Patient-Specific Analysis of the Influence of VMO Training on Patellofemoral Forces and Pressures John J. Elias, PhD Surya P. Rai, MS David M. Weinstein, MD David L. Walden, MD Medical Education and Research Institute of Colorado Colorado Springs, CO

Patellofemoral Pain Patellofemoral pain is frequently attributed to lateral malalignmentPatellofemoral pain is frequently attributed to lateral malalignment Lateral shift and tilt can increase pressure applied to lateral cartilageLateral shift and tilt can increase pressure applied to lateral cartilage Overloading cartilage can lead to cartilage degradation and painOverloading cartilage can lead to cartilage degradation and pain

VMO Training VMO resists lateral force and lateral tilt momentVMO resists lateral force and lateral tilt moment A weak VMO and delayed VMO activation can contribute to patellofemoral painA weak VMO and delayed VMO activation can contribute to patellofemoral pain Physical therapy regimens commonly emphasize training the VMOPhysical therapy regimens commonly emphasize training the VMO Stoller et al. Interactive Knee - Radiology ©1999 Primal Pictures Ltd.

Study Goals Create computational models representing patients with patellofemoral painCreate computational models representing patients with patellofemoral pain Characterize how eliminating VMO weakness and delayed VMO activation influence patellofemoral force and pressure distributionsCharacterize how eliminating VMO weakness and delayed VMO activation influence patellofemoral force and pressure distributions

Patient-Specific Models Obtained IRB approvalObtained IRB approval MRI images of knees at full extension and 45° [Cohen et al., Am J Sports Med 31:87-98]MRI images of knees at full extension and 45° [Cohen et al., Am J Sports Med 31:87-98] Reconstructed bone surfaces and cartilageReconstructed bone surfaces and cartilage Characterize orientation of quadriceps muscles and patella tendon [Delp et al. IEEE Trans Biomed Eng 37: , Farahmand et al. J Orthop Res. 16:136-43]Characterize orientation of quadriceps muscles and patella tendon [Delp et al. IEEE Trans Biomed Eng 37: , Farahmand et al. J Orthop Res. 16:136-43] Patella Tendon VMOVL VML RF VI

Patellofemoral Kinematics Rotate tibia about distal femurRotate tibia about distal femur Characterize patellofemoral alignment of flexed kneeCharacterize patellofemoral alignment of flexed knee Maintain patella apex within groove and orientation of lateral facetMaintain patella apex within groove and orientation of lateral facet Patella flexion proportional to tibiofemoral flexionPatella flexion proportional to tibiofemoral flexion

Patellofemoral Cartilage Identify cartilage lesions based on thicknessIdentify cartilage lesions based on thickness Model cartilage as springsModel cartilage as springs E norm = 4 MPa, E lesion = 1 MPa, ٧ = 0.45, h = thickness, d = compressionE norm = 4 MPa, E lesion = 1 MPa, ٧ = 0.45, h = thickness, d = compression ٧ k n = -E(1- ٧ )ln(1 – d/h) ٧٧ (1 + ٧ )(1 – 2 ٧ )d k s = 0.02×k n mm med lat 0 6 [Blankevoort et al. J Biomech 24: ]

Patellofemoral Loading VMOVMLVIRFVL Patellofemoral Pain 4%9%44%22%21% Pain Free 10%12%40%19%19% Delayed VMO 0%9%44%22%21% Quadriceps Force Distribution for 30 N-m Extension Moment [Makhsous et al. Med Sci Sports Exerc 36: , Zhang et al. J Orthop Res 21:565-71]

Discrete Element Analysis F M V = potential energy  = spring deformation u = displacement vector K = stiffness matrix R = force vector knkn cartilage springs V = 1 / 2  (k n  n 2 + k s  s 2 )dS V = 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: , Elias et al. J Biomech 39: , Elias et al. J Biomech 37: , Elias et al. Am J Sports Med 32:1202-8]

Resultant Force and Moment VMO Lat Force Lat Rot Lat Tilt Force applied by VMO decreases lateral force, lateral tilt moment and lateral rotation moment acting on patella

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

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

Cartilage Lesions Lesions identified for 5 kneesLesions identified for 5 knees Various locations on patella and femurVarious locations on patella and femur Lesions increased pressure, had little influence on effectiveness of VMO trainingLesions increased pressure, had little influence on effectiveness of VMO training Lesions Quads Distribution

Patellofemoral PainPain Free Lateral Medial MPa 40° Patella Lesion

Patellofemoral PainPain Free Lateral Medial MPa 80° Patella Lesion Trochlear Lesion

Conclusions Improving VMO function reduces force applied to lateral cartilage by decreasing lateral force and lateral tilt moment acting on patellaImproving VMO function reduces force applied to lateral cartilage by decreasing lateral force and lateral tilt moment acting on patella Decreasing lateral force, lateral tilt moment and lateral rotation moment acting on patella decrease maximum cartilage pressureDecreasing lateral force, lateral tilt moment and lateral rotation moment acting on patella decrease maximum cartilage pressure

Conclusions Cartilage lesions increase maximum cartilage pressureCartilage lesions increase maximum cartilage pressure Further modeling of lesions necessary to characterize influence on effectiveness of VMO trainingFurther modeling of lesions necessary to characterize influence on effectiveness of VMO training Results depend on accuracy of computational assumptionsResults depend on accuracy of computational assumptions

Acknowledgements Colorado Institute for TechnologyColorado Institute for Technology