Jennings Lm. Wear Of Knee Replacements Influence Of Kinematics And Design

THE UNIVERSITY OF LEEDS

Wear of Knee Replacements Influence of Kinematics and Design

Louise M Jennings, John Fisher

Institute of Medical and Biological Engineering

University of Leeds

VOCA Congress

September 2007


Two Types of Polyethylene Wear

1. Delamination or structural fatigue

2. Surface wear


Surface Wear in Knees

• Surface wear produces micron and sub micron size wear particles

• Accumulation in periprosthetic tissues leads to osteolysis

• Potential for long term failure we see in the hip occurring in the knee


Factors That Affect TKR Surface Wear in Current Polyethylenes

• Kinematic conditions

• Prosthesis design

• Simulator studies, 300 million cycles


Leeds ProSim Knee Simulator


Leeds ProSim Knee Simulator• Anatomical Mounting

• 6 degrees of freedom

• 4 controlled axesFemoral SideAxial LoadFlexion/Extension Tibial SideRotationDisplacement

• 2 passive axes

• Displacement or Force Control

Tibial Rotation

AP Displacement

Flexion/Extension

Axial Force

Abduction/Adduction

Medial/Lateral Translation


Kinematic Inputs

Femoral - relative to Tibia• ISO standard (ISO 14243)

– Axial Load – Flexion/Extension

Tibial - relative to Femur• Kinematics of the natural knee (Lafortune et al. 1992)

– IE Rotation – AP Displacement – AP Force (ISO)

-300

-200

-100

0

100

200

300

% Gait Cycle (HS-HS)

AF

(x

10

N)

/ A

P F

orc

e (

N)

-20

0

20

40

60

FE

, TR

(°) / AP

(mm

)

-300

-200

-100

0

100

200

300


AF

(x

10

N)

/ A

P F

orc

e (

N)

-20

0

20

40

60

FE

, TR

(°) / AP

(mm

)

-300

-200

-100

0

100

200

300


AF

(x

10

N)

/ A

P F

orc

e (

N)

-20

0

20

40

60

FE

, TR

(°) / AP

(mm

)


Effect of Bearing Materials

Fixed Bearing Knees - PFC, PFC Sigma

• Historic device and material: PFC 1020 - IR in air

• Current device and material: PFC Sigma 1020 GVF

• Potential material: PFC Sigma Marathon (5 MRad 1050

+ re melt with GP sterilisation)

• High kinematics (± 5° rotation, 10 mm displacement)

• 5 million cycles


Fixed Bearing Knees

PFC - IR in airPFC Sigma - IR under vacuum and foil packed

PFC Sigma Marathon

0

10

20

30

40

50

60

PFC 1020 gammaair

PFC Sigma 1020GVF

PFC SigmaMarathon

Wea

r R

ate

(mm

3 p

er

mil

lio

n c

yc

les

)


Effect of Kinematics on Wear

Fixed Bearing Knees


Effect of Kinematics

Fixed Bearing Knees - PFC Sigma 1020 GVF

High Kinematics

Intermediate Kinematics

Low Kinematics

AP Displacement

10 mm 5 mm 5 mm

IE Rotation ± 5 ° ± 5 ° ± 2.5 °


Fixed Bearing Knees

PFC Sigma 1020 GVF High KinematicsPFC Sigma 1020 GVF Intermediate Kinematics

PFC Sigma 1020 GVF Low Kinematics

0

5

10

15

20

25

30

High Int Low

Wea

r R

ate

(mm

3 p

er m

illio

n c

ycle

s)

McEwen et al J Biomechanics 2005


Effect of Kinematics

• Fixed Bearing knees : doubling the amount of

internal-external rotation and anterior-posterior

translation produced a five fold increase in wear rate

• Implications for young, high demand patients

• Wear is increased by more multidirectional

kinematics due to greater cross shear on the

polyethylene surface


Fixed Bearing Knees Multidirectional motion of the femoral component

relative to the tibial bearing surface

femoral rotation

a-p translation

tray

femoral

bearing

flexion- extension


Molecular Strain Hardening

UHMWPE exhibits molecular orientation in the principle direction of sliding (Pooley & Tabor 1972)

principle direction of sliding

stra

in s

ofte

ning

strain hardening

Orientation leads to increased strength parallel to sliding (‘hardening’) and reduced strength transverse to sliding (‘softening’) (Wang et al 1996)


Effect of Prosthesis Design


Effect of Prosthesis Design

Fixed Bearing PFC SigmaMobile Bearing PFC Sigma Rotating Platform

Fixed Bearing Mobile Bearing

Rotating Platform

• 5 million cycles


Fixed Bearing Knee PFC Sigma Mobile Bearing Knee PFC Sigma RP

Engineering Comparison: Matched Intermediate Kinematics

0

5

10

15

Fixed RP Mobile

Wea

r R

ate

(mm

3 p

er m

illi

on

cyc

les)

McEwen et al, J Biomechanics 2005


Fixed Bearing Knee PFC Sigma Mobile Bearing Knee PFC Sigma RP

Clinical Comparison High Kinematics

05

101520253035

Fixed AP 10mm RP Mobile AP 4mm

Wea

r R

ate

(mm

3 p

er m

illi

on

cyc

les)

McEwen et al, J Biomechanics 2005


Mobile Bearing Knees

Bearing rotation decoupled by allowing rotation at the tibial counterface

reduced rotation at the femoral counterface

Tibial counterface rotation in rotating platform (RP) mobile bearings is simple linear motion

tibialrotation

flexion- extension

femoral

bearing

tray


Kinematics

wea

r ra

te

Multidirectional motion accelerates UHMWPE wear

(Wang et al 1996,

Barbour et al 1999)

unidirectionalmultidirectional

Friction

vectors

Wear

path

— transverse— principal

FFR

Unidirectional

motion reduces wear Rotating platform

(Marrs et al 1999)

(Pooley et al 1962)

RPFixedHip


Wear DebrisVolumetric Concentration

0

20

40

60

80

100

120

PFC Sigmafixed

PFC SigmaRP

LCS RP Hip

Implant Design

Mean

% v

olu

metr

ic c

on

cen

trati

on

of

part

icle

s

<0.1

0.1 - 1.0

1.0-10

>10


Biological Activity and Osteolytic Potential

Implant Type Mean Wear Rate (mm3/106

cycles)

SBA

Specific

Biological Activity

FBA

Functional

Osteolytic

Potential

PFC Sigma 22.75 ± 5.95 0.3 6.8

PFC Sigma RP

5.2 ± 3.8 0.36 1.8

Hip 25.6 ± 5.3 0.69 17.6

Tipper et al, Soc Biomaterials 2005


Knee Design

• Rotating platform mobile bearings produced a substantial reduction in wear compared to fixed bearing knees

• Osteolytic potential of knees is less than hips, debris less reactive

• Osteolytic potential of rotating platform mobile bearing knees substantially reduced compared to fixed bearing knees

• However…


Lift Off

• Introduced adduction moment to tibial carriage

• 1 mm of lateral femoral condylar lift off during swing phase

• Associated M/L displacement

• Simulated for every gait cycle

Jennings et al ORS 2005


Effect of Lift Off Fixed Bearing PFC Sigma

Mobile Bearing PFC Sigma Rotating Platform

0

5

10

15

20

FixedIntermediateConditions

Fixed Lift Off RP Standard RP Lift Off

We

ar

Ra

te (

mm

3 /mill

ion

cyc

les)

p < 0.05 p < 0.05

p > 0.05


Lift off

• Femoral condylar lift off accelerated the wear of both fixed and mobile bearing knees

• Medial condyle displayed more wear damage under lift off conditions due to– Elevated contact stresses as lateral lift off

produced uneven loading of the bearing

– Acceleration of wear by cross shearing of

polyethylene in the medial/lateral direction


Conclusions

Rotating platform mobile bearing knees

reduced wear rate and functional osteolytic potential by a factor of four

However some of this benefit is lost if lift off and medial lateral shift occurs,

which increases wear in both designs

Surgery stability and soft tissue reconstruction is important in wear performance


Thank you

Co authorsHannah McEwen, Petra Barnett, Carol Bell,

Amir Kamali, Dan Auger, Richard Farrar,Joanne Tipper, Alison Galvin,

Mark Taylor, Martin Stone, Eileen Ingham

Research supported by EPSRC, ARC, DePuy

Jennings Lm. Wear Of Knee Replacements Influence Of Kinematics And Design

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