Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-12-27
2002-09-10
Sanders, Kriellion A. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S424000
Reexamination Certificate
active
06448315
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for the preparation of UHMWPE doped with an antioxidant, preferably vitamin E.
The invention also relates to an implant comprising UHMWPE doped with an antioxidant by using said method.
BACKGROUND ART
The majority of endoprosthetic joint replacements currently implanted in patients comprises a highly polished metal or ceramic component articulating on ultra high molecular weight polyethylene (UHMWPE) material. Although such combination of materials has been used over the last 30 years, the present clinical practice of using said prostheses in an increasing number of younger patients and older patients with longer life expectancy has generated renewed concern about the wear and durability of UHMWPE.
UHMWPE is a semicrystalline, linear homopolymer of ethylene, which is produced by stereospecific polymerization with Ziegler-Natta catalyst at low pressure (6-8 bar) and low temperature (66-80 C.). The synthesis of nascent UHMWPE results in a fine granular powder. The molecular weight and its distribution can be controlled by process parameters such as temperature, time and pressure. High wear resistance, high abrasion resistance, low coefficient of friction, high impact strength, excellent toughness, low density, biocompatibility and biostability are some of the properties that make UHMWPE an excellent material for implants. However, because of difficulties in fabrication by conventional techniques such as extrusion, injection molding or calendering, the use of UHMWPE has been limited. Due to the high molecular weight thereof, UHMWPE has a very high melt viscosity rendering it difficult or impractical to fabricate by the conventional melt processing techniques.
The major concern relating to the long term clinical performance of UHMWPE implants is adverse tissue reactions caused by the generation of UHMWPE debris. The particles of UHMWPE debris generated at the articulating surface are transported to the hard and soft tissues surrounding the joint. Billions of submicron particles are shed into the joint space leading to an inflammatory reaction with increased amount of joint fluid and pressure. Certain UHMWPE particles cause a macrophage mediated inflammatory response leading to bone resorption. When present around the neck of the femoral component, this type of bone resorption is a major cause for implant loosening, patient pain and the need for revision surgery.
In an effort to determine the cause of accelerated wear rates and premature component failures, studies have been conducted on material variables involved in component fabrication and utilization. Deformation and loosening of implants brought on by excessive wear rates has been correlated to &ggr;-irradiation sterilization in air causing chain scission, which lowers the wear resistance and accelerates the degradation process.
Sterilization by &ggr;-irradiation has been the method of choice for implants since about 1980. However, &ggr;-sterilization generates free radicals, which react in the presence of oxygen to almost exclusively form peroxyl radicals.
These free radicals and peroxyl radicals react with PE chains and each other to form oxidative degradation products and additional radical species. This cycle of oxidation product and radical species formation has been shown to occur over several years as oxidation levels continuously increase in components over this time period. The resulting formation of chain scission products creates shorter molecular chains, degrading the mechanical properties and performance of UHMWPE implants. Furthermore, the damage caused by &ggr;-irradiation does not require the implant to be exposed to levels of stress found in use. Instead, oxidative degradation of &ggr;-irradiated components may occur during storage prior to implantation.
A recognized method of minimizing the wear rate while avoiding long-term oxidation of UHMWPE implants is to induce controlled amounts of crosslinking in the UHMWPE either by chemical or radiation techniques. Studies of the effects of high dose &ggr;-irradiation of UHMWPE have been performed by H. McKellop, et al, at the J. Vernon Luck Orthopaedic Research Center, Orthopaedic Hospital, presented on the 44
th
Annual Meeting., Orthopaedic Research Society, in Mar. 16-19, 1998, New Orleans, La., USA. Extruded bars of UHMWPE were exposed to &ggr;-irradiation in air at doses from 3.3 to 100 Mrad and remelted by heating in air to 150 C. and holding at 150 C. for 5 hours and then slow cooling to room temperature. It was concluded that the wear rate of the radiation crosslinked UHMWPE was decreased markedly with increasing. dose compared to the UHMWPE controls without crosslinking. Radiation crosslinking caused little or no change in yield strength, but, the ultimate strength elongation to failure and impact strength decreased with increasing radiation dose, indicating that an optimum crosslinking dose would be one which provides a substantial reduction in wear while maintaining acceptable levels of other physical properties. Additionally, it was concluded that remelting of the irradiated UHMWPE substantially reduced the residual free radicals. This method therefore provides a practical means for the production of UHMWPE implants on an industrial scale.
Another method of preventing oxidation in the manufacturing of UHMWPE implants has been carried out by Stark C., Sun D. C., Yau S. S., Pereira P., Schmidig G., Wang A., Dumbleton J. H., and was reported on the “Combined Orthopaedic Research Societies Meeting”, Sep. 28-30, 1998, Hamamatsu, Japan. Said method involved a manufacturing process substantially oxygen-free from resin to final machining resulting in a product essentially free from oxidation.
Two other methods which have been recently employed are ion beam irradiation for a few minutes followed by annealing (remelting) at 150 C. (W. H. Harris, A. A. O. S., February., 1999, Anaheim, USA), and &ggr;-irradiation in an inert environment followed by so called stabilization for five days at 50 C. (Dumbleton et al., A. A. O. S., 1999, Annaheim, USA).
Yet another known method to prevent oxidation in UHMWPE orthopaedic implants was presented in J. Jpn Orthop. Assoc. 72 (8), 1998, and involves the addition of a small amount of vitamin E to the polymer. In this method, UHMWPE was mixed with vitamin E (DL-&agr;-Tocopherol) in an amount of 0.5 weight %. A 60×60×10 mm plate was formed by compression in a mold and treated by 2.5 Mrad &ggr;-radiation in an environment of air. After treatment the plate was allowed to stand in room temperature, for 60 days and was subsequently oxidized in air at 220 C. for 10 hours. Fatigue tests showed that flaking damage and crack under the surface were inhibited by the addition of vitamin E.
As stated above, due to the high molecular weight thereof, UHMWPE has a very high melt viscosity rendering it difficult or impractical to fabricate by the conventional melt processing techniques. Consequently, it is further extremely difficult to obtain a homogenous mixture of UHMWPE with other substances.
In U.S. Pat. No. 5,827,904 a composition for the manufacture of medical implants is shown, which consists of a polymeric material in powder form and a carotenoid doped into the polymer to produce a oxidation-resistant matrix for forming the implant. In the doping process the carotenoid is dissolved solved in an organic solvent, such as 2-propanol, cyclohexane, n-hexane, benzene, and the like.
Even if the organic solvent is present in the soaking of the material to be doped, the use of volatile organic solvents is dangerous as many organic solvents are flammable and explosive. Furthermore, many organic solvents are toxic or carcinogenic and most so if inhaled. In addition, expensive “explosion proof” equipment must be used, such as static electricity control systems, explosion vents, reinforced equipment, solvent recovery systems and the like.
A method involving an organic solvent for the preparation of antioxidant doped UHMWPE to be used in an implant may also result in difficulties
Bengtsson Peter
Lidgren Lars
Sj{overscore (o)}vall Peter
Wesslén Bengt
Bone Support AB
James Ray & Associates
Sanders Kriellion A.
LandOfFree
Method for the preparation of UHMWPE doped with an... 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 the preparation of UHMWPE doped with an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for the preparation of UHMWPE doped with an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2831380