Prof. Stéphane AVRIL

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Institut Mines-Télécomavril@emse.fravril@emse.fr

Mechanical characterization of arterial wall

Prof. Stéphane AVRIL

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Basics of arterial mechanics

Humphrey JD (2002) Cardiovascular Solid Mechanics: Cells, Tissues, and Organs, Springer-Verlag, NY

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Institut Mines-Télécomavril@emse.fr

Functional biomechanical behavior

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Material characterization and

constitutive modeling

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Institut Mines-Télécomavril@emse.fr 5

Application to the prediction of AA index of rupture?

dissection

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Context

 More and more aneurysms are detected at an early stage (incidence >8% for males >65 years old).

 An intervention is recommended if the aneurysm grows more >1cm/year or it is >5.5cm. This represents >90000 interventions per year in Europe and USA

 BUT:

• 25% aneurysms <5.5cm rupture : 15000 deaths ^** !

• 60% of aneurysms >5.5 cm never experience rupture!

 In summary: very high rate of inappropriate decisions and misprogramed surgical interventions!!

** Pape et al, Aortic Diameter ≥5.5 cm Is Not a Good Predictor of Type A Aortic Dissection Observations From the International Registry of Acute Aortic Dissection (IRAD), Circulation, 2007

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Methodology

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40 Patients with ATAA

Preoperative dynamic imaging

Dynamic CT Scanner

4D MRI

Collection of intraoperative aortic segment

Mechanical inflation tests

Histological Analysis

2014

2017

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Methodology

40 Patients with ATAA

Preoperative dynamic imaging

Dynamic CT Scanner

4D MRI

Collection of intraoperative aortic segment

Mechanical inflation tests

Histological Analysis

2014

2017

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Collection of the samples

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PREPARATION

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Institut Mines-Télécomavril@emse.fravril@emse.fr

Bulge inflation test

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III) Inflation test device. Speckle pattern is made before starting the

inflation test Circumferential

A x ia l

Romo et al. Journal of Biomechanics -2014.

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Undeformed Deformed

x y

Full-field measurements using

sDIC

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Rupture profiles

50% of aortas r uptured wi th an angle θ equal to 90 ° Bl ood flow

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Rupture risk estimation

Physiological Stress

Stretch Stretch/Stress Curve

Rupture Stress

E

_in-vitro

= Tangent elastic modulus

Cauch y Stress (MP a)

1 1.1 1.2 1.3

Physiological Stretch

1.4 1.5 1.6 1.7

Rupture Stretch

γ _stress = Physiological stress Rupture stress γ _stretch = Physiological stretch

Rupture stretch

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Correlation between the stretch-based rupture risk and the tangent elastic

modulus

Duprey A, et al. Biaxial rupture properties of ascending thoracic aortic aneurysms. Acta Biomaterialia 2016.

E _in-vitro (MPa)

Ru pture ris k

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SAINT-ETIENNE PROTOCOL

40 Patients with ATAA

Preoperative dynamic imaging

Dynamic CT Scanner

4D MRI

Collection of intraoperative aortic segment

Mechanical inflation tests

Histological Analysis

2014

2017

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Measurement of aortic DISTENSIBILITY

 Gated CT – acquisition and segmentation

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13 patients: Dynamic preoperative scanners during cardiac cycle ( 0.92 s) = 10 phases.

CT: (resolution 512x512, slice thickness of 0.5 mm)

1 phase superposition of 10 phases

Aorta

Systole

Diastole

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Measurement of aortic DISTENSIBILITY

 Aortic wall - 3D reconstruction from gated CT

13 patients: Dynamic preoperative scanners during cardiac cycle ( 0.92 s) = 10 phases.

CT: (resolution 512x512, slice thickness of 0.5 mm)

 Aasc

 Adesc

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 Aortic wall - 3D reconstruction from gated CT

 Vasc

Measurement of aortic DISTENSIBILITY

13 patients: Dynamic preoperative scanners during cardiac cycle ( 0.92 s) = 10 phases.

CT: (resolution 512x512, slice thickness of 0.5 mm)

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 The tangent elastic modulus can be derived using:

𝐸 _{𝒊𝒏−𝒗𝒊𝒗𝒐} = ^∅

ℎ𝐷 where

𝒉 : thickness of the aortic wall.

∅: Maximal diameter of aneurysm.

D: Distensibility.

 Access the in vivo thickness?

𝒉 = 𝒉 _𝟎

𝝀 _𝟏 ∗ 𝝀 _𝟐  𝒉 _𝟎 𝝀 _{𝒊𝒏 𝒗𝒊𝒗𝒐}

Measurement of aortic circumferential

STIFFNESS

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Results: stretch-based rupture risk vs E _in-vivo

E in -v iv o (MPa )

Rupture risk

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Summary

 2 ways of defining rupture:

 PWS – but unknown patient-specific strength

 γ _stretch correlated with in vivo circumferential stiffness

Higher distensibility  less risk because the aneurysm can more easily withstand volume variation

C. Martin et al., Acta Biomater. 9 (2013) 9392–9400

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Illustrative

Clinical applications

Development of

mechanobiological models

TOWARDS ATAA GROWTH

PREDICTIONS

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Prof. Stéphane AVRIL

Mechanical characterization of arterial wall