mobile bearing tka - university of...

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


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


Con

tact

Are

a (m

m2 )

"bearing" interface

"mobile" interface

50-50, 1BW

0.0

0.5

1.0

1.5

2.0


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


LCS PS (DePuy)Femoral Component

•PC substituting

Tibial Tray• rotating platform II


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


Res

istin

g To

rque

(Nm

)

LCS StdLCS PSStaticDynamic

50-50 vs 60-40 - 0° Flexion

0

2

4

6

8

10


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


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


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

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