mechanical properties & wear of uhmwpe of uhmwpe - mr... · 2009-07-06 · mechanical...
TRANSCRIPT
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Mechanical Properties & Wear of
UHMWPE
Mr P. Julian Owen
SpR SW Thames
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Introduction
• Biomechanics
– Material properties: Stress v Strain, Creep & Fatigue
– Tribology: Friction, Lubrication & Wear
• UHMWPE
– What is it?
– Manufacture
– Improvements
– In vivo behaviour
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Biomechanics 1
Stress v Strain
• Young's Modulus
– Elastic zone
• Ductile vs. Brittle
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Biomechanics 2
Creep & Fatigue
• Creep (‘Cold Flow’) :
– Prolonged constant load -> Progressive
deformation
– > 6mm UHMWPE to prevent creep
• Fatigue:
– ‘Failure with repetitive loading cycles at stress
below the ultimate tensile strength’
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Biomechanics 3
Tribology
Definition (OECD):
• ‘the science & technology of surfaces acting
upon one another in relative movement and
the problems associated therewith’
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Biomechanics 4
Tribology
• Friction
• Lubrication
• Wear
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Biomechanics 5
Friction
• Friction:
– ‘Resistance between 2 bodies when one slides
over the other’
– F = mN
– Not a function of contact area
• Coefficient of Friction (m)
– Human jt = 0.025
– CoCrMo Alloy on UHMWPE = 0.15
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Biomechanics 6
Lubrication
• ‘Decreases ‘m’ between 2 surfaces’
• Main mechanisms:
– Synovial joints
• Boundary
• Hydrostatic
– Artificial joints
• Boundary
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Biomechanics 7
Wear
• Main mechanisms:
– Abrasive
– Adhesive
– 3rd Body
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Biomechanics 8
Wear
• CoCrMo Alloy on UHMWPE
– Linear = 0.2mm/yr (cf. Ceramic = 0.03mm/yr)
– Volumetric = 100mm3/yr
– NB. Increases after 10 yrs in vivo
• Metal > Alumina > Zirconia
– Ceramics: ‘Wetability’ = ‘Boundary’
lubrication
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Biomechanics 9
Requirements for in Vivo Use
• Cope with NaCl ‘Corrosive’ environment
• Bio-compatibility
• Withstand & Transmit fluctuating muscle
forces - ‘Shock absorber’
• Minimal Wear
– Increased hardness/ coatings
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UHMWPE
History: Orthopaedic polymers 1
• Polytetraflouroethylene
• Charnley’s first polymer
• Low coefficient of friction
• Low resistance to creep
• Poor abrasive wear characteristics
• HMWPE
• Craven (Charnley co-worker) tested a sample from a
salesman of plastic gears
• First implanted 1962
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UHMWPE
History: Orthopaedic polymers 2
• Polyactal (Delrin & Celcon)
• Easier manufacture via injection moulding
• Higher failure rates than UHMWPE (p = 0.000007)
• High density Polyethylene v UHMWPE
– Lower M.Wt, Impact strength & toughness
– Worse abrasive wear characteristics
nb. NOT THE SAME MATERIAL
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UHMWPE 1
What is it?
• Ethylene (C2H4)70,000-210,000
• Density = 0.94g/mm3
• Melting point = 145 deg.c
• Other uses:
– Ski bottoms, cutting boards, coal chute liners
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UHMWPE 2
Synthesis
• Catalyst
– Ziegler/Natter coordination catalyst (TiCl4 + Al
alkyl)
• Powder
– 3 grades on the market
• +/- Calcium Stearate -> Minimises
yellowing
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UHMWPE 3
Manufacture 1
• Moulded
– Powder in mould -> Heat & Pressure
– No machine lines
– Can achieve high gloss finish
• Machined
– ‘Ram extruded’ powder into a cylindrical bar
– +/- Annealing
– Up to 15cm diameter
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UHMWPE 4
Manufacture 2
• Machined after Moulding
– Mould large sheets -> Machining
– Up to 2.5m x 2.5m x 20cm
NB. All methods raise temperature above
melting point, which can alter it’s
crystalline and physical properties.
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UHMWPE 5
Molecular Weight
• Reported molecular weights are nominal
– Underestimates
– Measured by the viscosity of a solution
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UHMWPE 6
Crystalinity 1
• 2 Phases of UHMWPE
– Crystalline
– Amorphous (Non-crystalline)
• Normally 60-75% crystalline
– Estimated with differential scanning
calorimetry
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UHMWPE 7
Crystalinity 2
• By heating UHMWPE above melting point
and recooling, crystalinity is decreased
• Can reverse with extremes of high pressure
& temperature
• Increased crystallinity -> Increased
resistance to fatigue crack propagation
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UHMWPE 8
Crystalinity 3
• Polymer Crystaline regions
– ‘Unit cells’
• form Lamellae
• Chain fold length 150 CH2 groups
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EM - Extruded Rod - Lamellae
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UHMWPE 9
Fusion Defects - Asperities
• Subsurface concentrations of non-
consolidated particles
• Lead to Cracks and Delamination
• If heated to melting & recooled -> particles
coalesce
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Photomicrograph - Asperity
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X5 Magnification - Multiple Asperities
Even Distribution
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X2 Magnification - Multiple Asperities
Uneven Distribution
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UHMWPE 10
Oxidation 1
• A chemical attack
– Chain scissions
– C=C double bond formation (Unsaturation)
– Oxygen incorporated in polymer
• Alters physical properties
– Low levels of chemical change -> high levels
of physical change
– Creates a brittle material -> Increased wear
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UHMWPE 11
Oxidation 2
• Gamma Sterilisation accelerates oxidation
– Breaks C-C/C-H bonds
– Interacts with oxygen to form ‘Free Radicals’
– Peak density then lies below the surface ->
Increased stresses associated with wear damage
• Increased wear (Pin on disc lab studies)
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UHMWPE 12
Oxidation 3
• Inert gas packaging:
– New
– ? Improved implant life
• Oxidation continues after implantation
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UHMWPE 13
Material Standards - US F648-84
• Ensures quality of UHMWPE used for TJR
– Ultimate tensile strength 27MPa
– Tensile yield strength 19MPa
– Elongation at break 200%
• Variability
– Yield Strengths 25%
– Elastic Modulus 35%
– Creep performance 400%
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UHMWPE 14
Room for Improvement?
• UHMWPE remains the ‘Gold standard’ in jt
replacement
• Some recent changes include:
– Composites
– Recrystalization
– Crosslinking
– Sterilisation & Packaging
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UHMWPE 15
Composites
• UHMWPE & Carbon fibre composite -
‘Poly Two’
– Increased creep resistance
– Reduced fatigue resistance
– No wear improvement
– Use discontinued
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UHMWPE 16
Recrysilization 1
• >250 deg c & >2800 A (atmospheres)
• Increases strength & Young’s modulus
• Not currently commercially viable
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UHMWPE 17
Recrysilization 2 - ’Hylemar’
• New process increasing crystalinity from 50
to 80%
– Increased tensile strength 30%
– Increases Young’s modulus 375%
– ? Behaviour - Under debate
• May detriment TJR’s with high contact &
subsurface shear stresses
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UHMWPE 18
Crosslinking
• Increased component stability
• ? Results
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UHMWPE 19
Sterilisation & Packaging
• Inert atmosphere gamma sterilisation &
packaging
– ? Alters wear properties
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UHMWPE 20
Polyethylene = weakness of TJR
• Wear
– Confirmed with retrieval specimens
• Polyethylene Reaction
– Polyethylene particles are found in
• Pseudo-membranes around failed implants
• Filling osteolytic cysts
– > Pathophysiological Hypothesis
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UHMWPE 21
The Future
• UHMWPE remains the material of choice
in TJR
• Lifetime is limited by polyethylene related
osteolytic processes
• Work must concentrate on reducing
polyethylene debris