2Summary of Failed S-ROM Prosthesis Total hip implant failed after six months in vivo.Patient (male, 60 yrs in age) indicated symptoms of pain and device failure to his surgeon.Howmedica SROM with a 42 mm neck and a 28 mm head. A +12mm skirt was used in this device. The acetabular liner was a Howmedica polyethylene shell with a 20mm inside diameter and a 54mm outside diameter.Upon retrieval, the surgeon noted a large amount of white fluid with black particulate in the hip joint. The surgeon noted that there was a substantial amount of corrosion at the Morse taper and that it had a burnished appearance.
3Typical Failure Analysis How is a failure analysis conducted?Collect medical report. Histological analysis and x-rays. What materials and design used?Visual observation of device. Note any irregularities.Optical micrographs to capture all damage on device. Comparison to pristine device.Chemical and mechanical analysis.Scanning electron microscopy to look for micromechanisms of fracture.
4Failure AnalysisOnce the failed device was explanted it was documented with both optical and electron microscopy.Clear evidence of burnishing, pitting, and crevice corrosion were present on the device. Especially prevalent in the region of the Morse taper.Scanning electron microscopy of the retrieval revealed intergranular attack and pitting associated with crevice corrosion and burnishing or scratching indicative of micromotion or fretting.
9Scientific Assessment FrettingInitial tolerance mismatchstresses associated with the long neck (+12 mm neck)Devices exceeding designed tolerances can lead to poor mechanical stability and may disrupt the interference fit required for long term structural integrity at the taper (Jacobs et al. 1998)Brown et al. (1995) has shown a correlation between neck extension and fretting corrosion. Longer necks contribute to higher bending moments and enhance relative motion between the head and stem. It is postulated that fretting leads to a continuous passive film breakdown and repassivaton leading to oxygen consumption within the crevice.The fractography of the failed device exhibits burnishing (associated with fretting), an etched microstructure associated with low pH, and pitting associated with crevice corrosion.
10Possible solutionsPossible alternatives to prevent corrosion in Co-Cr heads coupled with Ti stems:(I) use hardened Ti head on Ti stem(II) use a cobalt-on-cobalt system(III) use a ceramic head on Ti or Co stem(IV) eliminate fluid from tapered interface(V) use self-locking mechanism to prevent fretting
11Important Elements of the Case Corrosion occurs in all metal implants(Jacobs et al, JBJS, 1998).Corrosion is more prevalent in modular devices: corrosion observed in >30% of mixed alloy head/stem combinations vs. <6% all Cobalt alloy devices(Collier et al., Clin Orthop, 1995).Biomechanical stresses are developed at the taper junction. Serves as a source of crevice corrosion (Gilbert et al., JBMR, 1993).
12Orthopedic Metallic Implants AlloySpecFeCCrNiCoTiAlV316LF-138Bal0.03max17-19Co-castF-750.75max0.35 max27-301.0ForgedF-7990.35WroughtF-903.0max0.05-0.1519 -219-11F-670.5max0.1 maxTi6Al4VF-1360.25max0.08 max
13Taper Junction Source of relative motion--fretting Bending in the cone Bending of the long neck extension (skirt) with proximal-distal slippingBore angle too largeBore angle too small
14Crevice corrosionMicromotion between components results in fretting corrosion that can lead to initiation of crevice corrosion.Metallic implants rely on passive oxide film for protection from corrosion.Repetitive motion leads to continuous breakdown and repassivation.Repeated breakdown consumes oxygen in crevice and results in drop in pH--crevice corrosion.
15Crevice CorrosionFound in crevices or deep, narrow flaws (mismatch of components at interfaceCan arise from localized oxygen depletion and metal ion concentration gradientsOH-O2
16Mechanically Assisted Crevice Corrosion In the head-neck taper, tolerances are such that narrow crevices exist with fluid presentAt onset of loading, interfacial shear stresses are sufficient to fracture oxide filmUnpassivated metal is exposed to initially oxygen rich fluid. Oxidation occurs--depleting oxygen in crevice fluid--increases free metal ions--which attract Cl ions-->metal chloridesMetal chlorides react with water to form metal hydroxide and HCl--lowers pHCr2O3 is unstable below pH of 3-- results in active attack of CoCr alloy--etched appearance (intergranular attack)
18Corrosion BasicsMultifactorial problem--depends on geometry, metallurgy, stresses, solution chemistryDriven by two primary factors: thermodynamic driving forces (Oxidation/Reduction) and kinetic barriersAn electro-chemical attack resulting in material degradationExacerbated by mechanical and biological attackCompromises Material PropertiesMechanical IntegrityBiocompatibilityAesthetics
19Corrosion Basics Occurs mostly in ionic, aqueous environments Primarily a concern for metalsOxidation – Reduction Reaction:Loss of metalBecome ions in solutionsCombine with other species to form compound (oxides, hydroxides)M → Mn+ + ne-nH+ + ne- → nH
20Uniform AttackGeneral corrosion that is evenly distributed over entire corrosion regionRusting of iron, tarnishing of silverwareMost readily detectable (visual) and preventable (alloying)
21Galvanic CorrosionTwo different metals/alloys that are in close proximity in an electrolytic environmentDistinct tendencies toward oxidationCommon in orthopaedics – Modular implantsTitanium femoral stems coupled with CoCr headsM+N+M+e-N+nM = nM+ +ne-nN+ + ne- = NM+N+MNM+N+N+Metal 1Metal 2
22Crevice CorrosionFound in crevices or deep, narrow flaws (mismatch of components at interfaceCan arise from localized oxygen depletion and metal ion concentration gradientsOH-O2
23Pitting Corrosion Subset of Crevice Corrosion Formation of pits: local thickness reductionDifficult to detectO2O2O2OH-OH-OH-Cl-Cl-H+H+M+M+M+M+Cl-Cl-H+M+H+
24Intergranular Corrosion Preferential attack along grain boundariesResults from localized differences in chemistryCommon in SS, nickel some Al alloysSensitive Regionsprecipitates
25FrettingWear process due to relative motions in highly loaded devices exaggerated by corrosive environmentasperities of contacting surfaceDevice micromotionsLoadRelative Motion
26Environmental Factors Ion concentraionFluid velocitiesHuman Body – Conducive to CorrosionAcidic – High ionic (H+) concentrationAqueous (Blood, Synovium) – fluid flow37 C – Elevated Temperature
27Importance to Implants Mechanical PropertiesEnhanced risk of crack propagation and fatigue fractureBiocompatibility – Presence of metal ions triggers enhanced foreign body responseOsteolysis, implant looseningBlood clotting (thrombosis)
28Importance to Implants Long term stability of metal implants critical for patient health & survival:StentsArthoplastyFracture FixationPacemakers