| Today,the cardiovascular disease has become the "first killer" threatening the health of people around the world.Vascular stenting,with its advantages of slight trauma and effective treatment,has been become the first choice for the treatment of vascular stenosis.Postoperative complications have been an important problem in stent implantation and affect the stenting treatment outcome,some sever complication directly threaten the people’s life.Stent fracture(SF)has been recognized as one of the import cause of complications after stent implantation.At present researches on the causes of SF,the mechanism of SF causing postoperative complications and how to reduce the SF rate are incomplete.In order to improvement the effect of stenting,the concept of biodegradable stent is proposed.Biodegradable stent can degrade gradually until completely disappeared in vivo,compared with the traditional metal stent,it has obvious advantage.However,the biodegradable stent has not been widely used in clinic,how to control the stent degradation rate and support ability is a difficult problem restricting the application of biodegradable stent.At present,the theories and analysis methods of biodegradable stent mechanical properties are not complete,which cannot meet the needs of stent design.In this work the SF and the biodegradable stent mechanical properties are studied,the details of this work are as follows:1.A simulation model of stent expansion mechanical properties based on CT images is carried out.The result based on CT model is closer to the clinical outcome than that in the ideal simple vascular model.Based on the analysis results,a simplified method of establishing the diseased vessel model is proposed,which provides a basis for the accurate analysis of the mechanical behavior of the stent.2.An experiment of stent during expansion is developed to study the mechanism of SF.It is found that the SF during expansion is a typical ductile fracture,which caused by stent overexpansion and non-uniform expansion.Based on the experiment result a simulation model combined with the metal ductile damage constitutive model is carried out to study the process of SF.The results show that the stiffness of decreases after SF,the vascular stress concentrated in the SF location.At the same time,the configuration of fractured stent affects the releases effect of drug stent,which may lead to a decrease in drug concentration at the SF location.These are the possible causes of high complications after SF.3.An explicit-implicit coupling simulation method was applied to analyze the mechanical responses of cardiovascular stents considering the effect of vascular dynamical bending(VDB).It is found that SF occurs more frequently as a result of VDB with the predicted fracture position located in the bridging struts of the stent.These results are consistent with the reported data in clinical literatures.The results show that the effect of VDB has a significant impact on the fatigue performance,and is the main cause of stent fatigue failure,thus it is necessary to be considered when analyzing stent’s long-term mechanical properties.Meanwhile,the results show that the explicit-implicit coupling simulation is an efficient computational method and it can be applied to analyze the stent mechanical properties.4.The influence of stent type and length on stent fatigue performance is studied using numerical simulation method combing the stent application in clinic.The results show that the fatigue failure occurs more frequently in the open-cell design stent and long stent.These results are consistent with the reported data in clinical literatures and reveal the mechanism of stent fatigue fracture.Based on the fracture mechanic and reliability analysis theory a stent fatigue life prediction model is established in which the random factor is considering.Using this model the mean time of stent fatigue fracture is predicted,which is helpful for doctor to make the maintenance policy after stent implantation.A simulation method is carried out to study the SF process in vascular.Based on this method the predicted configuration of complete fractured stent is consistent with the clinical results.The results show the stent failure risk is reducing after SF.5.The implantation process and the mechanical responses under the effect of VDB for overlap stents are studied by finite element method.The results show the most of contact pressure concentrated occurred in the edge of overlapping struts and a little slip occurs between overlapping stent under the effect of VDB.From the simulation results,it suggested that SF easily occurs in the stent edge and the interaction between overlapping stent is the reason for SF under the condition of overlapping.The predicted results about fracture position of overlapping stents compared favorably with those reported in the literature.This work provides a fundamental understanding of overlapping stent’s mechanical properties.6.Based on the elastic-plastic mechanics and the uniform degradation behavior of magnesium alloy,a theoretical formula for calculating the support stiffness of biodegradable stent is established during the whole degradation process of stent.Based on this calculation formula,the support stiffness of biodegradable stent is predicted,which is consistent with the numerical calculation result and the literature data.The calculation formula established in this work can be used to analyze the supporting performance of biodegradable stents easily and quickly.In this work the mechanisms of SF during stent expansion,the stent fatigue failure and the overlap failure are studied,the SF phenomenon and regularity in clinic is revealed.This work is valuable for analyzing the fracture behavior of stent,reducing the risk of SF and optimizing the stent design.Meanwhile the theoretical formula for calculating the support stiffness can provide guidance and help for the design of biodegradable stent. |
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