The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "This invention relates to medical implants formed of a polymeric material such as ultra-high molecular weight polyethylene, with superior oxidation and wear resistance produced by a sequential irradiation and annealing process.
"Various polymer systems have been used for the preparation of artificial prostheses for biomedical use, particularly orthopedic applications. Among them, ultra-high molecular weight polyethylene is widely used for articulation surfaces in artificial knee, hip, and other joint replacements.
"All components must then go through a sterilization procedure prior to use, but usually after being packaged. There exists several sterilization methods which can be utilized for medical applications, such as the use of ethylene oxide, gas plasma, heat, or radiation. However, applying heat to a packaged polymeric medical product can destroy either the integrity of the packaging material (particularly the seal, which prevents bacteria from going into the package after the sterilization step) or the product itself.
"It has been recognized that regardless of the radiation type, the high energy beam causes generation of free radicals in polymers during radiation. It has also been recognized that the amount or number of free radicals generated is dependent upon the radiation dose received by the polymers and that the distribution of free radicals in the polymeric implant depends upon the geometry of the component, the type of polymer, the dose rate, and the type of radiation beam. The generation of free radicals can be described by the following reaction (which uses polyolefin and gamma ray irradiation for illustration):
"Depending on whether or not oxygen is present, primary free radicals r.cndot. will react with oxygen and the polymer according to the following reactions as described in 'Radiation Effects on Polymers,' edited by
"In the Presence of Oxygen
"In radiation in air, primary free radicals r.cndot. will react with oxygen to form peroxyl free radicals r0.sub.2., which then react with polyolefin (such as UHMWPE) to start the oxidative chain scission reactions (reactions 2 through 6). Through these reactions, material properties of the plastic, such as molecular weight, tensile and wear properties, are degraded.
"It has been found that the hydroperoxides (rOOH and POOH) formed in reactions 3 and 5 will slowly break down as shown in reaction 7 to initiate post-radiation degradation. Reactions and 9 represent termination steps of free radicals to form ester or carbon-carbon cross-links. Depending on the type of polymer, the extent of reactions 8 and 9 in relation to reactions 2 through 7 may vary. For irradiated UHMWPE, a value of 0.3 for the ratio of chain scission to cross-linking has been obtained, indicating that even though cross-linking is a dominant mechanism, a significant amount of chain scission occurs in irradiated polyethylene.
"By applying radiation in an inert atmosphere, since there is no oxidant present, the primary free radicals r.cndot. or secondary free radicals P.cndot. can only react with other neighboring free radicals to form carbon-carbon cross-links, according to reactions 10 through 12 below. If all the free radicals react through reactions 10 through 12, there will be no chain scission and there will be no molecular weight degradation. Furthermore, the extent of cross-linking is increased over the original polymer prior to irradiation. On the other hand, if not all the free radicals formed are combined through reactions 10, 11 and 12, then some free radicals will remain in the plastic component.
"In an Inert Atmosphere
"r.cndot.+polyolefin - - - P.cndot. (10)
"2r.cndot. - - - r-r (C-C cross-linking) (11)
"2P.cndot. - - - P-P (C-C cross-linking) (12)
"It is recognized that the fewer the free radicals, the better the polymer retains its physical properties over time. The greater the number of free radicals, the greater the degree of molecular weight and polymer property degradation will occur. Applicant has discovered that the extent of completion of free radical cross-linking reactions is dependent on the reaction rates and the time period given for reaction to occur."
As a supplement to the background information on this patent application, NewsRx correspondents also obtained the inventors' summary information for this patent application: "The present invention relates to a method for providing a polymeric material, such as UHMWPE, with superior oxidation resistance, mechanical strength and wear properties. For the purpose of illustration, UHMWPE will be used as an example to describe the invention. However, all the theories and processes described hereafter should also apply to other polymeric materials such as polypropylene, high density polyethylene, polyhydrocarbons, polyester, nylon, polyurethane, polycarbonates and poly(methylmethcrylate) unless otherwise stated. The method involves using a series of relatively low doses of radiation with an annealing process after each dose.
"As stated above, UHMWPE polymer is very stable and has very good resistance to aggressive media except for strong oxidizing acids. Upon irradiation, free radicals are formed which cause UHMWPE to become activated for chemical reactions and physical changes. Possible chemical reactions include reacting with oxygen, water, body fluids, and other chemical compounds while physical changes include density, crystallinity, color, and other physical properties. In the present invention, the sequential radiation and annealing process greatly improves the physical properties of UHMWPE when compared to applying the same total radiation dose in one step. Furthermore, this process does not employ stabilizers, antioxidants, or any other chemical compounds which may have potentially adverse effects in biomedical or orthopedic applications.
"It is also known that at relatively low dose levels (
"It has been found that polyethylene crystallinity increases continuously with increasing radiation-doses due to chain-scission (approximately 55% before radiation, increasing to 60% at 3.0 MRads, and to 65% at 10 MRads).
"As the crystallinity increases with increasing dose of radiation, more residual free radicals are created and stored in the extra crystalline regions, which makes it increasingly more difficult to eliminate free radicals by annealing below the melt temperature. However, treating above the melting temperature (re-melting) significantly alters the crystallinity and crystal morphology which leads to significant reduction in mechanical properties such as yield strength and ultimate tensile strength and creep resistance and these properties are important for the structural integrity of the implant.
"An orthopedic preformed material such as a rod, bar or compression molded sheet for the subsequent production of a medical implant such as an acetabular or tibial implant with improved wear resistance is made from a polyethylene material cross-linked at least twice by irradiation and thermally treated by annealing after each irradiation. The material is cross-linked by a total radiation dose of from about 2 MRads to 100 MRads and preferably between 5 MRads and 10 MRads. The incremental dose for each irradiation is between about 2 MRads and about 5 MRads. The weight average molecular weight of the material is over 400,000.
"The annealing takes place at a temperature greater than 25.degree. C., preferably between 110.degree. C. and 135.degree. C. but less than the melting point. Generally, the annealing takes place for a time and temperature selected to be at least equivalent to heating the irradiated material at 50.degree. C. for 144 hours as defined by Arrenhius' equation 14. The material is heated for at least about 4 hours and then cooled to room temperature for the subsequent irradiation in the series.
"By limiting the incremental dose to below 5 MRads and preferably below 3.5 MRads and following with annealing, the crystallinity will fluctuate between 55% and 60% (instead of 55-65%) and hence both the amount of chain-scission and residual free-radical concentration can be significantly reduced.
"The polyethylene of the present invention may be in the form of a preformed rod or sheet with a subsequent production of a medical implant with improved wear resistance. The preformed rod or sheet is cross-linked at least twice by irradiation and thermally treated by annealing after each radiation. The incremental dose for each radiation is preferably between about 2 and 5 MRads with the total dose between 2 and 100 MRads and preferably between 5 and 10 MRads.
"After each irradiation, the preformed material is annealed either in air or in an inner atmosphere at a temperature of greater than 25.degree. C. and preferably less than 135.degree. C. or the melting point. Preferably, the annealing takes place for a time and temperature selected to be at least equivalent to heating the irradiated material at 50.degree. C. for 144 hours as defined by Arrenhius' equation (14). Generally, each heat treatment lasts for at least 4 hours and preferably about 8 hours.
"The preformed polyethylene material is then machined into a medical implant or other device. If the irradiation process occurred in air, then the entire outer skin to about 2 mm deep is removed from the preform prior to machining the medical implant or other device. If the process was done in a vacuum or an inner atmosphere such a nitrogen, then the outer skin may be retained.
"The end-results of reduced chain-scission and free-radical concentration are improved mechanical properties, improved oxidation resistance and enhanced wear resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
"FIG. 1 shows the oxidation index profiles of the specimens of Example 8; and
"FIG. 2 shows the oxidation index profiles of the specimens of Example 11."
For additional information on this patent application, see: Wang, Aiguo; Dumbleton, John H.; Essner, Aaron; Yau, Shi-Shen. Sequentially Cross-Linked Polyethylene. Filed
Keywords for this news article include: Alkenes, Chalcogens, Free Radicals,
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