Biomaterials Used in
Orthopedic Implants
Magnesium Foam As a Bioresorbable
Implant
Presented By
Joe Barker
Orthopedics
Bone properties, Osteoconductivity,
& Biocompatibility
Bone Properties





Density – 2.3g/cm3
Tensile Strength – 3-20MPa
Compressive Strength – 15,000 psi
Shear Strength – 4,000 psi
Young’s Modulus – 10-40 MPa
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
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
Screw Types
CANNULATED
SCREW
 Screw Sizes
–
–
6.5mm X 102mm
4.5 X 12.5mm
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
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.
Screw Types
Transverse Screws Continued....
Orthopedic Materials:
Metals
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
Devices Were Metals Are
Used

Orthopedic devices
– Plates and screws, Pins and
Wires, rods (temporary)
– Total joints (permanent)
– Clips and staples
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
Stainless Steel





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Generally about 12% chromium (316L, Fe-Cr-NiMo)
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
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
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
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
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
Orthopedic Materials
Metallic Foams
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 metals:A 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.
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
Requirements for Porous
Implant





Pore Morphology (Spherical)
Pore Size (200m - 500m)
Porosity
High Purity (99.9%)
Biocompatibility
Why Magnesium?


Bioresorbable
Biocompatible
–
–

Osteoconductive
Osteoinductive
Properties of bone can be easily attained
using varying processing techniques
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
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
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
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
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/ys=C(/s)3/2; C=0.3
– E/Es=A(/s)2; A=1
Properties Attained from
Processing
With a porosity of  50%
– Density = 0.87g/cm3
– pl = 2.33MPa
– ys = 2.843 MPa
– E = 10.476GPa
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.
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
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
Comparisons
Material
Density
Youngs Tensile
Modulus Strength
Bone
2.3
10 – 40
3 – 20
Estimated
Cost
Ranking
Na
Stainless 7.9
Steel
Co
8.9
Alloys
Ti Alloys 4.5
196
290
1
211
345
4
105
200
3
Mg Foam 2.33
10.476
2.843
2
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 don’t have to walk around with a foreign body
inside your leg for the duration of your life, or you
don’t have to have a second surgery to remove the
implant
Conclusion Continued....
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
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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.
Questions
Any Questions about the project?