Patient-Specific Analysis of the Patient-Specific Analysis of the Influence of VMO Training on Influence of VMO Training on

Patellofemoral Forces and Patellofemoral Forces and PressuresPressures

John J. Elias, PhDSurya P. Rai, MS

David M. Weinstein, MDDavid L. Walden, MD

Medical Education and Research Institute of ColoradoColorado Springs, CO

Patellofemoral PainPatellofemoral Pain• Patellofemoral pain is frequently Patellofemoral pain is frequently

attributed to lateral malalignment attributed to lateral malalignment • Lateral shift and tilt can increase Lateral shift and tilt can increase

pressure applied to lateral cartilagepressure applied to lateral cartilage• Overloading cartilage can lead to Overloading cartilage can lead to

cartilage degradation cartilage degradation and painand pain

VMO TrainingVMO Training

• VMO resists lateral force and VMO resists lateral force and lateral tilt momentlateral tilt moment

• A weak VMO and delayed VMO A weak VMO and delayed VMO activation can contribute to activation can contribute to patellofemoral painpatellofemoral pain

• Physical therapy regimens Physical therapy regimens commonly emphasize training commonly emphasize training the VMOthe VMO

Stoller et al.Interactive Knee - Radiology©1999 Primal Pictures Ltd.

Study GoalsStudy Goals

• Create computational models Create computational models representing patients with representing patients with patellofemoral painpatellofemoral pain

• Characterize how eliminating VMO Characterize how eliminating VMO weakness and delayed VMO weakness and delayed VMO activation influence patellofemoral activation influence patellofemoral force and pressure distributionsforce and pressure distributions

Patient-Specific ModelsPatient-Specific Models• Obtained IRB approvalObtained IRB approval• MRI images of knees at MRI images of knees at

full extension and 45full extension and 45°° [Cohen [Cohen et al., Am J Sports Med 31:87-98]et al., Am J Sports Med 31:87-98]

• Reconstructed bone Reconstructed bone surfaces and cartilagesurfaces and cartilage

• Characterize orientation Characterize orientation of quadriceps muscles of quadriceps muscles and patella tendon and patella tendon [Delp et al. [Delp et al. IEEE Trans Biomed Eng 37: 557-567, IEEE Trans Biomed Eng 37: 557-567, Farahmand et Farahmand et al. J Orthop Res. 16:136-43al. J Orthop Res. 16:136-43]]

Patella Tendon

VMO VL

VML

RF

VI

Patellofemoral KinematicsPatellofemoral Kinematics

• Rotate tibia about distal femurRotate tibia about distal femur• Characterize patellofemoral Characterize patellofemoral

alignment of flexed kneealignment of flexed knee• Maintain patella apex within groove Maintain patella apex within groove

and orientation of lateral facetand orientation of lateral facet• Patella flexion Patella flexion

proportional to proportional to tibiofemoral flexiontibiofemoral flexion

Patellofemoral CartilagePatellofemoral Cartilage• Identify cartilage lesions based on Identify cartilage lesions based on

thicknessthickness• Model cartilage as springsModel cartilage as springs• EEnormnorm = 4 MPa, E = 4 MPa, Elesionlesion = 1 MPa, = 1 MPa, ٧٧ = 0.45, = 0.45,

h = thickness, d = compressionh = thickness, d = compressionkn = -E(1- ٧٧)ln(1 – d/h) (1 + ٧٧)(1 – 2 ٧٧)d

ks = 0.02×kn

mmmed lat

0

6

[Blankevoort et al. J Biomech 24: 1019-1031]

Patellofemoral LoadingPatellofemoral Loading

VMOVMO VMLVML VIVI RFRF VLVL

Patellofemoral PainPatellofemoral Pain 4%4% 9%9% 44%44% 22%22% 21%21%Pain FreePain Free 10%10% 12%12% 40%40% 19%19% 19%19%Delayed VMODelayed VMO 0%0% 9%9% 44%44% 22%22% 21%21%

Quadriceps Force Distribution for 30 N-m Extension Moment

[Makhsous et al. Med Sci Sports Exerc 36:1768-75, Zhang et al. J Orthop Res 21:565-71]

Discrete Element AnalysisDiscrete Element Analysis

FM

V = potential energy = spring deformationu = displacement vectorK = stiffness matrixR = force vector

kn

cartilage springs V = 1/2(knn

2+ kss

2)dSV = 1/2{u}T[K]{u}R = V/u = [K]{u}

quads forces

pat tendon forces

Resultant force and moment applied in 5 equal steps

[Elias and Cosgarea, Am J Sports Med 34:1478-85, Elias et al. J Biomech 39:865-72, Elias et al. J Biomech 37:295-302, Elias et al. Am J Sports Med 32:1202-8]

Resultant Force and MomentResultant Force and Moment

40

50

60

70

80

90

100

110

120

30 40 50 60 70 80 90Flexion Angle (degrees)

Late

ral F

orce

(N)

Delayed VMOPatellofmeoral PainPain Free

0.5

1

1.5

2

2.5

3

30 40 50 60 70 80 90Flexion Angle (degrees)

Late

ral T

ilt M

omen

t (N

-m)

Delayed VMOPatellofemoral PainPain Free

VMO

Lat Force

Lat Rot

Lat Tilt

Force applied by VMO decreases lateral force, lateral tilt moment and lateral rotation moment acting on patella

0

0.5

1

1.5

2

2.5

30 40 50 60 70 80 90Flexion Angle (degrees)

Late

ral R

otat

ion

Mom

ent (

N-m

)

Delayed VMOPatellofemoral PainPain Free

Force DistributionForce Distribution

Open symbols significantly different from Open symbols significantly different from Patellofemoral Pain case (p < 0.05)Patellofemoral Pain case (p < 0.05)

70%

72%

74%

76%

78%

80%

82%

84%

86%

30 40 50 60 70 80 90Flexion Angle (degrees)

Late

ral F

orce

Per

cent

age

Delayed VMOPatellofemoral PainPain Free

Maximum PressureMaximum Pressure

Open symbols significantly different from Open symbols significantly different from Patellofemoral Pain case (p < 0.05)Patellofemoral Pain case (p < 0.05)

2

3

4

5

6

7

8

30 40 50 60 70 80 90Flexion Angle (degrees)

Max

Pre

ssur

e (M

Pa)

Delayed VMOPatellofemoral PainPain Free

Cartilage LesionsCartilage Lesions• Lesions identified for 5 kneesLesions identified for 5 knees• Various locations on patella and Various locations on patella and

femurfemur• Lesions increased Lesions increased

pressure, had littlepressure, had littleinfluence on influence on effectiveness of VMOeffectiveness of VMOtrainingtraining

1.5

2

2.5

3

3.5

4

30 40 50 60 70 80 90Flexion Angle (degrees)

Max

imum

Pre

ssur

e (M

Pa)

Pain, With LesionsPain, No LesionsPain Free, With LesionsPain Free, No Lesions

Lesions

Quads Distribution

Patellofemoral Pain Pain Free

LateralMedial

0

2.5

1.25

MPa

40°Patella Lesion

Patellofemoral Pain Pain Free

LateralMedial

0

2

1

MPa

80°Patella Lesion

Trochlear Lesion

ConclusionsConclusions

• Improving VMO function reduces Improving VMO function reduces force applied to lateral cartilage by force applied to lateral cartilage by decreasing lateral force and lateral decreasing lateral force and lateral tilt moment acting on patellatilt moment acting on patella

• Decreasing lateral force, lateral tilt Decreasing lateral force, lateral tilt moment and lateral rotation moment moment and lateral rotation moment acting on patella decrease maximum acting on patella decrease maximum cartilage pressurecartilage pressure

ConclusionsConclusions

• Cartilage lesions increase maximum Cartilage lesions increase maximum cartilage pressurecartilage pressure

• Further modeling of lesions Further modeling of lesions necessary to characterize influence necessary to characterize influence on effectiveness of VMO trainingon effectiveness of VMO training

• Results depend on accuracy of Results depend on accuracy of computational assumptionscomputational assumptions

AcknowledgementsAcknowledgements

• Colorado Institute for TechnologyColorado Institute for Technology