all test cups showed no to low
initial oxidation rates; however, Du-
rasul, Crossfire, and ArCom did have
higher “as received” oxidation l evels
than did a standard reference poly-
ethylene. After accelerated aging,
the Longevity, C rossfire, and ArCom
liners demonstrated significantly
more oxidation than their as-
received counterparts (P < 0.01),
while the others had no change in
oxidation level. The Longevity liners
had the lowest initial oxidation lev-
el of the six test specimens, and the
authors thought that its increase in
oxidation after aging was not enough
to affect its mechanical properties.
Mechanical testing demonstrated
a range of UTS (34 to 59 MPa) and a
smaller range of tensile strength at
yield point (19 to 24 MPa) for the as-
received components (Table 2 ). Af-
ter aging, ArCom, Durasul, and
Crossfire liners demonstrated a de-
crease in UTS, and ArCom, Reflec-
tion, and Crossfire liners showed sig-
nificant differences in yield point

compared with their as-received
counterparts (Table 3). All materials
tested exceeded the ASTM standard
(27 MPa) for UTS and tensile stress
at yield point (19 MPa) in non–cross-
linked polyethylene.
Collier et al
34
found that the UTS
of the materials was stratified by ra-
diation dose; the components receiv-
ing >5 Mrad had significantly (P <
0.01) lower values than did the refer-
ence polyethylene before aging. In
contrast, the tensile strength a t y ield
point was stratified by the heating
method rather than radiation dose.
The components that were heated at
or above their melting temperature
showed significantly (P < 0.01) low-
er values than did the reference ma-
terial. The investigators concluded
that even intentionally cross-linked
polyethylene is not immune to oxi-
dation and free-radical formation;
the varying oxidation levels and sus-
ceptibility to oxidation after aging
were dependent on the processing
conditions. The authors also stated
that increasing the radiation dosages

appears to produce lower wear rates,
as reported by the manufacturers,
but also results in lower toughness.
The appendix of the article
34
in-
cludes a paragraph from each manu-
facturer regarding the rationale for
the manufacturing processes used in
the production of its components.
Subsequent to the 2003 publica-
tion of the study by Collier et al,
34
Alexander C. Gordon, MD, et al
Volume 14, Number 9, September 2006 517
Smith & Nephew increased the
cross-linking radiation dose from 5
to 10 Mrad; Stryker Howmedica Os-
teonics introduced a new cross-
linked polyethylene (X3) that is se-
quentially irradiated and annealed;
and Biomet began production of Ar-
Com XL, an intentionally cross-
linked and heat-stabilized version of
its direct compression-molded Ar-
Com product. No published clinical
or laboratory results are available on
these updated products.
Concern about the loss of fracture
toughness and more brittle nature of

highly cross-linked polyethylene has
been the topic of numerous reports.
Baker and colleagues
36-38
conducted
several studies to elucidate the fa-
tigue resistance and fracture tough-
ness of cross-linked polyethylene.
Their hypothesis was that the high-
er degree of cross-linking, leading to
a restriction of chain mobility in the
amorphous regions of polyethylene,
would decrease the plasticity of the
polymer, resulting in a material that
is less resistant to crack propagation.
The true stress at break point and
the resistance to crack propagation
were inversely related to the cross-
linking radiation dose and were at-
tributed to a decrease in plasticity at
the fracture tip. The results of these
studies suggest that for clinical situ-
ations in which stress concentra-
tions and surface defects may exist,
a lower degree of cross-linking may
be safer.
Clinical Evaluation and
Retrieval Analysis
Extensive laboratory data exist on
the wear resistance of cross-linked

polyethylene, but few clinical stud-
ies are available. The published stud-
ies (Table 4) are generally consistent
with the laboratory data, but long-
term follow-up is not yet available.
Three randomized, prospective eval-
uations of highly cross-linked poly-
ethylene with 2-year clinical follow-
up have been published.
39-41
In a study of cemented THA, Di-
Table 1
Cross-linked Material Tested by Collier et al
34
Material
(Manufacturer) Resin Fabrication
Radiation
Source
Dose to
Cross-link Annealing
ArCom (Biomet,
Warsaw, IN)
1900H Direct compression-molded
or machined from
molded bar
Gamma 2.5 to 4 Mrad None
Marathon
(DePuy,
Warsaw, IN)
1050 Machined from extruded

bar
Gamma 5 Mrad Above melt temperature
(150°C)
Reflection XLPE
(Smith &
Nephew,
Memphis, TN)
1050 Machined Gamma 5 Mrad
(subsequently
changed to 10
Mrad)
At melt temperature
(136°C)
Durasul
(Zimmer,
Warsaw, IN)
1050 Machined from
compression-molded
sheet
Electron
beam
9.5 Mrad Above-room-temperature
pre-heat before electron
beam; melt anneal;
controlled heat and
cooling rates; warm
irradiation with
adiabatic melting
Crossfire
(Stryker

Howmedica
Osteonics,
Mahwah, NJ)
1050 Machined Gamma 7.5 Mrad
(subsequently
irradiated
with 3 Mrad)
Below melt temperature
(>120°C)
Longevity
(Zimmer)
1050 Compression molded and
machined
Gamma 10 Mrad Above-room-temperature
pre-heat before electron
beam; process between
cold irradiation with
subsequent melt and
warm irradiation with
adiabatic melting
Adapted with permission from Collier JP, Currier BH, Kennedy FE, et al: Comparison of cross-linked polyethylene materials for
orthopaedic applications. Clin Orthop Relat Res 2003;414:289-304.
Highly Cross-linked Polyethylene in Total Hip Arthroplasty
518 Journal of the American Academy of Orthopaedic Surgeons
gas et al
39
compared Durasul with
conventional polyethylene sterilized
by gamma irradiation in nitrogen.
Head penetration into the liner was

evaluated with radiostereometric
analysis (RSA) at 1 and 2 years post-
operatively. Head penetration seen
on supine radiographs was similar
between groups at 1 and 2 years, but
was approximately 50% less in the
highly cross-linked group in the
same time period as seen on stand-
ing radiographs. Additionally, n o dif-
ference was found between groups
with respect to component migra-
tion or the appearance of radiolucent
lines. Most of the head penetration
in the first year was attributed to
plastic deformation of the socket.
In another study,
40
these same in-
vestigators presented their 3-year
data with the cemented liners and
introduced a new study of bilateral
hybrid THAs. The cemented cup
study results were similar to those
in the previous report, with lower
head penetration rates in the highly
cross-linked group at 3-year follow-
up. The hybrid hip study used an
RSA method to compare Longev-
ity liners with polyethylene that
was compression-molded, gamma-

irradiated in nitrogen, and implant-
ed into a cementless cup. During the
first year, head penetration rates of
the two polyethylenes were not sig-
nificantly different, but at 2 years,
significantly (P < 0.0005) less head
penetration was observed in the
cross-linked components. The au-
thors concluded that the similar ear-
ly head penetration rates generally
reflect creep and not wear.
Using a digital radiographic tech-
nique in a randomized, prospective
evaluation with 2-year follow-up,
Martell et al
41
compared Crossfire
with polyethylene irradiated in ni-
Table 2
Mechanical Properties of As-Received Acetabular Liners
Cross-linked
Material
Yield Point
(MPa)
Probability
Value*
Ultimate Tensile
Strength (MPa)
Probability
Value* Elongation (%)

Probability
Value*
ArCom 24 ± 0.8 P < 0.01 59 ± 4.7 0.3826 240 ± 38 P <0.01
Marathon 21 ± 0.5 P < 0.01 56 ± 7.0 0.1892 300 ± 14 P < 0.01
Reflection
XLPE
20 ± 1.3 P < 0.01 56 ± 7.1 0.1895 300 ± 20 P < 0.01
Crossfire 22 ± 1.0 0.8177 53 ± 5.3 P < 0.01 230 ± 17 P < 0.01
Durasul 19 ± 1.6 P < 0.01 34 ± 3.4 P < 0.01 330 ± 19 P < 0.01
Longevity 21 ± 1.1 0.0271 43 ± 5.3 P < 0.01 250 ± 25 P < 0.01
HSS
Reference
UHMWPE
35
21.7 ± 1.0 — 58 ± 4.7 — 380 ± 10 —
*Probability values are for the t-test between the cross-linked materials and the Hospital for Special Surgery (HSS) reference
ultra-high–molecular-weight polyethylene (UHMWPE).
Adapted with permission from Collier JP, Currier BH, Kennedy FE, et al: Comparison of cross-linked polyethylene materials for
orthopaedic applications. Clin Orthop Relat Res 2003;414:289-304.
Table 3
Mechanical Properties of Acetabular Liners After 28 Days of Artificial Aging
Cross-linked
Material
Yield Point
(MPa)
Probability
Value*
Ultimate Tensile
Strength (MPa)
Probability

Value* Elongation (%)
Probability
Value*
ArCom 25 ± 1.4 P < 0.01 40 ± 8.1 P < 0.01 300 ± 60 P < 0.01
Marathon 21 ± 1.5 0.3877 56 ± 5.7 0.8699 290 ± 14 P < 0.01
Reflection
XLPE
21 ± 1.4 0.0393 58 ± 7.3 0.2643 300 ± 37 0.9858
Crossfire 24 ± 1.3 P < 0.01 48 ± 7.2 P < 0.01 280 ± 37 P < 0.01
Durasul 20 ± 0.7 0.0279 30 ± 7.1 P < 0.01 280 ± 74 P < 0.01
Longevity 21 ± 1.0 0.1662 43 ± 9.8 0.8006 240 ± 35 0.0718
*Probability values are for the t-test between the “as received” and aged cross-linked material properties
Adapted with permission from Collier JP, Currier BH, Kennedy FE, et al: Comparison of cross-linked polyethylene materials for
orthopaedic applications. Clin Orthop Relat Res 2003;414:289-304.
Alexander C. Gordon, MD, et al
Volume 14, Number 9, September 2006 519
trogen and barrier-packaged. The au-
thors noted a marked (40% to 50%)
decrease in the two-dimensional lin-
ear, two-dimensional volumetric,
and three-dimensional linear wear
rates in the highly cross-linked
group. Head penetration seen in the
first year after implantation was
mostly caused by plastic deforma-
tion, not by true wear.
Heisel et al
42
performed a nonran-
domized study comparing Marathon

cross-linked polyethylene with con-
ventional polyethylene sterilized by
gamma irradiation in air; they found
an 81% decrease in volumetric wear
in the cross-linked group after 2
years. Using regression analysis to
control for the differences between
groups, they determined that the
type of polyethylene was the only
significant variable influencing vol-
umetric wear rates.
The study with the longest
follow-up to date, published by Dorr
et al,
43
compared Durasul with
gamma/nitrogen polyethylene after 5
years of clinical use. In a retrospec-
tive study of 37 Durasul hips
matched to historical controls, direct
radiographic measurements were
used to calculate the mean annual
head penetration rates; the investiga-
tors found that a digital measure-
ment technique did not provide accu-
rate data. The “bedding-in” period for
Durasul was approximately 2 years,
while that of the conventional poly-
ethylene was 1 year. From 2 to 5
years, the linear wear rate of Durasul

was approximately 50% less and the
annual head penetration rate was
60% to 75% less than conventional
polyethylene during the same period.
In a prospective, nonrandomized
study using RSA, Rohrl et al
44
com-
pared wear rates of cemented stems
articulating with either cemented
gamma/air or Crossfire polyethylene.
In contrast with other studies, the
bedding-in period was only 2 months,
and the wear rates were linear for
both groups thereafter. From 2 to 24
months, an 85% reduction in wear
was noted in the Crossfire group, and
cross-linked polyethylene demon-
strated significantly (P < 0.001) lower
wear rates than did gamma/air poly-
ethylene without increased migra-
tion or radiolucencies.
In all of the aforementioned stud-
ies, wear rates were lower for cross-
linked polyethylene than for controls.
Larger differences between conven-
tional and cross-linked polyethylene
were found when the controls were
gamma-sterilized in air versus in an
inert environment, again demonstrat-

ing the inferior wear characteristics
of gamma/air polyethylene.
In addition to the clinical studies,
in vivo behavior of highly cross-
linked polyethylene after a relative-
ly short service life has been studied
using retrieval analysis. Bradford et
al
45
studied 21 cross-linked Durasul
liners revised 2 to 24 months after
implantation. Pitting, scratches, and
surface cracking were common find-
ings, but no liners d emonstrated bur-
nishing or severe wear (Figure 8).
The authors postulated that the
cracking was likely the result of the
diminished ductility and fatigue re-
sistance of the polymer and conclud-
ed that the in vivo wear patterns of
highly cross-linked polyethylene dif-
fer from those occurring in a hip
simulator. The significance of this
finding is that hip simulators did not
accurately predict the in vivo perfor-
mance of a given material.
Other researchers attribute a dif-
ferent significance to the surface find-
ings of explanted cross-linked liners,
however. Muratoglu et al

46
also stud-
ied liners not revised for wear with a
service life of 2 weeks to 10 months.
They used a melt-recovery technique
to test their hypothesis that the sur-
face scratching represented plastic de-
formation, not true wear. Their most
common findings were light and
heavy surface scratches; a few spec-
imens had polished areas. The melt-
recovery process was used to recover
the machining marks, if present. Five
of the seven liners treated with this
process had complete or near-
complete recovery of the original ma-
chining marks. The authors con-
Table 4
Summary of Highly Cross-linked Polyethylene Clinical Studies
Author
Cross-linked
Polyethylene
(Fixation)
Conventional
Polyethylene
Follow-up
(years)
Wear Reduction of Cross-linked
Polyethylene
Digas et al

39
Durasul Gamma/Nitrogen 2 50% linear wear
Digas et al
40
Durasul
(cemented)
Gamma/Nitrogen 3 50% linear wear
Longevity
(hybrid)
Gamma/Nitrogen 2 62% linear wear
Martell et al
41
Crossfire Gamma/Nitrogen 2 40% to 50% linear wear
Heisel et al
42
Marathon Gamma/Air 2 81% volumetric wear
Dorr et al
43
Durasul Gamma/Nitrogen 5 50% linear wear;
60% to 75% less head penetration
Rohrl et al
44
Crossfire Gamma/Air 2 85% linear wear
Highly Cross-linked Polyethylene in Total Hip Arthroplasty
520 Journal of the American Academy of Orthopaedic Surgeons
cluded that the scratches did not
represent removal of material, as
would occur with wear, but were a re-
configuration of the surface second-
ary to third-body abrasion (Figure 9).

Bradford et al
47
also reported on a
single case of osteolysis associated
with a cross-linked Longevity poly-
ethylene liner used in a hybrid THA.
At the time of femoral component re-
vision, they noted loosening of the
stem within the cement mantle as
well as deformation of the polyethyl-
ene rim consistent with impinge-
ment. Although analysis showed the
interface membrane was laden with
micron-sized polyethylene particles,
other factors, such as PMMA and
metal debris from a loose stem, can
lead to accelerated polyethylene wear .
Future Directions
Current research involving highly
cross-linked polyethylene is focused
on methods to maintain the materi-
al’s wear resistance while improving
its mechanical properties. The re-
melting step currently used to pro-
cess much of the commercially
available cross-linked polyethylene
is known to decrease the amount of
crystallinity in the material, thus
contributing to a decline in mechan-
ical strength. Below–melt tempera-

ture annealing can help retain crys-
tallinity but may allow residual free
radicals.
48
Second-generation cross-
linked polyethylenes are being de-
veloped that are heated below their
melt temperature but use pharmaco-
logic additives, mechanical deforma-
tion, or sequential low-dose irradia-
tion and annealing to eliminate
residual free radicals.
Figure 9
Photographs showing damaged surfaces of explanted highly cross-linked liners (A1
and B1). Nearly full recovery of machining marks after application of melt-recovery
technique (A2 and B2). (Reproduced with permission from Muratoglu OK,
Greenbaum ES, Bragdon CR, Jasty M, Freiberg AA, Harris WH: Surface analysis
of early retrieved acetabular polyethylene liners: A comparison of conventional and
highly crosslinked polyethylenes. J Arthroplasty 2004;19:68-77.)
Figure 8
A, Low-magnification image (x55) of a highly cross-linked liner demonstrating normal and abnormal machining marks. B, Low-
magnification image (x55) demonstrating altered machining marks and surface cracks parallel to marks on a highly cross-linked
liner. C, High-magnification image (x550) demonstrating cracks perpendicular to machine marks. (Reproduced with permission
from Bradford L, Baker DA, Graham J, Chawan A, Ries MD, Pruitt LA: Wear and surface cracking in early retrieved highly cross-
linked polyethylene acetabular liners. J Bone Joint Surg Am 2004;86:1271-1282.)
Alexander C. Gordon, MD, et al
Volume 14, Number 9, September 2006 521
Muratoglu et al
49
studied two

methods to improve the oxidative
stability of nonmelted cross-linked
polyethylene, hoping to improve the
mechanical properties over irradiated
and melted polyethylene. Their re-
search used irradiated polyethylene
treated with either mechanical
deformation or the addition of
α-tocopherol (vitamin E doping).
Comparing these samples with irra-
diated and melted ones, they found
significantly improved UTS (P < 0.05)
and fatigue strength (P < 0.05) with
comparable wear rates. They thought
that the introduction of these mate-
rials would be particularly useful in
high-stress environments, such as to-
tal knee replacement. Dumbleton
and Manley
50
compared sequential
low-dose irradiated and annealed
(SXL) polyethylene with specimens
gamma-sterilized in nitrogen. They
found no difference in the mechani-
cal properties or susceptibility to ox-
idation between the groups but noted
a 97% wear reduction in the SXL
group. The authors concluded that
this method represents the next gen-

eration of cross-linked polyethylene,
with applications for both THA and
knee arthroplasty.
Summary
Most of the polyethylene used today
for THA has an elevated level of
cross-linking. Although the produc-
tion steps are similar between man-
ufacturers, small differences may
have a large clinical impact. The lab-
oratory data and early clinical trials
clearly support t he claims of the sub-
stantially improved wear resistance
of highly cross-linked polyethylene
over polyethylenes used in the past.
Despite this, data regarding the
lower fatigue resistance and retriev-
als demonstrating in vivo damage
modes that are different from those
found in hip simulators may temper
the enthusiasm for highly cross-
linked polyethylene. Compared with
the new generation of hard bearings,
cross-linked polyethylene offers nu-
merous advantages. The concerns
over potential ion toxicity with
metal-on-metal devices, and cata-
strophic failure with ceramic-on-
ceramic, are not issues with highly
cross-linked polyethylene. Addition-

ally, the modularity of polyethylene
with the availability of multiple
head sizes; anteverted, lipped, later-
alized, and constrained liners; and
more neck length options is certain-
ly advantageous. Although polyeth-
ylene wear is not eliminated, the
wear rates of the cross-linked mate-
rials may be low enough that clinical
osteolysis may no longer be a prob-
lem. Clinical failures may occur
with highly cross-linked polyethyl-
ene, but knowledge of the product
can help surgeons and industry act
accordingly if problems develop.
References
Evidence-based Medicine: Level I
and II randomized, prospective stud-
ies: references 39, 40, 41, 44, and 46.
The remaining studies are implant
analyses or cohort case studies.
Citation numbers printed in bold
type indicate references published
within the past 5 years.
1. Berry DJ, Harmsen WS, Cabanela ME,
Morrey BF: Twenty-five-year survivor-
ship of two thousand consecutive pri-
mary Charnley total hip replacements:
Factors affecting survivorship of acetab-
ular and femoral components. J Bone

Joint Surg Am 2002;84:171-177.
2. Maloney WJ, Schmalzried T, Harris
WH: Analysis of long-term cemented
total hip arthroplasty retrievals. Clin
Orthop Relat Res 2002;405:70-78.
3. Engh CA Jr, Claus AM, Hopper RH Jr,
Engh CA: Long-term results using the
anatomic medullary locking hip pros-
thesis. Clin Orthop Relat Res 2001;
393:137-146.
4. Parvizi J, Keisu KS, Hozack WJ, Shar-
key PF, Rothman RH: Primary total
hip arthroplasty with an uncemented
femoral component: A long-term
study of the Taperloc stem.
J Arthroplasty 2004;19:151-156.
5. D’Antonio JA, Capello WN, Manley
MT, Geesink R: Hydroxyapatite fem-
oral stems for total hip arthroplasty:
10- to 13-year followup. Clin Orthop
Relat Res 2001;393:101-111.
6. McKellop HA, Campbell P, Park S-H,
et al: The origin of submicron poly-
ethylene wear debris in total hip ar-
throplasty. Clin Orthop Relat Res
1995;311:3-20.
7. Schmalzried TP, Jasty M, Harris WH:
Periprosthetic bone loss in total hip
arthroplasty: Polyethylene wear de-
bris and the concept of the effective

joint space. J Bone Joint Surg Am
1992;74:849-863.
8. Li S: Polyethylene, in Callaghan JJ,
Rosenberg AG, Rubash HE (eds): The
Adult Hip. Philadelphia, PA:
Lippincott-Raven, 1998, pp 105-122.
9. Edidin AA, Herr MP, Villarraga ML,
Muth J, Yau SS, Kurtz SM: Accelerat-
ed aging studies of UHMWPE: I. Effect
of resin, processing, and radiation en-
vironment on resistance to mechani-
cal degradation. J Biomed Mater Res
2002;61:312-322.
10. Won CH, Rohatgi S, Kraay MJ, Gold-
berg VM, Rimnac CM: Effect of resin
type and manufacturing method on
wear of polyethylene tibial compo-
nents. Clin Orthop Relat Res 2000;
376:161-171.
11. Kurtz SM, Muratoglu OK, Evans M,
Edidin AA: Advances in the process-
ing, sterilization, and crosslinking of
ultra-high molecular weight polyeth-
ylene for total joint arthroplasty.
Biomaterials 1999;20:1659-1688.
12. Ritter MA: Direct compression mold-
ed polyethylene for total hip and knee
replacements. Clin Orthop Relat Res
2001;393:94-100.
13. Sutula LC, Collier JP, Saum KA, et al:

The Otto Aufranc Award: Impact of
gamma sterilization on clinical per-
formance of polyethylene in the hip.
ClinOrthopRelatRes1995;319:28-40.
14. McKellop H, Shen FW, Lu B, Camp-
bell P, Salovey R: Effect of steriliza-
tion method and other modifications
on the wear resistance of acetabular
cups made of ultra-high molecular
weight polyethylene: A hip-simulator
study. J Bone Joint Surg Am 2000;82:
1708-1725.
15. McKellop HA, Shen F-W, Campbell P,
Ota T: Effect of molecular weight, cal-
cium stearate, and sterilization meth-
ods on the wear of ultra high molecu-
lar weight polyethylene acetabular
cups in a hip joint simulator.
J Orthop Res 1999;17:329-339.
16. Sychterz CJ, Orishimo KF, Engh CA:
Sterilization and polyethylene wear:
Clinical studies to support laboratory
data. J Bone Joint Surg Am 2004;86:
1017-1022.
17. Livingston BJ, Chmell MJ, Spector M,
Highly Cross-linked Polyethylene in Total Hip Arthroplasty
522 Journal of the American Academy of Orthopaedic Surgeons
Poss R: Complications of total hip ar-
throplasty associated with the use of
an acetabular component with a Hy-

lamer liner. J Bone Joint Surg Am
1997;79:1529-1538.
18. Yamauchi K, Hasegawa Y, Iwasada S,
et al: Head penetration into Hylamer
acetabular liner sterilized by gamma
irradiation in air and in a nitrogen at-
mosphere. J Arthroplasty 2001;16:
463-470.
19. Collier JP, Bargmann LS, Currier BH,
Mayor MB, Currier JH, Bargmann BC:
An analysis of Hylamer and polyeth-
ylene bearings from retrieved acetab-
ular components. Orthopedics 1998;
21:865-871.
20. Pryor GA, Villar RN, Coleman N: Tis-
sue reaction and loosening of carbon-
reinforced polyethylene arthroplasties.
J Bone Joint Surg Br 1992;74:156-157.
21. Greenwald AS, Bauer TW, Ries MD,
Committee on Biomedical Engineer-
ing, Committee on Hip and Knee Ar-
thritis: New polys for old: Contribu-
tion or caveat? J Bone Joint Surg Am
2001;83(suppl 2):27-31.
22. Muratoglu OK, Merrill EW, Bragdon
CR, et al: Effect of radiation, heat, and
aging on in vitro wear resistance of
polyethylene. Clin Orthop Relat Res
2003;417:253-262.
23. Bhattacharyya S, Doherty AM, Wan-

nomae KK, et al: Severe in vivo oxida-
tion in a limited series of retrieved
highly-crosslinked UHMWPE acetab-
ular components with residual free
radicals. Trans Orthop Res Soc 2004;
29:276.
24. Chiesa R, Tanzi MC, Alfonsi S, Parac-
chini L, Moscatelli M, Cigada A: En-
hanced wear performance of highly
crosslinked UHMWPE for artificial
joints. J Biomed Mater Res 2000;50:
381-387.
25. Livermore J, Ilstrup D, Morrey B: Ef-
fect offemoral head sizeon wear ofthe
polyethylene acetabular component.
JBone Joint Surg Am1990;72:518-528.
26. Hermida JC, Bergula A, Chen P, Col-
well CW Jr, D’Lima DD: Comparison
of the wear rates of twenty-eight and
thirty-two-millimeter femoral heads
on cross-linked polyethylene acetab-
ular cups in a wear simulator. J Bone
Joint Surg Am 2003;85:2325-2331.
27. Muratoglu OK, Bragdon CR,
O’Connor DO, Jasty M, Harris WH: A
novel method of cross-linking ultra-
high-molecular-weight polyethylene
to improve wear, reduce oxidation,
and retain mechanical properties: Re-
cipient of the 1999 HAP Paul Award.

J Arthroplasty 2001;16:149-160.
28. M uratoglu OK, Bragdon CR, O’Connor
DO, et al: Larger diameter femoral
heads used in conjunction with a
highly cross-linked ultra-high molec-
ular weight polyethylene: A new con-
cept. J Arthroplasty 2001;16(suppl 1):
24-30.
29. McKellop H, Shen F-W, DiMaio W,
Lancaster JG: Wear of gamma-
crosslinked polyethylene acetabular
cups against roughened femoral balls.
Clin OrthopRelatRes1999;369:73-82.
30. Minakawa H, Stone MH, Wroblewski
BM, Lancaster JG, Ingham E, Fisher J:
Quantification of third-body damage
and its effect on UHMWPE wear with
different types of femoral head.
J Bone Joint Surg Br 1998;80:894-899.
31. Bragdon CR, Jasty M, Muratoglu OK,
O’Connor DO, Harris WH: Third-
body wear of highly cross-linked
polyethylene in a hip simulator.
J Arthroplasty 2003;18:553-561.
32. Taylor SK, Serekian P, Br uchalski P,
Manley M: The performance of
irradiation-crosslinked UHMWPE cups
under abrasive conditions throughout
hip joint simulation wear testing.
Trans Orthop Res Soc 1999;24:252.

33. Ingram JH, Fisher J, Stone M, Ingham
E: Effect of crosslinking on biological
activity of UHMWPE wear debris.
Trans Orthop Res Soc 2003;28:1439.
34. Collier JP, Currier BH, Kennedy FE, et
al: Comparison of cross-linked poly-
ethylene materials for orthopaedic ap-
plications. Clin Orthop Relat Res
2003;414:289-304.
35. Sanford WM, Saum KA: Accelerated
oxidative aging testing of UHMWPE.
Trans Orthop Res Soc 1995;119:20.
36. Baker DA, Bellare A, Pruitt L: The ef-
fects of degree of crosslinking on the
fatigue crack initiation and propaga-
tion resistance of orthopedic-grade
polyethylene. J Biomed Mater Res A
2003;66:146-154.
37. Baker DA, Hastings RS, Pruitt L:
Study of fatigue resistance of chemi-
cal and radiation crosslinked medical
grade ultrahigh molecular weight
polyethylene. J Biomed Mater Res
1999;46:573-581.
38. Baker DA, Hastings RS, Pruitt L: Com-
pression and tension fatigue resistance
of medical grade ultra high molecular
weight polyethylene: The effect of mor-
phology, sterilization, aging and tem-
perature. Polymer 2000;41:795-808.

39. Digas G, Kärrholm J, Thanner J, Mal-
chau H, Herberts P: Highly cross-linked
polyethylene in cemented THA: Ran-
domized study of 61 hips. Clin
Orthop Relat Res 2003;417:126-138.
40. Digas G, Kärrholm J, Thanner J, Mal-
chau H, Herberts P: The Otto Aufranc
Award: Highly cross-linked polyethyl-
ene in total hip arthroplasty: Random-
ized evaluation of penetration rate in
cemented and uncemented sockets us-
ing radiostereometric analysis. Clin
Orthop Relat Res 2004;429:6-16.
41. Martell JM, Verner JJ, Incavo SJ: Clin-
ical performance of a highly cross-
linked polyethylene at two years in
total hip arthroplasty: A randomized
prospective trial. J Arthroplasty
2003;18(suppl 1):55-59.
42. Heisel C, Silva M, Dela Rosa MA,
Schmalzried TP: Short-term in vivo
wear of cross-linked polyethylene.
JBone Joint Surg Am 2004;86:748-751.
43. Dorr LD, Wan Z, Shahrdar C, Sirianni
L, Boutary M, Yun A: Clinical perfor-
mance of a Durasul highly cross-linked
polyethylene acetabular liner for total
hip arthroplasty at five years. J Bone
Joint Surg Am 2005;87:1816-1821.
44. Rohrl S, Nivbrant B, Mingguo L, Hewitt

B: In vivo wear and migration of highly
cross-linked polyethylene cups: A ra-
diostereometry analysis study.
J Arthroplasty 2005;20:409-413.
45. Bradford L, Baker DA, Graham J, Cha-
wan A, Ries MD, Pruitt LA: Wear and
surface cracking in early retrieved
highly cross-linked polyethylene ace-
tabular liners. J Bone Joint Surg Am
2004;86:1271-1282.
46. Muratoglu OK, Greenbaum ES, Brag-
don CR, Jasty M, Freiberg AA, Harris
WH: Surface analysis of early re-
trieved acetabular polyethylene lin-
ers: A comparison of conventional
and highly crosslinked polyethylenes.
J Arthroplasty 2004;19:68-77.
47. Bradford L, Kurland R, Sankaran M,
Kim H, Pruitt LA, Ries MD: Early fail-
ure due to osteolysis associated with
contemporary highly cross-linked
ultra-high molecular weight polyeth-
ylene: A case report. J Bone Joint
Surg Am 2004;86:1051-1056.
48. Ries MD, Pruitt L: Effect of cross-
linking on the microstructure and
mechanical properties of ultra-high
molecular weight polyethylene. Clin
Orthop Relat Res 2005;440:149-156.
49. Muratoglu OK, Burroughs BR, Malhi

A, et al: Abstract: Two second gener-
ation highly crosslinked UHMWPEs
show improved mechanical proper-
ties and fatigue strength. Trans
Orthop Res Soc 2005;30:1661.
50. Dumbleton JH, Manley MT: Abstract:
Sequential irradiation annealed poly-
ethylene: The next generation of highly
crosslinked polyethylene. 72rd A nnual
Meeting Proceedings. Rosemont, IL:
American Academy of Orthopaedic
Surgeons, 2005, p 453.
Alexander C. Gordon, MD, et al
Volume 14, Number 9, September 2006 523