A. E. Craft, B. Aboud, DePuy Orthopaedics, Inc., Warsaw, IN; K. Karve, University of Massachusetts, Lowell, MA
Metal passivation is commonly used to remove exogenous iron and iron compounds from the surfaces of metallic materials, however a comparative study among passivation treatments, nitric acid and the more environmentally friendly citric acid based formulations, indicated citric acid may also serve as a depyrogenating agent. Failure to remove residual bacterial endotoxins from orthopedic implants can lead to an adverse biological response. This study showed the depyrogenation effects of a particular citric acid formulation reduced residual endotoxins by at least three orders of magnitude. Titanium alloy (ASTM F136, Ti-6Al-4V ELI) coupons were inoculated with a high potency Escherichia coli endotoxin. Following the guidelines of ASTM A967 for metal passivation, coupons were passivated with nitric acid or a citric acid based formulation. Passivation parameters were 60ºC for 30 minutes for all experimental media, using concentrations of 30% nitric acid (by volume), 10% (by weight) citric acid, or with 4% to 5% (by weight) citric acid with surfactant additives. An extraction method was developed to maximize and appropriately quantify the endotoxin before and after the passivation treatment. Quantification of endotoxin was performed using the kinetic chromogenic limulus amebocyte lysate assay.
Summary: Metal passivation is commonly used to remove exogenous iron and iron compounds from the surfaces of metallic materials, however a comparative study among passivation treatments, nitric acid and the more environmentally friendly citric acid based formulations, indicated citric acid may also serve as a depyrogenating agent. Failure to remove residual bacterial endotoxins from orthopedic implants can lead to an adverse biological response. This study showed the depyrogenation effects of a particular citric acid formulation reduced residual endotoxins by three orders of magnitude. Titanium alloy (ASTM F136, Ti-6Al-4V ELI) coupons were inoculated with a high potency Escherichia coli endotoxin. Following the guidelines of ASTM A967 for metal passivation, coupons were passivated with nitric acid or a citric acid based formulation. Passivation parameters were 60ºC for 30 minutes for all experimental media, using concentrations of 30% nitric acid (by volume), 9.5% (by weight) citric acid, or with 8% (by weight) citric acid with surfactant additives. Inoculation and extraction methods were developed to maximize and appropriately quantify the endotoxin before and after the passivation treatment. Quantification of endotoxin was performed using kinetic chromogenic limulus amebocyte lysate.
