Analytical methods for braided stents design and comparison with FEA

Modified Theoretical Model Predicts Radial Support Capacity of Polymer Braided Stents

BACKGROUND AND OBJECTIVE

Self-expanding polymer braided stents are expected to replace metallic stents in the treatment of Peripheral Arterial Disease, which seriously endangers human health. To restore the patency of blocked peripheral arteries with different properties and functions, the radial supporting capacity of the stent should be considered corresponding to the vessel. A theoretical model can be established as an effective method to study the radial supporting capacity of the stent which can shorten the stent design cycle and realize the customization of the stent according to lesion site. However, the classical model developed by Jedwab and Clerc of radial force is only limited to metallic braided stents, and the predictions for polymer braided stents are deviated.

METHODS

In this paper, based on the limitation of the J&C model for polymer braided stents, a modified radial force model for polymer braided stents was proposed, which considered the friction between monofilaments and the torsion of the monofilaments. And the modified model was verified by radial force tests of polymer braided stents with different structures and monofilaments.

RESULTS

Compared with the J&C model, the proposed modified model has better predictability for the radial force of polymer braided stents that prepared with different braided structure and polymer monofilaments. The root mean squared error of modified model is 0.041±0.026, while that of the J&C model is 0.246±0.111.

CONCLUSIONS

For polymer braided stents, the friction between the polymer monofilaments and the torsion of the monofilaments during the radial compression cannot be ignored. The radial force prediction accuracy of the modified model considering these factors was significantly improved. This work provides a research basis on the theoretical model of polymer braided stents, and improves the feasibility of rapid personalized customization of polymer braided stents.

Finite element analysis of braided dense-mesh stents for carotid artery stenosis

When braided dense-mesh stents are used to treat carotid stenosis, the structural mechanics of vascular stents, the contact mechanics with blood vessels, and the fluid mechanics in the blood environment need to be studied in depth to reduce the damage of stents to blood vessels and the incidence of in-stent restenosis. Three types of braided stents with 8, 16, and 24 strands and laser-cut stents with the corresponding size parameters were designed, and the bending behavior of each of these types of stent, deployment, and fluid dynamic analysis of the 24-strand braided stent were simulated. The results show that the bending stress of the 8-, 16-, and 24-strand braided stents is 46.33%, 50.24%, and 31.86% of that of their laser-cut counterparts. In addition, higher strand density of the braided stents was associated with greater bending stress; after the 24-strand braided stent was expanded within the stented carotid artery, the carotid stenosis rate was reduced from 81.52% to 46.33%. After stent implantation, the maximum stress on the vessel wall in a zero-pressure diastolic environment decreased from 0.34 to 0.20 MPa, the maximum pressure on the intravascular wall surface decreased from 4.89 to 3.98 kPa, the area of high-pressure region decreased, the wall shear force of the stenotic segment throat decreased, and blood flow increased in the stenosis segments. The braided stent had less bending stress and better flexibility than the laser-cut stent under the same stent size parameters; after the 24-strand braided stent was implanted into the stented vessel, it could effectively dilate the vessel, and the blood flow status was improved.

Reinforcement learning for patient-specific optimal stenting of intracranial aneurysms

Developing new capabilities to predict the risk of intracranial aneurysm rupture and to improve treatment outcomes in the follow-up of endovascular repair is of tremendous medical and societal interest, both to support decision-making and assessment of treatment options by medical doctors, and to improve the life quality and expectancy of patients. This study aims at identifying and characterizing novel flow-deviator stent devices through a high-fidelity computational framework that combines state-of-the-art numerical methods to accurately describe the mechanical exchanges between the blood flow, the aneurysm, and the flow-deviator and deep reinforcement learning algorithms to identify a new stent concepts enabling patient-specific treatment via accurate adjustment of the functional parameters in the implanted state.

Computational modeling of braided venous stents - Effect of design features and device-tissue interaction on stent performance

Designing venous stents with desired properties is challenging due to the partly conflicting performance criteria, e.g., enhancing flexibility may be at odds with increasing patency. To evaluate the effect of design parameters on the mechanical performance of braided stents, computational simulations are performed using finite element analysis. Model validation is performed through comparison with measurements. Considered design features are stent length, wire diameter, pick rate, number of wires, and stent end-type, being either open-ended or closed looped. Based on the requirements of venous stents, tests are defined to study the effect of design variations with respect to the following key performance criteria: chronic outward force, crush resistance, conformability, and foreshortening. Computational modeling is demonstrated to be a valuable tool in the design process through its ability of assessing sensitivities of various performance metrics to the design parameters. Additionally, it is shown, using computational modeling, that the interaction between a braided stent and its surrounding anatomy has a significant impact on its performance. Therefore, taking into account device-tissue interaction is crucial for the proper assessment of stent performance.

A hazardous boundary of Poly(L-lactic acid) braided stent design: Limited elastic deformability of polymer materials

Poly (L-lactic acid) (PLLA) braided stents, which are expected to replace metal stents, are promising in peripheral vascular therapy due to their superior biocompatibility. Although various design ideas have been proposed and investigated on metal stents, few researches are related to the design theory of PLLA braided stent. In this article, mechanical performance of PLLA braided stents with different parameters was systematically evaluated, and a design theory based on material properties was proposed. Different from metal materials, the risk of filament deformation beyond elastic zone should be evaluated and controlled in PLLA stent design. The findings were obtained through combination study of experiments and simulations. Design parameters, including pitch angle and stent diameter, played a crucial role in mechanical performance of PLLA braided stent. The deformation of PLLA stents with larger pitch angles and stent diameters was in elastic zone and thus presented better mechanical performance with satisfactory resilience. This work could provide meaningful suggestions for preparing bioresorbable braided stents with suitable design parameters.

Mechanical property analysis and design parameter optimization of a novel nitinol nasal stent based on numerical simulation

Background: A novel braided nasal stent is an effective alternative to nasal packing after septoplasty that can be used to manage the mucosal flap after septoplasty and expand the nasal cavity. This study aimed to investigate the influence of design parameters on the mechanical properties of the nasal stent for optimal performance. Methods: A braided nasal stent modeling method was proposed and 27 stent models with a range of different geometric parameters were built. The compression behavior and bending behavior of these stent models were numerically analyzed using a finite element method (FEM). The orthogonal test was used as an optimization method, and the optimized design variables of the stent with improved performance were obtained based on range analysis and weight grade method. Results: The reaction force and bending stiffness of the braided stent increased with the wire diameter, braiding density, and external stent diameter, while wire diameter resulted as the most important determining parameter. The external stent diameter had the greatest influence on the elongation deformation. The influence of design parameters on von-Mises stress distribution of bent stent models was visualized. The stent model with geometrical parameters of 25 mm external diameter, 30° braiding angle, and 0.13 mm wire diameter (A3B3C3) had a greater reaction force but a considerably smaller bending stiffness, which was the optimal combination of parameters. Conclusion: Firstly, among the three design parameters of braided stent models, wire diameter resulted as the most important parameter determining the reaction force and bending stiffness. Secondly, the external stent diameter significantly influenced the elongation deformation during the compression simulation. Finally, 25 mm external diameter, 30° braiding angle, and 0.13 mm wire diameter (A3B3C3) was the optimal combination of stent parameters according to the orthogonal test results.

A Finite Element Investigation on Material and Design Parameters of Ventricular Septal Defect Occluder Devices

Background and Objective: Ventricular septal defects (VSDs) are the most common form of congenital heart defects. The incidence of VSD accounts for 40% of all congenital heart defects (CHDs). With the development of interventional therapy technology, transcatheter VSD closure was introduced as an alternative to open heart surgery. Clinical trials of VSD occluders have yielded promising results, and with the development of new material technologies, biodegradable materials have been introduced into the application of occluders. At present, the research on the mechanical properties of occluders is focused on experimental and clinical trials, and numerical simulation is still a considerable challenge due to the braided nature of the VSD occluder. Finite element analysis (FEA) has proven to be a valid and efficient method to virtually investigate and optimize the mechanical behavior of minimally invasive devices. The objective of this study is to explore the axial resistive performance through experimental and computational testing, and to present the systematic evaluation of the effect of various material and braid parameters by FEA. Methods: In this study, an experimental test was used to investigate the axial resistive force (ARF) of VSD Nitinol occluders under axial displacement loading (ADL), then the corresponding numerical simulation was developed and compared with the experimental results to verify the effectiveness. Based on the above validation, numerical simulations of VSD occluders with different materials (polydioxanone (PDO) and Nitinol with different austenite moduli) and braid parameters (wire density, wire diameter, and angle between left and right discs) provided a clear presentation of mechanical behaviors that included the maximal axial resistive force (MARF), maximal axial displacement (MAD) and initial axial stiffness (IAS), the stress distribution and the maximum principal strain distribution of the device under ADL. Results: The results showed that: (1) In the experimental testing, the axial resistive force (ARF) of the tested occluder, caused by axial displacement loading (ADL), was recorded and it increased linearly from 0 to 4.91 N before reducing. Subsequent computational testing showed that a similar performance in the ARF was experienced, albeit that the peak value of ARF was smaller. (2) The investigated design parameters of wire density, wire diameter and the angle between the left and right discs demonstrated an effective improvement (7.59%, 9.48%, 1.28%, respectively, for MARF, and 1.28%, 1.80%, 3.07%, respectively, for IAS) for the mechanical performance for Nitinol occluders. (3) The most influencing factor was the material; the performance rose by 30% as the Nitinol austenite modulus (EA) increased by 10,000 MPa. The performance of Nitinol was better than that of PDO for certain wire diameters, and the performance improved more obviously (1.80% for Nitinol and 0.64% for PDO in IAS, 9.48% for Nitinol and 2.00% for PDO in MARF) with the increase in wire diameter. (4) For all of the models, the maximum stresses under ADL were distributed at the edge of the disc on the loaded side of the occluders. Conclusions: The experimental testing presented in the study showed that the mechanical performance of the Nitinol occluder and the MARF prove that it has sufficient ability to resist falling out from its intended placement. This study also represents the first experimentally validated computational model of braided occluders, and provides a perception of the influence of geometrical and material parameters in these systems. The results could further provide meaningful suggestions for the design of biodegradable VSD closure devices and to realize a series of applications for biodegradable materials in VSD.