METHOD FOR COMPRESSION MOLDING A CROSS-LINKED PREFORM

Abstract: A method for forming a plastic prosthesis bearing, net-shape bearing or near net-shape bearing is provided. The method includes providing a cross-linked plastic resin preform in a volume sufficient to accommodate the bearing and molding the cross-linked preform. The molding is accomplished by applying heat and pressure to form the preform into a desired solid plastic shape. The heating and pressure is sufficient to melt the plastic resin so that the plastic resin flows under pressure and quench remaining free-radicals.

Description: METHOD FOR COMPRESSION MOLDING A CROSS-LINKED PREFORM 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. Patent No. 5.414. 049. Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun. et al. 2. U.S. Patent No. 5,449, 745, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al. 3. U.S. Patent No. 5.543.471. Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun. et al. 4. U.S. Patent No. 5.650.485, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun. et al. 5. U.S. Patent No. 5.728.748. Non-Oxidizing Polvmeric Medical Implant, to Deh-Chuan Sun. et al. 6. U.S. Patent No. 4.586.995. Polvmer and Irradiation Treatment Method, to James C. Randall. 7. U.S. Patent No. 5.153,039, High Density Polvethylene Article with Oxygen Barrier Properties, to Jay P. Porter, et al. 8. U.S. Patent No. 5.508.319, High Melt Strength. Ethylene Polvmer. Process for Making It. and Use Thereof, to Anthonv J. DeNicola, Jr. et al. 9. U.S. Patent No. 3,352,818, Stability of Polvolefines, to Gerhard Meyer, et al. 10. U.S. Patent No. 5,577,368, Method for Improving Wear Resistance of Polymeric Bioimplantable Components, to John V. Hamilton, et al. 11. U.S. Patent 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. Patent No. 5,709,020, Method for Reducing the Generation of Wear Parti culates From an Implant, to David A. Pienowski, et al. 13. U.S. Patent No. 5.702.456, Implant Having Reduced Generation of Wear Particulates, to David A. Pienowski. 14. U.S. Patent No. 5,515,590, Method for Reducing the Generation of Wear Particulates From an Implant, David A. Pienowski. 15. U.S. Patent No. 5,593,719, Treatments to Reduce Frictional Wear Between Components Made of Ultra-High Molecular Weight Polyethylene and Metal Alloys, Geoffrey Dearnaley, et al. 16. U.S. Patent 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. Patent No. 5,014,494, Method of Sterilizing Medical Articles, to Robert D. George. 18. U.S. Patent No. 5,137,688, Irradiated Articles Molded From Polycarbonate - Polyamide Blends, to James L. DeRudder. 19. U.S. Patent No. 5,879,400, Melt-Irradiated Ultra High Molecular Weight Polyethylene Prosthetic Devices, to Edward W. Merrill et al. 20. U.S. Application No. 08/911,792, Process for Medical Implant of Cross-linked Ultrahigh Molecular Weight Polyethylene Having Improved Balance of Wear Properties and Oxidation, filed August 15, 1997, to Kenneth Ashley Saum, et al. Foreign Patents 21. E.P. Patent No. 0722973 Al, 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 a U.S. application filed simultaneously herewith 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 the U.S. application filed simultaneously herewith 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 Embodiments) 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 actual desired bearing configuration or blank that can be machined subsequently to produce the desired bearing geometry. It will be appreciated, however, that acceptable results may be achieved when either or both the irradiation process and the molding process are carried out under atmospheric conditions, i.e., with some oxygen present. This may well be the case for certain bearing configurations. One embodiment of the present invention relates to a method for forming a plastic prosthesis bearing, net-shape bearing, or near net-shape bearing. The method comprises the steps of providing cross-linked plastic resin preform in a . volume sufficient to accommodate the bearing and molding the cross-linked preform by applying heat and pressure to form the preform into a desired solid plastic shape. The heating is sufficient to melt the plastic resin preform so that it will flow under pressure. The providing step may comprise cross-linking a plastic resin stock form by subjecting it to ionizing radiation (about 2 to 50 Mrads, preferably about 5 to 10 Mrads) such as gamma radiation. Preferably, the bar stock is stored in a vacuum, oxygen-free container or low oxygen environment or heat processed at or above the melting point of the polymer to quench remaining free-radicals before it is cut into a preform shape. This preform may or may not be similar to the final net shape part. It need only be of a shape and size that will fit into the dies of compression molding equipment. The preform is placed into the compression dies. The molding chamber is preferably brought under vacuum and then subjected to heat (about 275 °F. to 500 °F, preferably about 400 °F) and pressure (about 1000 to 70,000 psi, more preferably about 1000 to about 2500 psi, and most preferably about 2500 psi). The heat, pressure, and molding time are such as to force the preform to conform to the shape of the dies. Additionally, in the instance where the preform contains free radicals, the preform is held at temperature for" sufficient time to allow quenching of the free radicals by forming more molecular chain cross-links. The press is then cooled to below the polymer melting point and the net shape bearing is removed from the compression molding press. A more complete understanding of the present invention can be obtained by referring to the following illustrative examples of the practice of the invention, which examples are not intended, however, to be unduly limitative of the invention. EXAMPLE 1 A ram extruded bar stock (PolyHi, Ft. Wayne, IN) of UHMWPE polymer resin (GUR 1050 from Ticona, Frankfurt Germany) was acquired. The bar stock was placed into a foil bag and placed under vacuum for a period of time sufficient to produce a substantially oxygen free environment. The foil bag and bar stock were then irradiated with gamma rays at a dose of about 5 to 10 Mrads. The free-radical containing bar stock was removed from the bag and machined to a preform of a size and mass suitable to be placed into a net shape compression molding die. The resulting machined preform was placed into a die cavity of the molding press. The cavity with the machined preform was evacuated to remove oxygen. About 2500 psi pressure was then applied and the temperature was raised to about 400 °F. for a time of about 40 minutes to quench free radicals. The mold was allowed to cool to room temperature where the resulting net shape molded part was removed from the die cavity. EXAMPLE 2 A ram extruded bar stock (PolyHi, Ft. Wayne, IN) of UHMWPE polymer resin (GUR 1050 from Ticona, Frankfurt Germany) was acquired. The bar stock was placed into a foil bag and placed under vacuum for a period of time sufficient to produce a substantially oxygen free environment. The foil bag and bar stock were then irradiated with gamma rays at a dose of about 5 to 10 Mrads. The free radical containing bar was removed from the bag and placed into a vacuum oven which was subsequently brought under vacuum. To quench substantially all free radicals, the temperature of the vacuum oven was raised to above the melting point of the bar stock, (about 310 °F) for about 24 hours and then brought to room temperature. The bar stock was then machined to form a preform having a size and mass suitable to be placed into a net shape compression molding die. The machined preform was placed into a die cavity of the molding die. The cavity with the machined preformed was evacuated to remove oxygen. About 2500 psi pressure was then applied and the temperature was raised to about 400 °F. for a time of about 40 minutes. The mold was allowed to cool to room temperature where the resulting net shape molded part was removed from the die cavity. It is thought that the method for forming a plastic prosthesis bearing, net-shape bearing or near net-shape bearing by the present invention and many of its • attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.

Claims: WHAT IS CLAIMED IS: 1. A method for forming a plastic prosthesis bearing, net-shape bearing or near net-shape bearing, the method comprising the steps of providing cross-linked plastic resin preform in a volume sufficient to 5 accommodate the bearing, and molding the cross-linked preform by applying heat and pressure to form the preform into a desired solid plastic shape, the heating being sufficient to melt the plastic resin so that it will flow under pressure. 2. The method of claim 1 in which the molding step involves 10 pressure from 1 ,000 to 70,000 psi. 3. The method of claim 2 in which the molding step involves pressure of about 2500 psi. 4. The method of claim 3 in which the molding step involves increasing the temperature to about 400 °F. 15 5. The method of claim 3 in which the molding step involves maintaining the increased temperature for a time of about 40 minutes. 6. The method of claim 1 in which the molding step is at a temperature of 275° F to 500° F. 7. The method of claim 6 in which the molding step is at a 20 pressure of about 2500 psi. 8. The method of claim 6 in which the molding step is at a temperature of about 400 °F. 9. The method of claim 8 in which the molding step involves maintaining the temperature of about 400 °F for a time of about 40 minutes. 25 10. The method of claim 1 in which the molding step involves transferring the preform to a molding press where the heat and pressure are applied, the molding press comprising a mold cavity in the shape of the desire net-shape or near net-shape bearing. 11. The method of claim 10 in which the molding step is at a '30 pressure of about 2500 psi. 12. The method of claim 10 in which the molding step involves a temperature greater than the melting point of the resin. 13. The method of claim 12 in which the molding step involves maintaining the temperature at or above the melting point for a time of about 40 minutes. 14. The method of claim 13 in which the molding step is at a temperature of abo'ut 400 ° F. 15. The method of claim 1 in which the plastic resin is an olefinic plastic resin. 16. The method of claim 1 in which the plastic resin is an ultra high molecular weight polyethylene. 17. The method of claim 1 further comprising the step of raising the temperature of the preform to above its melting point before the molding step. 18. The method of claim 17 in which the raising step involves a temperature of at least 310 °F. 19. The method of claim 1 further comprising the step of raising the temperature of the preform to above its melting point for about 24 hours before the molding step. 20. A method for forming a plastic prosthesis bearing, net-shape bearing or near net-shape bearing, the method comprising the steps of providing plastic resin in the form of a cross-linked preform, heating the preform within a mold to a temperature at or above the melting point of the resin and applying pressure for a time sufficient to form the preform into a desired solid plastic shape. 21. The method of claim 20 in which the pressure applying step ' involves pressure from 1,000 to 70,000 psi. 22. The method of claim 21 in which the pressure applying step involves pressure of about 2500 psi. 23. The method of claim 20 in which the pressure applying step involves maintaining the temperature at or above the melting point for about 40 minutes. 24. The method of claim 20 in which the heating step is at a temperature of 275° F to 500° F. 25. The method of claim 24 in which the heating step involves a temperature of about 400 °F. 26. The method of claim 20 further comprising the step of raising the temperature of the preform to about 310 °F before heating the preform in the mold. 27. The method of claim 26 in which the raising step includes the steps of maintaining the temperature for about 24 hours and cooling the preform. 28. A method for producing a net-shape or near net-shape ultra high molecular weight polyethylene prosthesis bearing comprising the steps of: obtaining a preform of cross-linked ultra high molecular weight polyethylene resin of sufficient volume to provide the net-shape or near net-shaped bearing, placing the preform in a press-mold defining the desired net-shape or near net-shape, operating the press mold to apply sufficient heat to cause the resin to flow under pressure and to apply sufficient pressure to form the net-shape or near net- shape bearing. 29. The method of claim 28 further comprising the step of raising the temperature of the preform to above its melting point before placing the preform in the press-mold. 30. The method of claim 29 in which the raising step involves a temperature of about 310 °F. 31. The method of claim 29 in which the raising step involves maintaining the temperature for about 24 hours.

Inventors: MCNULTY DONALD, SMITH TODD, MCNULTY, DONALD, SMITH, TODD
Applicants: DEPUY ORTHOPAEDICS INC, DEPUY ORTHOPAEDICS, INC.
Application year: 2000
Main classification: B29C43
IPC codes: A61F2, B29B11, B29C35, B29C43