Prediction of the vertebral compressive failure load

WCB2014, 6-11th July, Boston, MA

Prediction of the vertebral compressive failure load using finite element models accounting for the adjacent intervertebral discs Yongtao Lu1, Ghislain Maquer2, Oleg Museyko3, Klaus Püschel4, Klaus Engelke3, Philippe zysset2, Michael M. Morlock1, Gerd Huber1 1Institute

of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

2Institute

of Surgical Technology & Biomechanics, University of Bern, Bern, Switzerland

3Institute 4Department

of Medical Physics, University of Erlangen-Nuremberg, Erlangen, Germany

of Legal Medicine, University Hospital of Hamburg-Eppendorf, Hamburg, Germany

Introduction – 1/5 Medical background • Vertebral compression fracture: a big and serious health problem • More than 700,000 such fractures each year • Direct medical costs due to VCF about 1.1 billion, may rise 50% by 2025

Vertebral Compression Fracture (VCF)

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Introduction – 2/5 Current VCF diagnose tool

DXA measured aBMD measurement only account around 50% of the variability in VCF[Lochmüller et al., 2002] Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

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[Matsumoto et al., 2009]

Experiment failure load [kN]

Introduction – 3/5 Finite element technique y = 0.66x + 0.91 R2 = 0.78

FE predicted failure load [kN] [Dall‘Ara et al., 2011] [Dall‘Ara et al., 2011] Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

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Introduction – 4/5 Vertebral fracture type

V.S. A

B

C

[Garfin et al., 1998]

Single vertebra FE model not able to simulate the failure of vertebral endplates! Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

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Introduction – 5/5 Influence of disc on vertebral fracture

Models with PMMA

Models with disc

[Maquer et al., 2012]

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Research question:

Can the prediction of vertebral compressive strength be improved by accounting adjacent intervertebral discs in the FE models? Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

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Methods – 1/7 Mechanical testing

Finite element modelling

FE-PMMA model

FE-IVD model

Regression Vertebral failure loads

Analysis

Predicted failure loads

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Methods – 2/7 Mechanical testing • 13 spinal segments (T11/T12/L1) fractured under 4◦ wedge loading • Spinal facet joints removed • T11 and L1 augmented to ensure the fracture in T12

T11 L1

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Methods – 3/7 Finite element modelling 1. FE-PMMA model • Axial compression loading • No IVD, vertebral endplates embedded in PMMA 2. FE-IVD model • 4◦ forward bending followed by axial compression • IVD with hyperelastic behaviour Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

2

1 IVD

T12

IVD

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Methods – 4/7 Finite element modelling – vertebral body The anisotropic elasticplastic-damage model for vertebra - T12 [Schwiedrzik and Zysset, 2013]

[Maquer et al., 2012] Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

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Methods – 5/7 Finite element modelling - Intervertebral disc (IVD) • Geometry of IVD from adjacent vertebral bodies • Volume ratio of nucleus based cross-section of IVD

Procedure for generating the IVD mesh Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

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Methods – 6/7 Finite element modelling - intervertebral disc (IVD)  Hyperelastic IVD models

Two cross-fibres embedded in annulus Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

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Methods – 7/7 Finite element modelling - intervertebral disc (IVD)  IVD material properties

Moderately degenerated

Healthy

Component Nucleus pulposus

severely degenerated

Healthy

Moderately degenerated

Severely degenerated

C10 [MPa]

0.12

0.170

0.190

C01 [MPa]

0.03

0.041

0.045

D [Mpa-1]

0.0005

0.158

0.300

Annulus fibrosus

C10 = 0.1MPa, C20 = 2.5 MPa, D = 0.3 MPa-1, K1 = 1.8 MPa, K2 = 11.0

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Results Linear regression of failure load with FE predictions

Similar correlations with vertebral failure load were found for FE-PMMA and FE-IVD models! Institute of Biomechanics, TUHH Hamburg University of Technology, Hamburg, Germany

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Discussion Why the FE-IVD is not superior than FE-PMMA? • Lack of patient-specific IVD models Geometry of IVD was idealized and not based on MRI images Material behavior of IVD was simplified as hyperelastic Degeneration of IVD was simulated by discrete degrees of degeneration

• Some other reasons, e.g. bone models

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Conclusion

Finite element analysis of human vertebral bodies embedded in PMMA or loaded via the hyperelastic intervertebral discs provide equivalent predictions of experimental strength, i.e. Prediction ability(FE-PMMA) = Prediction ability (FE-IVD)

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More details on:

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Acknowledgements:

State of Hamburg

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