CROSSLINKING OF POLYETHYLENE FOR LOW WEAR USING RADIATION AND THERMAL TREATMENTS

Abstract: The present invention discloses methods for enhancing the wear-resistance of polymers, the resulting polymers, and in vivo implants made from such polymers. One aspect of this invention presents a method whereby a polymer is irradiated, preferably with gamma radiation, then thermally treated, such as by remelting or annealing. The resulting polymeric composition preferably has its most oxidized surface layer removed. Another aspect of the invention presents a general method for optimizing the wear resistance and desirable physical and/or chemical properties of a polymer by crosslinking and thermally treating it. The resulting polymeric composition is wear-resistant and may be fabricated into an in vivo implant.

Description: CROSSLINKING OF POLYETHYLENE FOR LOW WEAR USING RADIATION AND THERMAL TREATMENTS This application is based on U.S. provisional applications: serial number 60/017,852 filed on July 9, 1996; serial number 60/025,712 filed on September 10, 1996; and U.S. provisional application, "Crosslmking of Polyethylene for Low Wear Using Radiation and Thermal Treatments", of Fu- Wen Shen et al . , with attorney docket number 5910-108P3 (correct serial number not yet assigned), filed on April 29, 1997. Technical Field Of The Invention The present invention relates to polymers. It discloses methods for enhancing the wear-resistance of polymers by crosslmking and thermally treating them. The polymers disclosed herein are useful for making implants, for example, as components of artificial joints such as acetabular cups. Background Of The Invention Ultrahigh molecular weight polyethylene (hereinafter referred to as "UHMWPE") is commonly used to make prosthetic joints such as artificial hip joints. In recent years, it has become increasingly apparent that tissue necrosis and interface osteolysis, in response to UHMWPE wear debris, are primary contributors to the long-term loosening failure of prosthetic joints. For example, wear of acetabular cups of UHMWPE in artificial hip joints introduces many microscopic wear particles into the surrounding tissues. The reaction to tnese particles includes inflammation and deterioration of the tissues, particularly tne bone to which the prosthesis is anchored. Eventually, the prosthesis becomes pam- fully loose and must be replaced. Improving the wear resistance of the UHMWPE socket and, thereby, reducing the rate of production of wear debris would extend the useful life of artificial joints and permit them to be used successfully in younger patients. Conse- quently, numerous modifications physical properties of UHMWPE have been proposed to improve its wear resistance. UHMWPE components are known to undergo a spontaneous, post -fabrication increase in crystallmity and changes in other physical properties. {See e.g., Rimnac, CM., et al . , J Bone & Joint Surgery, 76 -A (7) : 1052-1056 (1994)}. These changes occur even in stored (non- implanted) cups after sterilization with gamma radiation, which initiates an ongoing process of chain scission, crosslmking, and oxidation or peroxidation involving the free radicals formed by the irradiation. These degradative changes may be accelerated by oxidative attack from the joint fluid and cyclic stresses applied during use. In an attempt to improve wear resistance, DePuy-DuPont Orthopaedics fabricated acetabular cups from conventionally extruded bar stock that previously had been subjected to heating and hydrostatic pressure that reduced fusion defects and increased the crystallmity, density, stiffness, hardness, yield strength, and increased the resistance to creep, oxidation and fatigue. Alternatively, silane cross-linked UHMWPE (XLP) has also been used to make acetabular cups for total hip replacements in goats. In this case, the number of in vivo debris particles appeared to be greater for XLP than conventional UHMWPE cup implants {Ferris, B.D., J . Exp . Path. , 71:367-373 (1990)}. Other modifications of UHMWPE have included: (a) reinforcement w th carbon fibers; and (b) post-processing treat- ments such as solid phase compression molding. Indeed, carbon fiber reinforced polyethylene and a hea -pressed polyetnylene have shown relatively poor wear resistance wnen used as the tibial components of total knee prosthesis. {See e.g., Rimnac, CM., et al . , Trans. Orthopaedic Research Society, 17:330 (1992)}. Recently, several companies have modified tne method of radiation sterilization to improve the wear resistance of UHMWPE components. This has typically involved packaging the polyethylene cups either in an inert gas (e.g., Howmedica, Inc.), in a partial vacuum (e.g., Johnson & Johnson, Inc.) or with an oxygen scavenger (e.g., Sulzer Orthopaedics, Inc.). Summary Of the Invention The present invention comprises two aspects: The first aspect of the invention presents a method for increasing the wear resistance of a polymer by crosslmking the polymer, followed by thermally treating the crosslmked polymer. Non- limiting examples of the thermal treatments are remelting or annealing. Preferably, the polymer is crosslmked by gamma irradiation in the solid state prior to being modified to a desired final form or shape of the final product. In the preferred embodiment, the surface layer of the crosslmked and thermally treated polymer, which is the most oxidized and least crosslmked part of the polymer, is removed, e.g., in the process of machining the final product out cf the irradiated bar and thermally treated bar or block. The radiation dose is also preferably adjusted so that the optimal dose occurs withm the solid polymer bar or block at the level of the bearing surface of the final product. Also presented are the polymers made from this method; methods for making products (e.g., in vivo implants) from these polymers; and the products (e.g., in vivo implants) made from these polymers. The second aspect of the invention provides a system- atic method for determining an optimal balance among wear resistance and other physical and/or chemical properties that are deemed important to the long-term performance of an implant in vi vo , and applying this optimal balance to determine the appropriate crosslmking and thermal treatment conditions for processing a polymer. A flowchart is provided as a non-limitmg illustration of the method for determining the optimal balance. Also provided are methods for treating polymers which apply the above appropriate crosslmking and thermal treatment conditions; the polymers produced by these methods; methods for making products (e.g., in vi vo implants) from these polymers,* and the products (e.g., in vi vo implants) made from these polymers. Brief Description Of The Drawings FIG. 1 presents the degree of crystallmity vs. depth at indicated doses for UHMWPE that was irradiated m a vacuum (i.e., a low-oxygen atmosphere). FIG. 2 presents the gel content vs. depth at indicated doses for UHMWPE that was irradiated in a vacuum (i.e., a low-oxygen atmosphere) . FIG. 3 presents the gel content vs. depth at indicated conditions for UHMWPE. FIG. 4 presents the degree of crystallmity vs. depth at indicated conditions for UHMWPE. FIG. 5 presents the gel content vs. depth at indicated conditions for UHMWPE. FIG. 6 presents the shape of the acetabular cup fabricated from the irradiated UHMWPE. FIG. 7 presents a schematic diagram of the hip joint simulator used in the wear tests. FIG. 8 presents the wear by volume loss of each cup of the four materials. Upper curves: 3.3 Mrad; Lower curves: 28 Mrad. FIG 9 presents the curves of the average volumetric wear and standard deviations of three cups of each material at each interval . FIG. 10 presents the oxidation profile as a function of depth at various aging times. FIG. 11 presents the oxidation profile as a function of depth at various aging times. FIG. 12 presents the oxidation profile as a function of depth at various aging times. FIG. 13 presents the oxidation profile as a function of depth at various aging times. FIG. 14 presents the oxidation profile as a function of depth for various materials. The specimens were stored in air for 5 months and then aged for 20 days at 80°C. FIG. 15 presents gel content as a function of depth at various aging times. FIG. 16 presents gel content as a function of depth at various aging times. FIG. 17 presents gel content as a function of depth at various aging times. FIG. 18 presents gel content as a function of depth at various aging times. FIG. 19 presents the degree of crystallmity as a function of depth after 30 days' aging. Fig. 20 shows the combined soak-corrected wear for the non-aged and aged cups. Fig. 21 shows the individual wear for cups irradiated at different doses. Fig. 22 shows the average wear rate versus radiation dose of non-remelted and remelted cups. Figs. 23A and 23B present the flowchart illustrating the optimization method of the present invention. Fig. 24 graphically shows the oxidation profiles for irradiated and remelted UHMWPE as a function of depth from the UHMWPE bar surface. Fig. 25 graphically shows the tensile strength at yield versus radiation dose of irradiated UHMWPE with or without remelting, and non- irradiated and not remelted UHMWPE. Fig. 26 graphically shows the tensile strength at preak versus radiation dose of irradiated UHMWPE with or without remelting, and non-irradiated and not remelted UHMWPE. Fig. 27 graphically shows the elongation at breax: versus radiation dose of irradiated UHMWPE with or without remelting, and non-irradiated and not remelted UHMWPE. Detailed Description Of The Invention Abbreviations used in this application are as follows: UHMW -- ultra-high molecular weight UHMWPE -- ultra-high molecular weight polyetnylene HMW -- high molecular weight HMWPE -- high molecular weight polyethylene The present invention contains two aspects. The first aspect of the invention provides methods for improving the wear resistance of a polymer by crosslmking (preferably the bearing surface of the polymer) and then thermally treating the polymer, and the resulting novel polymer. Preferably, the most oxidized surface of the polymer is also removed. Also presented are the methods for using the polymeric compositions for making products and the resulting products, e.g., m vi vo implants. Specific examples of this method are presented in the section: "I. First Aspect of the Invention : Polymeric Compositions with Increased Wear Resistance" and "I (A) Further Examples of the First Aspect of the Invention" , below. The method of the invention utilizes irradiation for crosslmking a polymer followed by thermal treatment to decrease the free radicals to produce a preformed polymeric composition. The term "preformed polymeric composition" means that the polymeric composition is not in a final desired shape or form (i.e., not a final product) . For example, where the final product of the preformed polymeric composition is an acetabular cup, irradiation and thermal treatment of the polymer could be performed at pre-acetabu- lar cup shape, such as when the preformed polymeric composition is in the form of a solid bar or block. A second aspect of the invention provides a systematic method (an example of which is illustrated in the flowcnart, below) for determining the optimal parameters for the above mentioned crosslmking and thermal treatment. This second aspect provides a method for determining the maximum possi- ble improvement in wear resistance, consistent with keeping the other physical and/or chemical propert(ιes) within the user's desired limits, with the least amount of trial and error testing. Once the optimal parameters (i.e., crosslmking conditions such as radiation dose when radia- tion is used to crosslink the polymer, and thermal treatment parameters) are determined by this method, the polymer will then be processed according to the optimal parameters. Thus, this protocol renders the development of a preformed polymeric composition with particular chemical/mechanical char- acteristics routine without resort to undue experimentation. Also presented are the methods for using the preformed polymeric composition for making products, and the products, e.g., m vi vo implants. In the present invention, the wear resistance of the polymer is improved by crosslmking. The crosslmking can be achieved by various methods known n the art, for example, by irradiation from a gamma radiation source or from an electron beam, or by photocrosslinking. The preferred method for crosslmking the polymer is by gamma irradiation. The polymer is preferably crosslmked in the form of an extruded bar or molded block. In the preferred method, the crosslmked polymer is subjected to thermal treatment such as by remelting (i.e., heateα above the melting temperature of the crosslmked polymer) or annealing (i.e., heated at below the melting temperature of the crosslmked polymer) to produce the preformed polymeric composition. In the preferred embodiment of both the first and second aspects of the invention, the outer layer of the resulting preformed polymeric composition, whicn is generally the most oxidized and least crosslmked and, thus, least wear resistant, is removed. For example, tne bearing surface of the preformed polymeric composition may be fashioned from inside, e.g., by machining away the surface of the irradiated and thermally treated composition before or during fashioning into the final product, e.g., into an implant . Bearing surfaces are surfaces which are in moving contact with another, e.g., m a sliding, pivoting, or rotating relationship to one another. Choices of Polymers The polymers are generally polyester, poly (methylmethacrylate) , nylon, polycarbonates, and polyhydrocarbons such as polyethylene, and polypropylene. High molecular weight (HMW) and ultra-high molecular weight (UHMW) polymers are preferred, such as HMW polyethylene (HMWPΞ) , UHMW polyethylene (UHMWPE), and UHMW polypropylene. HMW polymers have molecular weights ranging from about 10 5 grams per mole to ust below 10". UHMW polymers have molecu- lar weights equal to or higher than IO 6 grams per mole, preferably from IO 6 to about 10 7 . The polymers are generally between about 400,000 grams per mole to about 10,000,000 and are preferably polyolefinic materials. For implants, the preferred polymers are those that are wear resistant and have exceptional chemical resistance. UHMWPE is the most preferred polymer as it is known for these properties and is currently widely used to make acetabular cups for total hip prostheses and components of other joint replacements. Examples of UHMWPE are those having molecular weight ranging from about 1 to 8 x 10 s grams per mole, examples of which are: GUR 4150 or 4050 (Hoechst-Celanese Corporation, League City, Texas) with a weight average molecular weight of 5 to 6 x IO 6 grams per mole; GUR 4130 with a weight average molecular weight of 3 to 4 x 10 6 ; GUR 4120 or 4020 with a weight average molecular weight of 3 to 4 x 10 s ; RCH 1000 (Hoechst-Celanese Corp.) with a weight average of molecular weight of 4 x IO 6 and HiFax 1900 of 2 to 4 x IO 6 (HiMont, Elkton, Maryland) . Historically, companies which make implants have used polyethylenes such as HIFAX 1900, GUR 4020, GUR 4120 and GUR 4150 for making acetabular cups. Sterilization Methods All polymeric products must be sterilized by a suitable method prior to implanting in the human body. For the formed crosslinked and thermally treated polymeric compositions (i.e., the final products) of the present invention, it is preferable that the products be sterilized by a non-radiation based method, such as ethylene oxide or gas plasma, in order not to induce additional crosslinking and/or oxidation of the previously treated preformed polymeric composition. Compared to radiation sterilization, a non-radiation sterilization method has a minor effect on the other important physical characteristics of the product. Nevertheless, the method can be used in conjunction with radiation sterilization. If the final products are to be sterilized by an additional dose of radiation, it is preferable to take into account the effect of this additional radiation dose on the wear resistance and other properties of the polymer, in determining the optimum radiation dose used in the initial crosslinking. Furthermore, it is preferable that the radiation sterilization be done while the final product (e.g., in vivo implant) is packed m a suitaole low-oxygen atmosphere (e.g , partial vacuum, in an inert gas such as nitrogen, or with an oxygen scavenger included) in order to minimize oxidation of the surface layer of the final product during and after sterilization by irradiation. The dose ranges in this application do not take into account radiation sterilization. If radiation sterilization is used, then the dose ranges may have to be adjusted Such adjustment can be easily performed using the teachings herein For example, if after comparing the dose-response curves for wear with those for other important physical or chemical properties, it is determined that the optimal total radiation dose is 8 Mrad, and it is intended to sterilize the polymer with 2.5 Mrad gamma radiation (the minimum industrial standard sterilization dose) , then the initial radiation dose (before sterilization) should be 5.5 Mrad, such that the total dose (initial plus sterilization doses) will be 8 Mrad. These calculations are approximate, since the total crosslmking achieved will not be exactly equivalent to a single 8 Mrad dose. Nevertheless, the applicants have discovered that a high level of crossl king in the surface layer of a polymer markedly reduces the degradative effects of surface oxidation, i.e., that would otherwise occur if a non-pre- crosslmked polymer were irradiated the presence of oxygen (for example, see Fig. 3) . Methods for Characterizing the Polymers The degree of crystallmity can be determined using methods known in the art, e.g. by differential scanning calorimetry (DSC) , which is generally used to assess the crystallmity and melting behavior of a polymer. Wang, X. & Salovey, R., J. Aop. Polymer Sci., 34:593-599 (1987). Wide-angle X-ray scattering from the resulting polymer can also be used to further confirm the degree of crystallmity of the polymer, e g. as described m Spru ell, J.E., & Clark, E.S., m "Methods of Experimental-Physics" , L. Marton δ- C. Marton, Eds., Vol. 16, Part B, Academic Press, New Yorκ (1980) . Other methods for determining the degree of crystallmity of the resulting polymer may include Fourier Transform Infared Spectroscopy { Painter, P.C. et al . , "The Theory Of Vibrational Spectroscopy And Its Application To Polymeric Materials", John Wiley and Sons, New York, U.S.A. (1982)} and density measurement (ASTM D1505- 68) . Measurements of the gel content and swelling are generally used to characterize crosslink distributions m poly- mers , the procedure is described Ding, Z.Y., et al . , J . Polymer Sci., Polymer Chem., 29:1035-38 (1990). FTIR can also be used to assess the depth profiles of oxidation as well as other chemical changes such as unsaturation {Nagy, E.V., & Li , S., "A Fourier transform infrared technique for the evaluation of polyethylene orthopaedic bearing materials", Trans. Soc . for Biomateπals , 13:109 (1990); Shinde, A. & Salovey, R., J. Polymer Sci., Polym. Phvs . Ed. , 23 :1681-1689 (1985) } . Use of Crosslmked Polymers for Implants Another aspect of the invention presents a process for making implants using the preformed polymeric composition of the present invention. The preformed polymeric composition may be shaped, e.g., machined, into the appropriate implants using methods known in the art. Preferably, the shaping process, such as machining, removes the oxidized surface of the composition. Preformed Polymeric Compositions The preformed polymeric compositions of the present invention can be used in any situation where a polymer, especially UHMWPE, is called for, but especially in situations where high wear resistance is desired. More particu- larly, these preformed polymeric compositions are useful for making implants. Implants Made of Crosslmked Polymers An important aspect of this invention presents implants that are made with the above preformed polymeric composi- tions or according to the methods presented herein. In particular, the implants are produced from preformed polymeric composition are UHMW polymers crosslmked by gamma radiation followed by remelting or annealing, removing the oxidized surface layer and then fabricating into a final shape. The preformed polymeric composition of the present invention can be used to make implants for various parts of the body, such as components of a joint in the body. For example, in the hip joints, the preformed polymeric composition can be used to make the acetabular cup, or the insert or liner of the cup, or trunnion bearings (e.g. between the modular head and the stem) . In the knee joint, the preformed polymeric compsition can be used to make the tibial plateau (fe oro-tibial articulation), the patellar button (patello-femoral articulation) , and trunnion or other bear- mg components, depending on the design of the artificial knee joint. In the ankle joint, the preformed polymeric composition can be used to make the talar surface (tibio- talar articulation) and other bearing components. In the elbow joint, the preformed polymeric composition can be used to make the radio-numeral joint, ulno-humeral joint, and other bearing components. In the shoulder joint, the preformed polymeric composition can be used to make the glenoro-humeral articulation, and other bearing components. In the spine, the preformed polymeric composition can be used to make mtervertebral disk replacement and facet joint replacement. The preformed polymeric composition can also be made into temporo-mandibular joint (jaw) and finger joints. The above are by way of example, and are not meant to be limiting. The following discusses the first and second aspects of the invention in more detail . I_;_ First Aspect of the Invention* Polymeric Compositions with Increased Wear Resistance The first aspect of the invention provides preformed polymeric compositions which are wear resistant and useful for making in vivo implants. In this aspect, for polymers general, and more preferably UHMW and HMW polymers, and most preferably UHMWPE and HMWPE, the irradiation dose is preferably from about 1 to about 100 Mrad, and more preferably, from about 5 to about 25 Mrad, and most preferably from about 5 to about 10 Mrad. This most preferable range is baseo on achieving what the inventors have determined to be a reasonable balance between improved wear resistance and minimal degradation of other important physical properties. In vivo implants of the present invention, i.e., irradiated within the above dose ranges are expected to function in vivo without mechanical failure. The UHMWPE acetabular cups used by Oonishi et al . [in Radiat . Phys . Chem., 39: 495-504 (1992)] were irradiated to 100 Mrad and functioned m vivo without reported mechanical failure after as long as 26 years of clinical use. Furthermore, it is surprising that, as shown n the EXAMPLES, acetabular cups from the preformed polymeric composition prepared according to the present invention, but irradiated to much less than 100 Mrad, exhibited much higher wear resistance than reported by Oonishi et al . On the other hand, if a user is primarily concerned with reducing wear, and other physical properties are of secondary concern, then a higher dose than the above stipulated most preferable range (e.g., 5 to 10 Mrad) may be appropriate, or vice versa (as illustrated in the detailed examples m the following section) The optimum radiation dose is preferably based on the dose received at the level of the bearing surface in the final product. Gamma radiation is preferred. The irradiated polymer is then preferably remelted at or above melting temperature of the irradiated polymer, e.g., air. As used herein, the melting temperature of the crosslmked or irradiated polymer s identified from the peak of the melting endotherm as measured by DSC. Preferably, the remelting temperature is from about the melting temperature of the irradiated polymer to about 100°C to about 160"C above the melting temperature of the irradiated polymer; more preferably from about 40°C to about 80°C above the melting temperature of the irradiated polymer, ana most preferably from about l°C to about 60°C aoove the melting temperature of the irradiated polymer. For example, in the case of UHMWPE, the remelting temperature is preferably from about 136°C to about 300°C, more preferably from about 136°C to about 250°C, and most preferably from about 136°C to about 200°C. Specific conditions for remelting are described in EXAMPLES 1 and 2, below. Generally, in practice, the remelting temperature is inversely proportional to the remelting period. The polymer is preferably remelted over a period from about 1 hour to about 2 days, more preferably from about 1 hour to about 1 day, and most preferably from about 2 hours to about 12 hours Since, depending on the time and temperature applied, annealing can produce less of an effect than remelting on physical properties such as crystallmity, yield strength and ultimate strength, annealing may be used in place of remelting as a means for reducing the free radicals remaining m the polymer after irradiation crosslmking, in order to maintain these physical properties within limits required by the user. Thermal treatment, such as remelting or anneal - mg, removes free radicals and thereby improves long term wear resistance of the polymer. On the other nand, annealing is slower and thus takes longer than remelting, making it likely to be more expensive m industrial applications. The annealing temperature is preferably from about room temperature to below the melting temperature of the irradiated polymer; more preferably from about 90°C to about 1°C below the melting temperature of the irradiated polymer; and most preferably from about 60°C to about 1°C below the melting temperature of the irradiated polymer. For example, UHMWPE may be annealed at a temperature from about 25°C to about 135°C, preferably from about 50°C to about 135°C, and more preferably from about 80°C to about 135°C The annealing period is preferably from about 2 hours to aoout 7 days, and more preferably from about 7 hours to about 5 days, and most preferably from about 10 hours to about 2 days. Instead of using the above range of radiation dose as a criterion, the appropriate amount of crosslinking may be determined based on the degree of swelling, gel content, or molecular weight between crosslinks after thermal treatment. This altenative is based on the applicant's findings (detailed below) that acetabular cups made from UHMWPE falling within a preferred range of these physical parameters have reduced or non-detectable wear. The ranges of these physical parameters include one or more of the following: a degree of swelling of between about 1.7 to about 5.3; molecular weight between crosslinks of between about 400 to about 8400 g/mol; and a gel content of between about 95% to about 99%. A preferred polymer or final product has one or more, and preferably all, of the above characteristics. These para e- ters can also be used as starting points in the second aspect of the invention (as illustrated by the flowchart, discussed below) for determining the desired radiation dose to balance the improvement in wear resistance with other desired physical or chemical properties, sucn as polymer strength or stiffness. After crosslmking and thermal treatment, preferably, the most oxidized surface of the preformed polymeric composition is removed. The depth profiles of oxiαation of the preformed polymeric composition can be determined by methods known in the art, such as FTIR, described above and m EXAMPLES 3 and 6. In general, to remove the most oxidized surface, preferably a minimum of about 0.5 mm to 1.0 mm of the surface of preformed polymeric composition which is exposed to air is removed, e.g. by machining, before or while fashioning the preformed polymeric composition into the final product . I . (A) Further Examples of the First Aspect of the Invention As noted above, the most preferable range of dose for crosslinking radiation (i.e., from 5 to 10 Mad) was based on achieving what the inventors have determined to be a reasonable balance between improved wear resistance and minimal degradation of other important physical properties. The following examples illustrate applications of the present invention with altenative criteria for the optimal dose. These examples use m vivo implants as non-limiting examples of the products, and UHMWPE or HMWPE bar or block as a non- limitmg example of a starting material. In the first example, the user desires to achieve a minimum wear rate of the m vivo implant made from the UHMWPE and HMWPE, and the other physical or chemical properties are important but of lesser concern. In such a case, the user may choose to irradiate the UHMWPE and HMWPE bar or block between about 15 Mrad to about 20 Mrad (as shown by Fig. 22) . As discussed m the section "II (b) Application of the Flowchart" , below, GUR 4150 is representative of UHMWPE and HMWPE. The irradiated UHMWPE or HMWPE bar or block is further remelted or annealed at a temperature and time described "I .First Aspect of the Invention* Polymeric Compositions with Increased Wear Resistance" , aoove . In a second example, the user may wish to produce an UHMWPE which is as wear resistant as possible while meeting the tensile strength at break (ultimate) , tensile strength at yield, and elongation at break criteria of the standard specified by the American Society for Testing and Materials F-648 standard (hereinafter referred to as "ASTM F648") for UHMWPE for m vivo use. The information about this standard can be found in a current issue of the Annual Book of ASTM Standards, Medical Devices and Services, "Standard Specifi- cation for Ultra-High-Molecular-Weight Polyethylene Powder and Fabricated Form for Surgical Implants", American Society for Testing and Materials. The method of the second aspect of the present invention (as illustrated by the flowchart) may be used to adjust the crosslmking and thermal treatment parameters to meet any current ASTM F648 criteria. For example, to meet the 1996 ASTM F648 (F648-96) criteria for Type 1 or 2 UHMWPE, the UHMWPE must have: a tensile strength at break (ultimate) of at least 35 MPa (for Type 1) and 27 MPa (for Type 2) at 23°C and 5.08 cm/mm,* a tensile strength at yield of at least 21 MPa (Type 1) and 19 MPa (for Type 2) at 23°C, and 5.08 cm/min; and elongation at break of at least 300% at 5.08 cm/mm. The test conditions are described in ASTM D638, Type IV (Annual Book of ASTM Standards , American Society for Testing and Materials) . Alternatively, to meet the 1996 ASTM F648 criteria for Type 3 UHMWPE, the UHMWPE must have: a tensile strength at break (ultimate) of at least 27 MPa at 23°C and 5.08 cm/mm; a tensile strength at yield of at least 19 MPa at 23 °C, and 5.08 cm/mm; and elongation at break of at least 250% at The plots of mechanical properties vs irradiation dose for GUR 4150 (which is representative of Type 2 UHMWPE) (Figs. 25-27) show that, for all of the radiation doses between 5 to 25 Mrad, the above ASTM F648 criteria for Types 2 UHMWPE are fulfilled except for the elongation at break, which crosses the 300 limit at about 6 Mrad. Thus, if the ASTM F648 criteria are to be met for Types 2 UHMWPE, the maximum d.e , the most preferred) gamma radiation dose is about 6 Mrad. As illustrated in the second aspect of the invention (following section), the corresponding curves of wear and other physical properties vs. crossl king dose could be used to determine the preferred dose range for other Types of UHMWPE or for other polymers in general . II . Second Aspect of the Invention: Method for Optimizing Wear Resistance and Desirable Physical and/or Chemical Characteristics of a Polymeric Composition The second aspect of the invention uses the findings in this patent application (including those presented in the "EXAMPLES" section, below) to construct a method which allows one skilled in the art, to systematically identify the conditions necessary to routinely produce a polymer with an optimal balance of wear resistance and physical and/or chemical properties, with minimal additional testing and minimal trial and error. In one embodiment of this aspect of the invention, the optimizing method can be schematically illustrated in a flowchart. Once the optimal conditions have been determined by this method, the polymer can then be subjected to these conditions for processing. The present invention is based part, on the discovery that wear rate decreases with increasing radiation dose, and there is a maximum dose above which there is little or no additional improvement in wear, but higher doses might degrade other important physical and/or chemical properties of the polymer, such as yield or ultimate strength, elongation to failure, impact strength or fatigue resistance, as well as increasing the susceptibility to oxidation. Oxida- tion, m turn, is known to adversely affect one or more of these physical properties, and was shown to occur the examples below for UHMWPE crosslmked at a dose averaging about 28 Mrad if there had been no thermal treatment. Consequently, a polymer irradiated at a high radiation dose may exhibit improved wear resistance, but its other physical or chemical properties may fall outside of desirable or allowable limits, such as those specified by ASTM F648 for UHMWPE for in vi vo use. The method is also based in part on the discovery that, while other important physical properties (such as crystallmity or elongation to break) may be markedly affected by the amount of thermal treatment (e.g., remelting or annealing) applied to the polymer after irradiation crosslmking, the wear resistance is not markedly affected. This latter discovery permits reducing the amount of addi - tional testing required by the user in order to identify the crossl king dose which will provide the user's desired balance among wear resistance and other physical properties. This method is useful, e.g., m the case where performed polymeric composition made of UHMWPE is used for making in vivo implants, such as acetabular cups. II (a) Summary of the Steps of the Optimization Method Thus, the second aspect of the present invention provides a systematic method for optimizing the balance among wear resistance and other desired physical and/or chemical characteristics of a polymer. The steps in this method are summarized in the non- limiting example of the flowchart of Figs. 23A and 23B. In the flowchart and the following discussion, for ease of discussion, irradiation is used as an example of a crosslmking method, and implant is used as an example of the product that is made from the polymer. However, as discussed elsewhere in this application, other crosslmking methods and products may be used. Step 1 The process typically begins with the polymer in solid form, such as an extruded bar or block. Step 2. The bar is irradiated over a range of doses up to the maximum that is likely to produce a material with the desired wear resistance and physical and/or chemical properties. This irradiation may be done, for example, in the case of gamma radiation by means of a cobalt 60 gamma radiation facility as is presently used for industrial -scale sterilization of implants. Step 3 ■ The irradiated bars are then remelted. Applicants found that remelting of an irradiated polymer would substantially reduce the free radicals produced during irradiation, thus minimizing long-term oxidation and chain scission. By improving the polymeric composition's resistance to long-term oxidation, remelting also improves the polymeric composition's long-term resistance to wear. For further discussion of the subject, see EXAMPLES 2, 3, and 4, below Although the bar may be contained in a low-oxygen atmosphere during the remelting, this may not be essential since, even if the bar is remelted in ambient air, the resultant oxidation may affect only the surface layer of the polymer (e.g. in the following EXAMPLE section, Figs. 2, 5, and 24, show oxidation extending to about 1 mm deep) . In the preferred embodiment of the invention, the oxidized surface layer of the preformed polymeric composition will be removed, e.g., during subsequent machining of the products out of the treated bar. Step 4A The radiation dose is correlated with the wear resistance of the products made from the irradiated remelted polymeric composition, as determined in a wear test that adequately simulates the wear conditions of the products For example, if the polymeric composition will oe maαe into an implant, then the wear test should preferably adequately simulate the wear conditions of such implants m vi vo The correlation may be arrived at by plotting a dose-response curve for irradiation dose vs. wear Step 4B* Similarly, the radiation dose is correlated with each of the physical and/or chemical properties that may be markedly affected by the radiation dose and that might, in turn, substantially affect the performance of the implant m vivo , both for non-remelted and remelted polymer Again, the correlation may be arrived at by plotting a dose-response curve for irradiation dose vs. each of these physical and/or chemical properties. The user does not have to do dose vs. properties for each property that might be affected, but only those properties that are considered important for the proper functioning of the implant m vivo . Which of these properties are important for the intended application, and the limiting values of these properties, may vary for different polymeric compositions and for different types of applications (e.g., hip prostheses compared to knee prostheses) and must, therefore, be established by the user before applying the flowchart . Step 5 is the first attempt at optimization. The user may first decide on the desired amount of improvement m the wear resistance, i.e., the maximum wear rate that is permissible for the user's application. The dose-response curve for wear (Step 4A) then shows the minimum radiation dose necessary to provide this amount of improvement in wear resistance . Similarly, the dose response curves for tne other physical or chemical properties deemed critical or important (Step 4B) provide the values of these properties correspond- mg to the specific radiation dose identified in Step 4A as being necessary to provide the desired improvement in wear resistance. If each of these other physical or chemical properties are within allowable limits for the crosslmked and remelted polymer, then an optimal method has been lden- tified (Step 6) . In other words, the implant can be made by irradiating the solid polymer bar, remelting the bar and machining out the implant; the entire process being conducted such that the resulting implant has received the optimal dose at its bearing surface. Alternatively, the user may first decide on critical values for one or more properties, such as ultimate tensile strength, fatigue strength, etc., and then check the corresponding dose response curves for the remelted polymer for the maximum allowable dose, and then check the wear vs. dose curve to determine whether this dose gives sufficient improvement in wear (i.e., the user does not necessarily have to begin by choosing the desired amount of improvement in wear) . However, if not enough improvement in wear will be obtained while keeping these other chemical and physical properties within allowable limits, or conversely, if the dose required for the desired wear improvement causes one or more of these properties to be out of allowable limits, then the user can use a lower radiation dose (i.e., accept a higher wear rate) if he wishes to remelt the materials or, alternatively, annealing may be substituted for remelting (Step 7) . For a crosslmked material, annealing is less efficient than remelting in removing free radicals, but may cause less of a reduction in other important physical prop- erties. Whether annealing is a practical option will be apparent from the dose-response curves for the non-remelted and remelted polymers. That is, if the desired value of the property in question falls between the two curves (see for example, Figs. 25 and 26) , then a polymer with the desired limiting value may be produced by an annealing process w th an appropriate time/temperature combination. It is not necessary to generate additional wear dose- response curves for each of the many possible combinations of annealing time and temperature. It is expected that the radiation dose necessary to produce the desired reduction in wear that is determined from the wear dose-response curve for remelted polymer in Step 4A, will also apply to an annealed polymer produced in Step 7. Step 7: Anneal samples of a bar or block which have been irradiated to that dose that was identified in Step 4A as being necessary to provide the required improvement wear resistance, at various time/temperature combinations, to produce a polymer with the critical properties between those for non-remelted and remelted materials. Step 8 : The physical or chemical propert(ιes) of interest of the irradiated and annealed samples of the polymer are correlated with annealing times and temperatures. Step 9 : Using ultimate tensile strength as an example of the physical characteristic of interest, depending on the resul- tant curve for annealing time and/or temperature vs. ultimate strength, the radiation dose required to achieve the desired wear resistance identified in Step 4A (above) should produce a polymer with an ultimate strength within allowable limits . Similar consideration should be given to each of the other important physical and/or chemical properties by generation of individual curves of these properties versus annealing time and/or temperature. If each of these proper- ties s with allowable limits at a particular annealing time and temperature combination, then a suitable method has been identified (Step 10) . If an annealing process cannot be identified that maintains the properties within allowable limits, then the user may choose to accept a lower radiation dose (Step 11) , i.e., to accept less of an improvement in wear resistance. However, if a lower radiation dose (and, therefore, a greater wear rate) is acceptable, then the corresponding physical and chemical properties should again be checked for the remelted polymer (using the correlation arrived earlier m Step 4B) , since these may be withm limits at the lower radiation dose. If the properties are withm limits for the remelted polymer at the lower radiation dose, then remelting may be used instead of annealing to produce a polymer with the desired improvement in wear resistance (Step 6) . If not, then the user should proceed with annealing as before (Steps 7 through 10 or 11) but at this lower radiation dose. The user may wish to progressively reduce the required amount of radiation crosslinking (i.e., to accept still higher wear rate) until a dose is identified for which all of the other properties deemed essential are within the user's required limits. The resultant dose represents the maximum improvement in wear resistance obtainable withm the user's criteria. II (b) Example Applications of the Flowchart As starting points for the flowchart, the ranges for radiation doses, remelting and annealing temperatures and times described in section "I^ First Aspect of the Invention: Polymeric Compositions with Increased Wear Resistance" and "I (A) Further Examples of the First Aspect of the Invention" , above, can be used, with regard to poly- ers in general, UHMW and HMW polymers in particular, and HMWPE and UHMWPE especially. For ease of discussion, the following examples illustrate the application of the flowchart using UHMWPE (which also behaves similar to HMWPE) as an example of a polymer and an acetabular cup as an example of an implant. GUR 4150 is representative of such a class of UHMWPE. Similarly, the description uses gamma radiation as an example for crosslmking the polymer. These examples are meant to illustrate and not meant to limit the invention. The method described by the flowchart is applicable to other polymers, implants or other products made from such polymers, and crosslmking methods (examples of which are described elsewhere in this application) , and methods for making an implant or product out of the preformed polymeric composition. From the data provided by the EXAMPLES (following sections) a number of generalities were discovered that allowed simplification of the use of the flowchart, i.e., to minimize the amount of additional testing that would be required of a user wishing to apply the method to other polymers, or to the GUR4150 of the EXAMPLES but with various optimization criteria. To establish the critical curve for the reduced in vivo wear (Step 4A) , the UHMWPE bar or block is preferably irradiated in Step 2 and remelted in Step 3, m a manner and to a dose and temperature and time as described for UHMWPE in the section, "I . First Aspect of the Invention*. Polymeric Compositions with Increased Wear Resistance" and "I_ (A) Further Examples of the First Aspect of the Invention " , above . In step 4A, acetabular cups are machined out of the irradiated bar and wear tested under conditions suitably representative of the intended m vivo application (e.g., by the method described in the EXAMPLES section below) to estaplish a wear vs. radiation dose response curve for t.ie specific polymer. EXAMPLE 5 and F g 22 show a wear dose- response curve for gamma irradiated GUR 4150 UHMWPE Applicants discovered that it is not necessary to generate additional wear dose-response curves for eacn of the many possible combinations of annealing time and te per- ature. This follows from the results of EXAMPLE 2 Since annealing is done at a lower temperature than remelting and, therefore, has a less marked effect on physical properties m general, it can be expected that annealing will have even less of an effect on the wear resistance than remelting Another important aspect of the invention is the discovery that wear resistance of GUR 4150 was not markedly affected by remelting and, therefore, it is also not likely to be markedly affected by annealing time and temperature. Therefore, it is expected that the radiation dose necessary to produce the desired reduction in wear that is determined from the wear dose-response curve for remelted polymer in Step 4A, will also apply to an annealed polymer produceα in Step 7. Therfore, while the user needs to do his own tests to establish tensile strength vs dose etc., he can rely on the wear vs dose curve developed for the remelted material, rather than running an additional set of wear curves for each annealing condition. This represents a considerable saving in experimental costs, since the tensile strength tests typically may be completed in a few days (using common tensile test apparatus) , but the tests of wear vs dose require months to complete (and require highly specialized equipment and techniques available on only a handful of laboratories in the world) . Furthermore, if the user is working with GUR 4150, he can use the dose vs wear curve of Fig. 22 (as well as the plots of other mechanical properties, Figs. 25-27) without needing to run any wear or tensile tests. Finally, if he is working with another grade of UHMW polyethylene, he can probably use Fig. 22, since other tests have shown that the wear resistances of these materials are very similar to GUR 4150 for a given sterilization treatment. At the least, Fig. 22 establishes the range on which the user may focus his wear vs . dose experiments for other grades of UHMW polyethylene, to minimize the testing necessary to identify the optimum dose. For other polymers, comparable wear tests at each end of the range of interest for radiation dose could be applied to verify whether remelting or annealing also does not markedly affect their wear resistance. Nevertheless, GUR 4150 is representative of UHMWPEs, especially those useful for implants, in its physical and chemical properties, and applicants have observed that other UHMWPEs, of different molecular weights and w th or without calcium stearate, such as GUR 1020 (calcium stearate free, lower molecular weight grade) behaved similarly to GUR 4150 in their wear resistance after irradiation sterilization m air. McKellop, H. et al . , Trans. Society for Biomateπals, Vol. 20, pg . 43 (1997) . Further, it has been observed that, although the start- ing physical properties of HMWPE are different from those of UHMWPE, these differences will be substantially reduced after sufficient crosslinking. For example, they are almost equal after electron beam irradiation treatment to 300 kGy (30 Mrad), for properties like gel content, swelling and strength. Streicher, R. M., Beta-Gamma 1/89: 34-43, at p. 42, right col., fourth full paragraph. Even the wear properties were the same, after the differences in the molecular arrangement between HMWPE and UHMWPE were offset by the irradiation procedure. Thus, it is predicted that the find- ings based on GUR 4150 and the above discussion would be applicable to polymers in general, and to UHMW and HMV polymers, m particular, and especially HMWPE and UHMPE . Thus, the radiation, remelting and annealing ranges found for GUR 4150 can be applied to polymers in general, and more preferaoly to HMW and UHMW polymers, and most preferably to HMWPE and UHMWPE; and these ranges can be used at the very least, as starting points in the flowchart for determining the specific ranges for other polymers, and the data m the "EXAMPLES" section below will facilitate the user in arriv- mg at the proper conditions for GUR 4150, ASTM F648 Type 2 UHMWPE, and UHMWPE and HMWPE in general. The following examples illustrate the use of these generalities in conjunction with the flowchart. In the first example, if the user is working with GUR 4150, or an UHMWPE that satisfies ASTM F648 Type 2 criteria in general, then, based on Figs. 25-27, only the elongation will fall below the ASTM limit (i.e., 300%) over the dose range of interest, i.e., 0 to 25 Mrad, and this occurs at about 6 Mrad. Thus, the maximum allowable dose is 6 Mrad and, from the wear vs dose plot (Fig. 22) , it can be seen that a 6 Mrad dose will provide a wear rate of about 7 to 8 mm 3 per million cycles. This is about a 78% or more reduction over the 33.1 mm 3 per million cycles shown for non-remelted polyethene gamma irradiated to 3.1 Mrad in air. If this reduction in wear rate is sufficient for the user's purpose, then his goal is achieved. Note however, that the elongation vs dose plot (Fig. 27) shows virtually the same behavior whether the polyethylene is remelted or not, so if the above 78% reduction is not sufficient for the user's purpose, then the user would have no choice but to increase the radiation dose, as annealing is also not likely to affect the elongation to break, for the reasons discussed above. In a second example, a user requires a lower limit on tensile strength at break at 40 MPa, and wishes to produce a material with wear no more than 1 mm 3 per million cycles. The wear vs dose curve (Fig. 22) shows that a dose of about 15 Mrad is required to produce a polyethylene with the desired amount of wear resistance However, the tensile strength at fracture vs dose curve shows that the tensile strength at 15 Mrad for a remelted material is about 36 Mpa. Since this is below the user's acceptable limit of 40 Mpa, he can either use a smaller radiation dose and, therefore, accept a smaller improvement in wear rate (i.e., if he wishes to remelt his material) or he can try annealing instead of remelting since, depending on the time/temperature combination used, annealing can be expected to produce a polymer with a value of tensile strength between the limits indicated by the curves for non-remelted and remelted polymer (Figs. 25 and 26) . As shown on these figures, the tensile strength at 15 Mrad for a non-remelted material is about 46 Mpa, well above the user's limit of 40. So, with minimal trial and error, the user can identify an annealing time and temperature that, when applied to a polyethylene that has been exposed to 15 Mrad radiation, has a tensile strength of the required 40 Mpa. Again, based on the wear test results, the user knows that he does not have to re-do the wear vs dose curve for all of the various annealing treatments he tries, in order to identify the dose necessary to produce the desired improvement in wear resis- tance . Having described the invention, the following examples are presented to illustrate and support the invention, and are not to be construed as limiting the scope of the invention. EXAMPLES The nominal dose of radiation applied to implants at a commercial radiation facility typically varies withm a range. Therefore, in the following examples, the average gamma radiation doses are given, such as average gamma radiation doses of 3.3, 26.5, and 28 Mrad. The average of 3.3 Mrad was arrived at by averaging the minimum and maximum doses, e.g., a minimum of 3.28 and a maximum of 3.45 Mrad. Similarly, for example, the average of 26.5 was based on averaging a minimum of 25.14 and a maximum of 27.70 Mrad; and the average of 28 was based on averaging a minimum of 26.01 and a maximum of 30.30 Mrad. EXAMPLE 1 : EFFECT OF RADIATION ATMOSPHERE AND DOSE ON THE PHYSICAL PROPERTIES OF UHMWPE Experimental Details Commercial -grade UHMWPE extruded bars (GUR 4150, Poly Hi Solidur) , with a weight average molecular weight of 5-6 x 10 6 , were used as received. The 8mm thick specimens were cut from the bars and irradiated with gamma-rays at room temperature either in ambient air or in a vacuum chamcer at SteriGenics International (Tustin, CA) to average doses ranging from 3.3 to 250 Mrad. Radiation was delivered at a dose rate of 0.2 Mrad/hr. For 250 Mrad, the dose rate was 4 Mrad/hr. Cobalt-60 was used as a source of gamma radiation. A subset of the 8mm thick specimens that had been irradiated in vacuum was remelted in a vacuum oven by heating from room temperature to 145°C slowly (at about 0.3 °C/mm. ) and maintaining at 145°C for one hour. After remelting, the specimens were slowly cooled to room temperature. The physical properties of the disk specimens before and after irradiation and remelting were characterized by DSC, gel content analysis and FTIR. Gel Content Analysis The gel content of each material was analyzed as a function of depth from the surface. 100 μm thick sections (about 50 mg) were microtomed across the specimen. Extraction of the sol-fraction was performed by boiling in p- xylene for 24 hours, with 0.5 wt% of antioxidant (2,6-dι-t- butyl- 4 -methyl phenol) added to prevent oxidation. For highly oxidized sections from the surface layer, whicn tended to break up during boiling, the specimens were wrapped in PTFE membrane filter (0.5 μm pore size) to avoid loss of gel. After extraction, the specimens were de-swollen acetone and dried at 60°C m a vacuum oven to constant weight. The gel fraction was determined from the ratio of the weight of the dried-extracted material to tnat of the dry non-extracted material. Differential Scanning Calorimetry (DSC) For DSC measurements, samples were cored and microtomed into 200 μm thick sections across the depth. Specimens (-4 mg) were heated from 50°C at 10°C/mm in a differential scanning calorimeter (Perkin-Elmer DSC-4) to 170°C. The melting temperature was identified from the peax of the melting endotherm. Indium was used for calibration of the temperature and heat of fusion. The heat of fusion was determined by comparing the area under the melting endotherm to the area of fusion of an indium sample having a known heat of fusion of 28.4 J/g, and divided by 292 J/g, the heat of fusion of an ideal polyethylene crystal, to get the degree of crystallmity. Results and Discussion As shown Fig. 1, irradiation increased the crystallmity of the 8mm thick specimens of UHMWPE from about 55% to 60-66%, with considerable overlapping for the different doses. Similar changes were observed with the samples that were irradiated in air. The gel content (i.e., the extent of crosslmking) (Fig. 2) also increased with increasing radiation dosage. Importantly, crosslmking increased markedly moving from the surface into the middle of each specimen, reaching about 92% for the 3.3 Mrad dose. Apparently, the oxygen present in the vacuum chamber was sufficient to cause the increased oxidation and decreased crosslinking of the surface layer Thus, our method, i.e., of irradiating a bar and machining away the surface is more effective and efficient than use of a vacuum or other low oxygen atmosphere in producing a final product with minimal oxidation of the bearing surface. For reference (Fig. 3), chemically crosslmked polyethylene (PE) (1% peroxide, lrra- diated m air) (Shen, F.W. et al . J. of Polv. Sc . Part B: Poly Phvs 34:1063-1077 (1996)), which exhibits very low wear, has a gel content of about 90% at aoout 100 microns from the surface, rising to a maximum of nearly 100% m the center. In a second phase of this example, the 8mm thick disks that had been irradiated m vacuum were remelted by heating to 145°C for one hour, and slowly cooled. This reduced the peak melting temperature, the degree of crystallmity and the crystal size. For example (Fig. 4), the crystallmity of the 3.3 Mrad specimens was reduced from the range of 60- 65% to the range of 50-53% by remelting. In addition, during remelting, residual free radicals that were formed by the irradiation apparently recombmed and increased the total crosslmking (as evident from the increased gel content, Fig. 5) . Extinguishing free radicals in this manner, in turn, further reduces the oxidation that would otherwise occur when the cups are stored on the shelf or exposed to body fluids after implantation. The lower gel content (crosslmking) near the surface (Fig. 3) was due to oxidation of the surface layer at the time of irradiation. Thus, it can be expected that the polymer of the surface layer would have less wear resistance than that m the center of the specimen. In the method presented in this application, this gradient would not be present, since the surface layer would be removed during machining of the final implant from the irradiated bar or block. The crystallmity and gel content of the irradiated 8mm thick disks, with and without remelting, are compared m Figs. 4 and 5, respectively. EXAMPLE 2 : WEAR TESTING OF RADIATION CROSSLINKED CUPS WITH AND WITHOUT REMELTING Experimental Details Six extruded bars of UHMWPE (GUR 4150) , each 3 inches in diameter, were exposed to 3.3 or 28 Mrad of gamma radiation at a dose rate of 0.2 Mrad per hour in ambient air (SteriGenics, Inc., Tustin, CA) . Two bars for each radiation dose were then remelted by heating in an oven in ambient atmosphere from room temperature to 150 °C at about 0.3°C per minute and holding at 150°C for five hours, and then slow-cooling to room temperature. The crystallmity and gel content of these four materials were measured across the cross section of extra samples of each bar using differential scanning calorimetry (DSC) and gel content analysis. The results are summarized n Tables 1 and 2. Four sets of acetabular cups were machined from bars of each of the four materials at a commercial machining shop (Bradford and Meneghmi Manufacturing Co., Santa Fe Springs, CA) . Each cup had a 2 inch outer diameter (O.D.) and 1.26 inch inner diameter (I.D.), and 1 inch outer radius and 0.633 inch inner radius (Fig. 6) . Wear tests were run on two sets of three cups for each radiation dose that had been remelted, and two sets of three cups for each dose that had not been remelted. The bars were intentionally used with larger diameters than the final cups so that the process of machining away the outer 0.5 inches of each bar removed the most oxidized, most crystalline, least crosslmked surface layer which is about 0.5 to 1.0 mm thick. In this manner, the bearing surface of each cup consisted of material from near the center of the bar, i.e., the most crosslmked, least crystalline, least oxidized region, whicn is predicted to be the most wear resistant. Since acetabular cups used in patients must first be sterilized by some acceptable means, the test cups this study were sterilized prior to wear testing using ethylene oxide at the appropriate dose for clinical implants. Ethyl - ene oxide was chosen instead of additional gamma irradiation (e.g., 2.5-4 0 Mrad) in order to confine the results to the effects of the original 3.3 or 28 Mrad doses used to crosslink the materials. Prior to wear testing, the cups were pre-soaked m distilled water for three weeks to minimize additional fluid absorption during the wear test, thereby making the weight loss method for wear measurement more accurate For the wear test, the cups were enclosed in polyurethane molds and pressed into stainless steel holders (Fig. 7) Each holder was fitted with an acrylic chamber wall to contain the lubricant. The chambers were mounted on the hip simulator wear machine, with each cup bearing against a ball of cobalt -chromium alloy (conventional hip replacement femoral balls were used, with implant-quality surface finish) . The ball-cup pairs were subjected to a physiological cyclic load with a peak load of about 2000 Newtons (Paul, JP . , "Forces transmitted by joints in the human body" In Lubrication and Wear n Living and Artificial Human Joints. Proc Instn Mech Engrs 1967;181 Part 3J.8-15) and the cups were oscil- lated against the balls through a bi-axial 46° arc at 68 cycles per minute. Each test station on the simulator (Fig. 7) contains a self-centering unit 5, the acetabular cup 6, a dual axis offset drive block 7, a test chamber 8, serum lubricant 9 and a femoral ball 10. The arrow indicates the direction of the computer controlled simulated physiological load applied to the simulated hip joint. During the test, the bearing surfaces were kept immersed in bovine blood serum to simulate lubrication m the human body. Sodium azide at 0.2% was added to the serum to retard bacterial degradation, and 20 mM ethylene- diaminetetraacetic acid (EDTA) was added to prevent precipitation of calcium phosphate onto the surface of the balls (McKellop, H. and Lu, B., "Friction and Wear of Polyethylene-metal and Polyethylene-ceramic Hip Prostheses on a Joint Simulator, Transactions of the Fourth World Biomaterials Congress, Berlin, Apr. 1992, p. 118). A polyethylene skirt covered each test chamber to minimize airborne contaminants. At intervals of 250,000 cycles, the cups were removed from the machine, rinsed, inspected under light microscopy and replaced in fresh lubricant. At intervals of 500,000 cycles, the cups were removed, cleaned, dried and weighed to indicate the amount of wear. After inspection under light microscopy, the cups were replaced on the wear machine with fresh lubricant and testing was continued to a total of three million cycles. One million cycles s approximately the equivalent of one year's walking activity of a typical patient . The weight loss was corrected for the effects of fluid absorption (which masks wear) by increasing the apparent weight loss of the wear test cups by the mean weight gam of three control cups of each material that were also immersed in serum and cyclically loaded on a separate frame, but without oscillation. The corrected rate of weight loss was converted to volume loss by dividing by the approximate density of UHMWPE (0.94 gm/cc) . The mean weight loss (after soak correction) and the standard deviation was calculated for each of the four types of materials at each weighing interval . The wear rate of each cup was calculated by applying linear regression to the wear data for the entire three million cycles. The mean wear rates and standard deviations also were calculated for each type of material. Resul t s Fig. 8 shows the soak-corrected wear (volume loss) of three cups of each material as a function of wear cycles. Fig. 5 shows the average wear (volume loss) of three cups of each material as a function of wear cycles. The individual wear rates and the mean values for each type of material are listed m Table 3. The most wear occurred with the cups subjected to 3.3 Mrad without remelting. These averaged 21.1 mm 3 per million cycles. The wear of the cups subjected to 3.3 Mrad with remelting averaged 18.6 mm 3 per million cycles, or 12% lower wear than for the non-remelted 3.3 Mrad cups. The cups subjected to 28 Mrad had much lower wear rates than the 3.3 Mrad cups, and the rates were similar, whether or not the material had been remelted. That is, the average wear rate of the non- remelted 28 Mrad cups was about 1.2% that of the non-re- melted 3.3 Mrad controls, and the average wear rate of the remelted 28 Mrad cups was about 1.7% of the same controls. Discussion The results of the wear test clearly demonstrated the improved wear resistance of the UHMWPE acetabular cups that resulted from exposure to 28 Mrad gamma radiation. Apparently, the crosslmking generated by the higher radiation dose reduced the wear rates to less than a few percent of the control value (3.3 Mrad) . The minimum amount of wear debris necessary to induce clinically significant osteolysis and other problems in a specific patient has not been established, and it may vary among patients. Nevertheless, a material which reduces the wear rate to the very low levels exhibited by the 28 Mrad cups in this study would be very likely to provide a large margin of safety over currently used materials. The wear curves for both of the 28 Mrad specimens (Figs. 8 & 9) were slightly negative on the first weighing at 0.5 million cycles. This was most likely due to a slight under-correction for fluid absorption (that is, the wear test cups absorbed slightly more water than the soak controls, and the error between the two was greater than the weight loss due to wear, producing a negative wear value; If this assumption is correct, then the overall wear rates for the two 28 Mrad sets were somewhat smaller, and possibly closer together, than the values indicated m Table 3. EXAMPLE 3 : ARTIFICIAL AGING OF RADIATION-CROSSLINKED UHMWPE MATERIALS Six UHMWPE (GUR 4150) extruded bars (3" diameter) were gamma irradiated m air, three bars eacn at 3.3 or 28 Mrad, at a dose rate of 0.2 Mrad/hour. For each radiation dose, two bars were then remelted by heating m an oven at ambient atmosphere from room temperature to 150°C at about 0.3°C/mm, holding at 150°C for 5 hours and slowly cooling to room temperature, and the third bar was not remelted. A 13 mm (0.5 inch) layer of the outer diameter of the treated (remelted) and untreated (non-remelted) bars was machined away to remove the most oxidized, least crosslmked surface layer. The bars were used to produce specimens for the artificial agmg tests described here and for the wear tests described m EXAMPLE 2. To examine the effect of artificial agmg on these four materials (3.3 and 28 Mrad, remelted and not remelted), 8 mm thick disks were cut from these 2 inch diameter cores and were heated in an oven slowly ( - 0.2 'C/min) to 80 'C at ambient atmosphere and held at 80 "C for 10, 20 or 30 days. In addition, one acetabular cup for each of the four conditions (3.3 and 28 Mrad, remelted and not remelted) that had been fabricated at the same time as the wear test cups of EXAMPLE 2 and stored in air for about 5 months was cut into four pieces and aged at 80 'C for the same periods. The gel content analysis and DSC method are as described in EXAMPLE 1, above. Fourier Transform Infrared Spectroscopy (FTIR) FTIR measurements were performed on the above specimens. Segments about 5 mm wide were cut from each polyethylene specimen and the segments were microtomed into 200 μm thick slices. The oxidation profiles, as indicated by the carbonyl concentration, were measured using a Mattson Po- laris FTIR (model IR 10410) with a Spectra-Tech IR plan microscope. Spectra were collected in 100 μm steps from the surface to the middle of the specimen, using 64 scans summation at a resolution 16 cm "1 with a MCT (Mercury Cadmium Telluride) detector. The carbonyl group concentration was indicated by the ratio of the peak height of the ketone absorption band at 1717 cm 1 to the height of the reference band at 2022 cm "1 ( - CH 2 - vibration) . RESULTS The oxidation profiles as a function of depth are shown in Figs. 10-13. As shown m Fig. 10 for the 3.3 Mrad, non- remelted material, oxidation increased with increasing aging time. In contrast, the 3.3 Mrad, remelted material (Fig. 11) showed almost no oxidation for 10 and 20 days agmg, but some oxidation for 30 days agmg. However, the oxidation peak at the surface with remelting was about 50% of that at the surface without remelting (Fig. 10) . For the 28 Mrad, non-remelted UHMWPE (Fig. 12), the oxidation showed a greater increase with increasing agmg time than the 3.3 Mrad, un-remelted material. Again, oxidation was much lower with remelting, i.e., the 28 Mrad, remelted UHMWPE (Fig. 13) essentially exhibited no oxidation after 20 days agmg (Fig. 13) , and the oxidation peak at the surface after 30 αays was only about 1/3 that without remelting (Fig. 12). Similarly, with the acetabular cups stored air for 5 months and then aged for 20 days at 80°C, the remelted materials (3.3 or 28 Mrad) showed no oxidation (Fig 14), while the non-remelted cups (3.3 or 28 Mrad) showed substan- tial oxidation (Fig. 14), especially for 28 Mrad UHMWPE, and with a subsurface oxidation peak in both non-remelted materials . Since crosslmking of UHMWPE reduces its solubility, the percent of undissolved material (gel content) is an indirect indication of the amount of crosslmking. The gel content as a function of depth for various conditions are shown in Figs. 15 to 18. As shown in Fig. 15 for 3.3 Mrad, non-remelted material, the gel content (i.e., crosslmking) decreased with increasing aging time. There was a strong gradient of gel content in the highly oxidized surface regions after 30 days aging, i.e., increasing from a minimum on the surface to a maximum about 2 mm below the surface. Near the surface, the gel content was highest (91%) m the un-aged specimen, and decreased with increasing agmg time to less than about 5% in the same region for the 30 day aged specimen. In contrast, the remelted materials (Fig. 16) showed much less reduction in gel content in the surface regions than the non-remelted materials. That is, comparison of Fig. 17 (28 Mrad, non-remelted) and Fig. 18 (28 Mrad, remelted) showed that the remelted UHMWPE had much higher retention of gel content (i.e., crosslmking) . The results of the DSC measurements indicated the degree of crystallmity as a function of depth for various materials aged for 30 days at 80°C, as shown in Fig. 19. Near the surface, the degree of crystallmity was 83% for the 23 Mrad, non-remelted material after agmg, compared to 65% before aging. The high level of crystallmity and increased bπttleness of the surface zone of the aged ate- rial often resulted in fragmentation of a layer about 1 mm thick during microtoming. In contrast, the 28 Mrad remelted material showed less increase m crystallmity m the surface regions due to aging, and no brittle zone was observed. Similarly, due to aging, the 3.3 Mrad non-remelted material exhibited an increase m crystallmity from 60% to about 78%, and the surface layer was again brittle, although not as brittle as with the 28 Mrad, non-remelted material. DISCUSSION Irradiation of UHMWPE produces crossl king, chain scission and the formation of free radicals. If oxygen is present, it may react with the free radicals to form oxidized species, leading to additional chain scission (reduction in molecular weight) and an increase in crystallmity. Since polymer crystallites melt and become amorphous above the melting temperature, molecular chain movements and rotations are increased, favoring the recombination of free radicals. The results of the present experiments showed that remelting at 150°C apparently caused the residual free radicals to decay and/or to recombme to form crosslinks, leading to an increased gel content. Therefore, remelting is an effective way to extinguish free radicals, making the material less susceptible to long-term oxidation and potentially improving the long-term wear resistance, as evident from the results of the artificial agmg experiments, where there was much less oxidation of the remelted materials. For a crosslmked polymer, oxidative degradation cleaves the molecules and leads to a reduction gel content. This was evident in the present experiments from the reduced gel content after agmg, particularly with the non- remelted materials (Figs. 15 to 18) . That is, the distribution of oxidation, as indicated by the profiles measured by FTIR, was inverse to the gel content with the material; the higher the oxidation, the lower the gel content (crosslmking) Since remelting extinguishes free radicals and increases gel content, thereby reducing the susceptibility to oxidation, the remelted materials (3.3 and 28 Mrad) had a much greater gel content after artificial agmg than the non-remelted materials. An appropriate amount of crosslmking of UHMWPE can improve its wear resistance. The high level of crosslmking the UHMWPE caused by the 28 Mrad gamma irradiation, as evident from the high gel content (EXAMPLE 2) , apparently contributed to the much greater wear resistance exhibited by the acetabular cups tested in EXAMPLE 2. In addition, as shown in EXAMPLE 3, remelting of the irradiated UHMWPE markedly reduced the residual free radicals, rendering the material much more resistant to subsequent oxidation and, therefore, resistant to a reduction in crosslmking, which can be of substantial benefit for implants in long-term clinical use. EXAMPLE 4: WEAR TESTING OF IRRADIATED CUPS WITH AND WITHOUT ARTIFICIAL AGING Materials and Methods The wear testing of irradiated cups with and without remelting was described m EXAMPLE 2. Effects of artificial agmg on the physical properties of irradiated UHMWPE, with and without remelting, were described in EXAMPLE 3. To examine the resistance of crosslmked cups to thermal -induced oxidation, and the effect of such oxidation on the wear of irradiated cups with and without remelting, two acetabular cups for each of the four conditions (3.3 and 28 Mrad, remelted and not remelted) that had been wear tested for 3 million cycles as described in EXAMPLE 2, were heated in an oven slowly (- 0.2 °C/mm) to 80 °C at ambient atmosphere and held at 80 °C for 20 days, with one acetabular cup for each of the four conditions being stored m ambient air. The oxidation profile after 20-day aging for each condition was shown m Fig. 14, EXAMPLE 3. Prior to wear testing, the cups were pre-soaked in distilled water for four weeks to minimize additional fluid absorption during the wear test, thereby making the weight loss method for wear measurement more accurate. The details for the wear test were described m EXAMPLE 2. Results Fig. 20 shows the combined soak-corrected wear (volume loss) for the cups before aging (3.3 and 28 Mrad, remelted and not remelted) during the first 3 million cycles (same data as EXAMPLE 2) and for the same cups, after two cups of each material had been artificially aged, from 3 to 7 million cycles. The individual wear rates and the mean values for each type of material, calculated by linear regression, are listed in Table 4. All cups subjected to 3.3 Mrad with remelting showed comparable wear rates, whether or not the material had been remelted or remelted and aged. Wear was negligible for all of the cups subjected to 28 Mrad, whether or not these were remelted, and whether or not they were aged. Discussion The results of the wear test clearly demonstrated the improved wear resistance of the UHMWPE acetabular cups that resulted from exposure to 28 Mrad gamma radiation. Apparently, the minor oxidation at the surface (Fig. 14) of the highly crosslinked acetabular cups (28 Mrad, without remelting) induced by the artificial agmg, had very limited effect on the wear resistance. Although a substantial oxida- tion peak occurred about 0.4 mm below the surface, because of the very high wear resistance of the 28 Mrad cups, the total penetration due to wear was too shallow to reach this sub-surface oxidized zone, even after 4 million cycles. For the non-remelted 3.3 Mrad cups, subsurface oxida- tion, peaking at about 1 mm below the surface (Fig 14), occurred after agmg air at 80 °C for 20 days. Since the total depth of penetration of these cups was about 300 microns (at 7 million cycles) , the full effect of this subsurface oxidation would not become apparent until a much larger number of wear cycles. Nevertheless, the sub-surface oxidation in the non- remelted cups (EXAMPLE 3, particularly for the 28 Mrad specimen) leads to reduced molecular weight, a reduction in crosslmking (as indicated by gel content) and an increased crystallmity and brittleness, all of which can contribute to reductions in mechanical properties such as fatigue strength and, eventually, a reduction in wear resistance. Although remelting had no apparent effect on the wear resistance of the aged cups the present example, the elim a- tion of free radicals by remelting improves the long-term resistance to oxidation, thereby improving the long-term wear resistance in vivo. EXAMPLE 5: WEAR TESTING OF GAMMA-IRRADIATED UHMWPE WITH MULTIPLE DOSES Materials and Methods In EXAMPLE 2, we demonstrated the improved wear resistance of UHMWPE acetabular cups that resulted from exposure to 28 Mrad gamma radiation, as compared to cups irradiated to 3.3 Mrad. The average wear rate of the 28 Mrad cups was less than 2% of that of the 3.3 Mrad cups (i.e., a dose withm the normal 2.5 to 4.0 Mrad range used to sterilize implants) . To examine the wear as a function of radiation dose and, thereby, determine an optimum dose for reducing wear, extruded bars of GUR 4150 UHMWPE, 3" diameter x 15" long, were gamma irradiated air, three bars at each dose of 4.5, 9.5, 14.5, 20.2 or 24 Mrad (SteriGenics , Inc., Corona, CA) , at a dose rate of 0 45 Mrad/hour Additional bars were irradiated air to 50 or 100 Mrad (SteriGemcs Inc., Tust , CA) , at a dose rate of 0.67 Mrad/hour. For each radiation dose, two bars were then remelted by heating in an oven in ambient atmosphere from room temperature to 150 °C at about 0.3 °C/mιn, holding at 150 °C for 5 hours and then slowly-cooled to room temperature, with the third bar not being remelted. The irradiated-remelted bars were used to produce acetabular cups for the wear tests. Seven sets of acetabular cups were machined from the irradiated-remelted bars for each of the seven doses at a commercial machining shop (Bradford and Meneghim Manufacturing Co., Santa Fe Springs, CA) . Each cup had a 2" O.D. and 1.26" I.D., with 1" outer radius and 0.63" inner radius (Fig. 6). Wear tests were run on the remelted specimens, using two cups for each radiation dose from 4.5 to 24 Mrad, and one cup each for 50 and 100 Mrad. The bars were intentionally used with larger diameters than the final cups so that the process of machining away the outer layer of each bar, about 0.5 inch thick, effectively removed the most oxidized, most crystalline, least crosslmked surface layer (about 0.5 to 1.0 mm). In this manner the bearing surface of each cup consisted of material from near the center of the bar, i.e., the most crosslmked, least crystalline, least oxidized region, which was expected to be the most wear resistant . Because acetabular cups used in patients must first be sterilized by some acceptable means, the test cups m this study were sterilized prior to wear testing using ethylene oxide at the appropriate dose for clinical implants. Ethylene oxide was chosen instead of additional gamma irradiation (e.g., 2.5-4.0 Mrad) m order to focus the results on the effects of the radiation doses used to crosslink the materials. Prior to wear testing, the cups were pre-soaked in distilled water for four weeks to minimize additional fluid absorption during the wear test, thereby making the weight loss method for wear measurement more accurate The details for tne wear test method were described in EXAMPLE 2 Results Fig. 21 shows the soak-corrected wear (volume loss) of each material (three cups for 3.3 Mrad from EXAMPLE 2, two cups each for radiation dose from 4.5 to 24.5 Mrad, and one cup each for 50 and 100 Mrad) . The individual wear rates, determined by linear regression, and the mean values for each type of material are listed in Table 5. At about 2.1 million cycles, there was a temporary overloading of the test cups, due to a malfunction of the computer controller. Although this overload had only a minor effect on the wear rates of the cups, the cup irradiated to 100 Mrad cracked and was, therefore, removed from the test. Fig. 22 shows the average wear rate (volume loss from 1 to 5 million) of each type of material, that had been remelted (denoted m the figure by darkened circles) and that had not been re-melted (denoted m the figure by an open circle), as a function of dose. The wear of the cups subjected to 3.3 or 4.5 Mrad with remelting averaged 17.5 or 9.3 mm 3 per million cycles, respectively, showing about 13% or 54% lower wear than for the 3.3 Mrad non-remelted cups (20.1 mm 3 per million cycles) . In contrast, the wear rate of the 9.5 Mrad remelted cups averaged 2.2 mm 3 per million cycles, i.e., about 89% lower than for the 3.3 Mrad non-remelted cups. For radiation doses greater than 9.5 Mrad, minimal systematic wear occurred, such that, compared to that with 3.3 Mrad non-remelted cups, the wear rates were about 94% lower for the 14.5 Mrad remelted cups, and minimal wear (>99% reduction) for the 20.2 Mrad remelted cups. "Negative" wear rates were calculated for the cups given 24 Mrad or greater doses. Apparently, these cups absorbed more water than the soak control cups, and the error between the two was greater than the weight loss due to wear, giving a net gain in weight. Discussion The results clearly demonstrated that the wear resis- tance of UHMWPE acetabular cups were improved substantially with increasing radiation dose over the range of 4.5 to 9.5 Mrad (i.e., with increasing crosslinking), such that wear was too small to accurately quantify for doses exceeding about 20 Mrad. Since, in addition to improving wear resis- tance, radiation induced crosslinking may degrade other physical properties, such as elongation to failure and fatigue strength, the dose-response curve developed in the present example provides the opportunity to select an optimum dose, i.e., one that provides the desired amount of improvement in wear resistance with a minimum reduction in other physical properties. The procedure for arriving at the choice of dose for a particular in vivo application is described in this application. UHMWPE acetabular cups that had been compression molded and then exposed to 3.1 Mrad gamma radiation in air but were not thermally treated (i.e., typical of commercially used implants over the past two decades) , showed an approximate wear rate of 33.1 mm 3 /million cycle using the procedure of the wear test described in EXAMPLE 2 , above . When compared to these conventional UHMWPE acetabular cups, the acetabular cups of the present invention (i.e., irradiated bar stock, remelted and machined into cups) show the following percentage reduction in wear rate: for the 3.3 Mrad remelted acetabular cup from EXAMPLE 2, above (about 47% reduction in wear rate); 4.5 Mrad remelted acetabular cup from EXAMPLE 5, above (about 72% reduction in wear rate); 9.5 Mrad remelted acetabular cup from EXAMPLE 5, above (about 93% reduction in wear rate) . EXAMPLE 6: PHYSICAL CHARACTERIZATION OF GAMMA-IRRADIATED UHMWPE WITH OR WITHOUT REMELTING Materials and Methods The materials for physical characterization were the same as the wear tested materials described in EXAMPLE 5. The materials included UHMWPE extruded bars (3" in diameter) gamma irradiated to 3.3, 4.5, 9.5, 14.5, 20.2, 24, 50 and 100 Mrad, with or without remelting, and the non- irradiated bars 8 mm thick disks were cut out of irradiated bars with or without remelting, and sterilized with ethylene oxide. The specimens for DSC and swelling measurements were cut out of the center of the 8 mm thick disks. The DSC measurement for crystallmity and melting temperature with sample weighing about 4 mg was described in EXAMPLE 1 For swelling measurements, 1 mm thick sheet weighing about 0.5 gram was cut out of the center of the 8 mm thick disk, and extraction of the sol-fraction was performed in boiling p-xylene for 72 hours, with 0.5 wt % antioxidant (2 , 6 -di- -butyl -4 -methyl phenol) being added to prevent oxidation. After extraction, the gel was transferred to fresh p-xylene and allowed to equilibrate at 120°C for 2 hours. The swollen gel was then quickly transferred to a weighing bottle, covered and weighed. The data was obtained as the average of five measurements. After measurements, samples were deswollen in acetone and then dried at 60°C n a vacuum oven to a con- stant weight. The gel fraction was determined as the ratio of the weight of the dried extracted to the initial dry non- extracted network. The degree of swelling was calculated as the ratio of the weight of the swollen gel to the dried extracted gel. The degree of swelling was used to calculate the network chain density, number-average molecular weight between crosslinks and crosslink density, according to the theory of Flory and Rehner (Shen et al . , J. Polvm. Sci . , Polv Phvs . , 34:1063-1077 (1996)}. For examining the oxidation profiles of the extruded bars irradiated and remelted in air, a two hundred micron thick section was microtomed perpendicular to the bar surface and examined by FTIR as a function of depth from the bar surface. Results and Discussion The melting temperature and crystallmity for non- lrradiated, and irradiated (with and without remelting) materials are shown in Table 6. The degree of swelling, average molecular weight between crosslinks, crosslink density and gel content are shown in Table 7. After lrradi- ation, the melting temperature and crystallmity increased, ranging from 135.3 to 140.2 °C, and about 60 to 71%, respectively, over the dose range studied. Remelting of the irradiated bars resulted in reductions in the melting temperature and crystallmity, ranging from about 131 to 135°C, and about 51 to 53%, respectively. As shown m Table 7, with increasing radiation dose, the degree of swelling and average molecular weight between crosslinks decreased, while the crosslink density increased. The gel content, m general, increased with radiation dose, but reached a plateau region at about 9.5 Mrad. With remelting, the degree of swelling and average molecular weight between crosslinks for bars irradiated up to 9.5 Mrad were significantly reduced, but remained almost unchanged after 9.5 Mrad. The crosslink density increased, after remelting, with dose up to 9.5 Mrad and then remained almost unchanged. The gel content, generally, increased after remelting. The oxidation profiles for the 9.5 and 24 Mrad materials, after remelting at 150°C in air for 5 hours, as a function of depth from the bar surface are shown m Fig. 24. The results clearly showed that the oxidation drops tremendously within 1 mm, and the most oxidized layer is about 1 mm deep below the surface, after irradiation and remelting in air. EXAMPLE 7: TENSILE PROPERTIES OF GAMMA-IRRADIATED UHMWPE AT VARIOUS DOSES, WITH OR WITHOUT REMELTING Materials and Methods The materials for tensile test are the same as the wear tested materials described EXAMPLE 5, aoove The maten- als included UHMWPE extruded bars (3" diameter) gamma irradiated to 4.5, 9.5, 14.5, 20.2, and 24 Mrad, with or without remelting, and non-irradiated bars. Five tensile specimens each was machined out of the center of the 3" diameter bars according to ASTM F648-96 and D-638 (type IV) . Tensile tests were performed using an servo-hydraulic tensile test machine at speed of 2 inches/min. Results and Discussion The tensile strength at yield, elongation, and tensile strength (ultimate) at breaks are shown in Table 8. The average tensile properties as a function of radiation dose are shown in Figs. 25-27. The tensile strength at yield after irradiation was higher than that of non- irradiated material, and slightly increased with radiation dose. Remelting of the irradiated bars resulted in a reduction in tensile strength at yield, and the strength remained almost constant over the dose range studied (Fig. 25) . The tensile strength (ultimate) and elongation at break decreased with increasing doses (Figs. 26-27) . Remelting resulted in further reduction in ultimate tensile strength over the dose range. However, remelting had almost no effect on the elongation at break over the same dose range. All publications and patent applications mentioned in this Specification are herein incorporated by reference to the same extent as if each of them had been individually indicated to be incorporated by reference. Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding, it will be obvious that various modifications and changes which are with the skill of those skilled m the art are considered to fall with the scope of the appended claims. Future technological advance- ments which allows for obvious changes the basic invention herein are also with the claims. Table 1 3.3 Mrad Before remelting After remelting Distance peak melting degree of gel content peak melting degree of gel content from surface temperature crystallinity (%) temperature crystallinity (%) ( m ) O (%) CO (%) 34.8-35 135.3 59.6 91 131.4 52.5 94.7 35.8-36 135.4 60.2 91 131.5 51.2 94.7 36.8-37 135.3 60.5 91 131.4 51.9 94.7 37.8-38 135.3 60 91.1 131.3 52 95 (center) Table 2 28 Mrad Before remelting After remelting Distance peak melting degree of gel content peak melting degree of gel content from surface temperature crystallinity (%) temperature crystallinity (%) ( m ) CO (%) CO (%) 34.8-35 139.8 65.1 95.8 135 52 97.7 35.8-36 139.8 64.2 95.8 134.8 52.1 97.7 36.8-37 139.7 64.5 95.8 134.9 52.5 97.7 37.8-38 139.7 65.3 95.8 134.9 52.7 97.7 (center) Table 3 Cup# Material Wear Rate Mean Wear (mm 3 /million Rate ± Std cycles) Deviation Nil 33 Mrad 208 N16 Not 212 21 1 ±03 N17 remelted 214 R21 177 33 Mrad R26 201 Remelted 186 ± 13 R27 180 N35 28 Mrad 029 N31 Not 024 025 ±003 N32 remelted 024 R48 036 28 Mrad R45 035 Remelted 036 ±0001 R49 036 Table 4 0 - 3 Million Cycles 3 - 7 Million Cycles (non- aged) Cup# Material Wear Rate Mean Wear Wear Rate Mean Wear (mπvYmillion Rate ± Std Conditions (mπrYmillion Rate ± Std cycles) Deviation cvcles) Deviation Nil 33 Mrad 208 non-aged 212 - N16 Not 212 21 1 ±03 aged 215 218±05 N17 remelted 214 aged 222 R21 177 non-aged 175 - 33 Mrad R26 201 186± 13 aged 192 Remelted 198 ± 10 R27 180 aged 205 N35 28 Mrad 029 non-aged 003 - N31 Not 024 025 ± 003 aged -047 -071 ±03 N32 remelted 024 aged -093 R48 036 non-aged 047 - 28 Mrad R45 035 Remelted 036±0001 aged 008 - 006 ± 02 R49 036 aged -020 Table 5(1-5 million cycles) Cup # Material Wear Rate Mean Wear Rate ± SD (mmVmillion cycles) (mmVmillion cvcles) Nil 3.3 Mrad 20.46 N16 Not 19.32 20.12 ± 0.7 * N17 remelted 20.59 R21 17.04 R26 3.3 Mrad Remelted 180 17.51 ±048 * R27 17.49 RA2 9.93 4.5 Mrad Remelted 9.28 ±0.92 RA3 8.63 RB3 2.39 9.5 Mrad Remelted 2.22 ± 0.24 RB6 2.05 RC5 1.26 14.5 Mrad Remelted 1.17 ±013 RC6 1.08 RD1 0.26 20.2 Mrad Remelted 0.12 ±0.2 RD6 -0.02 RE3 -049 24 Mrad Remelted -0.59 ±0.13 RE4 -0.68 RF2 50 Mrad Remelted -0.8 - RG1 100 Mrad Remelted -6.88** - "The wear data of the 3.3 Mrad materials in Example 2. **The wear rate in the period of 1 - 2 million cycles. Table 6 Non-ienielted Remelted Samples Melting point Crystallmity Melting point Ciystal nity (°C) (%) (°C) (%) Non-irrad 1338 55 - - 33 Mrad 1353101 601104 1314101 518106 Ul 45 Mrad 1362102 658116 1316102 520113 Ul I 95 Mrad 1371 ±0 671122 1348102 533121 145 Mrad 1375102 696116 1350101 53 O± 15 202 Mrad 1374101 708128 1353101 521118 24 Mrad 1379103 680113 1352101 517112 50 Mrad 1389102 670113 135210 528102 100 Mrad 1402103 663127 1308102 523 ± 17 Table 7 Table 8 Materials Tensile Strength at Tensile Strength at Elongation at Yield (MPa) Break (MPa) Break (%) Non-irradiated 233 ±011 521 ±478 356123 Without remelting 45 Mrad 249 ±033 4691291 314 ± 12 95 Mrad 253 ±012 476 ± 276 251 r 8 145 Mrad 257 ±025 464 ± 120 213 ±5 202 Mrad 262 ± 027 402 ± 272 175 ±7 24 Mrad 264 ± 023 400 ± 542 164 ± 17 After remelting 45 Mrad 215 ±033 456 ±889 309120 95 Mrad 213 ±060 432 ± 280 25218 145 Mrad 218 ±029 368 ± 172 206 ±9 202 Mrad 2191018 3431361 185 -t 8 24 Mrad 21.7 ±0.25 3231281 160 ± 19

Claims: We cla im : 1. A preformed polymeric composition comprising a crosslmked thermally treated polymer. 2. The composition of claim 1, wherein the co posi- tion possesses one or more of the following characteristics: degree of swelling of between about 1.7 to about 5.3; molecular weight between crosslinks of between about 400 to about 8400 g/mol; and a gel content of between about 95 to about 99 %. 3. The composition of claim 1, wherein the preformed polymeric compsition is crosslmked by gamma radiation at a a dose from about 1 to about 100 Mrad. 4. The composition of claim 3, wherein the dose is from about 5 to about 25 Mrad. 5. An m vi vo implant comprising a crosslmked and remelted polymer. 6. A method for increasing the wear resistance of a preformed polymeric composition, comprising the steps of: (a) crosslinking the polymeric composition a solid state; and (b) subjecting the crosslmked polymeric composition to thermal treatmen . 7. The method of claim 6, further comprising the step of removing the most oxidized surface of the thermally treated crosslmked polymeric composition. 8. The method of claim 6, wherein the crosslinking is by gamma irradiation. 9. The method of claim 8, wherein the gamma lrradia- tion is at a dose of from about 1 to about 100 Mrad. 10. The method of claim 6, wherein the thermal treatment comprises remelting the crosslmked polymer. 11. The method of claim 10, wherein the remelting temperature is between the melt g temperature of the irradiated polymer to about 160°C above the melting temperature of the irradiated polymer. 12. The method of claim 6, wherein the resulting polymeric composition possesses one or more of the following characteristics: degree of swelling of between about 1.7 to about 5.3; molecular weight between crosslinks of between about 400 to about 8400 g/mol; and a gel content of between about 95 to about 99 %. 13. The method of claim 6, wherein the thermal treatment comprises annealing the crosslmked polymer. 14. The method of claim 13, wherein the annealing temperature is from about 90°C below to about 1°C below the meltmg temperature of the irradiated polymer. 15. A polymeric composition made from the steps of: (a) crosslmking a starting polymer in a solid state to form a crosslmked polymer; and (b) subjecting the crosslmked polymer to thermal treatment . 16. The polymeric composition of claim 15, further comprising the step of removing the most oxidized surface of the crosslmked polymeric composition. 17. The polymeric composition of claim 15, wherein the crossl king is by gamma irradiation. 18. The polymeric composition of claim 17, wherein the gamma irradiation is at a dose of from about 1 to about 100 Mrad. 19. The polymeric composition of claim 15, wherein the thermal treatment comprises remelting the crosslmked polymer . 20. The polymeric composition of claim 19, wherein the remelting temperature is between the meltmg temperature of the irradiated polymer to about 160°C above the meltmg temperature of the irradiated polymer. 21. The polymeric composition of claim 15, wherein the thermal treatment comprises annealing the crosslmked polymer . 22. The method of claim 21, wherein the annealing temperature is between about 90°C below to about 1°C below the meltmg temperature of the irradiated polymer. 23. A product made by the process of: (a) crossl king a preformed polymeric composition m a solid state; (b) subjecting the crosslmked polymeric composition to thermal treatment; and (c) fashioning the product from the crosslmked polymeric composition. 24. The product of claim 23, further comprising the step of removing the most oxidized surface of the crosslmked polymer. 25 The product of claim 24, wherein the proαuct is an m vivo implant. 26. A method for determining an optimal radiation dose and tnermal treatment for treating a polymer to increase its wear resistance, when made into a desired product, while maintaining its desirable physical and/or chemical properties, the method comprises the steps of (a) irradiating the polymer in the solid state over a range of radiation doses likely to produce the desirable wear resistance and physical and/or chemical properties, (b) remelting the polymer, (c) correlating the radiation doses with the wear rate of the desired product made from the irradiated remelted polymer using actual or simulated wear conditions for the desired product; (d) correlating the radiation doses with each of the physical and/or chemical properties of the desired product made from the irradiated remelted polymer using actual or simulated wear conditions for the desired product; (e) comparing the correlations in steps (c) and (d) to determine the optimal radiation dose which will produce a desirable wear rate while maintaining the desirable physical and/or chemical properties, if such a radiation dose is arrived at, use this optimal radiation dose for future treatment of the polymer; (f) if the optimal radiation dose cannot be arrived at m step (e) , then determining a dose that would produce a desirable wear rate based on the correlation of step (c) and annealing instead of remelting the polymer which has been irradiated to said dose; (g) correlating the physical and/or chemical properties of the desired product made from the irradiated and annealed polymer, using actual or simulated wear conditions for the desired product, with different annealing times and temperatures; (h) determining an annealing temperature and time which will provide the desirable wear rate and physical and/or chemical properties, if this is possible, then use the radiation dose and annealing conditions determined at this step for future treatment of the polymer, d) if step (h) does not provide the desirable wear rate and physical and/or chemical properties, then apply a lower radiation dose and repeat steps (c) to (i) or (h) until the optimal radiation dose and annealing conditions are determined or the steps confirm that no optimal radiation dose and annealing conditions can be obtained for the desired wear rate and physical and/or chemical properties. 27. The method of claim 26, wherein the irradiation is gamma irradiation between a range of about 1 to 100 Mrad; the remelting temperature is between the melt g temperature of the irradiated polymer to about 160 °C above the meltmg temperature of the irradiated polymer; and the annealing temperature is between about 90 °C below to about 1°C below the meltmg temperature of the irradiated polymer. 28. A polymer produced by irradiation and thermal treatment, wherein the radiation dose and remelting or annealing conditions are determinable by the method of claim 27. 29. An m vivo implant made from the polymer of claim 26, wherein the most oxidized surface of the polymer is removed . 30. A process for treating a polymer, wherein the process employs radiation dose and remelting or annealing conditions determ able by the steps of claim 26.

Inventors: MCKELLOP HARRY A, SALOVEY RONALD, SHEN FU-WEN, MCKELLOP, HARRY, A., SALOVEY, RONALD, SHEN, FU-WEN
Applicants: MCKELLOP HARRY A, ORTHOPAEDIC HOSPITAL, SALOVEY RONALD, SHEN FU WEN, UNIV SOUTHERN CALIFORNIA, MCKELLOP, HARRY, A., SALOVEY, RONALD, SHEN, FU-WEN, THE ORTHOPAEDIC HOSPITAL, THE UNIVERSITY OF SOUTHERN CALIFORNIA
Application year: 1997
Main classification: A61F2
IPC codes: A61F2, A61L27, B29C35, B29C43, B29C71, C08F110, C08J3, C08J5, C08L23, C08L101