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Mobile Bearing TKA
Knee JointComplex motion
•flexion/extension•sliding•rolling•endo/exorotation
Large joint loads•4 x body weight
Cartilage degeneration ⇒ osteoarthritis
femur
patella
tibia
femoralcomponent
tibialcomponent
poly
Total Knee Replacement•Resurface joint ends
•metal and plastic•Accommodate complexmotion
Fixed bearing•Single contact surface•Round on flat•High stresses ⇒ wear/failure•Balance conformity/constraint
Mobile Bearing Total Knees•↑ conformity, ↓ constraint•Two contact surfaces•Abnormal kinematics•Limited bearing motion
(Nilsson et al., 1991; Stiehl et al., 1997; Hartford et al., 2001)
•High conforming fixed bearing
Understanding mobility important to success
PFC SigmaRotating Platform TKA
TKA Mobile Bearing Designs
LCS (DePuy)
Hypothesis
•Friction at the “mobile” interface of a rotating platform total knee produces sufficient counter-torque to interfere with endo/exorotation.
LCS PS (DePuy)Femoral Component
•PC substituting
Tibial Tray• rotating platform II
Polyethylene Insert• 10.0 mm thickness
Finite Element MeshModel Geometry
• IGES
Metal Components•ECoCr >> Epoly• rigid Bezier surface•3-noded triangular
Polyethylene Insert• 8-noded brick
Polyethylene Material PropertiesE(σ) = 634.92 – 12.31σ – 3.61σ2 + 0.199σ3 – 0.00283σ4
0
10
20
30
40
50
60
0 0.2 0.4 0.6Strain (mm/mm)
Stre
ss (M
Pa)
Nonlinear model(Cripton, 1993)634.92
ν = 0.45
Dual SurfaceInteractions“Bearing” Interface
•FC/PE insert
“Mobile” Interface•TT/PE insert
Friction•µ = 0.089
Constraints/FreedomsFemoral Component
• free translation• free varus/valgusrotation
Tibial Tray• fixed translation•prescribed internalrotation
Loading Conditions
Variables Examined• resisting torque • contact area•contact stress • relative rotation
LoadAllocation
Axial Load(BW)
FlexionAngle (°)
InternalRotation (°)
50-50 1, 2, 3, 4 0 1060-40 1, 2, 3, 4 0 10
50-50 10,15,2030,60,90 10
Step Procedure1: TT/PE into contact
2: FC/PE into contact
3: 50 N load
4: PE freed
5: FC freed in translationand V/V rotation
6: Physiological load
Testing FixtureFemoral Component
•axial/torsional load•endo/exorotation• flexion
Tibial Tray•M/L and A/P•V/V
Condylar Load•equal•medially biased
0° Flexion
0
1
2
3
4
5
6
7
0 2 4 6 8 10Tibial Tray Rotation (deg)
Res
istin
g To
rque
(N-m
) 4 BW
3 BW
2 BW
1 BW
50-5060-40
0°
0
1
2
3
4
5
6
7
0 1 2 3 4x Body Weight
Res
istin
g To
rque
(N-m
)
50-5060-40
0° Flexion
0
1
2
3
4
5
6
7
1 2 3 4x Body Weight
Res
istin
g To
rque
(N-m
)
50-50 FEM50-50 EXP60-40 FEM60-40 EXP
50-50, 1 BW
0.0
0.5
1.0
1.5
2.0
0 15 30 45 60 75 90Flexion Angle (deg)
Torq
ue (N
-m)
FEMEXP
50-50, 0° Flexion
4 BW
1 BW
No rotation 10º rotation
50-50, 0° Flexion
A
P
ML
2 BW
28 mm
Stress Distribution•peripheral edge loading• long moment arm•FE/Exp match well
0° Flexion
0
200
400
600
800
1000
1200
0 1 2 3 4x Body Weight
Con
tact
Are
a (m
m2 )
'mobile' interface
'bearing' interface
50-5060-4050-5060-40
50-50, 1 BW
0
100
200
300
400
500
600
0 15 30 45 60 75 90Flexion Angle (deg)
Con
tact
Are
a (m
m2 )
"bearing" interface
"mobile" interface
50-50, 1BW
0.0
0.5
1.0
1.5
2.0
0 15 30 45 60 75 90Flexion Angle (deg)
Rel
ativ
e R
otat
ion
(deg
)
FEMEXP
FE vs Experimental Agreement
•Resisting torque• load allocation•axial load• flexion angle
•Contact stress distribution•peripheral edge loading
•Relative rotation
Summary
•50-50 and 60-40 same•Resisting torque ∝ axial load•20° flexion ⇒ FC radii transition
• less congruent•↓ contact area•↑ relative rotation
•Peripheral edge loading• large moment arm
Peak Torque•Present data: 5.98 ± 0.19 N-m•Taylor et al., 1998: 6-8 N-m
Design Improvements?•↓ peripheral stresses•↓ resisting torque
Loading Conditions•50-50 or 60-40•1, 2, 3, or 4 BW (1 BW = 686.5 N)•0, 15, 20, 30, 60, 90° flexion• internally rotated 10°•elastic ⇒ 4 BW, 50-50, 0° flexion
Variables Examined• resisting torque•contact stress•contact area• relative rotation
KinematicsFlexion
•60° walking (Lafortune et al., 1992 )• 90° stair ascent/decent (Andriacchi et al., 1980)
Internal/External Rotation•12° walking (Kettelkamp et al., 1970)
Posterior Translation•7 mm walking (Dennis et al., 2001)• 20 mm high flexion (Dennis et al., 2001)
KineticsAxial Load
•4 BW walking (Morrison, 1970)• 5 BW stair ascent/decent (Morrison, 1969)•Medially biased (Morrison, 1970)
Internal/External Torque•8 Nm walking (Li et al., 1993)
ArthritisCartilage degeneration ⇒ Pain ⇒ TKR
PurposeStudy the mobility and contact mechanics of rotating platform TKRs under functional loading conditions
MethodMobility ⇒ internal/external torque, rotationContact Mechanics ⇒ contact stress, areaParametric Evaluation
•physical experiments• finite element model
Walking Cycle
LCS Std (DePuy)Femoral Component
•PC sacrificing•CoCr
Tibial Tray• rotating platform•CoCr
Polyethylene Insert• 6.0 mm thickness
LCS PS (DePuy)Femoral Component
•PC substituting
Tibial Tray• rotating platform II
Polyethylene Insert• 6.0 mm thickness
Loading Conditions•Axial load 1, 2, 3, 4 BW (1 BW=687 N)•Load allocation 50-50 or 60-40•Flexion angle 0, 15, 30, 45, 60, 90°•Axial rotation ±10°•Lubrication bovine serum
Variables Examined•Resisting torque
•static/dynamic• Insert rotation lag•Contact distribution (Fuji)
Finite Element ModelModel Geometry
• IGES → PATRAN
Metal Components•ECoCr >> Epoly• rigid surface•3-noded triangular
Polyethylene Insert• nonlinear solid•8-noded brick
femoralcomponent
tibialtray
polyinsert
LCS Std
LCS PS
FE AnalysisABAQUS
•3D•Nonlinear
•Materially•Geometrically
•Large displacement•Multi-contact
Dual SurfaceInteractions•Bearing interface•Mobile interface•Friction, µ = 0.089
Constraints/Freedoms•Same DOF as testingfixture
•PE free to move
“bearing”
“mobile”
Loading Conditions•Axial load 1, 2, 3, 4 BW (1 BW=687 N)•Load allocation 50-50 or 60-40•Flexion angle 0, 15, 30, 45, 60, 90°•Axial rotation ±10°•Friction µ = 0.089
Variables Examined•Resisting torque • Contact stress• Insert rotation lag • Contact area
Full Walking Cycle Simulation• ISO 14243-1• Input waveforms
•axial load (medially biased)•axial torque•A/P forces• flexion angle
•Soft tissue constraints•A/P•axial rotation
0
500
1000
1500
2000
2500
3000
0 20 40 60 80 100Walking Cycle (%)
Axi
al L
oad
(N)
0
10
20
30
40
50
60
0 20 40 60 80 100Walking Cycle (%)
Flex
ion
Ang
le (°
)
-2-101234567
0 20 40 60 80 100
Walking Cycle (%)
Axi
al T
orqu
e (N
-m) internal (+)
external (-) -300-250-200-150-100
-500
50100150
0 20 40 60 80 100
Walking Cycle (%)
A/P
For
ce (N
)
anterior (+)
posterior (-)
ISO 14243-1
LCS Std vs PS - 50-50, 0° Flexion
LCS Std vs PS - 50-50, 0° Flexion
0
2
4
6
8
10
0 1 2 3 4x Body Weight
Res
istin
g To
rque
(Nm
)
LCS StdLCS PSStaticDynamic
50-50 vs 60-40 - 0° Flexion
0
2
4
6
8
10
1 2 3 4x Body Weight
Res
istin
g To
rque
(Nm
)50-5060-40
PS Exp StaticPS Exp DynamicPS FEM DynamicStd FEM Dynamic
Exp vs FEM - 0° Flexion
0
2
4
6
8
10
1 2 3 4x Body Weight
Res
istin
g To
rque
(Nm
)ExperimentalFEM
Std 50-50PS 50-50PS 60-40
LCS Standard - 3 BW, 50-50
0123456789
10
0 15 30 45 60 75 90Flexion Angle (°)
Res
istin
g To
rque
(Nm
)
Exp Static
Exp Dynamic
FEM Dynamic
LCS Standard
LCS PS - 50-50, 0° Flexion
50-50, 0° Flexion
0
200
400
600
800
1000
1200
1400
0 1 2 3 4x Body Weight
Con
tact
Are
a (m
m2 )
LCS StdLCS PS
mobile interface
bearing interface
LCS Standard - 50-50, 3 BW
0
200
400
600
800
1000
1200
1400
0 15 30 45 60 75 90Flexion Angle (°)
Con
tact
Are
a (m
m2 )
bearing interface
mobile interface
Lag vs Flexion Angle - 3 BW, 50-50
0.0
1.0
2.0
3.0
4.0
0 15 30 45 60 90Flexion Angle (°)
Inse
rt R
otat
ion
Lag
(°)
LCS StdLCS PS
Exp StaticExp DynamicFEM Dynamic
LCS Standard
-2
-1
0
1
2
3
4
5
6
0 20 40 60 80 100
% Walking Cycle
Axi
al R
otat
ion
(°)
endorotation (+)
exorotation (-)
Finite Element Model Validation|
•Resisting torque•axial load• load allocation• flexion angle
• Insert rotation lag•Contact stress distributions
Peak Torque•LCS Standard: 9.47 N-m (static)
5.51 N-m (dynamic)•Taylor et al., 1998: 6-8 N-m
Internal/External Rotation•LCS Standard: 6°•Normal knee: 12° (Kettelkamp et al., 1970)
Summary
•Resisting torque ∝ axial load•50-50 and 60-40 same•Mobile area > bearing area•25-30° flexion ⇒ FC radii transition
• less congruent, ↓ contact area•↑ insert rotation lag
• Insert rotation lag small•Contact stresses < 7 MPa
Comparison Std PSEdge loading •Contact stresses •Contact area
bearing •mobile •
Insert rotation lag •Resisting torque
static •dynamic •
mobile bearing
Backside wear•mobile bearing• fixed bearing
Evaluate other designs•capture mechanisms•PC substituting posts
Improve mobility•parametric designchanges
fixed bearing
LCS PS - 50-50, 0° Flexion
4 BW
1 BW
No rotation 10º rotation
Standard PS
FEA Full Gait Cycle Comparisons
Flanges
Insert Footprints
0
500
1000
1500
2000
2500
3000
0 20 40 60 80 100Walking Cycle (%)
Axi
al L
oad
(N)
0
10
20
30
40
50
60
0 20 40 60 80 100Walking Cycle (%)
Flex
ion
Ang
le (°
)
-2-101234567
0 20 40 60 80 100
Walking Cycle (%)
Axi
al T
orqu
e (N
-m) internal (+)
external (-) -300-250-200-150-100
-500
50100150
0 20 40 60 80 100
Walking Cycle (%)
A/P
For
ce (N
)
anterior (+)
posterior (-)
ISO 14243-1
-2
-1
0
1
2
3
4
5
0 20 40 60 80 100% Walking Cycle
Axia
l Rot
atio
n (d
eg)
StandardPS
external (-)
internal (+)
(a)
Axial Rotation
Lift-Off
0.0
0.5
1.0
1.5
2.0
0 20 40 60 80 100% Walking Cycle
Lifto
ff (m
m)
-5
0
5
10
15
20
Rol
lbac
k (m
m)
StandardPS
LiftoffRollback
(b)
Rollback & Lift-Off
0
5
10
15
20StandardPS
anterior posterior
(a)
medial profile
Bearing Surface Contact Stress
0
5
10
15
20StandardPS
anterior posterior
(b)
medial profile
Mobile Interface Contact Stress