BACKGROUND OF THE INVENTION
The present invention relates to polymeric compositions and methods of making the same for use in fabricating prosthetic medical devices, as well as prosthetic devices made at least partially therefrom with improved wear resistance.
Many prosthetic medical devices are implanted into load-bearing joints such as knees, hips, etc. As such, these prosthetic devices must be very strong and possess a high degree of wear resistance. Presently, the prosthetic medical device industry has utilized various metals and polymers and combinations thereof to fabricate prosthetic devices. Unfortunately, both metals and polymers have drawbacks. For example, metals such as stainless steel, tungsten and titanium, and alloys thereof, may succumb to the corrosive environment of the body and eventually begin to wear. Such wear may result in fine metallic particles being scraped away from the contact surface of the device and into surrounding tissue and bone which may potentially cause pathogenic problems. Polymers, such as polyethylene, polypropylene and nylons may also exhibit wear and may consequently produce particles which diffuse into tissue and bone. Both metallic and polymeric particles shed from these prosthetic medical devices are of concern because they may be inherently reactive with the tissue and bone they contact, thus possibly causing tissue degradation or necrosis.
Various methods have been devised attempting to reduce the wear rate of the load bearing prosthetic medical devices. For polymers, a common practice within the prosthetic medical device industry is to use cross-linked polymers and resins to form the medical device. Polymers are commonly cross-linked by chemical catalysis or irradiation exposure. Most cross-linking methodologies do result in greater wear resistance. However, indiscriminate or uncontrolled cross-linking may result in the formation of a weakened polymeric matrix, not capable of withstanding the enormous pressures placed on the devices in the patient resulting in degradative wear as described above.
Another difficulty conventionally encountered in the manufacturing process of prosthetic medical devices is that they cannot be formed by inexpensive injection molding techniques. Instead, these medical devices must be formed by extrusion, for example, which requires further machining into the finished article. Injection molding, on-the-other-hand, allows for the final article to be formed in virtually one step.
Therefore, a need exists within the prosthetic medical device industry to fabricate an improved polymeric prosthetic device possessing sufficient strength to withstand the stress and pressure imposed on it, yet resist wear so that foreign particles liberated from the prosthetic device do not cause health problems to the patient. There also exists a need to fabricate the devices inexpensively by injection molding. The present invention provides compositions, as well as methods of improving the wear resistance of prosthetic medical devices, by selectively cross-linking a polymeric resin using a controlled cross-linking process providing optimum strength and wear resistance, thus diminishing or eliminating the frequency by which foreign particles are liberated from the implanted prosthesis, thereby reducing the risk of compromising the patient's health. The present invention also provides compositions and methods of injection molding prosthetic medical devices thus rendering a less expensive, and more facile prosthetic medical device fabrication process.
SUMMARY OF THE INVENTION
It has been discovered that by selectively cross-linking components of a polymeric matrix used for prosthetic medical devices, a device can be fabricated that possesses the required strength and wear resistance and thereby avoids or reduces the level of polymeric material liberated from the device.
One aspect of the present invention provides for a polymeric composition containing mixtures of cross-linked and non-crosslinked polyolefinic resins blended together and ultimately formed into cured polymeric articles. Preferably, the resulting composition and fabricated article made from the blended polymeric material of the present invention contains cross-linked, linear and branched polyolefinic resins. The physical properties of the resulting compositions of the present invention have been found to exhibit an unexpectedly high degree of wear resistance and strength. As such, a preferred use of the composition of the present invention may be for prosthetic joints or components for devices for shoulders, elbows, ankles, wrists, fingers, jaws, hips, knees, vertebra, and other load-bearing orthopedic prosthetic medical devices. Other preferred prosthetic medical devices fabricated from the composition of the present invention include such articles as syringes, catheters and surgical implements requiring a high degree of wear resistance.
In another aspect of the present invention, a method of producing the polymeric composition is provided. In other aspects of the invention, methods for fabricating prosthetic medical devices made from the polymeric composition are provided.
A preferred embodiment of the present invention seeks to achieve a desirable balance of wear resistance and high tensile strength and toughness. A desirable balance is achieved by virtue of combining, in an integrated matrix, cross-linked and non-crosslinked polyolefinic polymers and resins. Once blended, the present invention provides for the mixture to be formed in any suitable manner or otherwise made into prosthetic medical devices. The finished articles preferably can then be processed and packaged for use alone or as components of prosthetic medical devices.
It has been discovered that certain compositions of the present invention allow for the blended polymeric mixture to be injection molded. This is possible because selected mixture combinations exhibit theological properties and characteristics which are amenable to injection molding. Such mixtures exhibit a relatively low viscosity providing a flowable liquid to be fed into injection molding equipment.
Another aspect of the present invention provides for the use of polyolefinic polymers and resins. Within the context of the present invention, a polymer is defined as an organic compound having repeating units of similar or different monomers. A resin is defined herein as a partially cured polymer having utility as a moldable material suitable for curing into a solid article. The polymers and resins of the present invention have molecular weights ranging from between 1,000 to 10,000,000. While the invention preferably uses polyolefinic polymers or resins, any polymer capable of being formed into, and used as, prosthetic devices may be used. Preferably, examples of such polyolefinic materials may be polyethylene (PE), polypropylene (PP), high molecular weight polypropylene (HMWPP), high molecular weight polyethylene (HMWPE), ultra high molecular weight polyethylene (UHMWPE) and ultra high molecular weight polypropylene (UHMWPP), high density polyethylene (HDPE), low density polyethylene (LDPE), high density polypropylene (HDPP) and low density polypropylene (LDPP). Other polymers and resins of the present invention may be polysilanes, polyurethanes, polyethers, polyamides, polyesters, polyalkyl acrylates, nylon, rubber and epoxy resins. It should be understood that the above list of polymers is not exhaustive, and other polymers may also be employed in the present invention.
A further aspect of the invention provides for the use of mixtures of polymers and resins, both cross-linked and non-crosslinked varieties, to form a single blended matrix. It is also emphasized that not every polymer or resin component of the present invention need participate in, or be responsible for, the structural integrity or physical characteristics of the resulting prosthetic medical device, but could also serve to improve processing and handling manipulations performed on the raw materials, intermediate articles and workpieces, as well as the finished devices.
Another aspect of the invention provides for the use of lubricants, dyes, stabilizers and other processing compounds to be incorporated into the polymeric matrix. These compounds enhance the polymeric mixture's manufacturing properties but do not necessarily contribute to the structural integrity of the final matrix.
In another aspect of the present invention, solid materials may be incorporated into the polymer or resin mixtures. Such solid materials may be, for example, chopped carbon or glass fiber or nanotubes, carbon black, graphite powder, talc, mica, polyamide fiber and other fillers commonly used in the polymer industry.
In another aspect of the invention, a process is provided whereby polyolefinic polymers or resins are sealed in a container preferably purged of most or all oxygen and filled with an inert atmosphere such as nitrogen. Preferably, a powdered form of the polyolefinic polymer resin is irradiated to effect a certain degree of cross-linking to the polyolefinic polymer or resin. Other cross-linking methods may also be utilized, such as those employed in U.S. Pat. Nos. 5,728,748, 5,650,485, 5,449,745, 5,414,049, 5,153,039, 5,160,464, 5,037,928 and U.S. Provisional Application No. 60/130,322, each of which is incorporated herein as if fully set forth. The irradiated material, now possessing a certain degree of cross-linking, is ready to be blended into a polymeric mixture, and processed into a prosthetic device. Optionally, the present invention also provides for further irradiation of the finished article or workpiece. Such subsequent irradiation may be necessary or desirable for further strengthening or sterilization of the finished article or workpiece.
In another aspect of the present invention the irradiated polyolefinic polymer or resin is blended with non-irradiated polyolefinic polymer or resin into a mixture which is then preferably processed and cured into either a finished article or unfinished stock article. The processed polymeric mixture may also be rendered as a powder or pellet, for example.