Early Failure of a Modular Hip Implant

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Case Study

Early Failure of a Modular Hip Implant
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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.

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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.

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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.

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• Burnishing/
• Fretting

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Burnishing/Fretting
Corrosion
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SEM Analysis of Taper
Intergranular attack
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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.

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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

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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 gt30 of mixed alloy
  head/stem combinations vs. lt6 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).

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Orthopedic Metallic Implants
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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

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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.

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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

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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--gtmetal 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)

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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

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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
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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)

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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
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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

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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
Cl-
M
H
Cl-
M
H
http//www.materialsengineer.com/dup20image/corro
sion2005b.jpg
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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_diagno
stic_photographs.htm
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Fretting

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

Load
Relative Motion
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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

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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)

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Importance to Implants

• Long term stability of metal implants critical
  for patient health survival
• Stents
• Arthoplasty
• Fracture Fixation
• Pacemakers