U.S. patent application number 11/982100 was filed with the patent office on 2008-06-05 for chemically crosslinked ultrahigh molecular weight polyethylene for artificial human joints.
Invention is credited to Harry A. McKellop, Ronald Salovey, Fu-Wen Shen.
Application Number | 20080133018 11/982100 |
Document ID | / |
Family ID | 27007631 |
Filed Date | 2008-06-05 |
United States Patent
Application |
20080133018 |
Kind Code |
A1 |
Salovey; Ronald ; et
al. |
June 5, 2008 |
Chemically crosslinked ultrahigh molecular weight polyethylene for
artificial human joints
Abstract
The present invention discloses a method for enhancing the
wear-resistance of polymers by crosslinking them, especially before
irradiation sterilization. In particular, this invention presents
the use of chemically crosslinked ultrahigh molecular weight
polyethylene in in vivo implants.
Inventors: |
Salovey; Ronald; (Rancho
Palos Verdes, CA) ; McKellop; Harry A.; (Los Angeles,
CA) ; Shen; Fu-Wen; (Los Angeles, CA) |
Correspondence
Address: |
Wean Khing Wong;Attorney at Law
PMB 210, 1441 Hungtington Drive
South Pasadena
CA
91030
US
|
Family ID: |
27007631 |
Appl. No.: |
11/982100 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10752167 |
Jan 3, 2004 |
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11982100 |
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10262869 |
Oct 3, 2002 |
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10752167 |
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09898192 |
Jul 2, 2001 |
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10262869 |
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09406305 |
Sep 27, 1999 |
6281264 |
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09898192 |
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08698638 |
Aug 15, 1996 |
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09406305 |
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08376953 |
Jan 20, 1995 |
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08698638 |
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Current U.S.
Class: |
623/18.11 ;
522/161; 525/333.7 |
Current CPC
Class: |
B29K 2105/24 20130101;
A61F 2002/3082 20130101; B29K 2995/0089 20130101; B29C 2035/085
20130101; A61F 2/468 20130101; A61F 2/32 20130101; B29K 2995/0087
20130101; A61F 2002/3611 20130101; A61F 2002/30233 20130101; B29L
2031/7532 20130101; C08J 5/00 20130101; B29B 13/08 20130101; B29C
43/16 20130101; A61F 2/30 20130101; C08J 7/0427 20200101; B29C
71/0063 20130101; A61F 2002/30879 20130101; A61F 2/34 20130101;
B29C 43/00 20130101; A61F 2/3094 20130101; C08J 2323/06 20130101;
A61L 27/16 20130101; B29K 2023/0683 20130101; A61F 2230/0069
20130101; A61L 27/16 20130101; A61F 2/30 20130101; C08L 23/06
20130101; A61L 27/16 20130101; C08L 23/06 20130101 |
Class at
Publication: |
623/18.11 ;
525/333.7; 522/161 |
International
Class: |
A61F 2/30 20060101
A61F002/30; C08F 10/02 20060101 C08F010/02; C08J 3/28 20060101
C08J003/28 |
Claims
1. A medical implant for use within a body, said implant being
formed of a crosslinked ultrahigh molecular weight polyethylene
having a polymeric structure of 51% crystallinity or less, so as to
increase wear resistance of said implant within the body.
2. In a medical implant having at least a first member and a
bearing component providing a bearing contact surface for the first
member, the bearing component made of crosslinked ultrahigh
molecular weight polyethylene and wherein said crosslinked
ultrahigh molecular weight polyethylene is characterized by a
polymeric structure of at least 97.5% gel content, as determined by
using boiling p-xylene.
3. The medical implant of claim 2, wherein crosslinking is achieved
according to the method selected from the group consisting of:
chemically crosslinking a polyethylene, irradiation crosslinking a
polyethylene, and photocrosslinking a polyethylene.
4. The medical implant of claim 3, wherein said implant is further
irradiated in the solid state for sterilization.
5. The medical implant of claim 4, wherein said implant is
irradiated in air at a sterilization dose.
6. The medical implant of claim 3, wherein said crosslinked
ultrahigh molecular weight polyethylene is further characterized by
its lack of shrinkage.
7. The medical implant of claim 2, wherein the implant is a hip
prosthesis, said first member is a femoral component and said
bearing component is an acetabular component.
8. The medical implant of claim 2, wherein the medical implant is a
prosthesis selected from the group consisting of: hip, knee, ankle,
elbow, jaw, shoulder, finger and spine prostheses.
9. A medical implant bearing component with improved wear
resistance for use within a joint prosthesis within a body, said
implant bearing component is made of a crosslinked ultrahigh
molecular weight polyethylene having a polymeric structure of at
least 97.5% gel content, as determined by using boiling p-xylene,
so as to increase wear resistance of said implant within the
body.
10. The medical implant bearing component of claim 9, wherein said
crosslinked ultrahigh molecular weight polyethylene is
characterized by a polymeric structure of about 3.4 degree of
swelling or less, as determined by using boiling p-xylene.
11. The medical implant bearing component of claim 9, wherein the
joint prosthesis is a hip prosthesis and said implant is an
acetabular component which cooperates with a femoral component.
12. The medical implant bearing component of claim 9, wherein the
joint prosthesis is selected from the group consisting of hip,
knee, ankle, elbow, jaw, shoulder, finger and spine prostheses.
13. The medical implant bearing component of claim 9, wherein said
ultrahigh molecular weight polyethylene is further characterized by
a smooth fracture surface, decreased tensile strength at break
point, and decreased Young's modulus compared to a corresponding
uncrosslinked ultrahigh molecular weight polyethylene.
14. The medical implant bearing component of claim 9, wherein
crosslinking is achieved according to the method selected from the
group consisting of chemically crosslinking a polyethylene,
irradiation crosslinking a polyethylene, and photocrosslinking a
polyethylene.
15. The medical implant bearing component of claim 14, wherein the
medical implant is further irradiated in air at a sterilization
dose in its solid state.
16. The medical implant bearing component of claim 15, wherein said
ultrahigh molecular weight polyethylene has about 45% crystallinity
or less, as determined by differential scanning calorimetry.
17. The medical implant bearing component of claim 16, wherein the
crosslinked ultrahigh molecular weight polyethylene is further
characterized by a lack of shrinkage.
18. The medical implant bearing component of claim 17, wherein said
implant is an orthopaedic bearing component for use in hip or knee
joint replacement.
19-143. (canceled)
Description
[0001] This is a continuation of co-pending U.S. patent application
Ser. No. 10/752,167, filed on Jan. 3, 2004, which is a division of
co-pending U.S. patent application Ser. No. 10/262,869, filed on
Oct. 3, 2002, entitled "CHEMICALLY CROSSLINKED ULTRAHIGH MOLECULAR
WEIGHT POLYETHYLENE FOR ARTIFICIAL HUMAN JOINTS", which is a
continuation of application Ser. No. 09/898,192, filed on Jul. 2,
2001 which is a continuation of application Ser. No. 09/406,305,
filed on Sep. 27, 1999, and issued as U.S. Pat. No. 6,281,264,
which is a continuation of application Ser. No. 08/698,638, filed
on Aug. 15, 1996 and now abandoned, which is a division of
application Ser. No. 08/376,953, filed on Jan. 20, 1995 and now
abandoned. The entire contents of the parent applications are
expressly incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to polymers. It discloses a
method for enhancing the wear-resistance of polymers, especially
polymers that are to be irradiated, by crosslinking the polymers.
The crosslinked polymers may be annealed to stabilize their size
shrinkage. The polymers disclosed herein are particularly useful
for making in vivo implants.
BACKGROUND OF THE INVENTION
[0003] Ultrahigh molecular weight polyethylene (hereinafter
referred to as "UHMW polyethylene") is commonly used to make
prosthetic joints such as artificial hip joints. In recent years,
it has become increasingly apparent that tissue necrosis and
interface osteolysis, in response to UHMW polyethylene wear debris,
are primary contributors to the long-term loosening failure of
prosthetic joints. For example, the process of wear of acetabular
cups of UHMW polyethylene in artificial hip joints introduces many
microscopic wear particles into the surrounding tissues. The
reaction of the body to these particles includes inflammation and
deterioration of the tissues, particularly the bone to which the
prosthesis is anchored. Eventually, the prosthesis becomes
painfully loose and must be revised. It is generally accepted by
orthopaedic surgeons and biomaterials scientists that the reaction
of tissue to wear debris is the chief cause of long-term failure of
such prostheses.
[0004] Laboratory experiments and examination of worn polyethylene
components, as used in acetabular cups of total hip prostheses,
after removal from patients, have shown that polyethylene wear in
vivo primarily involves three fundamental mechanisms: adhesive,
abrasive, and fatigue wear {Brown, K. J., et al., Plastics in
Medicine & Surgery Plastics & Rubber Institute, London,
2.1-2.5 (1975); Nusbaum, H. J. & Rose, R. M., J. Biomed.
Materials Res., 13:557-576 (1979); Rostoker, W., et al., J. Biomed.
Materials Res., 12:317-335 (1978); Swanson, S. A. V. & Freeman,
M. A. R., Chapter 3, "Friction, lubrication and wear.", The
Scientific Basis of Joint Replacement, Pittman Medical Publishing
Co., Ltd. (1977).}
[0005] Adhesive wear occurs when there is local bonding between
asperities on the polymer and the opposing (metal or ceramic)
counterface. If the ratio of the strength of the adhesive bond to
the cohesive strength of the polymer is great enough, the polymer
may be pulled into a fibril, finally breaking loose to form a wear
particle. Small wear particles (measuring microns or less) are
typically produced.
[0006] Abrasive wear occurs when asperities on the surface of the
femoral ball, or entrapped third-body particles, penetrate into the
softer polyethylene and cut or plow along the surface during
sliding. Debris may be immediately formed by a cutting process, or
material may be pushed to the side of the track by plastic
deformation, but remain an integral part of the surface.
[0007] Fatigue wear is dependent on cyclic stresses applied to the
polymer. As used herein, fatigue wear is an independent wear
mechanism involving crack formation and propagation within the
polymer. Cracks may form at the surface and coalesce, releasing
wear particles as large as several millimeters and leaving behind a
corresponding pit on the surface, or cracks may form a distance
below the surface and travel parallel to it, eventually causing
sloughing off of large parts of the surface.
[0008] There are gaps in the prior art regarding the contributions
of the above three basic mechanisms to the wear of polyethylene
cups in vivo. While numerous laboratory studies and analyses of
retrieved implants have provided valuable details on wear in vivo,
there is ongoing disagreement regarding which wear mechanisms
predominate and what are the controlling factors for wear.
[0009] However, it is clear that improving the wear resistance of
the UHMW polyethylene socket and, thereby, reducing the amount of
wear debris generated each year, would extend the useful life of
artificial joints and permit them to be used successfully in
younger patients. Consequently, numerous modifications in physical
properties of UHMW polyethylene have been proposed to improve its
wear resistance.
[0010] UHMW polyethylene components are known to undergo a
spontaneous, post-fabrication increase in crystallinity and changes
in other physical properties. {Grood, E. S., et al., J. Biomedical
Materials Res., 16:399-405 (1976); Kurth, J., et al., Trans. Third
World Biomaterials Congress, 589 (1988); Rimnac, C. M., et al., J.
Bone & Joint Surgery, 76-A(7):1052-1056 (1994)). These occur
even in stored (non-implanted) cups after sterilization with gamma
radiation which initiates an ongoing process of chain scission,
crosslinking, and oxidation or peroxidation involving free radical
formation. {Eyerer, P. & Ke, Y. C., J. Biomed. Materials Res.
18:1137-1151 (1984); Nusbaum, H. J. & Rose, R. M., J. Biomed.
Materials Res., 13:557-576 (1979); Roe, R. J., et al., J. Biomed.
Materials Res., 15:209-230 (1981); Shen, C. & Dumbleton, J. H.,
Wear, 30:349-364 (1974)}. These degradative changes may be
accelerated by oxidative attack from the joint fluid and cyclic
stresses applied during use. {Eyerer, P. & Ke, Y. C., J.
Biomed. Materials Res., supra; Grood, E. S., et al., J. Biomed.
Materials Res., supra; Rimnac, C. M., et al., ASTM Symposium on
Biomaterials' Mechanical Properties, Pittsburgh, May 5-6
(1992)}.
[0011] On the other hand, it has been reported that the best total
hip prosthesis for withstanding wear is one with an alumina head
and an irradiated UHMW polyethylene socket, as compared to a
un-irradiated socket. The irradiated socket had been irradiated
with 10.sup.8 rad of .gamma.-radiation, or about 40 times the usual
sterilization dose. {Oonishi, H., et al., Radiat. Phys. Chem.,
39(6):495-504 (1992)}. The usual average sterilization dose ranges
from 2.5 to 4.0 Mrad. Other investigators did not find any
significant reduction in the wear rates of UHMW polyethylene
acetabular cups which had been irradiated, in the solid phase, in
special atmospheres to reduce oxidation and encourage crosslinking.
{Ferris, B. D., J. Exp. Path., 71:367-373 (1990); Kurth, M., et
al., Trans. Third World Biomaterials Congress, 589 (1988); Roe, R.
J., et al., J. Biomed. Materials Res., 15:209-230 (1981); Rose, et
al., J. Bone & Joint Surgery, 62A(4):537-549 (1980); Streicher,
R. M., Plastics & Rubber Processing & Applications,
10:221-229 (1988)}.
[0012] Meanwhile, DePuy.DuPont Orthopaedic has fabricated
acetabular cups from conventionally extruded bar stock that has
previously been subjected to heating and hydrostatic pressure that
reduces fusion defects and increases the crystallinity, density,
stiffness, hardness, yield strength, and resistance to creep,
oxidation and fatigue. {U.S. Pat. No. 5,037,928, to Li, et al.,
Aug. 6, 1991; Huang, D. D. & Li, S., Trans. 38th Ann. Mtg.,
Orthop. Res. Soc., 17:4.03 (1992); Li, S. & Howard, E. G.,
Trans. 16th Ann. Society for Biomaterials Meeting, Charleston,
S.C., 190 (1990).) Silane cross-linked UHMW polyethylene (XLP) has
also been used to make acetabular cups for total hip replacements
in goats. In this case, the number of in vivo debris particles
appeared to be greater for XLP than conventional UHMW polyethylene
cup implants (Ferris, B. D., J. Exp. Path., 71:367-373 (1990)}.
[0013] other modifications of UHMW polyethylene have included: (a)
reinforcement with carbon fibers {"Poly Two Carbon-Polyethylene
Composite--A Carbon Fiber Reinforced Molded Ultra-High Molecular
Weight Polyethylene", Technical Report, Zimmer (a Bristol-Myers
Squibb Company), Warsaw (1977)}; and (b) post processing treatments
such as solid phase compression molding {Eyerer, P., Polyethylene,
Concise Encyclopedia of Medical & Dental Implant Materials,
Pergamon Press, Oxford, 271-280 (1990); Li, S., et al., Trans. 16th
Annual Society for Biomaterials Meeting, Charleston, S.C., 190
(1990); Seedhom, B. B., et al., Wear, 24:35-51 (1973); Zachariades,
A. E., Trans. Fourth World Biomaterials Congress, 623 (1992)}.
However, to date, none of these modifications has been demonstrated
to provide a significant reduction in the wear rates of acetabular
cups. Indeed, carbon fiber reinforced polyethylene and a
heat-pressed polyethylene have shown relatively poor wear
resistance when used as the tibial components of total knee
prosthesis. {Bartel, D. L., et al., J. Bone & Joint Surgery,
68-A(7):1041-1051 (1986); Conelly, G. M., et al., J. Orthop. Res.,
2:119-125 (1984); Wright, T. M., et al., J. Biomed. Materials Res.,
15: 719-730 (1981); Bloebaum, R. D., et al., Clin. Orthop.,
269:120-127 (1991); Goodman, S. & Lidgren, L., Acta Orthop.
Scand., 63(3) 358-364 (1992); Landy, M. M. & Walker, P. S., J.
Arthroplasty, Supplement, 3:S73-S85 (1988); Rimnac, C. M., et al.,
Trans. Orthopaedic Research Society, 17:330 (1992); Rimnac, C. M.
et al., "Chemical and mechanical degradation of UHMW polyethylene:
Preliminary report of an in vitro investigation," ASTM Symposium on
Biomaterials' Mechanical Properties, Pittsburgh, May 5-6
(1992)}.
SUMMARY OF THE INVENTION
[0014] one aspect of the invention presents a method for reducing
the crystallinity of a polymer so that it can better withstand
wear. An effective method for reducing the crystallinity of the
polymer is by crosslinking. For reduction of crystallinity, the
polymer may be irradiated in the melt or, preferably, chemically
crosslinked in the molten state. The method is particularly useful
for polymer which undergoes irradiation sterilization in the solid
state. It is advantageous if the crosslinked polymer is annealed to
stabilize its shrinkage.
[0015] Another aspect of the invention presents a method for making
in vivo implants based on the above treatment of the polymer.
[0016] Another aspect of the invention presents a polymer, made
from the above method, having an increased ability to withstand
wear.
[0017] Another aspect of the invention presents in vivo implants
made from the polymer described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 presents SEM micrographs of fracture surfaces of the
compression molded UHMW polyethylene (after irradiation) at
magnifications of (A).times.200 and (B).times.5000.
[0019] FIG. 2 presents SEM micrographs of fracture surfaces of
compression molded UHMW polyethylene crosslinked with 1 wt %
peroxide (after irradiation) at magnifications of (A).times.200 and
(B).times.5000.
[0020] FIG. 3 presents the geometry of the acetabular cup tested
for wear on the hip joint simulator used in EXAMPLE 2 below.
[0021] FIG. 4 presents a schematic diagram of the hip joint
simulator used in EXAMPLE 2 below.
[0022] FIG. 5 presents a graph comparing the amounts of wear of the
modified and unmodified UHMW polyethylene cups during a run lasting
a million cycles.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Abbreviations used in this application are as follows:
[0024] DSC--differential scanning calorimetry. [0025] FTIR--Fourier
Transform Infrared Spectroscopy [0026] SEM--scanning electron
microscopy [0027] UHMW--ultra-high molecular weight [0028]
UHMWPE--ultra-high molecular weight polyethylene, also referred to
as UHMW polyethylene [0029] WAXS--wide angle X-ray scattering
[0030] Cutting through the plethora of choices and confusion in the
art, applicants discovered that a low degree of crystallinity is a
major factor in increasing the ability of polyethylene to withstand
wear in vivo, contrary to the above teaching of DePuy.DuPont
Orthopaedic. Solid polymers that can crystallize generally contain
both crystalline and amorphous states. These two states have
different physical properties. The applicants believe that the
crystalline component of polymers is more brittle and less
wear-resistant than the amorphous component, the amorphous
component being more ductile and more wear-resistant.
[0031] In the present invention, the degree of crystallinity of the
polymer is preferably reduced by crosslinking. The crosslinking can
be achieved by various methods known in the art, for example, by
irradiation crosslinking of the molten polymer; photocrosslinking
of the molten polymer; and crosslinking of the polymer with a free
radical generating chemical. The preferred method is chemical
crosslinking. As indicated, if the crosslinking is to be achieved
by irradiation, the polymer should be irradiated in the melt,
unlike the above mentioned prior art irradiation methods, such as
in Oonishi et al. Applicants also discovered that such a
crosslinked polymer is useful for in vivo implant because it is
wear resistant. Such in vivo implant has not been envisioned by the
prior art. Moreover, since acetabular cups are routinely sterilized
by irradiation which increases the crystallinity of UHMW
polyethylene {Bhateja, S. K., J. Macromol. Sci. Phys., B22:159
(1983); Bhateja, S. K., et al., J. Polym. Sci., Polym. Phys. Ed.,
21:523 (1983); and Bhateja, S. K. & Andrews, E. H., J. Mater.
Sci., 20:2839 (1985)}, applicants realized that the irradiation in
fact makes the polymer more susceptible to wear, contrary to the
teaching of the prior art such as Oonishi et al, supra. By
crosslinking the polymer before sterilization by irradiation,
applicants' method mitigates the deleterious effects of
irradiation, such as chain scission. Applicants' method calls for
determination of the crystallinity after irradiation to adjust the
crosslinking conditions to reduce crystallinity. The polymer may
also be irradiated under certain conditions e.g., in nitrogen
atmosphere to reduce the immediate and subsequent amounts of
oxidation. Reducing oxidation increases the amount of crosslinking.
In producing acetabular cups, applicants discovered that both
uncrosslinked and crosslinked cups show shrinkage in size, but
crosslinked cups tend to shrink more than uncrosslinked cups. Thus,
the present invention also provides for annealing the crosslinked
polymer in order to shrink it to a stable size before reshaping the
polymer.
[0032] Most importantly, implants which are produced by the
foregoing methods of the invention are more wear resistant than
conventional untreated polymer. Thus, an example of the present
invention presents an UHMW polyethylene acetabular cup of a total
hip prosthesis which has been chemically crosslinked by a peroxide,
and then sterilized by irradiation, showing only one fifth of the
wear of a control cup after a simulated year of in vivo use.
Method for Treating the Polymers
[0033] One aspect of the invention presents a method for treating a
polymer to reduce its crystallinity to less than 45% to enable the
resulting polymer to better withstand wear. The polymer's
crystallinity is preferably reduced by crosslinking in the molten
state followed by cooling to the solid state. Preferably, the
crosslinking reduces the crystallinity of the polymer by about 10%
to 50%; more preferably, by about 10% to 40%; and most preferably,
by about 10% to 30% compared to an uncrosslinked polymer. For
example, the preferable degree of crystallinity of crosslinked UHMW
polyethylene is between about 24% to 44%; more preferably, between
29% to 44%; and most preferably, between about 34% to 44% After
sterilization by irradiation, the crosslinked polymer has a reduced
crystallinity compared to the uncrosslinked polymer. Preferably,
the irradiated crosslinked polymer possesses about 10% to 50%; more
preferably, about 10% to 40%; and most preferably, about 10% to 30%
less degree of crystallinity compared to the uncrosslinked but
irradiated polymer. For example, the preferable degree of
crystallinity of irradiated, crosslinked UHMW polyethylene is
between about 28% to 51%; more preferably, between about 33% to
51%; and most preferably, between about 39% to 51%. For example,
EXAMPLE 1, Table 1 below shows the degree of crystallinity for UHMW
polyethylene containing different weight percentage of peroxide. In
the following EXAMPLE 2, UHMW polyethylene which was crosslinked by
1% weight (wt) peroxide exhibited about 39.8% crystallinity, i.e.
about a 19% reduction in crystallinity compared to uncrosslinked
UHMW polyethylene which possessed about 49.2% crystallinity. After
gamma irradiation to an average dose of about 3.4 Mrad, the
crosslinked UHMW polyethylene exhibits about 42% crystallinity,
i.e., a reduction of about 25% in crystallinity compared to the
originally uncrosslinked but radiation sterilized UHMW polyethylene
which possessed about 55.8% crystallinity. Thus, it is contemplated
that after the usual sterilization dosage in the solid state, which
generally averages between 2.5 to 4.0 Mrad, the treated polymer
preferably possesses less than about 45% crystallinity, and more
preferably about 42% crystallinity or less. Also, the treated
polymer preferably possesses less than about 43%, more preferably
less than about 40%, crystallinity before irradiation in the solid
state.
[0034] If the polymer is to be molded, e.g. as an acetabular cup,
the polymer may be placed in the mold and crosslinked therein.
Further crosslinking examples are: (1) irradiation of the polymer
when it is in a molten state, e.g. UHMW polyethylene has been
crosslinked in the melt by electron beam irradiation; and molten
linear polyethylene has been irradiated with fast electrons
{Dijkstra, D. J. et al., Polymer, 30:866-709 (1989); Gielenz G.
& Jungnickle, B. J., Colloid & Polymer Sci., 260:742-753
(1982)}; the polymer may also be gamma-irradiated in the melt; and
(2) photocrosslinking of the polymer in the melt, e.g. polyethylene
and low-density polyethylene have been photocrosslinked {Chen, Y.
L. & Ranby, B., J. Polymer Sci.: Part A: Polymer Chemistry,
27:4051-4075, 4077-4086 (1989)}; Qu, B. J. & Ranby, B., J.
Applied Polymer Sci., 48:711-719 (1993)}.
Choices of Polymers
[0035] The polymers are generally polyhydrocarbons. Ductile
polymers that wear well are preferred. Examples of such polymers
include: polyethylene, polypropylene, polyester and polycarbonates.
For example, UHMW polymers may be used, such as UHMW polyethylene
and UHMW polypropylene. An UHMW polymer is a polymer having a
molecular weight (MW) of at least about a million.
[0036] For in vivo implants, the preferred polymers are those that
are wear resistant and have exceptional chemical resistance. UHMW
polyethylene is the most preferred polymer as it is known for these
properties and is currently widely used to make acetabular cups for
total hip prostheses. Examples of UHMW polyethylene are: Hostalen
GUR 415 medical grade UHMW polyethylene flake (Hoechst-Celanese
Corporation, League City, Tex.), with a weight average molecular
weight of 6.times.10.sup.6 MW; Hostalen GUR 412 with a weight
average molecular weight of between 2.5.times.10.sup.6 to
3.times.10.sup.6 MW; Hostalen GUR 413 of 3.times.10.sup.6 to
4.times.10.sup.6 MW; RCH 1000 (Hoechst-Celanese Corp.); and HiFax
1900 of 4.times.10.sup.6 MW (HiMont, Elkton, Md.). GUR 412, 413 and
415 are in the form of powder. RCH 1000 is usually available as
compression molded bars. Historically, companies which make
implants have used GUR 412 and GUR 415 for making acetabular cups.
Recently, Hoechst-Celanese Corp. changed the designation of GUR 415
to 4150 resin and indicated that 4150 HP was for use in medical
implants.
Methods for Characterizing the Polymers (Especially the
Determination of Their Crystallinity
[0037] The degree of crystallinity of the crosslinked polymer may
be determined after it has been crosslinked or molded. In case the
treated polymer is further irradiated, e.g., to sterilize the
polymer before its implant into humans, the degree of crystallinity
may be determined after irradiation, since irradiation effects
further crystallization of the polymer.
[0038] The degree of crystallinity can be determined using methods
known in the art, e.g. by differential scanning calorimetry (DSC),
which is generally used to assess the crystallinity and melting
behavior of a polymer. Wang, X. & Salovey, R., J. App. Polymer
Sci., 34: 593-599 (1987).
[0039] X-ray scattering from the resulting polymer can also be used
to further confirm the degree of crystallinity of the polymer, e.g.
as described in Spruiell, J. E., & Clark, E. S., in "Methods of
Experimental Physics", L. Marton & C. Marton, Eds., Vol. 16,
Part B, Academic Press, New York (1980). Swelling is generally used
to characterize crosslink distributions in polymers, the procedure
is described in Ding, Z. Y., et al., J. Polymer Sci., Polymer
Chem., 29: 1035-38 (1990). Another method for determining the
degree of crystallinity of the resulting polymer may include FTIR
(Painter, P. C. et al., "The Theory Of Vibrational Spectroscopy And
Its Application To Polymeric Materials", John Wiley and Sons, New
York, U.S.A. (1982)} and electron diffraction. FTIR assesses the
depth profiles of oxidation as well as other chemical changes such
as unsaturation (Nagy, E. V., & Li, S., "A Fourier transform
infrared technique for the evaluation of polyethylene orthopaedic
bearing materials", Trans. Soc. for Biomaterials, 13:109 (1990);
Shinde, A. & Salovey, R., J. Polymer Sci., Polym. Phys. Ed.,
23:1681-1689 (1985)}. A further method for determining the degree
of crystallinity of the resulting polymer may include density
measurement according to ASTM D1505-68.
Methods for Chemically Crosslinking the Polymers
[0040] The polymer is preferably chemically crosslinked to decrease
its crystallinity. Preferably, the crosslinking chemical, i.e. a
free radical generating chemical, has a long half-life at the
molding temperature of the chosen polymer. The molding temperature
is the temperature at which the polymer is molded. The molding
temperature is generally at or above the melting temperature of
polymer. If the crosslinking chemical has a long half-life at the
molding temperature, it will decompose slowly, and the resulting
free radicals can diffuse in the polymer to form a homogeneous
crosslinked network at the molding temperature. Thus, the molding
temperature is also preferably high enough to allow the flow of the
polymer to occur to distribute or diffuse the crosslinking chemical
and the resulting free radicals to form the homogeneous network.
For UHMW polyethylene, the molding temperature is between
150.degree. to 220.degree. C. and the molding time is between 1 to
3 hours; the preferable molding temperature and time being
170.degree. C. and 2 hours, respectively.
[0041] Thus, the crosslinking chemical may be any chemical that
decomposes at the molding temperature to form highly reactive
intermediates, free radicals, which would react with the polymers
to form the crosslinked network. Examples of free radical
generating chemicals are peroxides, peresters, azo compounds,
disulfides, dimethacrylates, tetrazenes, and divinyl benzene.
Examples of azo compounds are: azobis-isobutyronitride,
azobis-isobutyronitrile, and dimethylazodi isobutyrate. Examples of
peresters are t-butyl peracetate and t-butyl perbenzoate.
[0042] Preferably the polymer is crosslinked by treating it with an
organic peroxide. The preferable peroxides are
2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne (Lupersol 130,
Atochem Inc., Philadelphia, Pa.);
2,5-dimethyl-2,5-di-(t-butylperoxy)-hexane; t-butyl .alpha.-cumyl
peroxide; di-butyl peroxide; t-butyl hydroperoxide; benzoyl
peroxide; dichlorobenzoyl peroxide; dicumyl peroxide; di-tertiary
butyl peroxide; 2,5 dimethyl-2,5 di(peroxy benzoate) hexyne-3;
1,3-bis(t-butyl peroxy isopropyl) benzene; lauroyl peroxide;
di-t-amyl peroxide; 1,1-di-(t-butylperoxy) cyclohexane;
2,2-di-(t-butylperoxy)butane; and 2,2-di-(t-amylperoxy) propane.
The more preferred peroxide is
2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne. The preferred
peroxides have a half-life of between 2 minutes to 1 hour; and more
preferably, the half-life is between 5 minutes to 50 minutes at the
molding temperature.
[0043] Generally, between 0.2 to 5.0 wt % of peroxide is used; more
preferably, the range is between 0.5 to 3.0 wt % of peroxide; and
most preferably, the range is between 0.6 to 2 wt %.
[0044] The peroxide can be dissolved in an inert solvent before
being added to the polymer powder. The inert solvent preferably
evaporates before the polymer is molded. Examples of such inert
solvents are alcohol and acetone.
[0045] For convenience, the reaction between the polymer and the
crosslinking chemical, such as peroxide, can generally be carried
out at molding pressures. Generally, the reactants are incubated at
molding temperature, between 1 to 3 hours, and more preferably, for
about 2 hours.
[0046] The reaction mixture is preferably slowly heated to achieve
the molding temperature. After the incubation period, the
crosslinked polymer is preferably slowly cooled down to room
temperature. For example, the polymer may be left at room
temperature and allowed to cool on its own. Slow cooling allows the
formation of a stable crystalline structure.
[0047] The reaction parameters for crosslinking polymers with
peroxide, and the choices of peroxides, can be determined by one
skilled in the art. For example, a wide variety of peroxides are
available for reaction with polyolefins, and investigations of
their relative efficiencies have been reported {Lem, K. W. &
Han, C. D., J. Appl. Polym. Sci., 27:1367 (1982); Kampouris, E. M.
& Andreopoulos, A. G., J. Appl. Polym. Sci., 34:1209 (1987) and
Bremner, T. & Rudin, A. J. Appl. Polym. Sci., 49:785 (1993)}.
Differences in decomposition rates are perhaps the main factor in
selecting a particular peroxide for an intended application
{Bremner, T. & Rudin, A. J. Appl. Polym. Sci., 49:785 (1993)}.
Bremner and Rudin, id., compared three dialkyl peroxides on the
basis of their ability to increase the gel content, crosslinking
efficiency, and storage modulus of virgin polyethylene through a
crosslinking mechanism and found that they decreased in the order
of .alpha.,.alpha.-bis(tertiary butylperoxy)-p-diisopropyl
benzene>dicumyl peroxide>2,5-dimethyl-2,5-di-(tertiary
butylproxy)-hexyne-3 at the same active peroxide radical
concentrations and temperature.
[0048] More specifically, peroxide crosslinking of UHMW
polyethylene has also been reported {de Boer, J. & Pennings, A.
J., Makromol. Chem. Rapid Commun., 2:749 (1981); de Boer, J. &
Pennings, A. J., Polymer, 23:1944 (1982); de Boer, J., et al.,
Polymer, 25:513 (1984) and Narkis, M., et al., J. Macromol. Sci.
Phys., B 26:37, 58 (1987)}. de Boer et al. crosslinked UHMW
polyethylene in the melt at 180.degree. C. by means of
2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexyne-3 and found that
crosslinks and entanglements, whether trapped or not, contributed
to the same degree to the decrease in crystallinity of UHMW
polyethylene upon crosslinking {de Boer, J. & Pennings, A. J.,
Polymer, 23:1944 (1982)}. It was concluded that an almost
completely crosslinked (or gelled) material with high crystallinity
and good mechanical properties could be obtained by using as little
as 0.2-0.3 wt % of peroxide.
[0049] Some of the above references investigated the effect of
peroxide crosslinking on UHMW polyethylene, such as in lowering
crystallinity; and the effects of reaction parameters, such as
peroxide concentrations {de Boer, J. & Pennings, A. J.,
Polymer, 23:1944 (1982); Narkis, M., et al., J. Macromol. Sci.
Phys., B 26:37-58 (1987)}. However, these references do not address
the effect of peroxide crosslinking or the lowering of
crystallinity in relation to the wear property of the resulting
polymer. For example, de Boer and Pennings, in Polymer, 23:1944
(1982), were concerned with the effect of crosslinking on the
crystallization behavior and the tensile properties of UHMW
polyethylene. The authors found that tensile properties, such as
tensile strength at break point and Young's modulus, of the UHMW
polyethylene, showed a tendency to decrease with increasing
peroxide content.
[0050] Similarly, Narkis, M., et al., J. Macromol. Sci. Phys., B
26:37-58 (1987), separately determined the effects of irradiation
and peroxide on the crosslinking and degree of crystallinity of
UHMW polyethylene (Hostalen GUR 412), high molecular weight
polyethylene, and normal molecular weight polyethylene. However, M.
Narkis et al., did not study the inter-relationship of peroxide
crosslinking and irradiation, nor their effects on wear
resistance.
Use of Crosslinked Polymers for In Vivo Implants
[0051] Another aspect of the invention presents a process for
making in vivo implants using the above chemically crosslinked
polymer. Since in vivo implants are often irradiated to sterilize
them before implant, the present invention provides the further
step of selecting for implant use, a polymer with about 45%
crystallinity or less after irradiation sterilization. For
.gamma.-irradiation sterilization, the minimum dosage is generally
2.5 Mrad. The sterilization dosage generally falls between 2.5 and
4.0 Mrad. The preferable degree of crystallinity is between 25% to
45% crystallinity. In EXAMPLE 2 below, the polymer has about 39.8%
crystallinity after crosslinking; and about 42% crystallinity after
further irradiation with .gamma.-radiation to an average dose of
about 3.4 Mrad. Thus, the chemically crosslinked UHMW polymer
preferably possesses less than about 43% crystallinity before
irradiation in the solid state, and less than about 45%
crystallinity after irradiation with .gamma.-radiation to an
average dose of about 3.4 Mrad.
Annealing of Crosslinked Polymers
[0052] Applicants observed that both crosslinked and uncrosslinked
polymers tended to shrink, but the crosslinked polymer tended to
shrink more than the uncrosslinked polymer (see EXAMPLE 3 below).
Thus, the present invention further provides for annealing a
polymer to pre-shrink it to a size which will not shrink further
(i.e. stabilize the polymer's shrinkage or size). Thus, one aspect
of the invention provides for a method of: 1) crosslinking a
polymer, 2) selecting a crosslinked polymer of reduced
crystallinity, 3) annealing the polymer to stabilize its size.
Thus, the polymer can be molded at a size larger than desired, and
the molded polymer is then annealed to stabilize its size. After
size stabilization, the molded polymer is then resized, such as by
machining, into a product with the desired dimension.
[0053] The annealing temperature is preferably chosen to avoid
thermal oxidation of the crosslinked polymer which will increase
its crystallinity. Thus, the annealing temperature is preferably
below the melting point of the molded polymer before irradiation.
For example, the melting temperatures of molded UHMW polyethylene
and molded 1 wt % peroxide UHMW polyethylene are 132.6.degree. C.
and 122.3.degree. C., before irradiation, respectively. The
preferable annealing temperature for both these molded UHMW
polyethylenes is between 60.degree. C. to 120.degree. C., before
irradiation, and more preferably 100.degree. C. These temperatures
were determined by observation, based on experiments, of their
minimal effect on thermal oxidation of the molded polymers. The
annealing time is generally between 1 to 6 hours, and more
preferably between 2 to 4 hours. In the case of UHMW polyethylene,
the annealing time is preferably between 2 to 4 hours, and more
preferably about 2 hours.
[0054] To further avoid thermal oxidation of the crosslinked
polymer, the annealing is most preferably conducted in a vacuum
oven.
[0055] To ensure that the crosslinked and annealed polymer has the
desired degree of crystallinity, its degree of crystallinity is
preferably determined after the annealing process, using the
method(s) described previously.
Wear-Resistant Polymers
[0056] Another aspect of the invention presents a polymer with 45%
of crystallinity or less, in particular, after irradiation in the
solid state and/or annealing. In EXAMPLE 2 below, the polymer has
about 39.8% crystallinity after crosslinking; and about 42%
crystallinity, after further irradiation with .gamma.-radiation to
an average dose of about 3.4 Mrad; or about 40.8% crystallinity,
after crosslinking and annealing, but before irradiation in the
solid state.
[0057] The polymers of the present invention can be used in any
situation where a polymer, especially UHMW polyethylene, is called
for, but especially in situations where high wear resistance is
desired and irradiation of the solid polymer is called for. More
particularly, these polymers are useful for making in vivo
implants.
In Vivo Implants Made of Crosslinked Polymers
[0058] An important aspect of this invention presents in vivo
implants that are made with the above polymers or according to the
method presented herein. These implants are more wear resistant
than their untreated counterpart, especially after irradiation. In
particular, these in vivo implants are chemically crosslinked UHMW
polymers, which have been molded, annealed, and resized into the
shape of the implants. Further, the chemically crosslinked UHMW
polymer preferably possesses less than about 43% crystallinity
before irradiation in the solid state, and less than about 45%
crystallinity, after .gamma.-irradiation to an average dose of 3.4
Mrad, in the solid state. The modified polymer can be used to make
in vivo implants for various parts of the body, such as components
of a joint in the body. For example, in the hip joints, the
modified polymer can be used to make the acetabular cup, or the
insert or liner of the cup, or trunnion bearings (e.g. between the
modular head and the stem). In the knee joint, the modified polymer
can be used to make the tibial plateau (femoro-tibial
articulation), the patellar button (patello-femoral articulation),
and trunnion or other bearing components, depending on the design
of the artificial knee joint. In the ankle joint, the modified
polymer can be used to make the talar surface (tibio-talar
articulation) and other bearing components. In the elbow joint, the
modified polymer can be used to make the radio-humeral joint,
ulno-humeral joint, and other bearing components. In the shoulder
joint, the modified polymer can be used to make the glenoro-humeral
articulation, and other bearing components. In the spine, the
modified polymer can be used to make intervertebral disk
replacement and facet joint replacement. The modified polymer can
also be made into temporo-mandibular joint (jaw) and finger joints.
The above are by way of example, and are not meant to be
limiting.
[0059] Having described what the applicants believe their invention
to be, the following examples are presented to illustrate the
invention, and are not to be construed as limiting the scope of the
invention.
EXAMPLES
Example 1
Experimental Details
[0060] Commercial-grade UHMW polyethylene GUR 415 (from
Hoechst-Celanese Corporation, League City, Tex.), with a weight
average molecular weight of 6.times.10.sup.6, was used as received.
The peroxide used was
2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne (Lupersol 130,
Atochem Inc., Philadelphia, Pa.). The reason for choosing Lupersol
130 was its long half-life at elevated temperature. The peroxide
will decompose slowly, and the resultant free radicals can diffuse
in the specimen to form a homogeneous network at elevated
temperatures.
[0061] Mixing of the UHMW polyethylene and the peroxide was
accomplished by dispersing polyethylene powder in an acetone
solution of the peroxide and subsequently evaporating the solvent
{de Boer, J., et al., J. Polym. Sci., Polym. Phys. Ed., 14:187
(1976); de Boer, J. & Pennings, A. J., Makromol. Chem, Rapid
Commun., 2:749 (1981) and de Boer, J. & Pennings, A. J.,
Polymer, 23:1944 (1982)}. The mixed powder (22 g) was poured into
the mold cavity and then compression molded in a mold between two
stainless-steel plates at 120.degree. C. and ram pressure
11.times.10.sup.3 kPa for 10 minutes in order to evacuate the
trapped air in the powder. After pressing, the pressure was reduced
to 7.5.times.10.sup.3 kPa and the specimen was heated to
170.degree. C. by circulated heating oil. These conditions were
held for 2 hours. The half-life time of peroxide at 170.degree. C.
in dodecane is about 9 minutes. After 2 hours, pressure was
increased to 15.times.10.sup.3 kPa to avoid cavities in the
specimen and sink marks on the surface and the specimen was slowly
cooled in the mold to room temperature. The mold was in the shape
of an acetabular cup for a total hip prosthesis.
[0062] The specimens were irradiated with .gamma.-rays at room
temperature in air atmosphere by SteriGenics International (Tustin,
Calif.). Cobalt-60 was used as a source of gamma irradiation. The
radiation doses were delivered at a dose rate of 5 kGy/hr.
Specimens received doses to an average of about 34 kGy (i.e., an
average of about 3.4 Mrad).
[0063] The physical properties of specimens before and after
irradiation were characterized by DSC, equilibrium swelling, FTIR,
and WAXS measurements. Surface morphology was examined by SEM.
Results and Discussion
[0064] Before irradiation, the degree of crystallinity, peak
melting temperature, and recrystallization temperature for the
peroxide-free specimen are 49.2%, 132.6 and 115.5.degree. C.,
respectively. For a 1 wt % peroxide specimen, the degree of
crystallinity, peak melting temperature, and recrystallization
temperature are reduced to 39.8%, 122.3 and 110.1.degree. C.,
respectively. Peroxide crosslinking reactions are accompanied by
the decomposition of peroxide and abstraction of hydrogen atoms,
and the resulting combination of alkyl radicals to produce
carbon-carbon crosslinks. Generally, this reaction was performed
above the melting temperature of the polymer. Thus the crosslinking
step preceded the crystallization step. It was suggested that
crystallization from a crosslinked melt produced an imperfect
crystal, and crosslinks suppressed crystal growth, resulting in the
depression of melting temperature and a decreased crystallinity
(decreased crystallite size) {de Boer, J. et al., J. Polym. Sci.,
Polym. Phys. Ed., 14:187 (1976); de Boer, J. & Pennings, A. J.,
Makromol. Chem. Rapid Commun., 2:749 (1981); de Boer, J. &
Pennings, A. J., Polymer, 23:1944 (1982) and Narkis, M., et al., J.
Macromol. Sci. Phys., B26:37 (1987)}. Wide-angle x-ray scattering
shows that the degree of crystallinity, crystal perfection and size
decrease after peroxide crosslinking. For swelling measurement, the
peroxide-free specimen dissolves completely in boiling p-xylene.
The gel content, degree of swelling, and average molecular weight
between crosslinks for the 1 wt % peroxide specimen are 99.6%,
2.53, and 1322 (g/mol), respectively. Because of the extremely long
polymer chains in UHMW polyethylene, only a few crosslinks were
needed for gelation. In addition, an almost 100% gel can be
obtained by peroxide crosslinking because no chain scission occurs
by peroxide crosslinking.
[0065] After irradiation, the degree of crystallinity and peak
melting temperature for the peroxide-free specimen were increased
to 55.8% and 135.degree. C., respectively. It was suggested that
irradiation-induced scission of taut tie molecules permits
recrystallization of broken chains from the noncrystalline regions,
and results in an increase in the degree of crystallinity and an
increased perfection of existing folded chain crystallites {Narkis,
M., et al., J. Macromol. Sci. Phys., B26:37 (1987); Bhateja, S. K.,
J. Macromol. Sci. Phys., B22:159 (1983); Bhateja, S. K., et al., J.
Polym. Sci. Polym. Phys. Ed., 21:523 (1983); Kamel, I. &
Finegold, L., J. Polym. Sci., Polym. Phys. Ed., 23:2407 (1985);
Shinde, A. & Salovey, R., J. Polym. Sci., Polym. Phys. Ed.,
23:1681 (1985); Bhateja, S. K. & Andrews, E. H., J. Mater.
Sci., 20:2839 (1985); Minkova, L., Colloid Polym. Sci., 266:6
(1988); Minkova, L. & Mihailov, M., Colloid Polym. Sci.,
268:1018 (1990) and Zhao, Y., et al., J. Appl. Polym. Sci., 50:1797
(1993)}. The gel content after irradiation for the peroxide-free
specimen was 70.8%.
[0066] For the 1 wt % peroxide specimen, the degree of
crystallinity and peak melting temperature after irradiation were
increased to 42% (about 2% increase) and 125.1.degree. C.,
respectively. The gel content decreased to 97.5% after irradiation,
whereas, the degree of swelling and molecular weight between
crosslinks increased to 3.35 and 2782 (g/mol), respectively.
Apparently, irradiation-induced scission of taut tie molecules
resulted in a decreased gel content and an increased degree of
swelling. However, after peroxide crosslinking, the effect of
irradiation on network properties was mitigated. As a result of
peroxide crosslinking, radiation-induced chain scission becomes
less important in determining gel content. We suggest that peroxide
crosslinking reduces the effect of irradiation on the crosslinked
network because crosslinks introduced by peroxide crosslinking
stabilize chain fragments resulting from the scission of taut tie
molecules and suppress recrystallization of broken chains.
Wide-angle x-ray scattering showed that crystal perfection
increased after irradiation. It is suggested that crystal
perfection was improved by irradiation-induced scission of taut tie
molecules in the amorphous regions.
[0067] FTIR measurements showed that, after irradiation, the
carbonyl concentration significantly increased. This is because the
free radicals produced by irradiation reacted with oxygen dissolved
and/or diffused in the polymer. In addition, the carbonyl
concentration in irradiated peroxide-crosslinked samples was
higher, compared to the peroxide-free sample (after irradiation).
Peroxide, crosslinking produces tertiary carbons, therefore, the
concentration of tertiary carbons increases with increasing
peroxide concentration. Applicants believe that tertiary carbons
are more susceptible to oxidation during irradiation. Therefore,
carbonyl concentration in the irradiated peroxide-crosslinked
samples increased with increasing peroxide concentration.
[0068] After irradiation, scanning electron micrographs were taken
of the fracture surfaces of the peroxide-free and 1 wt % peroxide
specimens, compression molded at 170.degree. C. for 2 hours and
subsequently slowly cooled to room temperature. The micrographs are
shown in FIGS. 1 and 2, respectively. As shown in FIG. 1, a brittle
(rough) fracture boundary of size comparable to that of the
original UHMW polyethylene powder particles is observed. Close
examination (.times.5000 magnification) shows an oriented nodular
structure, composed of many smooth, submicron spheres. These
smooth, minute spheres are believed to correspond to those present
in the raw UHMW polyethylene powder and to form an aggregate. In
FIG. 2, peroxide crosslinked samples show a ductile (smooth)
fracture surface, compared to the rough fracture surface of
peroxide-free specimen. The difference in appearance of fracture
surfaces for peroxide-free and 1 wt % peroxide specimens is due to
the crystallinity difference. After irradiation, the degree of
crystallinity for the peroxide-free and 1 wt % peroxide specimens
were 55.8 and 42%, respectively. It is believed that the
peroxide-free specimen (55.8% crystallinity) suffered higher forces
and less deformation during fracturing process, leading to a sharp
break in the polymer.
[0069] The crosslinking experiment was also conducted with
different concentrations of Lupersol 130, using a smaller amount, 5
g, of GUR 415 and a smaller mold which was in the form of a disk.
It was observed that the degree of crystallinity of the crosslinked
polymer decreased with increased concentrations of Lupersol 130.
The result is shown in Table 1 below:
TABLE-US-00001 TABLE 1 wt % Crystallinity (%) Crystallinity (%)
Peroxide Before Irradiation After Irradiation 0 49.2 55.8 0.2 44.0
50.0 0.4 41.6 46.8 0.6 41.3 46.2 0.8 40.0 45.0 1.0 39.8 42.0 1.5
36.8 36.8 2.0 36.5 36.7
Conclusions
[0070] Peroxide crosslinking leads to a decrease in the degree of
crystallinity, peak melting temperatures, and recrystallization
temperatures for 1 wt % peroxide specimen. Irradiation produces
crosslinking in amorphous regions plus extensive scission of taut
tie molecules and leads to increased crystallinity and crystal
perfection, reduces gel content, and increases the degree of
swelling of a crosslinked network.
[0071] Peroxide crosslinking reduces the effect of irradiation on
the crosslinked network. This is because crosslinks introduced by
peroxide crosslinking can stabilize the chain fragments resulting
from the scission of taut tie molecules and suppress
recrystallization of broken chains.
[0072] FTIR measurements showed that, after irradiation, the
carbonyl concentration significantly increased. This is because the
free radicals produced by irradiation react with oxygen dissolved
and/or diffused in the polymer. In addition, carbonyl concentration
in the irradiated peroxide-crosslinked samples is higher, compared
to the peroxide-free sample (after irradiation). This is because
peroxide crosslinking introduces tertiary carbons which are more
susceptible to oxidation during irradiation, so that the carbonyl
concentration in the irradiated peroxide-crosslinked samples
increases.
[0073] Wide-angle x-ray scattering shows that crystal perfection
increases after irradiation. It is suggested that crystal
perfection is improved by irradiation-induced scission of taut tie
molecules in the amorphous regions.
[0074] The peroxide-free specimen shows brittle fracture because of
higher crystallinity (55.8%), whereas, the 1 wt % peroxide specimen
shows ductile fracture due to lower crystallinity (42%).
Example 2
Materials and Methods
[0075] In this example, the wear resistance of the polyethylenes
treated (modified) and untreated (unmodified) with peroxide in
EXAMPLE 2 were tested. The control (unmodified) and modified
polyethylenes were compression molded directly into the form of
acetabular cups. These were then exposed to an average of
approximately 3.4 Mrad of gamma radiation (SteriGenics
International, Tustin, Calif.), to simulate the condition of cups
that would be used in patients. Due to different amounts of
post-molding shrinkage, the internal surface of each cup was
machined to provide nearly identical internal diameters and
ball-to-cup clearances among the control and modified cups (FIG.
3). As shown in FIG. 3B, the cup's outer radius 1 is 24.5 mm, its
inner radius 2 is 16.1 mm, its height 3 is 29.8 mm, and its
diameter 4 is 49.0 mm
[0076] The cups were pre-soaked in distilled water for three weeks
prior to the wear test to minimize fluid absorption during the wear
test. The wear cups were mounted on the hip joint simulator,
including four cups of control polyethylene and three cups of
modified polyethylene. Each cup was held in a urethane mold and
mounted in a stainless steel test chamber, with a plexiglass wall
to contain the bovine serum lubricant. The lubricant had 0.2%
sodium azide added to retard bacterial degradation, and 20
milli-Molar ethylene-diaminetetraacetic acid (EDTA) to prevent
precipitation of calcium phosphate on the surfaces of the ball
(McKellop, H. & Lu, B., "Friction and Wear of
Polyethylene-Metal and Polyethylene-Ceramic Hip Prostheses on a
Joint Simulator, Fourth World Biomaterials Congress, Berlin, April
1992, 118). A polyethylene skirt covered each chamber to minimize
air-borne contamination. The cups were oscillated against highly
polished femoral balls of cast cobalt-chromium alloy, as used on
artificial hips. The simulator applied a Paul-type cyclic load at
one cycle per second {Paul, J. P., Proc. Instn. Mech. Engrs., 181,
Part 3J, 8-15, (1967)} with a 2000N peak, simulating the load on
the human hip during normal walking, and the cups were oscillated
through a bi-axial 46 degree arc at 68 cycles per minute. At
intervals of 250,000 cycles, the cups were removed from the wear
machine, rinsed, inspected and replaced with fresh lubricant. At
500,000 cycles and one million cycles, all of the cups were removed
from the wear simulator, cleaned, dried and weighed to determine
the weight loss due to wear. One million cycles is the equivalent
of about one year's use of a prosthetic hip in a patient. FIG. 4
presents a schematic diagram of the hip joint simulator. The arrow
indicates the direction of the computer controlled simulated
physiological load exerted on the simulated hip joint. The
simulator contains: a torque transducer 5, the acetabular cup 6, a
dual axis offset drive block 7, a test chamber 8, serum 9, and a
femoral head 10.
[0077] Three soak-correction acetabular cups of each material
(control and modified) were prepared in an identical manner, but
were not wear tested. These cups were mounted in a separate test
frame and a cyclic load, identical to that used in the wear test,
was applied. These soak-correction cups were cleaned and weighed
together with the wear test cups, and the average weight gain of
the correction cups was added to the apparent weight loss of the
wear test cups (i.e. to correct for fluid absorption by the wear
test cups that would obscure the weight loss due to wear).
Results and Discussion
[0078] Because of the apparent "negative" wear at 0.5 million
cycles (discussed below), the wear rates were calculated and
compared for all of the cups only for the interval from 0.5 to 1.0
million cycles. The four control polyethylene cups showed
comparable amounts of wear (FIG. 5), with an average corrected wear
rate of 19.19 (S.D.=2.38) milligrams per million cycles (Table 2).
This was within the range that applicants have measured for cups of
conventional UHMW polyethylene in a variety of studies that
applicants have run.
[0079] The wear was much lower for the modified cups (FIG. 5). As
shown in Table 2, the mean wear rate for the modified cups was 4.12
(S.D.=1.26) milligrams per million cycles, i.e. about one-fifth of
the wear of the control cups. This difference was statistically
significant at the level of p=0.0002).
TABLE-US-00002 TABLE 2 WEAR RATES FOR CONTROL AND MODIFIED
POLYETHYLENES (INTERVAL FROM 0.5 TO 1.0 MILLION CYCLES) MEAN WEAR
RATE CUP WEAR RATE (STANDARD MATERIAL NUMBER (mg/million cycles)
DEVIATION) CONTROL C2 21.67 19.19 POLYETHYLENE C3 16.78 (2.38) C4
17.57 C9 20.76 MODIFIED M4 4.08 4.12 POLYETHYLENE M5 2.88 (1.26) M7
5.39
[0080] For the data point at 0.5 million cycles, the corrected
weights were lower than the weights before the wear test. This was
most likely the result of the wear being very small, and the fluid
absorption by the test cups being slightly greater than the average
gain of the soak correction cups, such that the correction factor
did not entirely offset the fluid gain by the wear cups (giving an
apparent "negative" wear). A small difference in water absorption
rates between the wear cups and the correction cups could arise due
to differences in equilibrium temperatures (the wear cups were
typically at 35.degree. C. to 45.degree. C., whereas the soak
correction cups were at room temperature, about 20.degree. C.), due
to mechanical agitation of the serum during oscillation of the wear
test chambers, or other causes.
Example 3
[0081] During the wear test in the simulator described in EXAMPLE
2, it was discovered that the acetabular cups shrunk at simulated
human body temperature. In order to stabilize the shrinkage, in
this experiment (unrelated to EXAMPLE 2), the cups were annealed at
100.degree. C. in a vacuum oven for 2 hours. After annealing, the
total shrinkage in diameter for uncrosslinked and crosslinked cups
was approximately 1% and 2%, respectively. The degrees of
crystallinity of the annealed cups were determined by DSC. The
degree of crystallinity of the uncrosslinked polymer was unchanged,
whereas that of the crosslinked polymer was increased by
approximately 1%. To test for further shrinkage, the cups were
again put in the vacuum oven at 80.degree. C. for two hours, and no
further shrinkage was observed.
[0082] The present invention has been described with reference to
specific embodiments. However, this application is intended to
cover those changes and substitutions which may be made by those
skilled in the art without departing from the spirit and scope of
the appended claims.
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