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Antioxidant stabilized crosslinked ultra-high molecular weight polyethylene for medical device applications

Antioxidant stabilized crosslinked ultra-high molecular weight polyethylene for medical device applications description/claims

This application claims the benefit under Title 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 60/922,738, entitled AN ANTIOXIDANT STABILIZED CROSSLINKED ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE FOR MEDICAL DEVICE APPLICATIONS, filed on Apr. 10, 2007, the entire disclosure of which is expressly incorporated by reference herein.

1. Field of the Invention.

The present invention relates to crosslinked ultra-high molecular weight polyethylene and, particularly, to antioxidant stabilized, crosslinked ultra-high molecular weight polyethylene.

2. Description of the Related Art.

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

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

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

The present invention relates to a crosslinked UHMWPE and, particularly, an antioxidant stabilized, crosslinked UHMWPE. In one exemplary embodiment, an antioxidant is combined with UHMWPE prior to subjecting the UHMWPE to crosslinking irradiation. In one exemplary embodiment, the antioxidant is tocopherol. After the antioxidant is combined with the UHMWPE, the resulting blend may be formed into slabs, bar stock, and/or incorporated into a substrate, such as a metal, for example. The resulting product may then be subjected to crosslinking irradiation. In one exemplary embodiment, the UHMWPE blend is preheated prior to subjecting the same to crosslinking irradiation. Once irradiated, the UHMWPE blended product may be machined, packaged, and sterilized in accordance with conventional techniques.

In one exemplary embodiment, the formed UHMWPE/antioxidant blend may be subjected to multiple passes of crosslinking irradiation. By irradiating the blend in multiple passes, the maximum dose of radiation received by the UHMWPE blend at any one time is lessened. As a result, the maximum temperature of the UHMWPE blend reached during irradiation is correspondingly lessened. This allows for the UHMWPE to maintain a higher level of desirable mechanical properties and prevents substantial melting of the UHMWPE. In one exemplary embodiment, the UHMWPE is cooled after each individual pass of crosslinking irradiation. By allowing the UHMWPE blend to cool, the temperature at the time of subsequent irradiation is high enough to encourage the mobility of the individual polyethylene molecules, but is also low enough that the temperature increase experienced during irradiation is unlikely to substantially alter any desired material properties of the UHMWPE blend.

Advantageously, by incorporating an antioxidant, such as tocopherol, into the UHMWPE prior to subjecting the same to crosslinking irradiation, the UHMWPE may be stabilized without the need for post irradiation melt annealing or any other post-irradiation treatment to quench free radicals. Specifically, an antioxidant, such as tocopherol, acts as a free radical scavenger and, in particular, acts as an electron donor to stabilize free radicals. While tocopherol itself then becomes a free radical, tocopherol is a stable, substantially unreactive free radical. Additionally, because of the substantially reduced level of oxidation that occurs using a UHMWPE/antioxidant blend, the amount of oxidized material that must be removed to form a final, implantable medical component is reduced. As a result, the size of the stock material subjected to irradiation may be smaller in dimension, making it easier to handle and easier to manufacture into final medical components.

Moreover, by subjecting the UHMWPE/antioxidant blend to multiple passes of irradiation, the UHMWPE blend may be integrally incorporated onto a substrate prior to irradiation. Specifically, as a result of separating the total radiation dose into a plurality of individual passes, the temperature of the UHMWPE blend at the UHMWPE/substrate interface remains low enough that separation of the UHMWPE blend and the substrate is substantially prevented. Further, even after irradiation, some antioxidant remains unreacted within the UHMWPE blend, which may continue to quench free radicals throughout the lifetime of the medical component. Thus, even after the medical component is implanted, the antioxidant may continue to quench free radicals and further reduce the likelihood of additional oxidation.

In one form thereof, the present invention provides method for processing UHMWPE for use in medical applications, the method including the steps of: combining UHMWPE with an antioxidant to form a blend having 0.01 to 3.0 weight percent of the antioxidant, the UHMWPE having a melting point; processing the blend to consolidate, the consolidated blend having a melting point; preheating the consolidated blend to a preheat temperature below the melting point of the consolidated blend; and irradiating the consolidated blend while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend

In another form thereof, the present invention provides a crosslinked UHMWPE blend for use in medical implants prepared by a process including the steps of: combining UHMWPE with an antioxidant to form a blend having 0.1 to 3.0 weight percent antioxidant; processing the blend to consolidate the blend, the consolidated blend having a melting point; preheating the consolidated blend to a preheat temperature below the melting point of the consolidated blend; and irradiating the consolidated blend with a total irradiation dose of at least 100 kGy while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend.

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

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