Mechanical Properties & Wear of

UHMWPE

Mr P. Julian Owen

SpR SW Thames

Introduction

• Biomechanics

– Material properties: Stress v Strain, Creep & Fatigue

– Tribology: Friction, Lubrication & Wear

• UHMWPE

– What is it?

– Manufacture

– Improvements

– In vivo behaviour

Biomechanics 1

Stress v Strain

• Young's Modulus

– Elastic zone

• Ductile vs. Brittle

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’

Biomechanics 3

Tribology

Definition (OECD):

• ‘the science & technology of surfaces acting

upon one another in relative movement and

the problems associated therewith’

Biomechanics 4

Tribology

• Friction

• Lubrication

• Wear

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

Biomechanics 6

Lubrication

• ‘Decreases ‘m’ between 2 surfaces’

• Main mechanisms:

– Synovial joints

• Boundary

• Hydrostatic

– Artificial joints

• Boundary

Biomechanics 7

Wear

• Main mechanisms:

– Abrasive

– Adhesive

– 3rd Body

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

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

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

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

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

UHMWPE 2

Synthesis

• Catalyst

– Ziegler/Natter coordination catalyst (TiCl4 + Al

alkyl)

• Powder

– 3 grades on the market

• +/- Calcium Stearate -> Minimises

yellowing

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

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.

UHMWPE 5

Molecular Weight

• Reported molecular weights are nominal

– Underestimates

– Measured by the viscosity of a solution

UHMWPE 6

Crystalinity 1

• 2 Phases of UHMWPE

– Crystalline

– Amorphous (Non-crystalline)

• Normally 60-75% crystalline

– Estimated with differential scanning

calorimetry

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

UHMWPE 8

Crystalinity 3

• Polymer Crystaline regions

– ‘Unit cells’

• form Lamellae

• Chain fold length 150 CH2 groups

EM - Extruded Rod - Lamellae

UHMWPE 9

Fusion Defects - Asperities

• Subsurface concentrations of non-

consolidated particles

• Lead to Cracks and Delamination

• If heated to melting & recooled -> particles

coalesce

Photomicrograph - Asperity

X5 Magnification - Multiple Asperities

Even Distribution

X2 Magnification - Multiple Asperities

Uneven Distribution

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

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)

UHMWPE 12

Oxidation 3

• Inert gas packaging:

– New

– ? Improved implant life

• Oxidation continues after implantation

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%

UHMWPE 14

Room for Improvement?

• UHMWPE remains the ‘Gold standard’ in jt

replacement

• Some recent changes include:

– Composites

– Recrystalization

– Crosslinking

– Sterilisation & Packaging

UHMWPE 15

Composites

• UHMWPE & Carbon fibre composite -

‘Poly Two’

– Increased creep resistance

– Reduced fatigue resistance

– No wear improvement

– Use discontinued

UHMWPE 16

Recrysilization 1

• >250 deg c & >2800 A (atmospheres)

• Increases strength & Young’s modulus

• Not currently commercially viable

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

UHMWPE 18

Crosslinking

• Increased component stability

• ? Results

UHMWPE 19

Sterilisation & Packaging

• Inert atmosphere gamma sterilisation &

packaging

– ? Alters wear properties

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

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