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Key words: implants, fatigue, metal alloys, wire, residual stress
This project continues the work reported last year to identify and quantify the factors affecting the static mechanical and fatigue properties of the metal alloys commonly used in cardiovascular, orthopedic and dental implants. The alloys under study include 316L stainless steel, 6Al4V titanium, and several cobalt alloys, including L605, Elgiloy and MP35N, in both fine wire and bar stock form.
Last year, OST reported observing that the fatigue life of MP35N wire could be changed by two orders of magnitude by prestretching the wire an amount consistent with its "as-received" coldworked condition. Such cold working is common during the fabrication of interventional cardiology devices and is sometimes performed during surgery in orthopedic procedures. OST is completing those experiments by characterizing the hardness and grain structure of the virgin and prestretched wire and attempting to correlate the fatigue behavior with the results of stress-strain tests. In many cases, the static tests indicate little or no "work hardening," as represented by the stress-strain curve being almost horizontal post yield. However, microhardness measurements indicate that hardening is occurring, and manifesting itself in the change in fatigue behavior. As a side note, OST has observed that elongation-to-failure data on fine wires being reported by test labs and included in device submissions are inaccurate, owing primarily to lack of standardized methodology for measuring the actual plastic strain in areas of localized necking. Experiments using non-contact video extensometry and other methodologies are underway.
OST scientists have also initiated a full-factorial experiment to evaluate the effects of test frequency and physiological environment on the fatigue behavior of 316L and 6Al4V titanium alloys. These alloys were selected as being the most and least susceptible to corrosion fatigue among the commonly-used implant alloys. The results of this project should allow OST to develop definitive guidance regarding the experimental protocols for characterizing the fatigue behavior of these implant alloys and devices made from them. [Stds]
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Key words: particulates, biological response, wear debris
Loosening is the most common mode of failure of total hip and knee arthroplasties. The biological response to wear debris is complex, involving many cell types, including macrophages, which can phagocytize the wear debris. To date, in vitro studies have focused on the release of inflammatory mediators by phagocytic cells in the presence of particles. The current study uses real-time chemiluminescence to determine the generation of reactive oxygen species by human monocytederived macrophages in the presence of different concentrations of commercially pure titanium (CpTi), polystyrene (PS) and polymethylmethacrylate (PMMA) particles. The data indicate that CpTi elicits a weak extracellular (superoxide) response, but not an intracellular (hydrogen peroxide) response. PS and PMMA of matched particle number and diameter showed a dose response curve for both the intra- and extracellular reactions. Inhibitors of phagocytosis and superoxide suppressed the chemiluminescence reaction. The differences in the kinetic reactions between CpTi, PMMA and PS may indicate differences in the uptake and processing mechanisms of the ingested particles. [ProA]