Patent Number: 054854962
Section: summary

TECHNICAL FIELD This invention is directed to irradiation sterilizing of a medical device or product made of or containing biomaterial that undergoes substantial strength loss during use on gamma irradiation sterilizing under ambient conditions and has heretofore required gas sterilizing. BACKGROUND OF THE INVENTION Synthetic absorbable sutures composed of biodegradable biomaterials including polyglycolic acid (e.g., sold under the tradename Dexon), copolymer of glycolide and lactide (e.g., sold under the tradename Vicryl), poly-p-dioxanone (sold under the tradename PDSII), copolymer of glycolide and trimethylene carbonate (sold under the tradename Maxon) and copolymer of glycolide and epsilon-caprolactone (sold under the tradename Monocryl) are currently sterilized by gas (ethylene oxide) sterilization because of the known adverse effect of gamma irradiation sterilization on the mechanical properties and hydrolytic degradation rate of these biomaterials. Gas sterilization is time consuming and costly. The toxicity of residual amounts of ethylene oxide in medical devices and products has been a concern and degassing is a long and tedious, costly process. The medical industry has expressed a desire to replace ethylene oxide sterilization for absorbable biomaterials, e.g., absorbable sutures, with gamma irradiation sterilization if the latter method would not significantly increase the strength loss during use of the biomaterials. However, all reported data from conventional gamma irradiation of synthetic polymeric absorbable biomaterials indicates that gamma irradiation sterilization of synthetic polymeric absorbable biomaterials would be unacceptable. There is a similar problem for non-absorbable synthetic polymeric biomaterials, e.g., in the case of acetabular or tibia components of joint protheses made of ultra high molecular weight polyethylene, which are disadvantageously weakened by conventional gamma irradiation to the extent that gas sterilization has been required. SUMMARY OF THE INVENTION It has been discovered herein that medical devices or products composed of or containing absorbable as well as non-absorbable polymeric biomaterials that are significantly weakened by conventional gamma irradiation sterilization are provided with improved strength properties when gamma irradiation is carried out in the substantial absence of oxygen at very low temperatures. This discovery is embodied in a method for gamma irradiation sterilization of a medical device or product composed of or containing biomaterial that undergoes substantial strength loss after gamma irradiation sterilizing under ambient conditions and heretofore has required gas sterilizing, said method comprising gamma irradiating said medical device or product in the substantial absence of oxygen while said device is maintained at a temperature ranging from -180.degree. C. to -200.degree. C., thereby to sterilize said device. Preferably, vacuum treatment to provide 1.times.10.sup.-5 torr to about 1.times.10.sup.-7 torr, more preferably about 5.times.10.sup.-6 torr, is utilized to provide the required substantial absence of oxygen. Preferably, liquid nitrogen is utilized to provide the required temperature. The term "medical device or product" is used herein to mean a device or product for human body reconstruction or which is implanted in the body to control drug release. This term includes absorbable devices and products, e.g., absorbable sutures, absorbable clips, absorbable staples, absorbable pins, absorbable rods (for repairing broken bones), absorbable joints, absorbable vascular grafts, absorbable fabrics or meshes (e.g., for hernia repair), absorbable sponges, absorbable adhesives and absorbable drug control/release devices as well as non-absorbable devices and products, e.g., acetabular or tibia components of joint prostheses, and bone cement. The term "biomaterial" is used herein to mean a material which has properties which are adequate for human body reconstruction and/or drug control/release devices or products. The term includes absorbable materials, e.g., as in the case of absorbable sutures, as well as non-absorbable materials, e.g., in the case of prostheses components. The term "absorbable" is used herein to mean that the materials will be degraded and subsequently absorbed into a human body. The term "non-absorbable" is used herein to mean that the materials will not be degraded and subsequently absorbed into a human body. The term "sterilization" is used herein to mean treatment that achieves the killing of all types of microorganisms. The term "undergoes substantial strength loss after gamma irradiation sterilizing under ambient condition" is used herein to mean loss of at least 50% in tensile breaking force under ASTM specified conditions of 21.degree. C. and 65% relative humidity as determined using an Instron Universal Testing Machine with a crosshead speed of 10 mm/min, after 10 days in phosphate buffer solution (pH of 7.44) at 37.degree. C. after total gamma irradiation of 2 Mrad. The term "substantial absence of oxygen" means in an atmosphere containing the amount of oxygen remaining after the vacuum treatment described above.