2 Effect of Corrosion on the Body CompatibilityTissue responseLeach ratesToxicity
3 Corrosion Testing Two aspects of in vivo corrosion: How susceptible is implant material to corrosion in vivo?What is the effect of any corrosion (even very small amounts) on the body?
4 Device Susceptibility: Corrosion Performance Validation Selected corrosion tests used to validate medical devices:ASTM F Practice for Corrosion -Fatigue Testing of Metallic Implant MaterialsASTM F 1875 – Practice for Fretting Corrosion Testing of Modular Implant Interfaces: Hip Femoral Head-Bore and Cone Taper InterfaceASTM F 2129 – Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small ImplantsASTM G71 - Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in ElectrolytesASTM F 746 – Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials
6 Rest Potential Monitoring Addressed by several standardsISO 16429:2004Implants for surgery – Measurements of open-circuit potential to assess corrosion behaviour of metallic implantable materials and medical devices over extended time periodsASTM FStandard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant DevicesAlternative standardsISO 10271:2001 for dental materialsISO :2000
7 Rest Potential Monitoring Provides an opportunity to measure release of leachable substances, e.g., Ni, Cr, CoPeriodic solution analysis by ICP-MSNickel Leach Rate (μg cm-2t-1)Immersion time (hours)
8 Cyclic Potentiodynamic Polarization Preferred test methodASTM FStandard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant DevicesExtract potential dataRest potential (Er)Breakdown potential (Eb)Alternative test methodsISO 10271:2001 for dental materialsISO : not recommended
9 ASTM F 2129 General Procedure: Typically performed in saline environment at 37°CPBS, 0.9% NaCl, simulated bile, etc.Monitor rest potential (Er) for 1 hourPotentiodynamic polarization to 0.8 or 1 volt vs. SCEIf breakdown, record potential (Eb)Reverse potentiodynamic polarizationrecord repassivation potential (Ep)reformation of the passive layer
10 Cyclic Potentiodynamic Polarization No breakdownGood resistance to localized corrosionVertex Potential, EvPotential V (SCE)Rest Potential, ErCurrent mA cm-2
11 Cyclic Potentiodynamic Polarization Breakdown observedBreakdown potentialBreakdown Potential, EbPotential V (SCE)Rest potentialRepassivation potentialRest Potential, ErCurrent mA cm-2
12 Interpreting the Results Cyclic Potentiodynamic PolarizationASTM F is a deliberately aggressive testGeneral consensus that no breakdown up to 0.8 V (SCE) will provide sufficient resistance to localized corrosion in vivoBut if breakdown has been observedHow do we treat the data?How good is good enough?
13 Interpreting the Results Neither ASTM F 2129, nor the FDA (or other regulatory agencies) provide specific guidance as to what constitutes an acceptance criterionTwo approaches using EbCompare with threshold for ‘optimum corrosion resistance’Criterion is independent of material and environmentCompare with that of a predicate deviceAssumes suitable device is availableThe breakdown potential alone, however, is not a good measure of localized corrosion resistance
14 Interpreting the Results Er and Eb are not intrinsic properties of a metal or alloyFor a given alloy, Eb and Er are influenced by -The environment, e.g., pH, solution chemistry, temperatureSurface finish, e.g., mechanical polish vs. electropolishImmersion timeEb is also influenced by the test methodPotentiodynamic scan rateFaster scan rates can increase the measured value of Eb
15 Interpreting the Results Consider the gap between the breakdown potential and the rest potentialThus, a measure of an alloy’s susceptibility to localized corrosion is given by Eb - ErThe gap Eb - Er can be used to evaluate both pitting and crevice corrosion for a finished deviceBecause breakdown will occur at the most susceptible location whether it be a crevice or a pit-initiation site
16 ASTM F 2129 Example of Typical Data Presentation: Device Er Ezc Eb Ep Eb-ErEp-ErTest 1-305-337809420-1104725Test 2-410-442NB1200Test 3-376-393Test 4-311-350Test 5-420-4497403321160752Test 6-431-452Average-4047753761132739All potential values are in mVEr = rest potential Ezc = zero current potential Eb = breakdown potential Ep = repassivation potential Ev = vertex potential NB = no breakdown
17 Galvanic CorrosionPerform ASTM G 71 tests on galvanic couples and individual anodesMeasure and compare steady corrosion rates (current densities)Current increases of more than an order of magnitude are considered signficantAlso can compare coupled and un-coupled leach rates in longer-term leaching tests