CA2619502C

CA2619502C - Ultra high molecular weight polyethylene articles and methods of forming ultra high molecular weight polyethylene articles

Info

Publication number: CA2619502C
Application number: CA2619502A
Authority: CA
Inventors: Niels A. Abt; Werner Schneider-Storrer
Original assignee: Zimmer GmbH
Current assignee: Zimmer GmbH
Priority date: 2005-08-18
Filing date: 2005-08-18
Publication date: 2012-11-20
Anticipated expiration: 2025-08-18
Also published as: JP2009504283A; AU2005335669A1; DE602005017949D1; EP2277561A1; EP2792373A1; US7863348B2; EP2172229A1; US20170335074A1; US11015030B2; CA2619502A1; US20070059334A1; WO2007019874A1; US8673202B2; EP1924299A1; EP2277560A1; JP5314423B2; US20090118390A1; EP3170515A1; ES2441608T3; EP2277560B1

Abstract

The present invention generally provides implantable articles and methods of forming implantable articles from a crosslinked ultrahigh molecular weight polyethylene ("UHMWPE") blend stabilized with Vitamin E. The crosslinked UHMWPE blend may be prepared by combining the UHMWPE material and vitamin E
prior to irradiating the UHMWPE blend with electron beam radiation at a sufficient radiation dose rate to induce crosslinking. The crosslinked UHMWPE
blend may be incorporated into a variety of implants, and in particular, into endoprosthetic joint replacements

Description

ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE ARTICLES AND
METHODS OF FORMING ULTRA HIGH MOLECULAR WEIGHT
POLYETHYLENE ARTICLES

BACKGROUND
Many endoprosthetic joint replacements currently implanted in patients include a highly polished metal or ceramic component articulating on an ultra high molecular weight polyethylene (UFIMWPE)-material or blend. Wear and abrasion resistance, coefficient of friction, impact strength, toughness, density, biocompatibility and biostability are some of the properties that make UFIMWPE a suitable material for such implants. Although UFIMWPE
has been used in implants for many years, there is continuing interest in the wear and durability characteristics of implants incorporating U IIVIWPE.
One method employed to improve the durability and other physical characteristics of UFIMWPE implants has been to expose such implants to radiation, for example gamma radiation or electron beam radiation, to induce crosslinking in the UFIMWPE.
Similar radiation sources have also been used to sterilize UFIMWPE implants prior to distribution.
Despite the benefits of irradiating UHMWPE implants, the irradiation process may lead to increased rates of oxidation in the UHIVIWPE implant. In particular, irradiation has been shown to generate free radicals, which react in the presence of oxygen to form peroxyl radicals. These free radicals and peroxyl radicals may react with the polyethylene backbone and with each other to form oxidative degradation products and additional radical species.
This cycle of oxidation product and radical species formation may occur over several years (both prior to and after implantation) as oxidation levels in the implant increase.
One method that has been utilized to reduce oxidation in irradiated UHMWPE
materials is the addition of a stabilizing component to the UFIMWPE material to inhibit the oxidation cycle. However, the addition of a stabilizer or stabilizing components, such as vitamin E, to UFIMWPE prior to irradiation has been shown to have an adverse effect on crosslinking during irradiation. See Parth et al., "Studies on the effect of electron beam radiation on the molecular structure of ultra-high molecular weight polyethylene under the influence of a-tocopherol with respect to its application in medical implants," Journal of Materials Science: Materials In Medicine, 13 (2002), pgs. 917-921.
For this reason, the addition of stabilizers to UFIlVIWPE materials. after forming and irradiating via diffusion has been proposed. See e.g., PCT Published Application No. WO
2004/101009. However, the addition of stabilizers after irradiation has several limitations.
For example, vitamin E diffusion may provide a less uniform distribution of stabilizer in

2 UFIMWPE than pre-irradiation mixing. Diffusion of the vitamin E may also require separate irradiation steps to induce crosslinking prior to adding vitamin E and then to sterilize the implant after adding vitamin E.
Therefore, it would be beneficial to provide a method of forming a crosslinked UBMWPE material for use in implanted articles that overcomes one or more of these limitations.

SUMMARY
In one embodiment, the present invention provides an implantable article formed from a crosslinked ultrahigh molecular weight polyethylene ("UFIMWPE") blend. The crosslinked UHMWPE blend may be prepared by combining a UFIMWPE material with a stabilizer, such as vitamin E, and other optional additives reported herein to form a UHMWPE
blend, and then by irradiating the UFIMWPE blend with a suitable radiation source, such as electron beam radiation, at a sufficient radiation dose rate to induce crosslinking.
The resulting crosslinked UIIMWPE blend may have a swell ratio of less than about 4, and at least about 0.02 w/w% vitamin E is uniformly dispersed within at least a surface region of an article formed from the blend. According to this invention, the vitamin E may be uniformly distributed from the surface of the article to a depth of at least about 5 mm.
The crosslinked UBNIWPE blend of the present invention may be incorporated into a variety of implants, and in particular, into endoprosthetic joint replacements BRIEF DESCRIPTION OF FIGURES
Figs. 1A-1C are flow-charts illustrating methods of preparing UHMWPE implants according to embodiments of the present invention.

Figs. 2A-2B are flow-charts illustrating methods of preparing IJHIVIWPE
implants according to additional embodiments of the present invention.

Fig. 3 is a line graph illustrating the swell ratio of several UHMWPE samples, described in the Example, at various radiation dose rates.

Figs. 4A-4C are bar graphs illustrating the TVI (4A), swell ratio (4B) and soluble fraction (4C), of several UBMWPE samples.

Fig. 5 is a line graph illustrating the vitamin E concentration of several samples at a range of depths.

Fig. 6 is a prior art line graph showing the vitamin E index of samples prepared pursuant to U.S. Published Application No. 2004/0156879.

Fig. 7 is a line graph showing the oxidation levels of several UHMWPE samples at a range of depths.

3 Fig. 8 is a bar graph showing the tensile strength of several UHMWPE samples.
Fig. 9 is a bar graph showing the elongation percent at break of several UHMWPE
samples.
Fig. 10 is a bar graph showing the Charpy impact strength of several U MVv PE
samples.
DETAILED DESCRIPTION
UHMWPE is a semicrystalline, linear homopolymer of ethylene, which may be produced by stereospecific polymerization with a Ziegler-Natta catalyst at low pressure (6-8 bar) and low temperature (66-80 C.). The synthesis of nascent UID4WPE results in a fine granular powder. The molecular weight and its distribution can be controlled by process parameters such as temperature, time and pressure. UI'IMWPE generally has a molecular weight of at least about 2,000,000 g/mol.
Suitable UHII2WPE materials for use as raw materials in the present invention may be in the form of a powder or mixture of powders. The U IMWPE material may be prepared almost entirely from UHMWPE powder, or may be formed by combining UHMWPE
powder with other suitable polymer materials. In one embodiment, the UHMWPE material may include at least about 50 w/w% UHMWPE. Examples of suitable UHMWPE materials include GUR 1020 and GUR 1050 available from Ticona Engineering Polymers.
Suitable polymer materials for use in combination with the UHMWPE materials may include disentangled polyethylene, high pressure crystallized polyethylene and various other "super tough" polyethylene derivatives. In addition, biocompatible non-polyethylene polymers may also be suitable for use in certain embodiments.
Suitable additives to the UHMWPE material include radiopaque materials, antimicrobial materials such as silver ions, antibiotics, and microparticles and/or nanoparticles serving various functions. Preservatives, colorants and other conventional additives may also be used.
Suitable stabilizers for addition to the UHMWPE material generally include materials that can be added in an effective amount to the UHMWPE material in order to, at least in part, inhibit the oxidation cycle caused by irradiation of UHMWPE. Vitamin E
is particularly suitable for use in embodiments of the present invention. As used herein "vitamin E" refers generally to derivatives of tocopherol including a-tocopherol. Other suitable stabilizers may include phenolic antioxidants such as butylated hydroxytoluene, and ascorbic acid.
The vitamin E stabilizer and UHMWPE material may be combined via a number of known processes to form a U MWPE blend. Such processes include physical mixing,

4 mixing with the aid of a solvent, mixing with the aid of a solvent (e.g. Coe) under supercritical temperature and pressure conditions, and ultrasonic mixing.
Suitable mixing processes of these types are also described, for example, in U.S. Patent Nos.
6,448,315 and 6,277,390. In one embodiment, vitamin E is dissolved in ethanol and is drop-wise added to a powdered UHMWPE material while mixing. The ethanol may then be removed via a vacuum dryer or similar apparatus.

Figures IA-IC and 2A-2B are flowcharts illustrating methods for preparing implants from UHMWPE blends according to embodiments of the present invention. The general steps for processing the implant include a consolidating/compressing the UHMWPE
blend, cross-linking the UBMWPE blend, manufacturing an implant from the compressed UHMWPE
blend, packaging the implant, and sterilizing the packaged implant. As reflected in Figs. IA-C and 2A-2B, these steps may be carried out in varying order, in multiple steps, or simultaneously in accordance with embodiments of the present invention.
The UHMWPE blend may first be consolidated and/or compressed into suitable form for use as (oy as part of) a prosthetic device or other implant. Suitable compression and/or consolidation techniques include, for example, compression molding, direct compression molding, hot isostatic pressing, ram extrusion, high pressure crystallization, injection molding, sintering or other conventional methods of compressing and/or consolidating U MWPE. If desired, the compressed/consolidated UHMWPE blend may be further processed or manufactured by milling, machining, drilling, cutting, assembling with other components, and/or other manufacturing or pre-manufacturing steps conventionally employed to,manufacture implants from UHMWPE.
Prior to and/or after processing the implant as reported above, the UHMWPE
blend may be crosslinked by exposure to radiation at a high radiation dose and/or dose rate to form a crosslinked UHMWPE blend. In one embodiment, the UHMWPE blend may be exposed to electron beam radiation at a dose of at least about 25 kiloGrey, more particularly at least about 80 kiloGrey, and even more particularly at least about 95 kiloGrey. In another embodiment, the UHMWPE blend may be exposed to radiation at a dose rate of at least 1 MegaGrey per hour, more particularly at least about 15 MegaGrey per hour, and even more particularly about 18 MegaGrey per hour. In certain embodiments, the desired radiation dose may be achieved in a single exposure step at a high dose rate. In other embodiments, a series of high dose rate irradiation steps may be employed to expose the UHMWPE blend to a desired dose of radiation.

In certain embodiments, the radiation source is electron beam radiation.
Electron beam radiation exposure may be performed using conventionally available electron beam accelerators. One commercial source for such an accelerator is IBA
Technologies Group, Belgium. Suitable accelerators may produce an electron beam energy between about 2 and about 50 MeV, more particularly about 10 MeV, and are generally capable of accomplishing one or more of the radiation doses and/or dosage rates reported herein.
Electron beam exposure may be carried out in a generally inert atmosphere, including for example, an argon, nitrogen, vacuum, or oxygen scavenger atmosphere. Exposure may also be carried out in air under ambient conditions according to one embodiment. Gamma and x-ray radiation may also be suitable for use in alternate embodiments of the invention. The present invention need is not necessarily limited to a specific type of source of radiation.
Optionally, prior to and/or after electron beam irradiation, the UHMWPE blend may be subjected to one or more temperature treatments. In one embodiment, the UI{MWPE
blend may be heated above room temperature, more particularly above about 100 C, even more particularly between about 120 C and 130 C, prior to irradiation. U.S.
Patent No.
6,641,617 td Merril et al., reports methods of employing such temperature treatment steps in greater detail. In another embodiment, the UHMWPE blend may remain at room temperature or may even be cooled below room temperature, for example, below the glass transition temperature of the UHMWPE blend. After irradiation, the crosslinked UHMWPE
blend may be annealed at a temperature of up to about 200 C for up to about 72 hours, mort particularly at about 150 C for about 5 hours. Alternatively or additionally, the crosslinked 11HMWPE blend may be subjected to the mechanical annealing processes reported in U.S. Patent No. 6,853,772 to Muratoglu. In one embodiment, however, no pre-or post-irradiation temperature and/or annealing treatments are performed.

As part of the implant manufacturing process, additional components may be combined with the UHMWPE blend at any time during the process reported herein.
In one embodiment, tribological components such as metal and/or ceramic articulating components and/or preassembled bipolar components may be joined with the UHMWPE blend. In other embodiments, metal backing (e.g. plates or shields) may be added. In further embodiments, surface components such a trabecular metal, fiber metal, beats, Sulmesh coating, meshes, cancellous titanium, and/or metal or polymer coatings may be added to or joined with the UFIMWPE blend. Still further, radiomarkers or radiopacifiers such as tantalum, steel and/or titanium balls, wires, bolts or pegs may be added. Further yet, locking features such as rings, bolts, pegs, snaps and/or cements/adhesives may be added. These additional components may be used to form sandwich implant designs, radiomarked implants, metal-backed implants to prevent direct bone contact, functional growth surfaces, and/or implants with locking features.
A variety of implants, and in particular endoprosthetic joint replacements, may be prepared by employing the methods reported herein. Examples of such implants include artificial hips and knees, cups or liners for artificial hips and knees, spinal replacement disks, artificial shoulder, elbow, feet, ankle and finger joints, mandibles, and bearings of artificial hearts.
After manufacturing of the implant has been completed, it may be packaged and sterilized prior to distribution. Packaging is generally carried out using either gas permeable packaging or barrier packaging utilizing a reduced oxygen atmosphere. Because the presence of vitamin E in the U -IlVIWPE blend inhibits the oxidation cycle, conventional gas permeable packing may be suitable for embodiments of the present invention. Barrier packaging with an inert gas backfill (e.g. argon, nitrogen, oxygen scavenger) is also suitable.
As reflected in Figs. 1A-1C and 2A-2B, sterilization may be accomplished either by radiation exposure during crosslinking of the UIIMWPE blend, or as part of a separate processing step. A number of conventional sterilization techniques exist including gas plasma sterilization, ethylene oxide sterilization, gamma radiation sterilization and e-beam radiation. In the embodiments illustrated in Figs. IA, 1C and 2B, crosslinking is carried out prior to packaging. In the embodiments illustrated in Figs. 1B and 2A, sterilization and crosslinking are carried out by e-beam irradiation in a single step after packaging the implant.
Sterilization generally occurs after packaging. In certain embodiments, sterilization is carried out at the same time as crosslinking, and therefore utilizes e-beam radiation. In embodiments in which crosslinking occurs before sterilization, additional suitable sterilization methods include gamma irradiation (either inert or in air), gas plasma exposure or ethylene oxide exposure.

As further exemplified in the Examples set forth below, the crosslinked UEMWPE
blends produced according to embodiments of the present invention may have several beneficial characteristics. Notably, such blends exhibit lower levels of oxidation when compared to unstabilized UIIVIWPE materials, while still exhibiting suitable levels of crosslinking. The use of a high radiation dose rate or a series of high radiation dose rates, at least in part, contributes to improved crosslinking densities for the UIEVIWPE
blend, which is contrary to prior art reports that suggest that suitable crosslinking densities are difficult to achieve when irradiating stabilized UHMWPE blends.
Also, such UH1\4WPE blends may have a generally uniform distribution of vitamin E
at least a surface region of the blend. As used herein, the phrase "surface region" refers to a region of a crosslinked UIIMWPE blend extending from a surface of the blend to some depth or range of depths below the surface. For example, the implants formed from the crosslinked UHMWPE blend of certain embodiments may exhibit a substantially uniform distribution of vitamin E to a surface depth of at least 3 mm, more particularly, at least 5 mm. Other embodiments may exhibit a substantially uniform distribution of vitamin E to a surface depth of at least 10 mm, more particularly at least 15 mm, even more particularly at least 20 mm.
In further embodiments, the UHMWPE blend may have a substantially uniform distribution of vitamin E throughout the blend.

EXAMPLES
Table 1 sets forth the processing parameters for Samples A-I.

Vitamin Preheating Raw Irradiation Irradiation Irradiation Radiation Annealing SAMPLE Material E- Before Dose Dose Rate Environment Source Process Content Irradiation GUR w/w% C kGy A 1020 0.0 N/A 25-40 0.5 to 10 N2 Gamma N/A
(kGy/h) B 1020 0.0 N/A N/A N/A N/A N/A N/A
C 1020 0.1 N/A NIA N/A N/A N/A N/A
D 1020 0.1 N/A 25-40 0.5 to 10 N2 Gamma N/A
E 1020 0.0 120 95 (MG18 y/h) Air eBeam 155hC, F 1020 0.1 120 95 (MG 8 /h) Air eBeam 1501C, G 1020 0.1 N/A 95 (MGy/h) Air eBeam N/A
H 1020 0.1 N/A 95 0.5 to 10 Air Gamma NIA
(kGy/h) I 1050 0.0 120 95 (MG18 y/h) Air eBeam 1500C, As set forth in Table 1, GUR 1020 and GUR 1050 brand UHMWPE powders are available from Ticona GmbH, FrankfurtMain, DE. The vitamin E used for Samples C, D and F-H was a-tocopherol obtained from DSM Nutritional Products AG, Basel, Switzerland.

For Samples C, D and F-H, the a-tocopherol was dissolved in ethanol in a concentration of 50 g/l and mixed into the UHMWPE drop-wise using a Nauta-Vrieco brand screw-cone mixer. The ethanol was then removed from the UHMWPE blend in a vacuum dryer at 50 C for 6 hours, resulting in a UBMWPE blend having a concentration of a-tocopherol of about 0.1 w/w%. The resulting UHMWPE blend was then sintered for 7 hours at 220 C and 35 bar to produce UHMWPE plates having a thickness of 60 mm and a diameter of 600 mm. Homogeneity of the a-tocopherol in the UHMWPE blend was measured by standard HPLC methods and determined to vary up to +/-2% from the desired content.

Samples A, D and H were irradiated using a Studer IR-168 Gamma Irradiator utilizing a Co60 radiation source. Samples E-G and I were irradiated using a 10 MeV
Rhodotron electron accelerator available from IBA SA, Louvain-La-Neuve using a 120 kW
power setting.

RESULTS
Fig. 3 shows a line graph illustrating the swell ratio of unstabilized polyethylene versus a polyethylene blend stabilized with Vitamin E. The swell ratio is a useful indicator of the crosslinking density of a particular material. In particular, lower relative swell ratios are indicative of higher levels of crosslinking, and vice versa. The swell ratio was determined according to ASTM F2214-02. Specifically, 4-6 mm cubes of each of Samples H, F, G and E
were placed in a container filled with o-Xylene at 25 C and placed in a dynamical mechanical analyzer (DMA, DMA 7e available from Perkin Elmer) for 10 minutes.
A first sample height (HO) was then taken for each sample. The samples were then heated at a rate of 5K/min to a maintained temperature of 130 T. A second sample height (Hf) was then taken after 120 minutes at 130 C. The swell ratio was then calculated according to the following equation:

qs = (Hf/HO)3 The data points for the lower flat line include a swell ratio standard for unstabilized UHMWPE (obtained from the interlaboratory comparison in ASTM F2214-02 at a dose rate of 89 kGy) and unstabilized Sample E. These data points indicate that dose rates do not have a substantial effect on crosslink density. The data points for the upper descending line include Samples H, F and G. Notably, the increased irradiation dosage rates used for Samples F and G resulted in a decreased swell ratio when compared to sample H, and consequently, an increased crosslink density.

Figs. 4A-4C are three bar graphs illustrating several characteristics of Samples B, C, and E-H. Fig. 4A is a bar graph illustrating the trans-Vinylene Index (TVI) levels of the Samples. The TVI was determined by the method described in Muratoglu et al., "Identification and quantification of irradiation in UEMWPE through trans-vinylene yield."
TVI levels are an indicator of the radiation absorption efficiency of UHMWPE.
Fig. 4A
indicates that the Samples E and F, which were preheated before irradiation and annealed after irradiation, possessed higher radiation absorption efficiency than other samples.
Fig. 4B is a bar graph illustrating the swell ratio of the same samples reported in Fig.
4A. Notably, Sample H, which was gamma-irradiated, shows a higher swell ratio (and therefore lower crosslink density) than the e-beam irradiated Samples E, F and G.

Fig. 4C is a bar graph of the soluble fraction of the samples reported in Fig.
4A. The soluble fraction indicates the percentage of fully crosslinked material in the sample. The soluble fraction for each sample was determined in accordance with ASTM 2765-01.
Specifically, powdered UHMWPE was taken from a location 10 mm under the surface of the sample by a rasping technique. This sample was then weighed in a wire mesh and backfluxed for 12 hours in xylene. After backfluxing, the remaining gel portion was placed in a vacuum furnace and dried at a temperature of 140 C and a pressure of less than 200 mbar, and was then conditioned in an exsiccator before being weighted again.
The resulting gel portion and soluble portion was computed by weighing the sample before and after the procedure. Sample H, which was gamma irradiated, shows a higher soluble fraction than e-beam irradiated Samples E, F and G.

Fig. 5 is a line graph indicating the vitamin E content at a range of depths from the surface of Samples C, F, G and H. Fig. 5 indicates that a uniform vitamin E
concentration is maintained in each Sample in a surface region at least up to the measured depth of 20 mm.
This uniform distribution of vitamin E is particularly notable when compared to Prior Art Fig. 6 reported in U.S. Published Application No 2004/0156879, in which the vitamin E
index of diffused vitamin E samples steadily decreased as depth increased.
Fig. 7 is a line graph illustrating the oxidation levels of Samples E, F, G
and H.
Notably, oxidation levels at certain depths from the surface of the sample material were higher for Sample E (did not include vitamin E) and Sample H (gamma irradiated) as compared to Samples G and F (e-beam irradiated).
Figs. 8-10 are a series of bar graphs illustrating various mechanical properties of Samples A, D, E, F, G, H and I. Fig. 8 illustrates the mechanical strength of each sample, and generally indicates that the pre-heating and annealing processing methods utilized with samples E, F and I resulted in somewhat decreased mechanical strength as compared to the cold irradiation method used for Sample G and H. Fig. 9 illustrates the elongation percent at the breaking point of each sample. Fig. 10 illustrates the impact strength of each sample based on the Charpy impact scale (kJ/m2), and generally indicates that the presence of vitamin E increases the impact strength of crosslinked UH]VIWPE.

Claims

1. A method of forming a crosslinked ultrahigh molecular weight polyethylene blend comprising: combining vitamin E and an ultrahigh molecular weight polyethylene to form an ultrahigh molecular weight polyethylene blend prior to consolidation of the ultrahigh molecular weight polyethylene blend, in which the blend comprises the vitamin E in a substantially uniform distribution throughout the ultrahigh molecular weight polyethylene;
consolidating the ultrahigh molecular weight polyethylene blend; and irradiating the consolidated, ultrahigh molecular weight polyethylene blend with electron-beam radiation at an absorbed dose of at least 60 kiloGrey and a dose rate of at least 1 MegaGrey per hour to form the crosslinked ultrahigh-molecular weight polyethylene blend having a swell ratio of less than 4.

2. The method of claim 1 wherein the absorbed dose is at least about 90 kiloGrey and the dose rate is at least 10 MegaGrey per hour.

3. The method of claim I or 2 wherein the amount of vitamin E in the blend is between about 0.02 w/w % and about 2.0 w/w %.

4. The method of claim 1 or 2 wherein the amount of vitamin E in the blend is between about 0.05 and about 0.4 w/w %.

5. The method of any one of claims 1 to 4 further comprising the step of heating the ultrahigh molecular weight polyethylene blend prior to, after, or both prior to and after irradiating.

6. The method of any one of claims 1 to 5 further comprising the step of forming the crosslinked ultrahigh-molecular weight polyethylene blend into an implant.

7. The method of claim 6 further comprising the step of sterilizing the implant during or subsequent to irradiating.

8. The method of claim 7 wherein the sterilizing step comprises contacting the implant with electron-beam radiation, gamma radiation, gas plasma or ethylene oxide.

9. The method of claim 6 further comprising the step of packaging the implant.

10. The method of claim 7 wherein the sterilizing step occurs during, after or both during and after packaging the implant.

11. A method of forming a sterilized packaged implant comprising: combining vitamin E and an ultrahigh molecular weight polyethylene to form an ultrahigh molecular weight polyethylene blend prior to consolidation of the ultrahigh molecular weight polyethylene blend, in which the blend comprises the vitamin E in a substantially uniform distribution throughout the ultrahigh molecular weight polyethylene; consolidating the ultrahigh molecular weight polyethylene blend; forming an implant from the consolidated ultrahigh molecular weight polyethylene blend; packaging the implant; and irradiating the packaged implant with electron-beam radiation at an absorbed dose of at least 60 kiloGrey and a dose rate of at least I MegaGrey per hour, wherein the irradiated implant has a swell ratio of less than 4.

12. The method of claim 11 wherein the implant is packaged in a gas permeable package or a oxygen-deprived barrier package.

13. A method comprising: mixing an ultrahigh molecular weight polyethylene and vitamin E to provide a substantially uniform blend of vitamin E throughout the ultrahigh molecular weight polyethylene; consolidating the substantially uniform blend of ultrahigh molecular weight polyethylene and vitamin E; and inducing cross-linking of the consolidated blend by irradiating the consolidated blend with electron-beam radiation at an absorbed dose of at least 60 kiloGrey and a dose rate of at least 15 MegaGrey per hour to provide a cross-linked material.

14. The method of claim 13, further comprising: processing the cross-linked material to provide an implant; packaging the implant; and sterilizing the packaged implant.

15. The method of claim 14, in which the sterilizing step comprises exposing the packaged implant to radiation.

16. A method comprising: a first step comprising combining vitamin E and an ultrahigh molecular weight polyethylene powder to form a substantially uniform blend of the vitamin E throughout the ultrahigh molecular weight polyethylene powder; and a second step following the first step, the second step comprising irradiating the substantially uniform blend with electron-beam radiation at an absorbed dose of at least 60 kiloGrey and a dose rate of at least 15 MegaGrey per hour to form a crosslinked blend.

17. The method of claim 16, further comprising consolidating the substantially uniform blend of the vitamin E and the ultrahigh molecular weight polyethylene powder after the first step and before the second step.

18. An article comprising a blend of an ultrahigh molecular weight polyethylene and vitamin E, in which the vitamin E is present in a substantially uniform distribution throughout the ultrahigh molecular weight polyethylene, in which the blend has a swell ratio of less than about 3.5 after crosslinking, in which the blend is prepared by a process comprising combining the vitamin E and the ultrahigh molecular weight polyethylene;
blending the vitamin E and the ultrahigh molecular weight polyethylene until a substantially uniform distribution of vitamin E is present throughout the ultrahigh molecular weight polyethylene to provide a blend;
crosslinking the blend using electron beam radiation at an absorbed dose of at least about 95 kilo Grey and a dose rate of at least about 15 MegaGrey per hour; and annealing the crosslinked blend at a temperature up to about 200°C.

19. The article of claim 18, in which mechanical annealing is used to anneal the crosslinked blend.

20. The article of claim 18 or 19 wherein the blend further comprises at least one additional polymeric material.

21. The article of any one of claims 18 to 20 further comprising an additional component joined to the article to form an implant.

22. The article of any one of claims 18 to 21 wherein the article comprises at least portions of an artificial hip, hip liner, knee liner, disk replacement, shoulder, elbow, foot, ankle, finger, mandible or bearings in artificial heart.

23. The article of any one of claims 18 to 22, in which the vitamin E is present between 0.02 w/w% and about 2.0 w/w%.

24. A method of forming a crosslinked ultrahigh molecular weight polyethylene blend comprising:
combining an amount of vitamin E and an ultrahigh molecular weight polyethylene to form an ultrahigh molecular weight polyethylene blend; and irradiating the ultrahigh molecular weight polyethylene blend with electron beam radiation at an absorbed dose of at least 60 kiloGrey and a dose rate of at least 1 MegaGrey per hour to crosslink said ultrahigh molecular weight polyethylene blend.

25. The method of claim 24 wherein the amount of vitamin E combined with the blend is between about 0.02 w/w% and about 2.0 w/w%.

26. The method of claim 24 wherein the amount of vitamin E combined with the blend is between about 0.05 w/w% and about 0.4 w/w%.

27. The method of any one of claims 24 to 26 wherein at least some of the vitamin E is uniformly dispersed within a surface region of the blend and the surface region extends from an exposed surface of the blend to a depth of at least about 5 millimeters from the surface of the blend.

28. The method of claim 27 wherein the surface region extends from an exposed surface of the blend to a depth of at least about 15 millimeters from the surface of the blend. 29. The method of claim 27 wherein at least about 0.02 percent by weight of the vitamin

E is uniformly dispersed within the surface region of the blend.

30. The method of claim 28 wherein at least about 0.04 percent by weight of the vitamin E is uniformly dispersed within the surface region of the blend.

31. The method of any one of claims 24 to 26 wherein the vitamin E is dispersed substantially uniformly throughout the blend.

32. The method of any one of claims 24 to 31 further including preheating the ultrahigh molecular weight polyethylene blend to above about room temperature prior to irradiation.

33. The method of any one of claims 24 to 32 further including preheating the ultrahigh molecular weight polyethylene blend to between about 120°C and about 130°C prior to irradiation.

34. The method of any one of claims 24 to 33 wherein the ultrahigh molecular weight polyethylene blend is annealed after irradiation at a temperature up to about 200°C.

35. An implantable article comprising:
a crosslinked ultrahigh molecular weight polyethylene blend wherein the blend is produced by:
combining an amount of vitamin E and an ultrahigh molecular weight polyethylene to form an ultrahigh molecular weight polyethylene blend; and irradiating the ultrahigh molecular weight polyethylene blend with electron beam radiation at an absorbed dose of at least 60 kiloGrey and a dose rate of at least 1 MegaGrey per hour to crosslink said ultrahigh molecular weight polyethylene blend.

36. The implantable article of claim 35 wherein the amount of vitamin E
combined with the blend is between about 0.02 w/w% and about 2.0 w/w%.

37. The implantable article of claim 35 wherein the amount of vitamin E
combined with the blend is between about 0.05 and about 0.4 w/w%.

38. The implantable article of any one of claims 35 to 37 wherein at least some of the vitamin E is uniformly dispersed within a surface region of the blend and the surface region extends from an exposed surface of the blend to a depth of at least about 5 millimeters from the surface of the blend.

39. The implantable article of claim 38 wherein the surface region extends from an exposed surface of the blend to a depth of at least about 15 millimeters from the surface of the blend.

40. The implantable article of claim 38 wherein at least about 0.02 percent by weight of the vitamin E is uniformly dispersed within the surface region of the blend.

41. The implantable article of claim 38 wherein at least about 0.04 percent by weight of the vitamin E is uniformly dispersed within the surface region of the blend.

42. The implantable article of any one of claims 35 to 37 wherein the vitamin E is dispersed substantially uniformly throughout the blend.

43. The implantable article of any one of claims 35 to 42 wherein the ultrahigh molecular weight polyethylene blend is preheated prior to irradiation to above about room temperature.

44. The implantable article of any one of claims 35 to 43 wherein the ultrahigh molecular weight polyethylene blend is preheated prior to irradiation to between about 120°C and about 130°C.

45. The implantable article of any one of claims 35 to 44 wherein the ultrahigh molecular weight polyethylene blend is annealed after irradiation at a temperature up to about 200°C.

46. The implantable article of any one of claims 35 to 45 wherein the ultrahigh molecular weight polyethylene blend is annealed after irradiation at a temperature of about 150°C.