A Computational Framework Examining the Mechanical Behaviour of Bare and Polymer-Covered Self-Expanding Laser-Cut Stents

Abstract

Polymer covered stents have demonstrated promising clinical outcomes with improved patency rates compared to traditional bare-metal stents, however little is known on the mechanical implication of stent covering. In the present work, a combined experimental-computational investigation was carried out to determine the role of a polymeric cover on the biomechanical performance of self-expanding laser-cut stents. Experimental bench top tests were conducted on bare and covered versions of a commercial stent to evaluate the radial, axial and bending response. In parallel, a computational framework with a novel covering strategy was developed that accurately predicts stent mechanical performance, and provides further insight into covered stent mechanics by considering different stent geometries and polymer materials. Results show that stent covering causes increased initial axial stiffness and substantial radial stiffening at small crimp diameters as the cover folds and self-contacts. It was also shown that use of a stiffer polymeric covering material caused significant alterations to the radial and axial response, highlighting the importance of considering the mechanical properties of the combined cover and stent.

Recommendations for finite element modelling of nickel-titanium stents—Verification and validation activities

“…Common applications of FE models of Ni-Ti stents could be classified as (i) modelling of standard mechanical bench tests (e.g. axial tension/compression and radial compression test [ 21 , 27 ]), (ii) predictions of deployment either in an idealised straight vessel or in patient-specific realistic anatomies [ 15 , 16 ], and (iii) predictions of fatigue behaviour [ 14 ]. These applications were classified in three COUs and relevant VV activities were selected accordingly:…”

“…COU-1 : Radial compression is often used for deriving the material parameters for Ni-Ti [ 40 ] or evaluating the functional performance of different stent designs through relevant mechanical parameters (e.g., radial strength, radial resistive force, chronic outward force, stress field induced in the device) [ 27 ], as advised in the ASTM F3067 standard [ 41 ]. For COU-1, calculation verification was carried out by examining mesh discretization, temporal discretization, and element integration selection, while validation of the model inputs was performed through a detailed sensitivity analysis on both geometric and material model parameters.…”

“…The stable time increment in models of this type ranges from 4E-06 s to 1E-08 s [ 16 , 28 ], with these values depending greatly on the boundary conditions and loading rate in use. While a minimum density of 4 × 4 hexahedral elements across the stent strut (e.g., width × thickness) has been suggested [ 17 , 19 , 27 ] with linear reduced integration [ 15 , 16 , 23 , 27 ], the details of mesh density and element type details are sometimes not stated [ 20 ] or have been selected based on previous literature [ 23 ], with the procedure and outcomes of the verification activities rarely made available [ 21 , 29 , 30 ]. Also, there is substantial variation in how both geometric and material model inputs are derived.…”

“…As this information can be commercially sensitive, many authors have been forced to reconstruct the geometry through imaging techniques such as optical microscopy or micro-computer tomography (micro-CT) [ 13 – 15 , 19 ], which might add substantial uncertainty. To model the superelastic behaviour of Ni-Ti, the constitutive law from Auricchio and Taylor [ 31 ] is universally used [ 14 – 16 , 19 – 25 , 27 , 28 , 32 – 34 ], with this model now available as a built-in option in several commercial software packages such as Abaqus, ANSYS and COMSOL Multiphysics [ 35 ]. However, this constitutive model presents challenges as it requires up to 15 independent parameters to be determined [ 36 ].…”

Superficial femoral artery stenting: Impact of stent design and overlapping on the local hemodynamics

“…Among the endovascular treatments, the implantation of selfexpanding Nitinol stents is one of the most widespread approaches [3]. Two main classes of selfexpanding stents are adopted for the femoral region: (i) laser-cut stents, which are characterized by a wide variety of features like strut shape, thickness, length, and width, and interconnecting segments (or links); and (ii) wire braided stents, characterized by sets of interlacing wires running in clockwise and counterclockwise directions [4,5].…”