Two novel metal alloys, Ti-13Nb-13Zr and Zr-2.5Nb, have been engineered for applications in orthopedic implants because of their favorable mechanical properties, corrosion resistance, and compatibility with bone and tissue. These alloys also have the ability to form a hard, abrasion-resistant, ceramic surface layer upon oxidative heat treatment (diffusion hardening, DH). Previous studies have indicated that these and other ceramics cause limited hemolysis and exhibit remarkable structural integrity after extended exposure to physiological environments. Such observations suggest that DH Ti-13Nb-13Zr and ZrO2/Zr-2.5Nb could be used successfully as components in blood-contacting devices. Materials intended for such applications must possess properties that do not elicit adverse physiological responses, such as the initiation of the coagulation cascade or thrombus formation. In the present study measurements of prekallikrein activation, fibrinogen adsorption from diluted human plasma, and the strength of fibrinogen attachment as judged by residence-time experiments were performed to evaluate the potential hemocompatibility of these materials. The results of the prekallikrein activation and fibrinogen-retention studies correlated well with two electrochemical properties of the alloys, the open circuit potential and reciprocal polarization resistance. The results indicate that both the original and treated Ti and Zr alloys activate prekallikrein and adsorb as well as retain fibrinogen in amounts similar to other materials used as components of blood-contacting devices. On the basis of these studies, these alloys appear to be promising candidates for cardiovascular applications and merit further investigation.
Modular connections have been commonly and successfully utilized in orthopaedic implant systems for the last 15 or so years, particularly at the head/neck junction in total hip arthroplasty (THA). However, recent retrieval studies have shown that some of the tapered junctions between femoral heads and stems in total hip arthoplasty can be prone to fretting corrosion, and may be a cause for concern in the longevity of implants. Fretting corrosion, which may release metallic products (particulate debris and ions) into the joint space, is a complex phenomenon in which the interplay between mechanically induced interfacial micromotion (fretting) and electrochemical corrosive activity play an important role, along with materials selection and processes. This suggests that interfacial fretting corrosion at modular implant interfaces can be significantly affected by the design variables of the modular junction. The working hypothesis of this study was that different designs of the modular head/stem combination of femoral hip prostheses exhibit different release of fretting corrosion metal products during fatigue testing used to simulate ten years of in vivo service. Three designs of femoral head (Co-Cr-Mo alloy) and stem (Ti-6Al-4V alloy) combinations were investigated in this study. The study included detailed taper metrology followed by environmental fatigue testing of the tapered junctions. The results of this study showed that important taper design differences do exist in the three constructs tested, and these differences manifested in different fretting corrosion behavior.
Using potentiodynamic polarization and electrochemical impedance techniques, the electrochemical behavior of carbon-fiber/polyetheretherketone (C/PEEK) composite material was studied primarily in lactated Ringer's solution with and without a stable, fast reacting redox couple (0.01M K4[Fe(CN)6] + 0.01M K3[Fe(CN)6]), at 37°C. For comparison, the spontaneous passivation of stainless steel 316L, Co-Cr-Mo alloy, and Ti-6Al-4V was also investigated in these electrolytes. It was found that the rate of total electrochemical interaction (corrosion + electron exchange) between a spontaneously passivated metal and the environment can be considerably smaller than the rate of simple electron exchange between the carbon-fiber composite and the environment. Considering the excellent biocompatibility of carbon, this finding seems to indicate the important role of protective passive films on metals, rather than the clinical significance of higher electron exchange rates in general. When the protective passive layer on the metals is damaged or removed mechanically, the undesirable effect of substantially increased corrosion rates can be observed, particularly in environments with low redox activity. While galvanic corrosion may also occur between the mechanically depassivated and the passive sites of implant metal surfaces, more considerable galvanic corrosion can be expected if the metal undergoing fretting wear is in contact with a carbon-fiber composite, depending on the anodic/cathodic surface area ratio and on the redox properties of the environment. Additionally, the repassivation of the damaged metal surface may not take place effectively. Being the most susceptible to localized corrosion, stainless steel 316L, even in the passivated condition, may show accelrated pitting corrosion if coupled with carbon-fiber composites.
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