Chemically crosslinked ultrahigh molecular weight polyethylene for artificial human joints

a technology of polyethylene and high molecular weight, applied in the field of polymers, can solve the problems of adhesive wear, many microscopic wear particles into the surrounding tissues, long-term failure of such prostheses, etc., and achieve the effects of reducing the crystallinity of the polymer

Inactive Publication Date: 2008-06-05
SALOVEY RONALD +2
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0014]one aspect of the invention presents a method for reducing the crystallinity of a polymer so that it can better withstand wear. An effective method for reducing the crystallinity of the polymer is by crosslinking. For reduction of crystallinity, the polymer may be irradiated in the melt or, preferably, chemically crosslinked in the molten state. The method is particularly useful for polymer which undergoes irradiation sterilization in the solid state. It is advantageous if the crosslinked polymer is annealed to stabilize its shrinkage.

Problems solved by technology

For example, the process of wear of acetabular cups of UHMW polyethylene in artificial hip joints introduces many microscopic wear particles into the surrounding tissues.
It is generally accepted by orthopaedic surgeons and biomaterials scientists that the reaction of tissue to wear debris is the chief cause of long-term failure of such prostheses.
Adhesive wear occurs when there is local bonding between asperities on the polymer and the opposing (metal or ceramic) counterface.
Abrasive wear occurs when asperities on the surface of the femoral ball, or entrapped third-body particles, penetrate into the softer polyethylene and cut or plow along the surface during sliding.
Cracks may form at the surface and coalesce, releasing wear particles as large as several millimeters and leaving behind a corresponding pit on the surface, or cracks may form a distance below the surface and travel parallel to it, eventually causing sloughing off of large parts of the surface.
There are gaps in the prior art regarding the contributions of the above three basic mechanisms to the wear of polyethylene cups in vivo.
While numerous laboratory studies and analyses of retrieved implants have provided valuable details on wear in vivo, there is ongoing disagreement regarding which wear mechanisms predominate and what are the controlling factors for wear.
However, to date, none of these modifications has been demonstrated to provide a significant reduction in the wear rates of acetabular cups.
Indeed, carbon fiber reinforced polyethylene and a heat-pressed polyethylene have shown relatively poor wear resistance when used as the tibial components of total knee prosthesis.

Method used

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  • Chemically crosslinked ultrahigh molecular weight polyethylene for artificial human joints
  • Chemically crosslinked ultrahigh molecular weight polyethylene for artificial human joints
  • Chemically crosslinked ultrahigh molecular weight polyethylene for artificial human joints

Examples

Experimental program
Comparison scheme
Effect test

example 1

Experimental Details

[0060]Commercial-grade UHMW polyethylene GUR 415 (from Hoechst-Celanese Corporation, League City, Tex.), with a weight average molecular weight of 6×106, was used as received. The peroxide used was 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne (Lupersol 130, Atochem Inc., Philadelphia, Pa.). The reason for choosing Lupersol 130 was its long half-life at elevated temperature. The peroxide will decompose slowly, and the resultant free radicals can diffuse in the specimen to form a homogeneous network at elevated temperatures.

[0061]Mixing of the UHMW polyethylene and the peroxide was accomplished by dispersing polyethylene powder in an acetone solution of the peroxide and subsequently evaporating the solvent {de Boer, J., et al., J. Polym. Sci., Polym. Phys. Ed., 14:187 (1976); de Boer, J. & Pennings, A. J., Makromol. Chem, Rapid Commun., 2:749 (1981) and de Boer, J. & Pennings, A. J., Polymer, 23:1944 (1982)}. The mixed powder (22 g) was poured into the mold cavi...

example 2

Materials and Methods

[0075]In this example, the wear resistance of the polyethylenes treated (modified) and untreated (unmodified) with peroxide in EXAMPLE 2 were tested. The control (unmodified) and modified polyethylenes were compression molded directly into the form of acetabular cups. These were then exposed to an average of approximately 3.4 Mrad of gamma radiation (SteriGenics International, Tustin, Calif.), to simulate the condition of cups that would be used in patients. Due to different amounts of post-molding shrinkage, the internal surface of each cup was machined to provide nearly identical internal diameters and ball-to-cup clearances among the control and modified cups (FIG. 3). As shown in FIG. 3B, the cup's outer radius 1 is 24.5 mm, its inner radius 2 is 16.1 mm, its height 3 is 29.8 mm, and its diameter 4 is 49.0 mm

[0076]The cups were pre-soaked in distilled water for three weeks prior to the wear test to minimize fluid absorption during the wear test. The wear cup...

example 3

[0081]During the wear test in the simulator described in EXAMPLE 2, it was discovered that the acetabular cups shrunk at simulated human body temperature. In order to stabilize the shrinkage, in this experiment (unrelated to EXAMPLE 2), the cups were annealed at 100° C. in a vacuum oven for 2 hours. After annealing, the total shrinkage in diameter for uncrosslinked and crosslinked cups was approximately 1% and 2%, respectively. The degrees of crystallinity of the annealed cups were determined by DSC. The degree of crystallinity of the uncrosslinked polymer was unchanged, whereas that of the crosslinked polymer was increased by approximately 1%. To test for further shrinkage, the cups were again put in the vacuum oven at 80° C. for two hours, and no further shrinkage was observed.

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Abstract

The present invention discloses a method for enhancing the wear-resistance of polymers by crosslinking them, especially before irradiation sterilization. In particular, this invention presents the use of chemically crosslinked ultrahigh molecular weight polyethylene in in vivo implants.

Description

[0001]This is a continuation of co-pending U.S. patent application Ser. No. 10 / 752,167, filed on Jan. 3, 2004, which is a division of co-pending U.S. patent application Ser. No. 10 / 262,869, filed on Oct. 3, 2002, entitled “CHEMICALLY CROSSLINKED ULTRAHIGH MOLECULAR WEIGHT POLYETHYLENE FOR ARTIFICIAL HUMAN JOINTS”, which is a continuation of application Ser. No. 09 / 898,192, filed on Jul. 2, 2001 which is a continuation of application Ser. No. 09 / 406,305, filed on Sep. 27, 1999, and issued as U.S. Pat. No. 6,281,264, which is a continuation of application Ser. No. 08 / 698,638, filed on Aug. 15, 1996 and now abandoned, which is a division of application Ser. No. 08 / 376,953, filed on Jan. 20, 1995 and now abandoned. The entire contents of the parent applications are expressly incorporated herein by reference.TECHNICAL FIELD OF THE INVENTION[0002]The present invention relates to polymers. It discloses a method for enhancing the wear-resistance of polymers, especially polymers that are to ...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): A61F2/30C08F10/02C08J3/28A61F2/00A61L27/00A61F2/32A61F2/34A61F2/36A61F2/46A61L27/16B29B13/08B29C35/08B29C43/00B29C43/16B29C71/00C08F2/46C08J5/00C08J7/00
CPCA61F2/30C08J7/047A61F2/32A61F2/34A61F2/468A61F2002/30233A61F2002/3082A61F2002/30879A61F2002/3611A61F2230/0069A61L27/16B29B13/08B29C43/00B29C43/16B29C71/0063B29C2035/085B29K2023/0683B29K2105/24B29K2995/0087B29K2995/0089B29L2031/7532C08J5/00C08J2323/06A61F2/3094C08L23/06C08J7/0427
Inventor SALOVEY, RONALDMCKELLOP, HARRY A.SHEN, FU-WEN
Owner SALOVEY RONALD
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