AU2014274546B2 - An antioxidant stabilized crosslinked ultra-high molecular weight polyethylene for medical device applications - Google Patents

Abstract

The present invention provides a method for processing a blend comprising UHMWPE for use in medical applications. The method comprising processing a blend comprising UHMWPE and 0.01 to 3.0 weight percent of an antioxidant to consolidate the blend, the consolidated blend having a melting point. Preheating the consolidated blend to a preheat temperature above room temperature and below the melting point of the consolidated blend. Irradiating the consolidated blend while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend. The present invention also provides a crosslinked UHMWPE blend for use in medical implants prepared by a process comprising the steps of processing a blend comprising UHMWPE and 0.01 to 3.0 weight percent of an antioxidant to consolidate the blend, the consolidated blend having a melting point. Preheating the consolidated blend to a preheat temperature above room temperature and below the melting point of the consolidated blend, irradiating the consolidated blend with a total irradiation dose of at least 100 kGy while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend.

Description

AN ANTIOXIDANT STABILIZED CROSSLINKED ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE FOR MEDICAL DEVICE APPLICATIONS

Cross-Reference To Related Applications [0001] The present application is a divisional application from Australian Patent Application No. 2013200780 filed on 8 February 2013, the entire disclosure of which is incorporated herein by reference.

BACKGROUND 1. Field of the Invention.

[0002] The present invention relates to crosslinked ultra-high molecular weight polyethylene and, particularly, to antioxidant stabilized, crosslinked ultra-high molecular weight polyethylene. 2. Description of the Related Art.

[0003] Ultra-high molecular weight polyethylene (UHMWPE) is commonly utilized in medical device applications. In order to beneficially alter the material properties of UHMWPE and decrease its wear rate, UHMWPE may be crosslinked. For example, UHMWPE may be subjected to electron beam irradiation, gamma irradiation, or x-ray irradiation, causing chain scissions of the individual polyethylene molecules as well as the breaking of C-H bonds to form free radicals on the polymer chains. While free radicals on adjacent polymer chains may bond together to form crosslinked UHMWPE, some free radicals may remain in the UHMWPE following irradiation, which could potentially combine with oxygen, causing oxidation of the UHMWPE.

[0004] Oxidation detrimentally affects the material properties of UHMWPE and may also increase its wear rate. To help eliminate the free radicals that are formed during irradiation and that may continue to exist thereafter, UHMWPE may be melt annealed by heating the crosslinked UHMWPE to a temperature in excess of its melting point. By increasing the temperature of the UHMWPE above its melting point, the mobility of the individual polyethylene molecules is significantly increased, facilitating additional crosslinking of the polyethylene molecules and the quenching of free radicals.

[0005] While melt annealing irradiated, crosslinked UHMWPE helps to eliminate free radicals and reduce the potential for later oxidation of the UHMWPE, the melt annealing could potentially reduce other mechanical properties of the UHMWPE.

[0005a] A reference herein to a patent document or other matter winch is given as prior art is not to be taken as an admission or a suggestion that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

SUMMARY

[0005b] Viewed from one aspect, the present invention provides a method for processing UHMWPE for use in medical applications, the method comprising the steps of: processing a blend comprising UHMWPE and 0.01 to 3.0 weight percent of an antioxidant to consolidate blend having a melting point; preheating a consolidated blend comprising UHMWPE and 0.01 to 3.0 weight percent of an antioxidant, the consolidated blend having a melting point, to a preheat temperature above room temperature and below the melting point of the consolidated blend; and irradiating the consolidated blend while maintaining the consolidated blend at a temperature below' the melting point of the consolidated.

[0005c] Viewed from another aspect, the present invention provides a cross! inked UHMWPE blend for use in medical implants prepared by a process comprising the steps of: processing a blend comprising UHMWPE and 0.01 to 3.0 weight percent of an antioxidant to consolidate the blend, the consolidated blend having a melting point; preheating a consolidated blend comprising UHMWPE and 0.01 to 3.0 weight percent of an antioxidant, the consolidated blend having a melting point, to a preheat temperature above room temperature and below the melting point of the consolidated blend; and irradiating the consolidated blend with a total irradiation dose of at least 100 kGv while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend.

[0005d] Where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereto. ) [θί)0ό] The present invention relates to a crosslinked UHMWPE and, particularly, an antioxidant stabilized, erossSinked UHMWPE, In one exemplary embodiment, an antioxidant is combined with UHMWPE prior to subjecting the UHMWPE to crosslinking irradiation. In one exemplary embodiment, the antioxidant is tocopherol After the antioxidant is combined with the UHMWPE, the resulting blend may be formed into slabs, bar stock, and/or incorporated into a substrate, such as a metal, for example. The resulting product may then be subjected to crosslinking irradiation, in one exemplary embodiment, the UHMWPE blend is preheated prior to subjecting She same to ctossltokmg irradiation. Once irradiated, tire UHMWPE blended product may be machined, packaged, and sterilized in accordance with conventional techniques.

[00(17] in one exemplary embodiment, the formed UHMWPE/arttioxidant blend may be subjected to multiple passes of crosslinking irradiation. By irradiating the blend in multiple passes, the maximum dose of radiation recei ved by the U HMWPE blend at any one time is lessened, As a result, the maximum temperature of the UHMWPE biend reached during irradiation as correspondingly lessened, This allows for the UHMWPE to maintain a higher level of desirable mechanical properties and prevents substantial melting of the UHMWPE, In one exemplary embodiment, the UHMWPE is cooled after each individual pass of crosslinking irradiation. By allowing the UHMWPE blend to cool, the temperature at the time of subsequent irradiation is high enough to encourage the mobility of the individual polyethylene molecules, but is also low enough that the temperature increase experienced during irradiation is unlikely to substantially alter any desired material properties of the UHMWPE blend, [0008] Advantageously, by incorporating an antioxidant:, such as tocopherol, into the UHMWPE prior to subjecting the same to crosslinking irradiation, the UHMWPE may be stabilized without tire treed for post irradiation melt annealing or any other post-irradiation treatment to quench free radicals. Specifically, an antioxidant, such as tocopherol acts as a free radical scavenger and, in particular', acts as an electron donor to stabilize free radicals, While tocopherol itself then becomes a free radical, tocopherol is a stable, substantially unreactive free radical- Additionally, because of the substantially reduced level of oxidation that occurs using a UHMWPE/antioxidant biend, the amount of oxidized material that must be removed to form a final implantable medical component is reduced. As a result, the size of the stock material subjected to irradiation maybe smaller in dimension, making it easier to handle and easier to manufacture into final medical components, [Θ009] Moreover, by subjecting the UHMWPE/autioxidant blend to multiple passes of irradiation, the UHMWPE blend may be integrally incorporated onto a substrate prior to irradiation.

Specifically, as a result of separating the total radiation dose into a plurality of individual passes, the temperature of the UHMWPE biend at the UHMWPE/substrate interface remains Sow enough that separation of the UHMWPE blend and the substrate is substantially prevented. Further, even after irradiation, some antioxidant remains unreacted within die UHMWPE blend, which may continue to quench free radicals throughout the lifetime of the medical component. Thus, evert after the medical component is Implanted, the antioxidant may continue to quench free radicals and further reduce the likelihood of additional oxidation, 10010] In one form thereof, the present invention provides method for processing UHMWPE for use in medical applications, the method including the steps of; combining UHMWPE with an antioxidant to form a blend having 0,01 to 3.0 weight percent of the antioxidant, the UHMWPE having a melting point; processing the blend to consolidate, the consolidated blend having a melting point; preheating the consolidated blend to a preheat temperature below the melting point of the consolidated blend; and irradiating the consolidated blend while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend [SOU] In another form thereof, the present invention provides a crosslinked UHMWPE blend for use in medical implants prepared by a process including tire steps of: combining UHMWPE with an antioxidant to form a blend having 0.1 to 3,0 weight percent antioxidant; processing the blend to consolidate the blend, the, consolidated biend having a melting point; preheating the consolidated blend to a preheat temperature below the inciting point of the consolidated blend; and irradiating the consolidated blend with a total irradiation dose of at least 100 kOy while maintaining the consolidated blend at a temperature below the inciting point of the consolidated blend,

BRIEF..PESCKIFrcQN OP THE DRAWING

[0012] Use above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: [0013] Fig, 1 is schematic depicting exemplary processes for preparing and using the crosslinked UHMWPE blends of the present invention; and [0014] Fig, 2 is a perspective view of an exemplary medical implant formed form a UHMWPE blend and a substrate.

[0015] 'Hie exemplifications set out herein illustrate embodiments of the invention atid such exemplifications are not to be construed as limiting the scope of the invention in any manner, [0016] Referring to Fig, 1, UHMWPE is combined with an antioxidant to create a UHMWPE/amioxidant blend (the “UHMWPE blend"). Once combined, the UHMWPE blend may be processed to fabricate the same into a desired form, Once formed, the UHMWPE blend may be preheated and subjected to cross-linking irradiation, The crosslinked UHMWPE blend may then be subjected to machining, packaging, and sterilization, [0017] To create the UHMWPE/antioxidant blend, any medical grade UHMWPE powder may be utilized, For example, GUR 1050 and GUR 1020 powders, both commercially available from Tieona, having North American headquarters located in Florence, Kentucky, may be used.

Similarly, while any antioxidant, such as Vitamin C, lycopene, honey, phenolic antioxidants, amine antioxidants, bydroquinone, beta-carotene, ascorbic acid, CoQ-enzyroe, and derivatives thereof, may be used, the UHMWPE blend referred to herein is a UHMWPE/tocopherol, U, Vitamin E, blend. Additionally, as any tocopherol may be used in conjunction with the present invention, such as d-. a-tocopherol, d,Fa-tocopherol, or a-tocopherol acetate, unless otherwise specifically stated herein, the term "tocopherol” in its generic form refers to all tocopherols, However, the synthetic form, dtl-α-tocopherol, is the most commonly used, [I301S] In combining UHMWPE and tocopherol, any mechanism and/or process achieving a substantially homogenous blend of the components may be utilized, in one exemplary embodiment, solvent blending is utilized. In solvent blending, tocopherol is mixed wish a volatile solvent to lower she viscosity of the tocopherol and facilitate homogenous blending of the tocopherol with the UHMWPE, Once the tocopherol is mixed with the solvent, the tocopheroi/soSvemt mixture may be combined with the UHMWPE, such as with a corse mixer. The solvent is then evaporated, leaving only the UHMWPE/tocopherol blend, In another exemplary' embodiment, tocopherol may be blended with UHMWPE by precision coating or atomization. For example, tocopherol may be precision coated onto the UHMWPE powder using a MP-1 MULTI-PROCESSOR Fluid Bed connected to a laboratory' module Precision Cosier available from Niro Inc. of Columbia, Maryland. MULTI-PROCESSOR™ is a trademark of Niro Inc, [¢1019] in another exemplary' embodiment, low intensity mixing may be used. Low intensity, i.e. Sow shear, mixing may be performed using a Diosna P100 Granulator, available front Diosna GmbH of Osnabruek, Germany, a subsidiary of Multimixing S.A. in another exemplary' embodiment, high shear mixing may be used. High shear mixing of UHMWPE and tocopherol may be achieved using a RYG2B or a ROST High Intensity Mixer, both commercially available from Hindi Machines of Gurnee, Illinois. Alternatively, high shear mixing may be achieved using a Collette ULTIMAPRO™ 75 One Pot Processor available from Niro, Inc, of Columbia, Maryland. ULTIMAPRO™ is a trademark of Niro, Inc. EMsed on the results of testing the above identified methods useful for combining UHMWPE and tocopherol, high shear mixing appears to provide favorable results, including an acceptable homogeneity and a low number of indications, ί.ε., areas of high tocopherol concentrations relative to the surrounding areas as determined by visual inspection under ultraviolet light or by chemical measurements, such as infrared spectroscopy or gas chromatography. Additionally, in other exemplary embodiments, the fluidized bed, emulsion polymerization, electrostatic precipitation, wetting or coating of particles, and/or masier batch blending may he used to combine the UHMWPE and tocopherol.

[002¾) irrespective of the method used to combine the UHMWPE and tocopherol to form the UHMWPE blend, the components are combined in ratios necessary to achieve a tocopherol concentration of between 0.01 weight percent (wl %) and 3 wt. %, In exemplary embodiments, the tocopherol concentration may be. as low' as 0.01 wt, %, 0.05 wt, %, and 0,1 wt %, or as high as 0.6 wt. %, 0.8 wt. %, and 1,0 wt. %, for example. In determining the appropriate amount of tocopherol, two competing concerns exist. Specifically, the amount selected must be high enough to quench free radicals in the UHMWPE, but must aiso be low enough to allow' sufficient crosslinking so as to maintain acceptable wear properties of the UHMWPE. In one exemplary embodiment, a range of tocopherol from 0,1 to 0.6 wt. % is used to successfully quench free radicals while still maintaining acceptable wear properties, [0(121] Once the UHMWPE blend is substantially homogenously blended and die amount of tocopherol is determined to be within an acceptable range, the UHMW PE blend is processed to consolidate the UHMWPE blend, as radicated at Step 12 of Fig. 1, The UHMWPE blend may be processed by compression molding, net shape molding, injection molding, extrusion, monoblock formation, fiber, melt spinning, blow molding, solution spinning, Slot isostatic pressing, high pressure crystallization, and films. In one exemplary embodiment, as indicated at Step 16 of Fig. 1, foe UHMWPE blend is compression molded Into the form of a slab, in another exemplary' embodiment, indicated at Step 14 of Fig. 1. the UHMW'PE blend may be compression molded into a substrate, as described its further detail below, For example, the. UHMWPE blend may be compression molded into a roughened surface by macroscopic mechanically interlocking the UHMWPE biend with features formed at the roughened surface of the substrate, Similarly, the UHMWPE blend may be molded into another polymer or another antioxidant stabilized polymer. Alternatively, the UHMWPE blend may be net shape molded into the shape of the final orthopedic component at Step 15 of Fig, i, in this embodiment, if the final orthopedic component includes a substrate, the UHMWPE blend is net shape molded into the substrate at Step 15 and is processed in the same manner as a UHMW'PE blend compression molded into a substrate at Step 14, as described in deiai; below, in contrast, if the UHMWPE blend is net shape molded at Step 15, but is not net shape molded into a substrate, the. component is then processed in the same manner as a UHMWPE blend compression molded into a slab at Step 16, as described in detail below, [0022] In one exemplary embodiment, the substrate may be a highly porous biomateriai useful as a bone substitute, cell receptive material, tissue receptive material, an osteoconductive material, and/or an osteoinductive material, A highly porous biomateriai ittay have a porosity as low as 55, 65. or 75 percent or as high as 80, 85, or 90 percent, An example of such a material is produced using Trabecular Metal™ technology generally available from Zimmer, Inc., or Warsaw, Indiana, Trabecular Metal™ is a trademark of Zimmer Technology, Inc. Such a material may be formed from a reticulated vitreous carbon foam substrate which is infiltrated and coated with a biocompatible metal, such as tantalum, etc,, by a chemical vapor deposition ("CVD1') process in the manner disclosed in detail in U.S. Patent No, 5,282,861, the entire disclosure of which is expressly incorporated herein by reference. In addition to tantalum, all porous coating and other metals such as niobium, tivanium, cancellous structured titanium, or alloys of tantalum and niobium with one another or with other metals may also be used.

[0823] After processing, the UHMWPE biend may be heated to a temperature below the melting point of the UHMWPE blend to relieve any residual stresses that may have been formed during processing and to provide additional dimensional stability. In one exemplary embodiment, the melting point of the UHMWPE blend is determined according to standard methods using differential scanning calorimetry, Heating the UHMWPE blend below' the inciting point creates a more homogenous mixture and increases the final crystallinity. In one exemplary embodiment, she UHMWPE blend is heated to a temperature below its niching {joint, e.g., between 80° Celsius (C) and 140‘C, and held isothermally for six hours, in other exemplary embodiments, the UHMWPE may be heated to a temperature as low as 80°C, 9G3C, 95nC, or I00°C or as high as ilO°C, 115°C, 120°C, and ]26®C. In other exemplary embodiments the temperature may be held for as short as 0.5 hours, 1,0 hours, 1.5 hours, or 2,0 hours or as long as 3,0 hours, 4,0 hours, 5,0 hours, or 6.0 hours. In another exemplary embodiment, the UHMWPE blend is heated after irradiation, described below, to provide similar benefits to the UHMWPE biend.

[0824] Irrespective of whether the UHMWPE blend is heated to a temperature Ssciow the melting point of die UHMWPE blend to relieve any residua! stress, the processed UHMWPE blend is preheated at Steps 18, 20 of Fig. 1 i n preparation for receiving crosslinking irradiation, In one exemplary embodiment, the processed UHMWPE blend may be preheated to any temperature between room temperature, approximately 23°C, up to the melting {joint of the UHMWPE blend, approximately 140°C, In another exemplary embodiment, the UHMWPE blend is preheated to a temperature between 60°C and 13G°C, in other exemplary embodiments, the UHMWPE blend may be heated to a temperature as low as 60°C, 70°C, 80eC, 90e€, or l GOT or as high as i 10('C, 120°C, J30°C, 135°C, 140°C, By preheating the processed UHMWPE blend before irradiation, the material properties of the resulting irradiated UHMWPE blend are affected, Thus, the material properties for a UHMWPE biend irradiated at a relatively cold, e,g„ approximately 40“C, temperature are substantially different than the material properties for a UHMWPE blend Irradiated at a relatively warm, s.g„ approximately 120°C to approximately 140SC, temperature.

[Ms251 However, while the material properties of a UHMWPE blend irradiated at a lower temperature may be superior, the wear properties, fatigue properties, oxidation level, and ires radical concentration are all negatively affected. In eotstrasi, while irradiation of a UHMWPE blend at a higher temperature may slightly diminish the material properties, it also results in a higher crosslinking efficiency due to higher chairs mobility and adiabatic, melting, Additionally, by irradiating at a higher temperature, a greater number of crosslinks are formed, Thus, there are less free radicals in the UHMWPE blend and less tocopherol is consumed by reacting with the free radicals during irradiation and immediately thereafter. As a result, a greater amount of tocopherol remains in the blend that may react with free radicals during the UHMWPE blend's lifecycle, he,, after irradiation, This, it! turn, increases the overall oxidative stability of the UHMWPE blend, [00261 Referring specifically to Step 18, when the UHMWPE blend and its associated substrate are irradiated, Me substrate may rapidly increase in temperature, Thus, the temperature increase of the substrate should be taken itiio account when determining the preheat temperature of the UHMWPE blend and substrate, In one exemplary embodiment, a UHMWPE blend formed to a highly porous substrate manufactured using Trabecular Metal™ technology is preheated to a temperature between 40“C and 12G°C prior to subjecting the substrate and UHMWPE blend to crosslinking irradiation, A further consideration that may impact the preheat temjsrature is the material used to form any tooling that may contact the UHMWPE blend and substrate during irradiation. For example, a holder used to retain the UHMWPE blend and substrate in a desired position during irradiation may rapidly increase in teEnperature at a faster rate than the UHMWPE blend, In order to «substantially eliminate this concern, the tooling should have a heat capacity substantially equal to or greater than the heat capacity of the UHMWPE blend. In one exemplary embodiment, the UHMWPE blend has a heat capacity substantially between 1,9 j/g °C and 10 J/g °C. Titus, polyether ether ketone, for example, having a heat capacity of approximately 2,8 J/g C'C, may be used to form the tooling, Alternative materials that may be used to form the cooling also include carlron fiber and other composites.

[0027] After the desired preheat temperature of the UHMWPE blend is achieved, the UHMWPE blend is subsequently irradiated at Steps 26, 28 to induce crossiirtkitsg of the UHMWPE. Thus, as used herein, "crosslinking irradiation" refers to exposing the UHMWPE blend to ionizing irradiation to form free radicals which tnay later combine to form crosslinks. The irradiation may be performed in air at atmospheric pressure, in a vacuum chamber at a pressure substantially less then atmospheric pressure, or in an inert environment, t.e,, in an argon environ mem. lor example. The irradiation is, in one exemplary embodiment, electron beam irradiation, hi another exemplary embodiment, the irradiation is gamma irradiation, in yet another exemplary embodiment, steps rb, 28 do not requite irradiation, but instead utilize silane crosslinking. In one exemplary embodiment, erossltnkirtg is induced by exposing the UHMWPE blend to a total radiation dose between about 25 kGy and 1,000 . kGv. In another exemplary embodiment, erosslinking is induced by exposing the 11HMW PE blend to a total radiation dose between about 50 kGy anti 250 kGy in air, 1 hese doses are higher than doses commonly used to crosslink UHMWPE due ict the presence of tocopherol in the UHMWPE blend, Specifically, the tocopherol reacts with some of the polyethylene chains that became free radicals during iirEtdiation, As a result, a higher irradiation dose must be administered to the UHMWPE blend to achieve the same levei of crosslinkmg that would occur at a lower dose in standard UHMWPE, U„ UHMWPE absent att antioxidant [0028] However, the higher irradiation dose needed to crosslink the UHMWPE blend to the same level tts UHMWPE absent an antioxidant: may cause a greater temperature increase in the UHMWPE blend. Thus, if the entire irradiation dose is administered to the UHMWPE blend at once, the UHMWPE blend may be heated above the melting point: of the UHMWPE blend, approximately i 40C'C, and result in melt annealing of the UHMWPE blend. Therefore, prior to irradiating the UHMWPE blend, a determination is made at Steps 22, 24 comparing the total crosslinking Irradiation dose to be. administered to the UHMWPE blend to the maximum individual dose of radiation that can be administered to the UHMWPE blend without: raising the temperature of the UHMWPE blend near to and/or above its melting point.

[0029] Thus, If the total crosslinking irradiation dose determined in Steps 2 a 24 is less then the maximum individual crossiinJdng dose that cat! fee administered without raising the temperature of the UHMWPE near to and/or above the Enelting point, the UHMWPE is irradiated artd the total erossltnkirtg irradiation dose identified at Steps 22,24 is administered m air. In one exemplary embodiment, the maximum individual crossiinking dose is between about 50 kGy and 1000 kGy. in one exemplar)' embodiment, the maximum individual crosslittking dose is 150 kGy for the UHMWPE blend alone {Step 24) and is 100 kGy for the UHMWPE blend and substrate combination f Step 22), However, the maximum individual crosslinking dose may be any dose that does not cause the UHMWPE blend to increase in temperature above the inciting point of the UHMWPE biend. Additionally, the maximum individual crosslinking dose may be dependent on the type of irradiation used, Thus, the maximum individual crosslinking dose for electron beam irradiation may be different than the maximum individual cross!inking dose for gamma irradiation, in one exemplary embodiment of the UHMWPE blend and substrate, a heat sink may be attached to the substrate to dissipate heat therefrom and allow for the use of a higher individual irradiation dose, ire,, allow for a higher dose to be administered in a single pass. Further, in addition to the type of irradiation used, the dose rate, temperature at which the dose is administered, the amount of time between doses, and the level of tocopherol in the UHMWPE blend, may also affect the maximum individual erosslinking dose.

[883¾] If, at step 24, the total erosslinking dose for the UHMWPE blend is determined to exceed the maximum individual dose of approximately 150 kGy, multiple irradiation passes are required. Similarly, if, at .Step 22, the total erosslinking dose for the UHMWPE blend and substrate exceeds the maximum individual dose of approximately 100 k.Gy, multiple irradiation passes are required, 'The lower maximum individual dose for the UHMWPE blend and substrate results front the greater potential temperature increase of the substrate during irradiation. This potential temperature increase may be sufficient to melt or otherwise significantly alter tne UHMWPE blend along the UHMWPE blend/substrate interface. For example, as a result of the different coefficients of thermal expansion between the UHMWPE blend and the substrate, cracking may occur in the UHMWPE blend if irradiated at an individual dose in excess of the maximum individual dose, [88311 For electron beam irradiation, the preheat temperature and dose level per pass are interdependent variables that are controlled by the specific heat of the materials being irradiated.

The substrate material may beat to a significantly higher level than the polymer at. the same irradiation dose level if the specific heat of the substrate is substantially lower than the specific heat of the polymer. The final temperature of the materials achieved during the irradiation can be controlled by a judicious choice of dose level per pass and preheat temperature, so that temperatures are high enough to promote erosslinking in the presence of tocopherol, but: low enough to prevent substantia! melting of the UHMWPE blend. Further, while paitial melting may, m some embodiments, be desired, the final temperature should be low enough to prevent substantial melting and yet be high enough mat free radical levels ate reduced below' the levels that would be present it no heating during irradiation had occurred. The propensity for cracking is most likely due to a combination of effects related to the weakness of the UHMWPE blend arid expansion differences between the UHMWPE blend and substrate, whereas complete melting of the UHMWPE blend in die region near the substrate is due to overheating of the substrate.

[0032] if it is determined in Step 24 that multiple irradiation passes are required, as set forth above, then the first dose of irradiation administered in Step 28 should be less than ISO kGy. in one exemplary embodiment, the total irradiation dose determined in Step 24 is divided into equal individual irradiation doses, each less than 150 kGy, For example, if the total irradiation dose determined in Step 24 is 200 kGy, individual doses of 100 kGy each may be administered, in another exemplary embodiment, at least two of ilte individual irradiation doses are unetfu&I and all ol the individual irradiation doses do not exceeded 150 kGy, e,g,, a iota; erosslmking dose of 200 kGy is dividing into a first individual dose of 150 kGy and a second individual dose of 50 kGy, [0033] Similarly, if it is determined in Step 22 that multiple irradiation passes are required, then the first dose of irradiation administered in Step 26 should be less than ;00 kGy. in one exemplajy embodiment, the total irradiation dose determined in Step 22 ts divided into equal, individual irradiation doses, each less than 1 GO kGy. For example if the total irradiation dose determined in Step 22 is 150 kGy, individual doses of 75 kGy each may be administered, In another exemplary embodiment, at least two of the individual irradiation doses are unequal and all of the individual irradiation doses do not exceed 100 kGy, e.g., a total crosslinking dose of 150 kGy is divided into a first individual dose of 100 kGy and a second individual dose of 50 kGy, [0034] Further, in the UHMWPE biend/substrate embodiment, by irradiating the UHMWPE blend first, i.e., directing the electron beam to contact the UHMWPE blend prior to contacting the substrate, the resulting UHMWPE blend has characteristics similar to an irradiated UHMWPE blend without the substrate anti is generally suitable for normal applications, In contrast, by irradiating she substrate first, i.e,, directing the electron beam to contact the substrate prior to contacting the UHMWPE blend, the resulting UHMWPE blend has characteristics that are substantially different than an irradiated UHMWPE biend without the substrate, Additionally, the differences, such as decreased crystallinity, are more pronounced near the UHMWPE biend/substrate interface and decrease as the UHMWPE blend moves away from the substrate. {0035] In the event multiple irradiation passes are required as described in detail above, the temperature of the UHMWPE blend or UHMWPE blend and substrate may be equilibrated to the preheat temperature in Steps 30, 32, between the administration of the individual doses. Specifically, as a result of the first individual irradiation dose increasing the temperature of the UHMWPE bleed, immediately administering another individual irradiation dose may significantly alter the material properties of the UHMWPE blend, melt the UHMWPE, or cause other detrimental effects. In one exemplary embodiment, after the first individual irradiation dose is administered, the UHMWPE blend or UHMWPE blend and substrate are removed and placed in an oven. The overt is set to maintain the temperature at the preheat temperature, i.e,, the temperature used in Steps 18,20 as described in detail above, and the UHMWPE blend or UHMWPE blend and substrate are placed within the oven to slowly cool until reaching the preheat temperature. Once the preheat temperature is reached, the UHMWPE biend or UHMWPE blend and substrate are removed and the next individual irradiation dose administered. In the event further individual irradiation doses ate required, she temperature equilibration process is repeated, [0836] In another exemplary embodiment, selective shielding is used to protect certain areas of the UHMWPE blend from exposure to the irradiation and substantially prevent or lessen the resulting temperature increase of the UHMWPE blend. Additionally, selective shielding may be used to help ensure that an even dose of irradiation is received by the UHMWPE blend, in one embodiment, a shield, such as a metallic shield, is placed In the path of the irradiation to attenuate the radiation dose received in the shielded area, while allowing the full effect of the irradiation dose in areas where higher temperatures can be tolerated, in one embodiment, the use of selective shielding allows for the total crosslinking irradiation dose to be administered in a single pass, reducing the need to administer the total erossiinking irradiation dose over multiple passes, [0837] Additionally, selective shielding of the irradiation may be used to prevent the metallic substrate from excessive heating doe to the differences in specific hears between the substrate and UHMWPE blend. The shielding could, its one exemplary embodiment, be designed so that the UHMWPE blend receives a substantially full irradiation dose, while lessening the irradiation penetration so 'hat a reduced dose is received at the substrate. As a result, the temperature increase of the substrate due to irradiation absorption is decreased, Tills allows lor !he use of higher dose levels per pass, eliminating the need for multiple passes to achieve higher dose levels and thus higher levels of crosslinking, in some embodiments, single pass irradiation is advantageous since it is a more efficient manufacturing process, and the resulting mechanical properties of the erosslinked material may also he desirable. Specific aspects anti methods of irradiation shielding are disclosed in U,S. Patent No, 6,365,089, entitled METHOD FOR CRQSSLINKfNG UHMWPE IN AN ORTHOPEDIC IMPLANT, issued on April 2, 2002, die entire disclosure of which is expressly incorporated by reference herein, [0038] In another exemplary embodiment, tocopherol is not added at Step 10, as discussed in detail above. Instead, tocopherol is diffused into the UHMWPE by placing the UHMWPE in a tocopherol bath after the UHMWPE has been irradiated in accordance with standard erossiinking irradiation techniques. However, as a result of administering the erossiinking irradiation prior to the addition of tocopherol, the present embodiment does not allow for the administration of a higher erossiinking irradiation dose, as discussed above. Additionally, the mechanical properties achieved by adding tocopherol prior to administering crossiinking irradiation appear to be superior ίο diffusing tcjcopherol into the UHMWPE after the erossimking irradiation has been administered.

[0Ό391 Once the total crosslinking irradiation dose has been administered to the UHMYv PE blend, the UHMWPE blend may be machined in Step 34 into a medical product, such as an orthopedic implant, according to customary techniques, such as milling, boring, drilling, cutting, and CNC (Computer Numerical Control) machining. For example, the UHMWPE blend may be machined into a hip, knee, ankle, shoulder, elbow, finger, denial, or spinal implant. Additionally, the UHMWPE blend may be assembled to other components for form a medical device, However, ii die UHMWPE blend is processed at Step 12 in Fig, 1 by net shape molding, which is identified above as a potential processing method, the need to machine the UHMWPE blend at Step 34 is substantially eliminated, Specifically, if the UHMWPE blend is processed by net shape molding, the UHMWPE blend is formed to the final shape, i.e„ the shape of the desired medical product, at Step 12, which may shea be assembled to other components to form the linal medical device. Referring to Fig. 2, an exemplary medical implant 100 is shown including UHMWPE blend 102 and substrate 104, As shown in Fig. 2, UHMWPE blend 102 is intefdigitated with substrate 104 in a similar manner as described in U,S, Patent Application Serial No, 11/055,322, entitled -'MODULAR POROUS IMPLANT", filed February 10, 2002, and U.S, Patent No, 6,087,553, entitled "IMPLANTABLE METALLIC OPEN-CELLED LATTICE/POLYETHYLENE COMPOSITE MATERIAL AND DEVICES", issued on July 11, 2000, tbs entire disclosures of which are expressly incorporated byreference herein.

[004(1] The medical product may be packaged at Step 36 and sterilized at Ste-ρ 38. In one exemplary embodiment, the medical product is sterilized using gas plasma, in another exemplary' embodiment, the medical product is sterilized using ethylene oxide, In yet another exemplary embodiment, the medical product is sterilized using dry heat sterilization. Additionally, testing has indicated that surface sterilization techniques, such as gas plasma, dry heat, gamma radiation, ionizing radiation, autoclaving, supercritical fluid technique, and ethylene oxide, provide sufficient sterilization of the medical product, even if the UHMWPE blend is secured to a substrate. Specifically, the surface sterilization techniques have proven to sufficiently sterilize the UHMWPE hiend/substrate interface. In another exemplary embodiment, gamma irradiation may be used to sterilize she medical product. However, in this embodiment, it is believed that a higher tocopherol concentration would be necessary in order for enough tocopherol to be available to quench free radicals after the sterilization irradtation was performed, [SM)41 ] While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures front the present disclosure as come within known or customary practice in !he art to which this invention pertains and which fall within the limits of the appended claims.

Examples [0042] The following non-limiting Examples illustrate various features and characteristics of the present invention, which is not to be construed as limited thereto. The following abbreviations are used throughout the Examples unless otherwise indicated, TABLE I Abbreviations

[6843] Throughout the various Examples, irradiated UHMWPE blends are used, which have been irradiated according to one of three different irradiation methods, As set forth above, differences m the irradiation conditions and techniques may affect the resulting material properties of the UHMWPE blend. Therefore, in order to property analyse and compare the results set forth in the Examples anti corresponding Tables, each of the irradiated UHMWPE blends used in the Examples are identified as having been irradiated according to the one of the methods set forth below in Table 2. Additionally, the electron beam source is calibrated by performing dosimetry at low irradiation doses and then parametrically determining the activation of the electron beam source needed to achieve higher doses, As a result, at higher irradiation doses, differences may exist between the actual dose and the parametrically determined dose, which may cause differences in the material properties of the irradiated UHMWPE blends, TABLE 2 Irmdi ah on Method s

Example 1

Feasibility Study of a-Tocopherol Acetate [1)1144] Use feasibility of blending a-tocopherol acetate with UHMWPE was investigated, a- tocopherol acetate was obtained from OSM Nutritional Products, Ltd. of Geleen, Netherlands and medical grade UHMWPE powder GUR 1050 was obtained from Tseona, having North American headquarters located in Florence, Kentucky. Isopropano: was then added to the «'tocopherol acetate as a diluent and the α-tocopherol acetate was solvent blended with the UHMWPE powder. The blending continued until two different UHMWPE/ α-tocopherol acetate blends were obtained, one UHMWPE blend having 0,05 wt. % α-tocopheroi acetate and the other UHMWPE blend having 0,5 wt % α-tocopheroi acetate, Each of the UHMWPE blends were then compression molded Us form four one-inch-thick pucks. Two pucks of each UHMWPE blend, i.e., two pucks of the UHMW} E blend having 0,Q5 wt. % α-tocopheroi acetate and two pucks of die UHMWPE blend having 0,5 wt, % «'tocopherol acetate, were preheated to 12CFC in a Grieve convection oven, available from The Grieve. Corporation of Round Lake, Illinois, The pucks were held at 120°C for 8 hours, ALer the expiration of 8 hours, the pucks were irradiated at 10 MeV, 50 kGy-m/min dose rate at 65 kGy and 100 kGy dose at lotion Industries Canada Inc. located in. Post Coquitlam, BC, Canada, [5)045] The remaining two pucks of each UHMWPE biend, i.e,, two pucks of the UHMW'PE b:end having 0,05 wt % α-tocopheroi acetate and two pucks of the UHMWPE blend having 0.5 wt. % »-tocopherol acetate, were heated to 40^ overnight. The next snoming, the remaining two pmJcs of each UHMWPE blend were irradiated at 10 MeV, 50 kGy-m/rain dose rate at 100 kGy dose at lotron industries Canada Inc, located in Port Coquitlam, SC, Canada, [0946] After irradiation, ail of the pucks were cut in half and a him was cut from the center of each puck. The films were then subjected to P1TR analysis using a Broker Optics F11¾ Spectrometer, available from Broker Optics of Billerica, Massachusetts, Both halves of each puck were then machined into flat sheets approximately 1/8 inch thick. One half of the flat sheets were immediately subjected to FFIR, The other half of the flat sheets were then subjected to acceletated aging in accordance with the American Society for Testing and Materials (AS 1M) Standard F-200o, Standard Practice for Accelerated Aging of Ultra-High Molecular W'eight Polyethylene after Gamma Irradiation in Air. Tensile specimens formed irons the flat sheets were subjected to accelerated aging and were then subjected to FTIR analysis. The OI and wt. % of n-toeophero! acetate were determined from the FTIR results, set forth below in TABLES 3 and 4. However, there were interference peaks in the FFIR results that prevented measurement of 01 for the 0.5 wt, %, 65 kGy, imaged sample, TABLE 3 FTIR Results

TABLE.4 DM dati ve ΰ'.-kx of UHMWPE Blend v< Eh 0.50

[0047J The FTIR results revealed that the 01 of die UHMWPE biend having 0.05 wt. % a-tocopherol acetate was generally higher than the OI of the UHMWPE blend having 0.5Q wt, % a-tocopherol acetate. This is believed to be iiecau.se these samples still contained e-tocopherol acetate after irradiation. As a result, the α-tocopherol acetate was still available in these samples to react with free radicals attd reduce the oxidative degradation of the UHMWPE blend. Additionally, the FFIR results showed that virtually no α-tocopheroi acetate was left after irradiation of the UHMWPE blend having 0.05 wt. % a-toeopfcerol acetate anti that about one-third of the n-tocopberol acetate was left, after irradiation of the UHMWPE blend having 0,5 wt. % cs-tocopheroi acetate. Further, as shown in TABLE 5 below, tensile properties were similar for both the UHMWPE blends that were subjected to accelerated aging and the UHMWPE blends that were not subjected to accelerated aging. Finally, the FTIR results suggested that the UHMWPE blends containing ct-tocopherol acetate have similar stabilization properties, s.e„ a similar ability to prevent oxidative degeneration, as UHMWPE blends containing similar concentration of d.l-a-tocopberoi. TABLE 5.

Mechanical Pnorgsrties

Chemical Properties of UHMWPE Blended with Tocopherol [0848J The chemical properties of d/i-u,-tocopherol mechanically blended with a UHMWPE powder which was slab molded into bars and electron beam irradiated were investigated. To perform this investigation, Design Expert: 6,0,1.0 software, obtained from btat'Ease, Iso, Minneapolis, MN, was utilized to setup a modified fractional factorial Design of Experiment (DQEj, The DOE evaluated five different variables: UHMWPE resin type, wt. % of d/i-a-tocopheroJ, preheat temperature, dose rate, and irradiation dose. 10049] GUR 1050 and GUR 1020 medical grade UHMWPE powders were obtained from Tieona, having North American headquarters in Florence, Kentucky, d/ki-tocopheroi was obtained ftom DSM Nutritional Products, Ltd, of Geleen, Netherlands. The GUR 1050 and GUR ΙΟο,ϋ were separately mechanically blended with the d/l-n-cocopheroi by low intensity blending vising a Diosna PI00 Granulator, available ftom Diosna GmbH of Gsnabriick, Germany, a subsidiary of Multimixiog S.A. Both the GUR 1050 and the GUR 1020 resins were mixed with the d/l-a-toeopherol in several batches to create UHMWPE blends of both testa types having 0,2 wt, %, 0.5 wt. %, and 1,0 wt. % dd-a-tocopherol. Each batch of blended material was compression molded into a slab and cut into bars of various sizes. Each of the resulting bars was then preheated by hunting to a preheat temperature in a Grieve convection oven, available from The Grieve Corporation of Round Lake.

Illinois, The; preheat temperature was selected fro™ 4G°C, 10®°C, 110°C and 122.2°C, as set forth in TABLE δ below.

[0050] After being preheated, the DHMWPE blend bars were electron beam irradiated according to Method C. set forth in TABLE 2 above, at a selected dose rate until a selected total irradiation dose was administered, The dose rate was selected from 75 icGy-m/inin, 155 kGy-ru/min, and 240 kGy-tn/min and the total irradiation dose was selected from 90 kGy, 120 kGy, 150 kGy, and 200 kGy, The portion of each bar wees then mierotomed into 200 micron thick films, These films were then subjected to FUR analysis on a Broker Optics FUR spectrometer, available from Broker Optics of Billerica, Massachusetts, lire FFIK results were analyzed to determine the \ El, wt, % cli ct tocopherol, tbs ΟΪ, and the TVf, The VEI and wt, % d/l-a-tocopherol were determined by calculating the ratio of the area under the d/1-a-toeopheroi peak at 1275-1245 cm 1 on the resulting FTIR chart to the area under the polyethylene peak at 1392-1330 cm 1 and at 1985-1850 cot , The Oi was determined by calculating the ratio of the area under the carbonyl peak on the FF1R chart at 1765-1680cm''1 to the area of the polyethylene peak at 1392-1330 cm'!. Tire TV! was determined by calculating the ratio of the area on the FTIR chart under the vinyl peak at 980-947 cnfi! to the area under the polyethylene peak at 1392-1330 cm \ [0051] After !he initial VEI, wt. % d/l-a-tocopherol and TV! were determined from the FTIR analysis of the Ain films, each of the thin films were accelerated aged according to A3TM Standard F-2003, Standard Practice for Accelerated Aging of Ultra-High Molecular Ά eight Polyethylene amor Gamma irradiation in Air. The accelerated aged films were again subjected to FTIR analysts on a Broker Optics FTIR spectrometer, available from Broker Optics of Billerica, Massachusetts, ibe resulting FTIR charts were analyzed to determine VEI, wt, % d/l-a-tocopherol, Oi, and FVI according to the methods set forth above, Once subjected to FTIR analysis, the aged files were placed in boiling hexane and allowed to retnain there for 24 hours to extract the d/3-u-tocopherol. After extraction of the d/l-a-tocopherol, the aged films were again subjected to FTIR analysts on the Broker Optics FT IR spectrometer. The resulting El IR ehttrt was then analyzed to determine the OI in accordance with the method set forth above, The additional FTIR analysis was peri tan ted to eliminate the d/l-a-tocopherol peak from interfering with the oxidation peaks. An analysis of the results set; forth in TABLE 6 below indicate that selecting a warmer preheat temperature may result in a lower OI and may also result hi some of the d/i-a-locopherol remaining in the UHMW PE after irradiation, TABLE 6

FTIR Results of Eradiated UHMWPE Blended wjth.d/|-{?.:.^£SSfei?AP.I

mmida.!

Free Radical Concentrations tti UHMWPE Bfended.¾jill.4'.·.ELiLLfeiUf Si [0852] The impact of meehanieaiiy blending d/l-ct-tocopherol with UHMWPE powder on free radical concentration of electron beam irradiated UHMWPE blend molded pocks was investigated. To perform this investigation, Design Expert 6.0.10 software, obtained front Stat-Ease, Inc. Minneapolis, MN, was utilized to setup a modified central composite Design of Experiment iDOE;, The DOE evaluated five factors: preheat temperature, dose rate, irradiation dose, d/l-ct-tocopheroi concentration, and predetermined hold tone, t.s., the time elapsed between removal of the UHMVi PE blend from the oven until the initiation of electron beam irradiation.

[0053] GUR 1050 medical grade UHMWPE powder was obtained from 1 icons, having North American headquarters in Florence, Kentucky, d/3-a-tocophero! was obtained from DSM Nutritional Products, Lid, of Geleen, Netherlands. The GUR 1050 UHMWPE power was mechanically blended with the d/i-o-tocophero] by high intensity blending using an Etrich Mixer, available from Flinch Machines, Inc. of Gurnee, Olinois, The GUR 1050 resin was mixed with the d/l-o-tocopherol in several batches to create UHMWPE blends having between 0.14 and 0.x4 wt, % d/l-a-tocopherol, as set forth below in TABLE 7. 10054] Each of the UHMWPE blends were then compression molded into 2.5 inch diameter and 1 inch thick pucks. Each of the resulting pucks was then preheated by heating in a Grieve convection oven, available front The Grieve Corporation of Round Lake, Illinois, to a preheat temperature. The preheat temperature was selected from between 85nL and 115 C, as set forth in TABLE 7 below. The pucks were then removed from the convection oven and held lor a predetermined fteriod of time ranging between 7 minutes anti 2 i minutes, as set forth in FABLE / below. After the expiration of the predetermined bold time, the pucks were electron beam irradiated utilizing Method A of TABLE 2. The pucks were irradiated at a dose rate selected from, between 30 kGyun/ntin and 75 kGy-m/ntin until a total dose selected from between 160 kGy and 190 kGy was administered, as set forth in TABLE 7 below. Cylindrical cores approximately 1 inch long were machined from the pucks, llte cylindrical cores were then analyzed using a Broker RMX/EPR (electron paramagnetic resonance) spectrometer, which has a detection limit of 0.01 X 10 spins/gratn and is available from Broker Optics of Billerica, Massachusetts. The resulting analysis indicated that preheat temperature, percent d/l-a-ioeopheroi, and dose level were ail significant factors in determining the resulting free radical concentration of the UHMWPE biend. Specifically, preheat temperature and d/I-a-tceopfaerol concentration had a negative correlation with the free radical concentration, while the total dose had a positive correlation with the free radical concentration. TABLE 7

Free Radical Concentration of UHMWPE Blends After .yan.oug..Proc^sjBE

(0055] The mechanical properties of d/l-a-tocopherol mechanically blended with a UHMW PE powder which was slab molded into bars and electron beam irradiated were investigated. To perform this investigation, Design Expert 6,0,10 software, obtained from Stat-Ense, Inc, Minneapolis, MN, was utilized to setup a modified fractional factorial Design of Experiment (DOE), Use DOE evaluated five different variables: UHMWPE resin type, weight percent of d/3-a-tocopberol, preheat, temperature, dose rate, and irradiation dose.

[0056] GUR 1050 and GUR 1020 medical grade UHMWPE powders were obtained from Ticona, having North American headquarters in Rorence, Kentucky, d/l-a-tocopherol was obtained from DSM Nutritional Products, Ltd, of Geleen, Netherlands. The GUR 1050 and GUR 1020 were separately mechanically blended with the d/l-a-tocopherol by low intensity blending using a Dsosna P100 Granulator, available from Diosna GmbH of Gsnabriick, Germany, a subsidiary of Multimixing S.A. Both the GUR 1050 and the GUR 1020 resins were mixed with the d/l-a-tocopherol in several batches to create UHMWPE blends of both resin types having 0.2 wt. %, 0,5 wt, %, and LG wt. % d/l-a-tocopherol, Each batch of blended material was compression molded into a slab and cut into bars, Each of die resulting bars was then preheated by heating the bars in a Grieve convection oven, available front The Grieve Corporation of Round Lake, Illinois, to a preheat temperature. The preheat temperature was selected from 4G®C, lOO^C, 1 iO°C and 122.2 C, as set torch In TABLE 8 below, |O0S7j After being preheated, she UHMWPE blend bars were electron beam irradiated according to Method C, set forth in TABLE 2 above, at a selected dose rate until a selected total irradiation dose was administered. The dose rate was selected from 75 kGy-m/min, 155 kGy-m/min, and a4G kGy-m/τηίη and the total irradiation dose was selected from 90 kGy, 120 kGy, 150 kGy, aOO kCry, and 250 kGy. Tyne V tensile specimens, as defined by the American Society for Testing and Materials (ASTM) Standard D638, Standard Test Method for Tensile Properties of Plashes, were machined from each of the UHMWPE blend bars, The Type- V tensile specimens were then subjected to ultimate tensile elongation, UTS, and YS testing in accordance with ASTM Standard 0638. Szod specimens were also machined front each of the UHMWPE blend bars and tested for izod impact strength according to ASTM Standard D256, Standard Test Methods for Determining the food Pendulum Impact Resistance of Plastics, Dynamic mechanical analysis (DMA) specimens were also machined from each of the UHMWPE blend bars and tested using a Model DMA 2980 Dynamic Mechanical Analyzer from FA instruments of Now Castle, Delaware, [¢058] An analysis of the results Indicates that the total irradiation dose had an influence on the Hod impact strength, ultimate tensile elongation, and yield strength of the UHMWPE blends. Additionally, the preheat temperature had an influence on the ultimate tensile strength and yield strength. In contrast, the weight percent of d/l-ts-tocopherol had an influence on ultimate tensile elongation and the dynamic mechanical analysis, Additional results from foe testing are set forth below in TABLE 8. TABLE 8

[0059] The wear properties of IJHMWPB mechanically blended with d J-n-tocopheroI and exposed to electron beam irradiation was investigated, To perform this investigation, Design Expert 6.0,10 software, obtained from Stat-Ease, Inc. Minneapolis, MN, was utilised to setup a modified central composite Design of Experiment (DOE), The DOti evaluated five different variables. preheat temperature, dose rate, tote! dose administered, d,i-o-tocophsrol concentration, sad cooling period, t.e., the elapsed time from end of the preheat until initial exposure to irradiation.

[CM}6@] GUR 1050 medie&amp;f grade UHMWPE powder was obtained from Ticona, having North American headquarters in Florence, Kentucky, d/1 -α-tocopherol was obtained from DSM Nutritional Products, Ltd of Geieen, Netherlands. Hie GUR 10S0 was mechanically mixed with the d/i-a-tocopherol using a High intensity Mixer, available front b.irieh Machines oi: Gurnee, Illinois. The GUR 1050 resin was mixed with the d/l-a-tocopherol in several batches to create UHMWPE blends having a selected wt. % of d/l-a-tocopherol, Fhe wt. % of d/3-o-tocopheroi was selected horn 0.14 wt. %, 0.19 wt, %, and 0.24 wt, % d/l-a-tocopherol. Each of the blends were then consolidated and formed into 2,5 inch diasneter and 1 inch thick pucks. Each of the resulting pocks was then preheated by heating the pucks in a Grieve convection oven, available from The Grieve Corporation of Round Lake, Illinois, to a preheat temperature. The preheat temperature was selected from 85 C, 10Q°C, and i 15XA as set forth in TABLE 9 below, [04)61] After being preheated, the UHMWPE blend pucks were then removed from the convection oven for a cooling period. Use cooling period was selected from ? minutes, 14 minuses, and 2i minutes, as set forth in TABLE 9 below, Fhe, pocks were then election beam irradiated according to Method A, set forth in TABLE 2 above, at a selected doss rate until a selected total irradiation dose was administered. The dose rate was selected from 30 kGy-m/roin, 52.5 kGy-m/min, and 75 kGy-m/mm and the total irradiation dose was selected from 160 kGy, 1 /5 kGy, and 190 kGy, 10862] Pin-on-disc (POD) specimens in tits form cylinders having a 9 tnm diameter and 13 mm thickness were then machined from the UHMWPE blend pucks. A bidirectional pin-on-dise wear tester was then used to measure the wear rate of UHMWPE pins articulating against polished cobalt-chrorne discs lubricated by 100¾ bovine serum, These measurements were made ir. accordance with the teachings of Bragdon, C.R., et ah, in A new ρίη-οη-disk wear testmgjnejh.od for sj.mnlatmg went of polyethylene on cobalt-chrome alloy its total hip arthroplasty, published in the Journal of Arthroplasty, Vol. 16, Issue 5, 2001., on pages 658-65, the entire disclosure of which is expressly incorporated by reference herein. The bidirectional motion for the pin-on-disc wear tester was generated by a computer controlled XY table, available from the Compumotor Division of Parker Hannifin of Cleveland, Ohio, 'which was programmed to move in a 10 mm by 5 mm rectangular pattern. Affixed atop the XY table was a basin containing six cobalt-chrome discs polished to an implant quality finish. The XY table and basin were mounted on a servo-hydraulic MIS machine, available from MTS of Eden Prairie, Minnesota. The MTS machine then loaded she IJHMWPB blend pin specimens against the polished cobalt-chrome discs.

[0Θ63] The MTS machine was programmed to produce a Paul-type curve in synchronization with the motion of the XY table. A Paui-type curre is explained in detail m Forces.Tran^itUlgfl.BjiJsiRiS in the Human Body by J,P, Paul and published by in the Proceedings institution of Mechanical Engineers at Voi. 1 SI, Part 37, pages 8-13, the entire disclosure of which is expressly incorporated by reference herein. The peak load of the Paul-type loading curve corresponded to a peak contact pressure of 6,5 MPa between each of the UHMWPE pin specimens and the cobalt-chrome discs. Tests were conducted at 2 Ha to a total of ], 128 x 1QJ cycles. Analysis of the results indicated that the wear properties are affected by both the concentration of d/J-ct- tocopherol and the total irradiation dose. Specifically, tire results indicated that ittereasing the d/l-a-tocopherol concentration increased the wear rate of the UHMWPE blends, while increasing the total irradiation dose decreased the wear rate of the UHMWPE blends. Additionally, the results indicated that both dose rate and the cooling period had substantially no impact on the wear rate of the UHMWPE, TABLE 9

Example 6

Temperature Variations at the UHMWPE Blsnd/Substrate Itjtgrfagg.

[0(164] GUR 1050 medical grade UHMWPE powder was obtained from Ticona, having

North American headquarters in Florence, Kentucky, ii/l-a-locopherol was obtained from DSM Nutritional Products, Ltd of Geken, Netherlands. The GUR 1050 was mechanically blended with tire d/l-K-toeopherol using a High Intensity Miser, available from Eirieh Machines of Gurnee. Illinois. The GUR 1050 resin was mixed with the d/l-a-tocopherol to create a UHMWPE blend having 0.2 wt, % d/l-a-tocopherol, [0065] A portion of the UHMWPE blend was then compression molded into a block. Another portion of the UHMWPE biend was compression molded into a substrate to create a preterm. The substrate was a 70 rara diameter porous metal substrate la the form of a near-net shape acetabular shell. The porous metal substrate was produced using Trabecular MetalTvf technology generally available from Zimmer, Inc,, of Warsaw, Indiana, and described in detail above, lists process was repeated to create five different preforms. The preforms were then individually heated to a preheat temperature in a Grieve convection oven, available from Fhe Grieve Corporation of Round Lake, Illinois. The preheat temperature was selected from 1G0BC, 120°C, and 125'C. Once heated to the selected preheat temperature, the preforms were irradiated using Method B, set forth in 1 ABLE i above, until a total irradiation dose was received. The total irradiation dose was selected from 50 kGv, 75 kGy, and 150 kGy. Additionally, the UHMWPE block was heated to a preheat tenrperature of IGG'C and irradiated using Method B until a total irradiation dose of 150 kGy was received by the UHMWPE block, [0066] "The temperature of the preforms was measured at the UHMWPE blend/substrate interface, at a point in the UHMWPE biend adjacent to die UHMWPE blend/substrate interface, and at a point, in the center of the UHMWPE blend. Each of the temperature measures were taken using a Type 1 thermocouple, Additionally, the temperature at the center of the UHMWPE blend block was also measured using a Type J thermocouple, Based on the results, the presence of a porous substrate resulted in higher temperature readings in the UHMWPE blend, This is likely a result of substrate reaching a higher maximum temperature than the UHMWPE during irradiation.

Effect of Substrate Orientation on UHMWPE„ jjiend [0067] QUR 1050 medical grade UHMWPE powder was obtained from Ticona, having North American headquarters in Florence, Kentucky, d/3-a-tocopheroi was obtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. The GUR 1050 was .mechanically blended with the d/1-α-tocephero! using a High Intensity Mixer, available from Eirich Machines of Gurnee, Illinois, The GUR 1050 resin was mixed with the d/l-a-tocopherol to create a UHMWPE blend hav.ng 0.5 wt. % d/1- a- tocopherol.

[0068] A portion of the UHMWPE blend was compression molded into a substrate to create a preform. The substrate was a 70 mm diameter porous metal substrate in the fonit oi a near-net shape acetabular shell. The porous metal substrate was produced using Trabecular Metal™ technology generally available from Tlmmer, Enc,( of Warsaw, Indiana, and described in detail above. This process was repeated to create Ihree different preforms, The preforms weirs then heated in a convection oven to a preheat temperature of 110°C for a minimum of 12 hours. Two of the pteforms were dten irradiated using Method A, as set forth in FABLE a above, with the substrate of one of the preforms facing the irradiation source and the substrate of the other preform facing away from the irradiation source, With tire preforms in these positions, they were exposed to a first. 100 kGy dose of irradiation. The preforms were then allowed to sit in ambient air for 20 minutes. After the expiration of 20 minutes, the preforms were exposed to a second, 100 kGy dose of irradiation, for a total Irradiation dose of 200 kGy.

[0869] The remaining preform was irradiated using Method B, as set forth in TABLE 2 abos'e, with the substrate of the preform facing the irradiation source. With the preform in this position, the preform was exposed to a first, 100 kGy dose of irradiation. The preform was then placed in a convection oven which maintained a constant temperature of 1UPC. After the expiration of lout hours, the preform was removed from the convection oven and exposed to a second, 100 kGy dose of irradiation, for a total irradiation dose of 200 kGy.

[1)070J Each of the preforms was then cut through the center and the substrate removed. The UHMWPE biend w'as then microtomed and subjected to FT1R aoalysis using a Broker FUR Spectrometer, available from Broker Optics ox Billerica, Massachusetts, to determine the TV! of the UHMWPE blend. This analysis was performed on the thickest part of (he specimens. A sample of die UHMWPE blend was then subjected to DSC using a TA Instruments Q1000, available from 1A Instruments of New Castle, Delaware, to determine (he percent crystallinity of the UHMWPE blend. This analysis was repeated for samples of the UHMWPE blend taken from different locations.

[CH)71] In both of the monobiocks shat were irradiated wish the substrate facing the irradiation source, a band of discoloration, i.e., translucerice, can be seen along the edge of the UHMWPE blend that interfaced with the substrate, As shown in TABLE i 1 below, the FTIR analysis showed a substantial decline in the TV! of the UHMWPE blend at a point Just past the interface between the UHMWPE blend and the substrate. Additionally, the percent crystallinity at a point in the center oi the UHMWPE blend was approximately 59%. The percent crystallinity decreased as the UHMWP&amp; blend approached the interface with the substrate, with the percent crystallinity reaching ^8% in the translucent region near the UHMWPE biend/substrate interface, as shown in TABLE 12 below. In the preform that was irradiated with the substrate facing away from die irradiation source, the TVI of the UHMWPE blend was substantially more uniform throughout the UHMWPE blend and the percent crystallinity varied by only 2.2%. This may he a result of more uniform cross!inking occurring in the preform in which the substrate faced away front the irradiation source during irradiation. TABLE 1 i

Comparison of TVI in UHMWPE Blend

TABLE 12

Example Jg

Effect of Irradiation Doss on UHMWPE Blend [0072] Design Expert 6,0,10 software, obtained front Stat-Ease, Inc, Minneapolis, MN, was utilized to setup a central composite response surface Design of Experiment. (DOE), The DQh evaluated three different variables: d,l-a-toeopherol concentration, preheat temperature, total irradiation dose administered, and irradiation dose per pass, [€073] GUR 1050 medical grade UHMWPE powder was obtained from Ticona, having North American headquarters in Florence, Kentucky, d/Ta-tocopherol was obtained from D5M Nutritional Products, Ltd of Geleen, Netherlands, The GUR 1050 was mechanically mixed with the d/3-α-tocopheroi using a High Intensity Mixer, available from Eiricb Machines of Gurnee, Illinois, The GUR 1050 resin was mixed with the d/i-cs-tocophe'fol in several batches to create UHMWPE blends having a selected wt. % of d/i-a-tocophcrol. The wt. % of d/Tct-toeopherol was selected from 0.10 wi, %, 0.20 wt, %, 0.35 wt, %, 0:50 wt. %, and 0,60 wt. % d/l-o-tocopheroL Each of the blends was then compression molded into a substrate to create a preform. The substrate was a 70 mm outer diameter porous metal substrate in the form of a near-net shape acetabular shell, The porous metal substrate was produced using Trabecular Metal™ technology generally available from Zimmer, lac., of Warsaw, Indiana, and described in detail above.

[0074] The resulting preforms were then placed inside a piece of expandable beamed polyethylene terephthalate sleeving and vacuum sealed inside an aluminum-metallized plastic film pouch, such a pooch formed from a polyethylene terephthalate resin, such as Mylar®, which has been coated with a metal, such as aluminum, to reduce gas diffusion rates through the film. Mylar is a registered trademark of DuPont. Teijin Films U.S, Limited Partnership of Wilmington, Delaware, The preforms remained in this condition until they were removed in preparation for exposing the preforms to irradiation. Prior to irradiation, each of the resulting preforms was preheated by heating She preforms in a Grieve convection oven, available from The Grieve Corporation of Round lutke, Illinois, to a preheat temperature, which was held fora minimum of 32 hours. The preheat temperature was selected from 60°C, 70°C, 85°C, 100°C, and i lt7€. as set forth in TABLE 13 below.

[067S} The preforms were then exposed to a selected total irradiation dose according to Method 8, as set forth above in TABLE 2, The total irradiation dose was selected from 133 ktjy, ISO kGy, 3 3 kGy, 200 kGy, and 217 kGy. Additionally, the total irradiation dose was divided and administered to the preforms in either two equal passes or three equal passes, which are combined ίο achieve the total irradiation dose. Specifically, tite preforms indicated to be "Block 1" In 7 ABLE i3 below received the total irradiation dose in two equal passes, while the preforms indicated to be Block 2 in TABLE 13 received the total irradiation dose in three equal passes.

[0076] After irradiation, each of the UHMWPE blends was separated from the substrate and three Ρίη-οη-Disc (POD) specimens in the shape of cylinders having a 9 mm diameter and 13 mm thickness were then machined from the UHMWPE blend pucks. A bidirectional ρίη-οη-disc wear tester was then used to measure the wear rate of UHMWPE pins articulating against polished cobait-chrome discs lubricated by 100% bovine scrusn, These measurements were made in accordance with the teachings of Bragdon, C.R., et al„ in A new of polyethylene on cobalt-chrome alloy in total hip arthroplasty, published in the Journal of

Arthroplasty, Vol. S 6, Issue S, 2001, on pages 658-65, the entire disclosure of which is expressly incorporated by reference hereit!, The bidirectional motion for the pirt-on-disc wear tester was generated by a computer controlled XY table, available from the Compumotor Division of Parker .

Hannifin of Cleveland, Ohio, which was programmed to move in a 10 mm by 5 mm rectangular pattern. Affixed atop the XY table was a basin containing six cobalt-chrome discs podsned to an implant quality finish. The XY table and basin were mounted on a servo-hydraulic MTS machine,

available from MTS of Eden Prairie, Minnesota. The MTS machine (hen loaded the UHMWPE biend pin specimens against the polished cobalt-chrome discs, [6077] The MI'S machine was programmed to produce a Paul-type curve [2] in synchronization with the motion of the XY table. A Paul-type curve is explained in detail in Forces.TraMprbted.By.

Joints in the Human Body by j.P. Paul and published in the Proceedings Institution of Mechanical Engineers at Vol. 181, Part 37, pages S-15, the entire disclosure of which is expressly incorporated by reference herein. The peak load of the Paul-type loading curve corresponded to a peak contact pressure of 6,5 MPa between each of the UHMWPE pin specimen and die cobalt-chrome discs,

Tests were conducted at 2, Hz to a total of 1,128 x 106 cycles.

[867§] The remaining portions of the UHMWPE blends were cut in half to form microtome films that were subjected to ΕΎΓΕ analysts utilizing a Broker Optics FTIR Spectrometer, available from Bother Optics of Billerica, Massachusetts. The films were then accelerated aged according to ASTM

Standard F2003, Standard Guide for Accelerated Aging of Ultra-High Molecular Weight Polyethylene, The Oi of the post-aged films was then measured, [0079] Once the measurements were taken, the post-aged films were placed tit boding hexane for 24 hours to extract any d/l-a-tocopherol remaining in the films. The percentage of d/l-a-tocopherol extracted from the UHMWPE blend Fdms was then determined, The remaining UHMWPE blend from the monoblock was then machined into 1/16” flats and Type V tensile specimens, as defined by A STM Standard D638, Standard Test Method for Tensile Properties of Plastics, were machined from the flats, [Θ080] An analysis of the results, set forth below in TABLE 13, indicated that wear increased with a Sower total irradiation dose or with a higher concentration of d/i-a-tocopherol. Additionally, the d/l-a-toeopherol concentration had a significant impact on alt;mate tensile elongation, The yield strength was affected the most by the preheat temperature, whereas UTS was affected the most by the total irradiation dose and d/l-a-tocopherol concentration. The Oi was decreased with higher preheat temperatures and higher concentration of d/l-a-tocopherol. Although ;he percentage of <&amp;3-α-tocopherol decreased after irradiation and aging, a significant amount of d/l-tr-tocopherol still remained in the UHMWPE blend after irradiation and aging, TABLE 13

Effect of Irradiation Dose on UHMWPE Blend

. Example 9

Elution in Deionized Waigr [00S1] The amount of d/l-a-tocopherol eluted from UHMWPE blends formed into consolidated pucks was investigated over a period of B weeks, GUR 1050 medical grade UHMAVPE powder was obtained from Ticona, having North American headquarters in Florence, Kentucky, d/l-a-tocopherol was obtained front DSM Nutritional Products, Ltd of Geleen, Netherlands, Fhe GUR 1050 was mechanically mixed with the d/l-a-tocopherol using a High Intensity Mixer, available from Eirich Machines of Gurnee, Illinois. The GUR 1050 resin was mixed with the d/l-a- tocopherol to cteste a UHMWPE blend having 0.25 wt. % of d/l-a-tocopherol. The UHMWPE blend was them compression molded into a series of 2.5 inch diameter and 1,5 inch thick pucks, [01)82] The pucks were preheated in a Grieve convection oven, available from Ihe Grieve Corporation of Round Lake, Illinois, to a preheat temperature. Ihe preheat temperature was selected from 8S°C and 115°C , Once preheated, ihe pucks were then exposed to a selected total irradiation dose according to Method A, as set forth above in TABLE 2, Fhe total irradiation dose was selected from 160 kGy and 190 kGv. One centimeter cubes were then machined from the pucks and placed in glass jars containing 100 mi of deionized water, The jars were then sealed using Teflon® seals and caps, available from E.I, DuPont Nemours and Company. Teflon® is a registered trademark of IL L DuPont Nemours and Company of 1007 Market Street, Wilmington Delaware, [¢¢83] Each of the glass jars was then placed in a water bath that was thermostatically held at a test: temperature, The test temperature was selected from 37°C and 70^0. At two week intervals, aliquots of extract solution were taken from each jar and assayed using 297 ran wavelength ultraviolet Sight to determine the concentration of d/1-a-tocopheroS, Absorption measurements were made using 10 mm quartz cuvettes and deionized water as the reference material. Once the assay was completed, the test aliquots were returned to the glass jars. This analysis was repeated for a total of S3 days. As the results set forth below in TABLE 14 indicate, no eluted d/l-a-tocopherai was detected In the UHMWPE blend cubes that were soaked in deionized water maintained at 37 C. Additionally, no definitive elution of d/l-a-loeopherol was detected in the UHMWPb blend cubes that were soaked in deionized water maintained at 7Q°C. For example, the results showed that the antioxidant leached from 2 grams of the crosslinked UHMWPE in 1 GO milliliters of 31 degree Celsius water after 53 days resulted in an extraction solution absorbance at 297 nanometers was no greater than 0.01 units from foe reference water absorbance, TABLE 14

Elution of d/l-a-toeopherol in ISeionized Water

Example 10

Color Measurement of UHMWPE Blend Samples [¢084] GUR 1050 medical grade UHMWPE powder was obtained from Ticona, having North American headquarters in Florence, Kentucky, d/3-a-tocopheroi was obtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. The GUR 10S0 was mechanically blended with the d/l-a-toeopherol using a High Intensity Mixer, available from Eiricn Machines of Gurnee, lilinois, 1 he GUR 1050 resin was mixed with the d/l-a-toeopherol to create a UHMWPE blend having tess than 0.5 wt. % d/l-a-toeopherol and was compression molded. The compression molded UHMWPE blend was then sectioned and subjected to analysis with a spectrophotometer to determine the coles of the UHMWPE blend. Additionally, consolidated UHMWPE powder absent tocopherol was also subjected to analysis with a spectrophotometer to determine the color of the consolidated UHMWPE absent tocopherol, [0085] Specifically, a Color Checker 545 Portable Spectrophotometer hand held unit, available front X-Rite Incorporated of Grand Rapids, Michigan, was used to test the material samples, This device uses a system ilSurmnattt DOS and has a degree observer, i.e., the placement of the device relative to die sample being tested, of 10 degrees. The device was calibrated using a calibration tile and the average results per reading were recorded for comparison with the test samples. Each of the samples were then subjected to analysts.

[ΘΘ86j The results of each individual analysis were displayed on the device using the L*;s b* (CEELAB) color space definition system. This system describes all colors visible to the human eye by providing the lightness of the color, the position of the color between red/magenta and green, and the position of the color between yellow and blue. T hese results are displayed as L*, havmg a value from 0, which corresponds to black, to 100, which corresponds to white, a*, where a negative value indicates green and a positive value indicates red/magenta, and b*, where a negative value indicates blue and a positive value indicates yellow.

[0087] Based on the results of the testing, set forth in TABLE 15 below, the UHMWPE blend having tocopherol exhibited a yellowish color, TABLE 15

Color Measurements of UHMWPE and UHMWPE w/Tocopherol

[9088] GUR 1050 medical gratis UHMWPE powder was obtained from Ticona, having North American headquarters in Florence, Kentucky, d/i-re-toeopheroi was obtained from DisM Nutritional Products, Ltd of Geleen, Netherlands. The GUR 1050 was mechanically blended with the d/1-α-tocopherol using a High intensity Mixer, available from Eitich Machines of Gurnee. Illinois. The GUR 1050 resin was mixed with the d/l-a-tocopherol to create UHMWPE blends having 0,2,0.5, or 1.0 weight percent d/1-α-tocopherol. The UHMWPE blends were then compression molded to form pucks that were then machined to font! cubes having 5 inn! sides. The UHMWPE cubes were then heated to a preheat temperature selected from 40 5C, 100 ,5C. and 110 3C. Once heated to the selected preheat temperature, the UHMWPE blends were irradiated using Method C, set forth in TABLE 2 above, until a total irradiation dose was received. The total irradiation dose was selected from of 90 kGy, 120 kGy, 150 kGy, and 200 kGy.

[0089] The resulting UHMWPE blend cubes were then studied to investigate the polymer network parameters of the UHMWPE blend by measuring the materials' swell ratio (qs) with a Swell Ratio Tester (SRT), Cambridge Polymer Group (Boston, MA), in accordance with ASTM F-xTlT-iU.. Knowing qs, the Fiory interaction parameter (χ.ι), the molar volume of the solvent (cpi), and the specific volume of the solvent (v), the crosslink density («,) and the molecular weight between crosslinks (Mc) of the material were calculated according the following equations:

(06903- Additionally, the swell ratio in stabilized o-xylene at 130 ®C was measured in the compression molded direction, The results of the testing are set forth in 1 ABwE 16 below, For example, it was found that a UHMWPE blend having nominally 1,0 % weight percent ofd/1-α-toeopherol when preheated to nominally 40 'C and subsequently electron beam crosslinked with a total dose of nominally 200 kGv has a qs less than about 4.3, a ux more than about 0,090 and a M; iess than about 11,142, It was also found that a UHMWPE blend having nominally 1,0 % weight percent of d/l-a-tocopherol when preheated to nominally 1 i 0 °C and subsequently electron beam crosslinked with a total dose of nominally 200 kGy has a qs less than about 3.6, a i>x more than about 0,117 and a Mc less than about 8,577.

[0091] Also, it was found that a UHMWPE blend having nominally 0.5 % weight percent of d/l-u-tocopheroi when preheated to nominally 40 UC and subsequently electron beam crosslinked with a total dose of nominally 200 kGy has a qs less than about 3,8, a ux more than about 0.; 19 and a M-less than about 8,421. It was also found ihat a UHMWPE blend having nominally 0.5 %? weight percent of d/l-a-tocopherol when preheated to nominally 110 JC and subsequently electron beam crosslinked with a total dose of nominally 200 kGy has a q, less than about 3.6, a u* more than about 0.109 and a Mc less than about 9,166, [0092] Further, it was found that a UHMWPE blend having nominally 0.2 % weight percent of d/i-o-iocopheroi when preheated to nominally 40 °C and subsequently electron beam crosslinked with a total dose of nominally 200 kGy has a qs less than about 2,8, a t>s more than about 0,187 and a ML less than about 5,351, ft was also found that the UHMWPE blend having nominal]v 0,2 % weight percent of d/l-a-tocopherol when preheated to nominally 110 “C and subsequently electron beam crosslinked with a total dose of nominally 200 kGy has a % less than about 3,0, a u, more than about 0.164 and a Mt. less than about 6,097, [0093] Additionally, it was found that under some conditions the crosslinked UHMWPE blend exhibited a crosslink density of less than 0.200 moles/dm5, Under other conditions, the crosslinked UHMWPE blend having at least 0,1 weight percent antioxidant exhibited a crosslink density of less than 0,190 nroles/dnr\ Further, under certain conditions, the crosslinked UHMWPE biend having at least 0,1 weight percent antioxidant exhibited a crosslink density of more than Q.aOO iiKaes/dm and had a molecular weight between crosslinks of less than 11,200 daltons. TABLE 16

Swell J:gio. DossEnk Dennis. and

Claims (40)

1. The claims defining the invention are as follows:

   1. A method for processing UHMWPE for use in medical applications, the method comprising the steps of: preheating a consolidated blend comprising UHMWPE and 0.01 to 3.0 weight percent of an antioxidant, the consolidated blend having a melting point, to a preheat temperature above room temperature and belowr the melting point of the consolidated blend; and irradiating the consolidated blend while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend.
2. 2. The method of Claim 1, wherein the consolidated blend is a substantially homogeneous blend of the UHMWPE and the antioxidant.
3. 3. The method of any one of Claims 1 to 2, wherein the antioxidant is selected from the group consisting of tocopherol, Vitamin C, lycopene, honey, phenolic antioxidants, amine antioxidants, hydroquinone, beta-carotene, ascorbic acid, CoQ-enzyme, and derivatives thereof.
4. 4. The method of any one of Claims 1 to 3, wherein the antioxidant is d,l-a-toc-opherol.
5. 5. The method of any one of Claims 1 to 4, wherein the consolidated blend is formed by a process comprising at least one of compression molding, net shape molding, injection molding, extrusion, monoblock formation, fiber, melt spinning, blow molding, solution spinning, hot isostatic pressing, high pressure crystallization, and films.
6. 6. The method of any one of Claims 1 to 4, wherein the consolidated blend is formed by a process comprising compression molding the blend to extend at least partially into a porous substrate.
7. 7. The method of Claim 6, wherein the substrate comprises a highly porous biomaterial useful as at least one of a bone substrate, a cell receptive material, a tissue receptive material, an osteoconductive material, and an osteoinductive material.
8. 8. The method of Claim 6, wherein the substrate comprises a highly porous biomaterial useful as at least one of a bone substitute, a cell receptive material, and a tissue receptive material having a porosity of at least 55 percent.
9. 9. The method of any one of Claims 1 to 4, wherein the consolidated blend is formed by a process comprising compression molding the blend into a roughened surface on the substrate having macroscopic mechanically interlocking features.
10. 10. The method of any one of Claims 1 to 9, wherein the melting point of the consolidated blend is determined by differential scanning calorimetry.
11. 11. The method of any one of Claims 1 to 10, wherein said preheating step further comprises preheating the consolidated blend to a preheat temperature substantially between 60 degrees Celsius and 130 degrees Celsius.
12. 12. The method of any one of Claims 1 to 11, further comprising the additional step of machining the consolidated blend to form a medical product.
13. 13. The method of any one of Claims 1 to 12, further comprising the additional steps of: identifying a total crosslinking irradiation dose to be received by the consolidated blend; identifying a maximum individual irradiation dose to be received by the consolidated blend while maintaining the consolidated blend below the melting point of the consolidated blend; and wherein the step of irradiating the consolidating blend further comprises irradiating the consolidated blend with a first irradiation dose that is one of equal to and less than the maximum individual irradiation dose.
14. 14. The method of Claim 13, further comprising the additional step of irradiating the consolidated blend with at least subsequent irradiation dose that is one of equal to and less than the maximum individual irradiation dose.
15. 15. The method of Claim 13 or 14, wherein said step of irradiating the consolidated blend further comprises irradiating the consolidated blend with at least one of electron beam irradiation, gamma irradiation, and x-ray irradiation.
16. 16. The method of any one of Claims 13 to 15, wherein the total crosslinking irradiation dose is substantially between 50 kGy and 1000 kGy.
17. 17. The method of any one of Claims 13 to 15, wherein the total crosslinking irradiation dose is substantially between 100 kGy and 250 kGy.
18. 18. The method of any one of Claims 13 to 17, further comprising, after said first irradiating step, the additional steps of: equilibrating the temperature of the consolidated blend to the preheat temperature; and irradiating the consolidating blend with a second irradiation dose that is one of equal to and less than the lesser of the difference between the total crosslinking irradiation dose and the first irradiation dose and the maximum individual irradiation dose for the consolidated blend.
19. 19. The method of any one of Claims 1 to 18, further comprising the additional step of sterilizing the UHMWPE blend by at least one of gas plasma sterilization, ethylene oxide sterilization, gamma sterilization, ionizing irradiation sterilization, autocalving, and supercritical fluid techniques.
20. 20. The method of any one of Claims 1 to 19, further comprising, after said irradiating step, the additional step of heat treating the consolidated blend.
21. 21. The method of any one of Claims 1 to 20, further comprising, before said irradiating step, the additional step of shielding at least a portion of the consolidated blend.
22. 22. A crosshnked UHMWPE blend for use in medical implants prepared by a process comprising: preheating a consolidated blend comprising UHMWPE and 0.01 to 3.0 weight percent of an antioxidant, the consolidated blend having a melting point, to a preheat temperature above room temperature and below the melting point of the consolidated blend; and irradiating the consolidated blend with a total irradiation dose of at least 100 kGy while maintaining the consolidated blend at a temperature below' the melting point of the consolidated blend.
23. 23. The crosslinked UHMWPE blend of Claim 22, wherein the crosslinked UHMWPE blend has an ultimate tensile elongation of at least 250 percent.
24. 24. The crosslinked UHMWPE blend of Claim 22 or 23, wherein the crosslinked UHMWPE blend has tensile yield strength of at least 21 mega pascals.
25. 25. The crosslinked UHMWPE blend of any one of Claims 22 to 24, wherein the crosslinked UHMWPE blend has an ultimate tensile strength of at least 45 mega pascals.
26. 26. The crosslinked UHMWPE blend of any one of Claims 22 to 25, wherein the crosslinked UHMWPE blend has pin-on-disc wear rate of less than 2.75 mg/Mc.
27. 27. The crossimked UHMWPE blend of any one of Claims 22 to 26, wherein the _ 'y crosslinked UHMWPE blend has an izod impact strength of at least 53 kJ/nr.
28. 28. The crosslinked UHMWPE blend of any one of Claims 22 to 27, wherein the crosslinked UHMWPE blend is molded into at least one of a substrate, a polymer, and a antioxidant stabilized polymer.
29. 29. The crosslinked UHMWPE blend of any one of Claims 22 to 28, wherein the crosslinked UHMWPE blend has an oxidation index, the oxidation index remaining below 0.1 after accelerated aging for two weeks in an oxygen atmosphere at a pressure of 72 pounds per square inch and a temperature of 70 degrees Celsius.
30. 30. The crosslinked UHMWPE blend of any one of Claims 22 to29, wherein the antioxidant leached from 2 grams of the crosslinked UHMWPE in 100 milliliters of 37 degree Celsius water after 53 days results in an extraction solution absorbance at 297 nanometers that is no greater than 0.01 units from the reference water absorbance.
31. 31. The crossimked UHMWPE blend of any one of Claims 22 to 29, wherein the antioxidant leached from 2 grams of the crosslinked UHMWPE in 100 milliliters of 70 degree Celsius water after 53 days results in an extraction solution absorbance at 297 nanometers that is no greater than 0.01 units from the reference water absorbance.
32. 32. The crosslinked UHMWPE blend of any one of Claims 22 to 31, wherein the crosslinked UHMWPE blend has an oxidation index, the oxidation index being less than 0.1 percent and the crosslinked UHMWPE blend has a free radical level measured by electron spin resonance that is less than 1 x 101' spins per gram.
33. 33. The crossimked UHMWPE blend of any one of Claims 22 to 31, wherein the crosslinked UHMWPE blend has an oxidation index, the oxidation index being less than 0.1 percent, and wherein the crossimked UHMWPE exhibits a crosslink density of more than 0.200 moles/drn3.
34. 34. The crosslinked UHMWPE blend of any one of Claims 22 to 33, wherein the crosslinked UHMWPE blend has at least 0.1 weight percent antioxidant and exhibits a pin-on-disc wear rate of less than 2.0 mg/Me.
35. 35. The crosslinked UHMWPE blend of any one of Claims 22 to 34, wherein the crosslinked UHMWPE exhibits a crosslink density7 of more than 0.200 moles/dm3.
36. 36. The crosslinked UHMWPE blend of any one of Claims 22 to 35, wherein the crosslinked UHMWPE blend has at least 0.1 weight percent antioxidant and exhibits a crosslink density of more than 0.190 moles/dm".
37. 37. The crosslinked UHMWPE blend of any one of Claims 22 to 34, wherein the crosslinked UHMWPE blend has at least 0.1 weight percent antioxidant, exhibits a crosslink density of more than 0.200 moles/dmJ, and has a molecular weight between crosslinks of less than 11,200 daltons.
38. 38. The crosslinked UHMWPE blend of any one of Claims 22 to 37, wherein the UHMWPE blend has an L* value, an a* value, and a b* value as measured by a colormeter, wherein the L* values is greater than 90, the a* value is greater than negative 6, and the b* value is greater than 17.
39. 39. The crosslinked UHMWPE blend of any one of Claims 22 to 38, wherein the process further comprises the additional step of molding the consolidated blend into at least one of a porous substrate, a polymer, and a polymer containing an antioxidant.
40. 40. The crosslinked UHMWPE blend of any one of Claims 22 to 39, wherein the crosslinked UHMWPE blend has a free radical level measured by electron spin resonance that is less than 1 x 101' spins per gram.