Biomaterials Used in Orthopedic Implants

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Biomaterials Used in Orthopedic Implants

• Magnesium Foam As a Bioresorbable Implant
• Presented By
• Joe Barker

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Orthopedics

• Bone properties, Osteoconductivity,
  Biocompatibility

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Bone Properties

• Density 2.3g/cm3
• Tensile Strength 3-20MPa
• Compressive Strength 15,000 psi
• Shear Strength 4,000 psi
• Youngs Modulus 10-40 MPa

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Orthopedic Terms
Osteoconductive The property of a material that
allows for the possible integration of new bone
with the host bone.
Osteoinductive Characteristic in materials that
promote new bone growth.
Bioresorbable The ability of a material to be
entirely adsorbed by the body.
Trochanter The second segment of the leg, after
the coxa and before the femur
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Screw Types

• OBLIQUE SCREWS
• In subtrochanteric and high femoral fractures
  oblique screws may be required to be inserted up
  the femoral neck
• Screws are 4.5mmX150mm

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Screw Types

• CANNULATED SCREW
• Screw Sizes
• 6.5mm X 102mm
• 4.5 X 12.5mm

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Screw Types

• CANNULATED SCREW Continued...
• A bulbous ended nail with cannulated 12.5 mm
  screws is shown here successfully stabilizing a
  subtrochanteric non-union of the femur following
  a failed Gamma nail

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Screw Types

• TRANSVERSE SCREWS
• In most subtrochanteric and upper femoral
  fractures it is much easier to insert transverse
  screws in the upper femur, than use oblique
  screws up the neck of the femur.

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Screw Types
Transverse Screws Continued....
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Orthopedic Materials

• Metals

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Metals For Implants

• Must be corrosion resistant
• Mechanical properties must be appropriate for the
  desired application
• Areas subjected to cyclic loading must have good
  fatigue properties -- implant materials cannot
  heal themselves

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Devices Were Metals Are Used

• Orthopedic devices
• Plates and screws, Pins and Wires, rods
  (temporary)
• Total joints (permanent)
• Clips and staples

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Metals Used in Implants

• Three main categories of metals for orthopedic
  implants
• stainless steels
• cobalt-chromium alloys
• titanium alloys
• Material looked at in this project
• Magnesium Foam

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Stainless Steel

• Generally about 12 chromium (316L, Fe-Cr-Ni-Mo)
• High elastic modulus, rigid
• Low resistance to stress corrosion cracking,
  pitting and crevice corrosion, better for
  temporary use
• Corrosion accelerates fatigue crack growth rate
  in saline (and in vivo)
• Intergranular corrosion at chromium poor grain
  boundaries -- leads to cracking and failure
• Wear fragments - found in adjacent giant cells

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Cobalt Based Alloys

• Co-Cr-Mo
• Used for many years in dental implants more
  recently used in artificial joints
• good corrosion resistance
• Co-Cr-Ni-Mo
• Typically used for stems of highly loaded
  implants, such as hip and knee arthroplasty
• Very high fatigue strengths, high elastic modulus
• High degree of corrosion resistance in salt water
  when under stress
• Poor frictional properties with itself or any
  other material
• Molybdenum is added to produce finer grains

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Titanium and Titanium Alloys

• High strength to weight ratio
• Density of 4.5 g/cm3 compared to 7.9 g/cm3 for
  316 SS
• Modulus of elasticity for alloys is about 110 GPa
• Not as strong as stainless steel or cobalt based
  alloys, but has a higher specific strength or
  strength per density
• Low modulus of elasticity - does not match bone
  causing stress shielding

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Titanium Alloys

• Co-Ni-Cr-Mo-Ti, Ti6A4V
• Poor shear strength which makes it undesirable
  for bone screws or plates
• Tends to seize when in sliding contact with
  itself or other metals
• Poor surface wear properties - may be improved
  with surface treatments such as nitriding and
  oxidizing

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Best Performance

• Titanium has the best biocompatibility of the
  three.
• Metal of choice where tissue or direct bone
  contact required (endosseous dental implants or
  porous un-cemented orthopedic implants)
• Corrosion resistance due to formation of a solid
  oxide layer on surface (TiO2) -- leads to
  passivation of the material

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Orthopedic Materials

• Metallic Foams

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Metallic Foam

• Types of metallic foams
• Solid metal foam is a generalized term for a
  material starting from a liquid-metal foam that
  was restricted morphology with closed, round
  cells.
• Cellular metalsA metallic body in which a
  gaseous void is introduced.
• Porous metal Special type of cellular metal with
  certain types of voids, usually round in shape
  and isolated from each other.
• Metal Sponges A morphology of cellular metals
  with interconnected voids.

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Magnesium Foam

• The type of Magnesium foam used in this study
  would be classified as a porous metal.
• Why Foam?
• Open cellular structure permits ingrowths of
  new-bone tissue and transport of the body fluids
• Strength Modulus can be adjusted through
  porosity to match natural bone properties

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Requirements for Porous Implant

• Pore Morphology (Spherical)
• Pore Size (200?m - 500?m)
• Porosity
• High Purity (99.9)
• Biocompatibility

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Why Magnesium?

• Bioresorbable
• Biocompatible
• Osteoconductive
• Osteoinductive
• Properties of bone can be easily attained using
  varying processing techniques

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Processing the Mg by Powder Metallurgy Techniques

• Powder
• Mg powder
• 99.9 purity
• particle size ?180?m
• Binder Ammonium Bicarbonate
• Spherical Shape
• 99.0 purity
• Size between 200?m 500?m

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Processing the Mg by Powder Metallurgy Techniques

• Processing Steps
• Blend powders until a homogenous mixture is
  attained.
• Uniaxially press at 100MPa into green compacts
• Heat treat at 200ºC for 5hrs, for binder burnout
• Sinter at 500ºC for 2hrs

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Results From Processing

• Optical Micrograph of Porous Mg
• Small isolated micropores distributed in the wall
  of the interpenetrated macropores.
• The micropores are on the order of microns, while
  the macropores are in the range of 200?m 500?m

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Results of Processing

• SEM Micrograph of Mg
• Micropores result from the volume shrinkage
  during sintering and are to small for bone growth
• Macropores are made on the appropriate size level
  to promote the ingrowths of new-bone tissues and
  transport of body fluid

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Determination of Mechanical Properties

• The Stress Strain Curve shows a large plateau
  region
• From this you can see that the plateau stress of
  the Mg foam is roughly 2.33 MPa
• Using Gibson Ashby model the following
  properties can be attained
• ?pl/?ysC(?/?s)3/2 C0.3
• E/EsA(?/?s)2 A1

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Properties Attained from Processing

• With a porosity of ? 50
• Density 0.87g/cm3
• ?pl 2.33MPa
• ?ys 2.843 MPa
• E 10.476GPa

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Adsorption and Toxicity

• Adsorption Rates for Mg
• The bone will adsorb around 40 of the Mg in the
  screw per year.
• From this the lifetime of the screw would be
  between 5 7 years before no traces are left.
• Toxicity
• Recommended dosage of Mg per day is 350mg
• 60 of Mg in the body is found in bones
• In recent studies, a diet rich in Mg resulted in
  increases in bone density in postmenopausal women
• Relatively low toxicity issues, but in vivo
  testing would clarify.

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Cost of Materials

• Price of Mg powder for particle size ? 200?m
• 56.24 for 1Kg
• Price of Binder powder within set size limits
• 40.66 for 1Kg
• Cost for smallest screw size 4.5mm X 12.5mm
• 0.07
• Cost for largest screw size 6.5mm X 150mm
• 0.50

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Processing Costs

• Processing features
• Simple Uniaxially pressed operation
• No need for mass production
• No continuous processing (more costly)
• No need for multiple large industrial scale
  facilities
• Less workers and utility costs

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Comparisons
Material Density Youngs Modulus Tensile Strength Estimated Cost Ranking
Bone 2.3 10 40 3 20 Na
Stainless Steel 7.9 196 290 1
Co Alloys 8.9 211 345 4
Ti Alloys 4.5 105 200 3
Mg Foam 2.33 10.476 2.843 2
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Conclusions

• Mg Foam has mechanical properties better suited
  for bone substitute than the other commercial
  products
• You have the advantage of Mg being bioresorbable
  and osteoconductive.
• Promotes new bone growth
• Is completely replaced by new bone, making the
  bone stronger
• You dont have to walk around with a foreign body
  inside your leg for the duration of your life, or
  you dont have to have a second surgery to remove
  the implant

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Conclusion Continued....

• Processing cost is comparative to products all
  ready in use
• Toxicity issues are small since the amounts of Mg
  is low, but medical workups would be advised
  before implantation
• Mg Foam is a viable option for use in a screw
  implant.

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Questions

• Any Questions about the project?