In-Vivo Corrosion and Fretting of Modular TI-6AL-4V/CO-CR-MO Hip Prostheses: The Influence of Microstructure and Design Parameters

Abstract

The purpose of this study was to evaluate the incidence of corrosion and fretting in 48 retrieved titanium-6aluminum-4vanadium and/or cobalt-chromium-molybdenum modular total hip prosthesis with respect to alloy material microstructure and design parameters. The results revealed vastly different performance results for the wide array of microstructures examined. Severe corrosion/fretting was seen in 100% of as-cast, 24% of low carbon wrought, 9% of high carbon wrought and 5% of solution heat treated cobaltchrome. Severe corrosion/fretting was observed in 60% of Ti-6Al-4V components.Design features which allow for fluid entry and stagnation, amplification of contact pressure and/or increased micromotion were also shown to play a role. 75% of prosthesis with high femoral head-trunnion offset exhibited poor performance compared to 15% with a low offset. Large femoral heads (>32mm) did not exhibit poor corrosion or fretting. Implantation time was not sufficient to cause poor performance; 54% of prosthesis with greater than 10 years in-vivo demonstrated none or mild corrosion/fretting. v Effect of heat treatments on Ti-6Al-4V microstructure. Effect of heat treatments on Ti-6Al-4V mechanical propertiesFracture toughness values for equiaxed and highly transformed microstructuresTi-6Al-4V ASTM mechanical and compositional requirements Summary of modular total hip prosthesis retrieval studies during the past 25 years. As-received prostheses catalogingGeneral prostheses feature cataloging Macroscopic Examination. Overall implant examination, i.e. non-surface specific. Porous Surface Coating TypesFemoral head and femoral stem dimensional variables Modular junction dimensional variablesCorrosion and Fretting Scoring Criteria Worst fretting or corrosion score distribution by moment arm length.Worst fretting/corrosion score distribution for either head or trunnion by implantation time in-vivo. Cobalt-Carbon phase diagram illustrating the lever rule principle that dictates the fraction AB/AC of the constituent phases which will be liquid at the sintering temperature AC. i.e. an increase in carbon content will decrease the temperature required for adhering porous coatings.Porous coated as-cast cobalt-chrome stem Effect of Cooling Rates and Highest Temperature on Ti-6Al-4V Microstructure Finite element analysis predicting greater wear at the trunnion surface as femoral head is increased from 32mm to 56mm.Material, contact conditions and environmental variables governing potential for fretting wear.Pitting corrosion phenomena and associated reactions and surface features. Ion channeling contrast enhanced image of taper subsurface at 6,000X magnification, revealing differences in morphology near the surface.FIB trench subsurface regions selected for energy dispersive spectroscopy analysis.Cobalt, chromium, and molybdenum elemental weight percent at different depths as determined by EDS.(Left to right) 1. FIB sample pull out of as-cast carbide in matrix. 2. Areas from which EDS and TEM diffraction patterns were taken. 3. TEM im...

The Distribution and Severity of Corrosion Damage at Eight Distinct Zones of Metallic Femoral Stem Implants

“…Since there was no particular hypothesis about the relative level of corrosion at the eight zones, 28 pairwise comparisons were carried out to exhaust the entire pairwise comparison of the zones. The distal region of the medial quadrant was found to have the highest odds of receiving a higher corrosion score which is aligned with the previous findings in the literature that identified the distal region [19,20,27] and the medial quadrant [7,10,16,28] having the highest corrosion scores. Also, this study shows that the distal region of all the four quadrants had more corrosion damage in comparison with the proximal region of those quadrants.…”

“…Based on this method, eight distinct zones of the retrieved stem tapers were scored individually. Fretting wear was not scored because it has been reported by several studies that fretting may be masked by corrosion damage; and therefore, hard to visually identify [14,15,19,20]. Also, it is thought that the severity of fretting in Goldberg's method cannot be measured consistently because the pitch of the machined threads over the taper surface varies among different stem designs [14,21].…”