Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – High energy or particulate radiation
Reexamination Certificate
2001-05-15
2003-09-30
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
High energy or particulate radiation
C264S322000, C264S494000
Reexamination Certificate
active
06627141
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to improved bearings for use in orthopaedic implant prosthesis and particularly to methods for making ultra high molecular weight polyethylene (hereinafter, UHMWPE) bearings by molding a cross-linked preform by the application of sufficient heat and pressure.
Such UHMWPE resin is commonly used for bearings in hip, knee, shoulder and elbow prostheses. Typically, the bearings may be formed by direct compression molding processes or by machining the required bearing shapes from mill shapes such as sheet or bar stock. Typically, the stock or the molded bearings are irradiated and subsequently heat treated or heat annealed. The irradiation generates molecular cross-links and free radicals. The free radicals are subsequently eliminated by the heat treating processes.
Reference is made to a number of prior art references as follows: U.S. Patents:
1. U.S. Pat. No. 5,414,049, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
2. U.S. Pat. No. 5,449,745, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
3. U.S. Pat. No. 5,543,471, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
4. U.S. Pat. No. 5,650,485, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
5. U.S. Pat. No. 5,728,748, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
6. U.S. Pat. No. 4,586,995, Polymer and Irradiation Treatment Method, to James C. Randall.
7. U.S. Pat. No. 5,153,039, High Density Polyethylene Article with Oxygen Barrier Properties, to Jay P. Porter, et al.
8. U.S. Pat. No. 5,508,319, High Melt Strength, Ethylene Polymer, Process for Making It, and Use Thereof, to Anthony J. DeNicola, Jr. et al.
9. U.S. Pat. No. 3,352,818, Stability of Polyolefines, to Gerhard Meyer, et al.
10. U.S. Pat. No. 5,577,368, Method for Improving Wear Resistance of Polymeric Bioimplantable Components, to John V. Hamilton, et al.
11. U.S. Pat. No. 5,753,182, Method for Reducing the Number of Free Radicals Present in Ultrahigh Molecular Weight Polyethylene Orthopedic Components, to Joel Higgins.
12. U.S. Pat. No. 5,709,020, Method for Reducing the Generation of Wear Particulates From an Implant, to David A. Pienowski, et al.
13. U.S. Pat. No. 5,702,456, Implant Having Reduced Generation of Wear Particulates, to David A. Pienowski.
14. U.S. Pat. No. 5,515,590, Method for Reducing the Generation of Wear Particulates From an Implant, David A. Pienowski.
15. U.S. Pat. No. 5,593,719, Treatments to Reduce Frictional Wear Between Components Made of Ultra-High Molecular Weight Polyethylene and Metal Alloys, Geoffrey Deamaley, et al.
16. U.S. Pat. No. 4,366,618, Bone Connective Prosthesis Adapted to Maximize Strength and Durability of Prostheses—Bone Cement Interface; and Methods of Forming Same, to Simon Raab.
17. U.S. Pat. No. 5,014,494, Method of Sterilizing Medical Articles, to Robert D. George.
18. U.S. Pat. No. 5,137,688, Irradiated Articles Molded From Polycarbonate'Polyamide Blends, to James L. DeRudder.
19. U.S. Pat. No. 5,879,400, Melt-Irradiated Ultra High Molecular Weight Polyethylene Prosthetic Devices, to Edward W. Merrill et al.
20. U.S. Pat. No. 6,017,975, Process for Medical Implant of Cross linked Ultrahigh Molecular Weight Polyethylene Having Improved Balance of Wear Properties and Oxidation, to Kenneth Ashley Saum, et al. Foreign Patents
21. E.P. Patent No. 0722973 A1, Chemically Crosslinked Ultrahigh Molecular Weight Polyethylene for Artificial Human Joints, to Ron Salovey, et al.
22. W.O. Patent No. 97/29793, Radiation and Melt Treated Ultra High Molecular Weight Polyethylene Prosthetic Devices, to W. Merrill, et al.
23. W.O. Patent No. 98/01085, Crosslinking of Polyethylene for Low Wear Using Radiation and Thermal Treatments, to Fu-Wen Shen, et al.
The above references teach the general concepts involved in forming or consolidating UHMWPE resin directly into a part or a stock form from which the part is made, gamma or other irradiation of the part or the stock form and subsequent heat treating (annealing or remelting) of the part or stock form. The disclosures of these above-listed references are incorporated herein for purposes of establishing the nature of UHMWPE resin, the irradiation steps and options and the heat treating steps and options. Applicant also incorporates by reference U.S. application Ser. No. 09/328,080, filed Jun. 8, 1999, titled CROSS-LINKED MOLDED PLASTIC BEARINGS naming Todd Smith and Donald McNulty as coinventors.
SUMMARY OF THE INVENTION
The present invention is directed to a method for molding a bearing from a preform which has previously been cross-linked by obtaining such a preform and placing it in a press mold defining the desired bearing shape or near net shape and applying heat and pressure in the mold to form the bearing. The preform may be produced from irradiated polymer resin to form cross-links and free radicals followed by a molding process as taught by U.S. application Ser. No. 09/328,080, filed Jun. 8, 1999, titled CROSS-LINKED MOLDED PLASTIC BEARINGS naming Todd Smith and Donald McNulty as coinventors incorporated herein. In some embodiments, the preforms may be made from consolidated UHMWPE stock which has been irradiated. Also, in some embodiments, the irradiated UHMWPE preforms may already be heat treated to quench the free radicals.
The present invention is designed to reduce the amount of wear debris generated from UHMWPE bearings. Such wear debris has been associated with bone and soft tissue deterioration leading to implant loosening, which may necessitate even revision surgery. The present invention is also to improve the wear resistance of UHMWPE bearings.
Additional features of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The present invention involves producing a cross-linked orthopaedic implant, such as a bearing component by molding a work piece which has been cross-linked by the application of heat and pressure in a mold defining the shape or near net shape of the bearing. The process of the present invention may be used to mold net-shape bearings directly to provide a finish which is satisfactory for an articular (bearing) surface. The term preform is intended to define a plastic resin work piece that is derived from an unfinished plastic resin stock form, unlimiting examples of which include billets, bar stock, and sheets. It is appreciated that the preform of the present invention may be formed from a wide variety of crude or processed plastic resins suitable for use in orthopaedics, that can be converted by manufacture, processing, or combination into a finished bearing, net-shape bearing, or near net-shape bearing. The term “net-shape” is intended to define a shape that is the final shape of the bearing to be used in the implant. “Near net-shape” requires some degree of machining to produce the final bearing. It is believed that smoother bearing surfaces can be made by molding processes than by machining processes.
The preform of the present invention is formed from a plastic resin stock form that has been irradiated previously to form cross-links. The stock form may be formed from an olefinic resin and preferably an UHMWPE resin. Preferably, the stock form is irradiated with gamma rays at a dose of about 2 to 50 Mrads. This stock form may then be machined into a preform of a size and mass suitable to be placed into a net shape compression molding die. The irradiation process may be performed under vacuum, in an inert atmosphere to eliminate oxygen, or in an oxygen reduced atmosphere. It is also appreciated that the present disclosure contemplates radiating the preform.
The molding process, which may be performed under vacuum or oxygen free atmosphere, may involve pressures from 1,000 to 70,000 psi and set point temperatures of 275° to 500° Fahrenheit. The resulting work piece is either the
McNulty Donald E.
Smith Todd
Barnes & Thornburg
DePuy Orthopaedics, Inc.
Tentoni Leo B.
LandOfFree
Method for molding a cross-linked preform does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for molding a cross-linked preform, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for molding a cross-linked preform will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3004063