Corrosion

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Early Failure of a Modular Hip Implant Case Study

Summary of Failed S-ROM Prosthesis: 

Summary 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.

Typical Failure Analysis: 

Typical 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.

Failure Analysis: 

Failure Analysis Once 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.

Slide 5: 

Burnishing/ Fretting

Slide 6: 

Burnishing/Fretting Corrosion

SEM Analysis of Taper: 

SEM Analysis of Taper Intergranular attack

Scientific Assessment : 

Scientific Assessment Fretting Initial tolerance mismatch stresses 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.

Possible solutions: 

Possible solutions Possible 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

Important Elements of the Case: 

Important 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).

Orthopedic Metallic Implants: 

Orthopedic Metallic Implants Alloy Spec Fe C Cr Ni Co Ti Al V 316L F-138 Bal 0.03 max 17-19 13-15.5 Co-cast F-75 0.75max 0.35 max 27-30 1.0 Bal Forged F-799 0.75max 0.35 max 27-30 1.0 Bal Wrought F-90 3.0max 0.05- 0.15 19 -21 9-11 Bal Ti F-67 0.5max 0.1 max Bal Ti6Al4V F-136 0.25max 0.08 max Bal 5.5-6.5 3.5-4.5

Taper Junction: 

Taper Junction Source of relative motion--fretting Bending in the cone Bending of the long neck extension (skirt) with proximal-distal slipping Bore angle too large Bore angle too small

Crevice corrosion: 

Crevice corrosion Micromotion 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.

Crevice Corrosion : 

Crevice Corrosion Found in crevices or deep, narrow flaws (mismatch of components at interface Can arise from localized oxygen depletion and metal ion concentration gradients OH - OH - OH - OH - O 2 O 2 O 2

Mechanically Assisted Crevice Corrosion: 

Mechanically Assisted Crevice Corrosion In the head-neck taper, tolerances are such that narrow crevices exist with fluid present At onset of loading, interfacial shear stresses are sufficient to fracture oxide film Unpassivated metal is exposed to initially oxygen rich fluid. Oxidation occurs--depleting oxygen in crevice fluid--increases free metal ions--which attract Cl ions-->metal chlorides Metal chlorides react with water to form metal hydroxide and HCl--lowers pH Cr2O3 is unstable below pH of 3-- results in active attack of CoCr alloy--etched appearance (intergranular attack)

Corrosion Basics: 

Corrosion Basics Multifactorial problem--depends on geometry, metallurgy, stresses, solution chemistry Driven by two primary factors: thermodynamic driving forces (Oxidation/Reduction) and kinetic barriers An electro-chemical attack resulting in material degradation Exacerbated by mechanical and biological attack Compromises Material Properties Mechanical Integrity Biocompatibility Aesthetics

Corrosion Basics: 

Corrosion Basics Occurs mostly in ionic, aqueous environments Primarily a concern for metals Oxidation – Reduction Reaction: Loss of metal Become ions in solutions Combine with other species to form compound (oxides, hydroxides) M → M n+ + ne- nH + + ne- → nH

Uniform Attack: 

General corrosion that is evenly distributed over entire corrosion region Rusting of iron, tarnishing of silverware Most readily detectable (visual) and preventable (alloying) Uniform Attack

Galvanic Corrosion: 

Galvanic Corrosion Two different metals/alloys that are in close proximity in an electrolytic environment Distinct tendencies toward oxidation Common in orthopaedics – Modular implants Titanium femoral stems coupled with CoCr heads e - Metal 1 Metal 2 M N M + M + M + M + N + N + N + N + N + nM = nM + +ne- nN + + ne- = N

Crevice Corrosion : 

Crevice Corrosion Found in crevices or deep, narrow flaws (mismatch of components at interface Can arise from localized oxygen depletion and metal ion concentration gradients OH - OH - OH - OH - O 2 O 2 O 2

Pitting Corrosion: 

Pitting Corrosion Subset of Crevice Corrosion Formation of pits: local thickness reduction Difficult to detect OH - OH - OH - H + H + H + M + Cl - Cl - Cl - Cl - M + M + M + M + H + O 2 O 2 O 2 http://www.materialsengineer.com/dup%20image/corrosion%2005b .jpg

Intergranular Corrosion: 

Intergranular Corrosion Preferential attack along grain boundaries Results from localized differences in chemistry Common in SS, nickel some Al alloys http://www.corrosionresolutions.com/example_diagnostic_photographs.htm Sensitive Regions precipitates

Fretting: 

Fretting Wear process due to relative motions in highly loaded devices exaggerated by corrosive environment asperities of contacting surface Device micromotions Load Relative Motion

Environmental Factors: 

Environmental Factors Ion concentraion Fluid velocities Human Body – Conducive to Corrosion Acidic – High ionic (H + ) concentration Aqueous (Blood, Synovium) – fluid flow 37 C – Elevated Temperature

Importance to Implants: 

Importance to Implants Mechanical Properties Enhanced risk of crack propagation and fatigue fracture Biocompatibility – Presence of metal ions triggers enhanced foreign body response Osteolysis, implant loosening Blood clotting (thrombosis)

Importance to Implants: 

Importance to Implants Long term stability of metal implants critical for patient health & survival: Stents Arthoplasty Fracture Fixation Pacemakers