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Early Failure of a Modular Hip Implant PowerPoint PPT Presentation


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Case Study. Early Failure of a Modular Hip Implant. 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.

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Early Failure of a Modular Hip Implant

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Early failure of a modular hip implant

Case Study

Early Failure of a Modular Hip Implant


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.


Early failure of a modular hip implant

Burnishing/

Fretting


Early failure of a modular hip implant

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


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 corrosion1

Crevice Corrosion

OH-

O2

OH-

O2

O2

OH-

OH-

  • Found in crevices or deep, narrow flaws (mismatch of components at interface

    • Can arise from localized oxygen depletion and metal ion concentration gradients


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

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 basics1

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 → Mn+ + ne-

  • nH+ + ne- → nH


Uniform attack

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)


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

M+

N+

M+

e-

N+

nM = nM+ +ne-

nN+ + ne- = N

M+

N+

M

N

M+

N+

N+

Metal 1

Metal 2


Crevice corrosion2

Crevice Corrosion

OH-

O2

OH-

O2

O2

OH-

OH-

  • Found in crevices or deep, narrow flaws (mismatch of components at interface

    • Can arise from localized oxygen depletion and metal ion concentration gradients


Pitting corrosion

Pitting Corrosion

  • Subset of Crevice Corrosion

    • Formation of pits: local thickness reduction

    • Difficult to detect

O2

O2

O2

OH-

OH-

OH-

Cl-

Cl-

H+

H+

M+

M+

M+

M+

Cl-

Cl-

H+

M+

H+

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

Sensitive Regions

precipitates

http://www.corrosionresolutions.com/example_diagnostic_photographs.htm


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 implants1

Importance to Implants

  • Long term stability of metal implants critical for patient health & survival:

    • Stents

    • Arthoplasty

    • Fracture Fixation

    • Pacemakers


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