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Non-oxidizing polymeric medical implant

a medical implant and non-oxidizing technology, applied in the field of medical implants, can solve the problems of anti-oxidation, employ stabilizers, etc., and achieve the effect of preventing further oxidation and superior oxidation resistan

Inactive Publication Date: 2005-03-17
HOWMEDICA OSTEONICS CORP
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

As stated above, while UHMWPE polymer is very stable and has very good resistance to aggressive media except for strong oxidizing acids. Upon sterilization radiation, 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 a new sterilization radiation process greatly reduces the adverse effects caused by a conventional radiation process. Furthermore, this new sterilization process does not employ stabilizers, antioxidants, or any other chemical compounds which may have potential adverse effects in biomedical or orthopedic applications.
It is therefore an object of the invention to provide a polymeric orthopedic implant having superior oxidation resistance after irradiation.
It is still another object of the invention to provide a method for manufacturing such an implant from the resin powder thereof through the final sterilization step so that the implant may thereafter be exposed to air without degradation due to oxidation.
These and other objects are achieved by a method for producing a polymeric medical implant including the steps of placing the polymeric resin in a sealed container and removing a substantial portion of the oxygen from the container. After a predetermined time, the container is repressurized with an inert gas such as nitrogen, argon, helium or neon. The resin is thereafter transferred to a forming device which normally melts and forms the resin in an oxygen reduced atmosphere to produce a polymeric raw material. The polymeric raw material, such as UHMWPE is then machined to an implant such as a tibial tray or a liner for an acetabular cup. The finished part is then sealed into a package in an oxygen reduced atmosphere. The package is of an airtight nature to prevent oxygen or moisture from entering after the package is sealed. The then packaged implant is radiation sterilized and then heat treated for the predetermined time and temperature sufficient to form cross-links between free radicals of the neighboring polymeric chains. This prevents further oxidation once the implant is removed from the package.

Problems solved by technology

Furthermore, this new sterilization process does not employ stabilizers, antioxidants, or any other chemical compounds which may have potential adverse effects in biomedical or orthopedic applications.

Method used

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  • Non-oxidizing polymeric medical implant

Examples

Experimental program
Comparison scheme
Effect test

example 1

Two sets of 1-mm-thick UHMWPE sheets prepared by Methods A through D above were oven aged in air at 80° C. for 11 and 23 days respectively. After these sheets were cooled in room temperature, a thin film specimen of about 100 microns in thickness was cut from each of the 1-mm-thick aged UHMWPE sheets and placed in an IR window for a standard FTIR (A Nicolet 710 FTIR system was used) transmission run. A total of 32 spectra (scans) was collected and averaged. To determine the extent of oxidation, the IR absorption peaks in the frequency range of between 1660 and 1800 cm−1, corresponding to carbonyl (C—O) functional groups, were integrated for the peak area. The peak area is proportional to the amount of oxidized UHMWPE in the specimen. To correct for difference in specimen thickness, the integrated peak area was then normalized to the specimen thickness, by dividing by the area of the 1463 cm−1 (methyl) peak which is proportional to the specimen thickness. The obtained ratio was defi...

example 2

Two sets of 1-mm-thick UHMWPE sheets prepared by Methods B through D cited in the Sample Preparation were oven aged in air at 80° C. for 11 and 23 days respectively. After these sheets were cooled in room temperature, six tensile specimens with a dumbbell shape according to ASTM D638 (Type IV) were cut from each of the 1-mm-thick aged UHMWPE sheets. A standard tensile test was performed for each specimen at a speed of 2 inches / mm. Another set of 1-mm-thick UHMWPE sheets prepared by Methods B through D cited in the Sample Preparation, but without oven aging, were also evaluated by the same tensile test method for comparison. Tensile breaking strength results (average of six tests for each condition) are shown in Table 2:

TABLE 2Tensile BreakingSampleStrength, psiMethod B / not oven aged6510Method B / 11 day oven aging5227Method B / 23 day oven aging3192Method C / not oven aged6875Method C / 11 day oven aging6400Method C / 23 day oven aging6004Method D / not oven aged6941Method D / 11 day oven agin...

example 3

Two sets of 1-mm-thick UHMWPE sheets prepared by Methods B and Method D cited in the Sample Preparation were oven aged in air at 80° C. for 11 and 23 days respectively. After these sheets were cooled in room temperature, samples cut from sheets were characterized by a high temperature gel permeation chromatograph (GPC) column for molecular weight distribution. The samples were dissolved in hot trichlorobenzene (TCB). They were then run in the aforementioned solvent at 1.2 ml / mm. using a Jordi Gel Mixed Bed Column, 50 cm×10.0 mm ID., at a column oven temperature of 145° C. on the Waters 15° C. Chromatograph. The injection size was 250 uL of a 0.1% solution. An antioxidant (N-phenyl-2-naphthylamine) was added to all high temperature GPC samples to prevent polymer deterioration.

Prior to sample runs, the column was calibrated using narrow MW polystyrene standards. Since the samples were only partially soluble in the solvent due to cross-linking, the so-determined molecular weight dis...

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Abstract

A medical implant made of polymeric material having an increased oxidation resistance is formed by a method including the steps of placing a resin powder in a sealed container. A substantial portion of the oxygen is removed from the sealed contained by either a vacuum, an oxygen absorbent or by flushing with inert gas. The container is then repressurized with a gas such as nitrogen, argon, helium or neon so that long term storage may be possible. On use, the resin in transferred to a forming device which both melts and forms the resin in an oxygen reduced atmosphere to produce a polymeric raw material such as a rod or bar stock. The medical implant is then formed from this raw material annealed and sealed in an airtight package in an oxygen reduced atmosphere. The implant is then radiation sterilized and thereafter annealed in the package for a predetermined time and temperature sufficient to form cross-links between any free radicals in neighboring polymeric chains.

Description

BACKGROUND OF THE INVENTION This invention relates to medical implants formed of a polymeric material such as ultra-high molecular weight polyethylene, with superior oxidation resistance upon irradiation and a method for making the same. 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 and hip replacements. Ultra-high molecular weight polyethylene (UHMWPE) has been defined as those linear polyethylenes which have a relative viscosity of 2.3 or greater at a solution concentration of 0.05% at 135° C. in decahydronaphthalene. The nominal weight—average molecular weight is at least 400,000 and up to 10,000,000 and usually from three to six million. The manufacturing process begins with the polymer being supplied as fine powder which is consolidated into various forms, such as rods a...

Claims

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Application Information

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IPC IPC(8): A61L2/08A61L27/00A61L15/60A61L27/16A61L29/04A61L31/00A61L31/04A61L33/00B29C35/08B29C43/00B29C43/16C08F2/46
CPCA61L2/08Y10S522/915A61L2/082A61L2/087A61L15/60A61L27/16A61L29/041A61L31/048B29C35/0805B29C43/00B29C43/16B29C2035/085B29K2023/0683B29K2105/253B29K2995/0087B29K2995/0089B29L2031/7532A61L2/081Y10S525/937Y10S623/901C08L23/06C08L23/04
Inventor SUN, DEH-CHUANSTARK, CASPER F.
Owner HOWMEDICA OSTEONICS CORP
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