Crosslinking of polyethylene for low wear using radiation...

Details Crosslinking of polyethylene for low wear using radiation... Crosslinking of polyethylene for low wear using radiation...

1999-01-06

2001-05-08

Berman, Susan W. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C522S154000, C522S157000, C522S161000, C522S163000, C522S164000, C523S113000, C523S115000, C525S937000, C526S352000, C526S351000, C623S018110, C623S019140, C623S020230, C623S021180, C623S022290, C623S023480

Reexamination Certificate

active

06228900

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to polymers. It discloses methods for enhancing the wear-resistance of polymers by crosslinking 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 these particles includes inflammation and deterioration of the tissues, particularly the bone to which the prostheis is anchored. Eventually, the prosthesis becomes painfully 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. Consequently, numerous modifications in physical properties of UHMWPE have been proposed to improve its wear resistance.
UHMWPE components are known to undergo a spontaneous, post-fabrication increase in crystallinity and changes in other physical properties. {See e.g., Rimnac, C. M., et al.,
J. Bone & Joint Surgery,
76-A(7):1052-1056 (1994)}. These chances occur even in stored (non-implanted) cups after sterilization with gamma radiation, which initiates an ongoing process of chain scission, crosslinking, 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 crystallinity, 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 with carbon fibers; and (b) post-processing treatments such as solid phase compression molding. 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. {See e.g., Rimnac, C. M., et al.,
Trans. Orthopaedic Research Society,
17:330 (1992)}.
Recently, several companies have modified the 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 crosslinking the polymer, followed by thermally treating the crosslinked polymer. Non-limiting examples of the thermal treatments are remelting or annealing. Preferably, the polymer is crosslinked 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 crosslinked and thermally treated polymer, which is the most oxidized and least crosslinked part of the polymer, is removed, e.g., in the process of machining the final product out of the irradiated bar and thermally treated bar or block. The radiation dose is also preferably adjusted so that the optimal dose occurs within 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 systematic 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 vivo, and applying this optimal balance to determine the appropriate crosslinking and thermal treatment conditions for processing a polymer. A flowchart is provided as a non-limiting illustration of the method for determining the optimal balance. Also provided are methods for treating polymers which apply the above appropriate crosslinking and thermal treatment conditions; the polymers produced by these methods; methods for making products (e.g., in vivo implants) from these polymers; and the products (e.g.,in vivo implants) made from these polymers.


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