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Research Papers

Simplified Multistage Computational Approach to Assess the Fatigue Behavior of a Niti Transcatheter Aortic Valve During In Vitro Tests: A Proof-of-Concept Study

[+] Author and Article Information
Lorenza Petrini

Department of Civil and
Environmental Engineering,
Politecnico di Milano,
Milano 20133, Italy

Elena Dordoni, Dario Allegretti, Francesco Migliavacca

Laboratory of Biological Structure Mechanics,
Department of Chemistry,
Materials and Chemical
Engineering “Giulio Natta,”
Politecnico di Milano,
Milano 20133, Italy

Desiree Pott, Maximilian Kütting

Department of Cardiovascular Engineering,
Institute of Applied Medical Engineering,
Helmholtz Institute,
RWTH Aachen University,
Aachen 52062, Germany

Giancarlo Pennati

Laboratory of Biological Structure Mechanics,
Department of Chemistry,
Materials and Chemical
Engineering “Giulio Natta,”
Politecnico di Milano,
Piazza Leonardo da Vinci 32,
Milano 20133, Italy
e-mail: giancarlo.pennati@polimi.it

1Authors contributed equally.

2Corresponding author.

Manuscript received July 29, 2016; final manuscript received January 9, 2017; published online May 3, 2017. Assoc. Editor: Marc Horner.

J. Med. Devices 11(2), 021009 (May 03, 2017) (11 pages) Paper No: MED-16-1282; doi: 10.1115/1.4035791 History: Received July 29, 2016; Revised January 09, 2017

Nowadays, transcatheter aortic valve (TAV) replacement is an alternative to surgical therapy in selected high risk patients for the treatment of aortic stenosis. However, left ventricular contraction determines a severe cyclic loading for the implanted stent-frame, undermining its long-term durability. Technical standards indicate in vitro tests as a suitable approach for the assessment of TAV fatigue behavior: generally, they do not specify test methods but require to test TAV in the worst loading conditions. The most critical conditions could be different according to the specific valve design, hence the compartment where deploying the valve has to be properly identified. A fast and reliable computational methodology could significantly help to face this issue. In this paper, a numerical approach to analyze Nickel-Titanium TAV stent-frame behavior during in vitro durability tests is proposed. A simplified multistage strategy was adopted where, in each stage, only two of the three involved components are considered. As a proof-of-concept, the method was applied to a TAV prototype. Despite its simplifications, the developed computational framework gave useful insights into the stent-frame failures behavior during a fatigue test. Numerical results agree with experimental findings. In particular, the most dangerous condition was identified among a number of experimental tests, where different compartments and pressure gradients were investigated. The specific failure location was also correctly recognized. In conclusion, the presented methodology provides a tool to support the choice of proper testing conditions for the in vitro assessment of TAV fatigue behavior.

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Figures

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Fig. 1

Experimental setup and tested valve prototype: (a) self-expandable transcatheter aortic valve within the silicone compartment, (b) multichambers durability tester with a detail of the mounted compartment and valve, (c) sketch of the durability tester describing the main components, and (d) typical pressure waveforms (TOP and BOTTOM) applied during a durability test, with a maximum pressure drop (Δp) of about 100 mmHg (13.3 kPa)

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Fig. 2

Boundary conditions for fatigue simulations (for sake of visualization clearness, a third of the full geometry is shown): instead of considering all the three valve components and the sole internal negative pressure as in reality (a), the leaflets are not considered and the reaction forces exerted by the leaflets on the stent-frame are applied while the pressure acts only on the compartment inner surface (b)

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Fig. 3

Numerical simulations workflow: Boxes and ellipses represent geometrical models and FE analyses (FEA), respectively. At the end of the process, the mean and alternating strains throughout the stent-frame are calculated to be used in the constant-life diagram for fatigue evaluation.

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Fig. 4

Creation of the aortic valve stent-frame FE model: (a) 2D planar sketch for the laser cutting process, (b) 3D valve model in the postlaser cutting configuration, (c) surfaces used to simulate the valve expansion process

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Fig. 5

Comparison between (a) the expanded stent-frame model and (b) the real expanded valve configuration in terms of diameters measured (mm) at five different levels

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Fig. 6

(a) Average experimental tensile result versus computational fit and (b) material model parameters NiTi

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Fig. 7

Crimp and deployment of the valve stent-frame model inside the compartment: (a) undeformed configuration with fully expanded valve, silicone compartment and cylindrical rigid surface for crimp—the imposed boundary conditions are indicated, (b) stent-frame crimped configuration, (c) after self-expansion inside the compartment, and (d) stand-alone model of the deformed valve stent-frame after the deployment inside the silicone compartment

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Fig. 8

Simulation to deduce the forces acting on the stent frame due to closing pressure, as transmitted by the leaflets: (a) deformed valve stent-frame model as after the deployment inside the silicone compartment, with added PU leaflets almost closed—the portion used for the simulation of leaflets closure is highlighted, (b) boundary conditions for the simulation of leaflet closure against a fixed rigid wall, (c) valve configuration after simulation of leaflets coaptation, and (d) the forces acting on the stent are evaluated to be used for the fatigue analysis

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Fig. 9

Crimping results. Numerical results in terms of equivalent Von Mises strain, with valve crimped at 14 mm (a) and 10 mm OD (b). Boxes detail the most stressed zones, corresponding to the bottom leaflet attachment tip and V-strut tip. (c) Images of the most critical zones (dashed lines) throughout the valve, before (left) and after (right) a crimping test down to 10 mm OD and subsequent self-expansion: the valve does not recover its original shape.

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Fig. 10

Alternating equivalent strain values (εa) through the stent-frame, in case of different compartments: low (LO) and high (HO),oversizing, standard (SS) lower (LS) and higher (HS) stiffness. Peculiar stent-frame zones (TLA = top leaflets attachment edge, MLA = medium leaflets attachment edge, V-S = V-strut tip, BLA = bottom leaflets attachment edge) are shown in detail, with bolded boxes corresponding to areas where εa reach the maximum values.

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Fig. 11

Numerical results obtained for Δp = 100 mmHg: Constant-life diagrams for low (a) and high oversizing (b) compartments with different stiffness (LS, SS, HS). εa = alternating strains, εm = mean strains, Ecomp = compartment elastic modulus.

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Fig. 12

(a) Constant-life diagram for the simulations mimicking the two in vitro investigated cases (HO-LS: compartment with high oversizing and soft compartment, LO-SS: compartment with low oversizing and standard material) and increased pressure drop across the leaflets (Δp = 150 mmHg = 20 kPa). For HO-LS case, some points of the stent-frame reach very high values of alternating strains, indicating a very likely fatigue failure. (b) Stent frame failure as experimentally observed: the fracture location, at the upper leaflet attachment edge, corresponds to the most risky points indicated by FEA. εa = alternating strains, εm = mean strains.

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