jennings lm. wear of knee replacements influence of kinematics and design
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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
THE UNIVERSITY OF LEEDS
Two Types of Polyethylene Wear
1. Delamination or structural fatigue
2. Surface wear
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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
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Factors That Affect TKR Surface Wear in Current Polyethylenes
• Kinematic conditions
• Prosthesis design
• Simulator studies, 300 million cycles
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Leeds ProSim Knee Simulator
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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
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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
% 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
% Gait Cycle (HS-HS)
AF
(x
10
N)
/ A
P F
orc
e (
N)
-20
0
20
40
60
FE
, TR
(°) / AP
(mm
)
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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
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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
)
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Effect of Kinematics on Wear
Fixed Bearing Knees
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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 °
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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
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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
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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
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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)
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Effect of Prosthesis Design
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Effect of Prosthesis Design
Fixed Bearing PFC SigmaMobile Bearing PFC Sigma Rotating Platform
Fixed Bearing Mobile Bearing
Rotating Platform
• 5 million cycles
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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
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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
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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
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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
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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
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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
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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…
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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
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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
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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
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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
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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