1,720,969 research outputs found
The innovation incubator: six years of achievements
No full textSince 2005, more than 500 foreign students have chosen Lecco campus for their education enrolling to our Master of Science degrees, not to mention exchange and double-degree students. In this context all the international students are involved in the “Incubator of innovation” program created thanks to the collaboration between University, ICE (Istituto Commercio Estero), Unioncamere and the Chambers of Commerce of Lecco and Como, with the purpose of training high-level students with a strong interest in technology and Italian managerial culture. The aims of this paper is to describe and highlight the achievement of five year of international exchange, active since the academic year 2005/2006 at the Politecnico di Milano – Polo Territoriale di Lecco
Numerical simulations of fatigue for two overlapped stents in peripheral arteries.
No full textA typical endovascular procedure to restore blood flow perfusion in stenotic peripheral arteries consists in balloon angioplasty and deployment of self-expanding Nitinol stents. However, movements during gait produce large and cyclic deformations of the superficial femoral artery, increasing the risk of fatigue fracture of implanted stents. Fatigue failure typically occurs in cases of very extended lesions, which often require the use of two or more overlapping stents. In this study, finite element models were used to study the fatigue behavior of overlapped Nitinol stents. Two different approaches were adopted: i) simulating the deployment of a stent into another stent and
ii) using a single stent with double stiffness in the overlapping region. Simulations of cyclic axial compression of stents deployed in a simplified cylindrical model of a peripheral artery were then performed (Ansys Inc., Canonsburg, PA, USA). Fatigue risk
was assessed on the basis of amplitudes (εI a) and mean values (εI m) of the cyclic first principal strain through the stent. Similar results were found in both approaches: strain concentrates in the links close to the overlapping portion where the abrupt change in
stiffness causes higher cyclic compression (εI a = 0.19%), while in the overlapping region the value of εI a is lower (= 0.15%), due to a higher stiffness. These findings help to explain the high incidence of stent fracture observed in various clinical trials located close to the overlapping portion. Moreover, this study demonstrates the possibility of using a simplified model with a region with double stiffness, reducing significantly the computational time
Finite element analyses of in vivo fatigue behavior of peripheral stents: effect of plaque features.
No full textFatigue resistance of Nitinol peripheral stents implanted into femoropopliteal arteries is a critical issue due to the particular biomechanical environment of this district. Hip and knee joint movements associated with patient’ daily activities expose the superficial femoral artery, and therefore the implanted stents, to large and cyclic deformations. These loadings, combined with the cyclic loading due to the arterial blood pressure, may cause fatigue fracture of stents. The stress/strain field throughout the stent is likely affected by plaque features. In this study, finite element simulations of angioplasty, stenting and subsequent in vivo loading conditions (cyclic pressure and axial compression) have been developed in different stenotic vessel models. A model of a stent, resembling the geometry of a commercial peripheral stent, has been reconstructed. A parametric model of stenotic artery has been developed, described by vessel inner diameter, obstruction ratio, length, plaque asymmetry and sharpness. The results, analyzed in terms of amplitudes (ε1a) and mean values (ε1m) of the first principal strain through the stent, showed that: i) cyclic pressure effects are negligible if compared with axial compression ones; ii) replicating a realistic vessel morphology is fundamental, since plaque features affect fatigue resistance of the stent with the same loading conditions
Fatigue behaviour of Nitinol peripheral stents: The role of plaque shape studied with computational structural analyses
No full textFatigue resistance of Nitinol stents implanted into femoro-popliteal arteries is a critical issue for the particular biomechanical environment of this district. Hip and knee joint movements due to the cyclic daily activity expose the superficial femoral artery (SFA), and therefore the implanted stents, to quite large and cyclic deformations influencing stent fatigue resistance. Objective of this work is to provide a tool based on finite element analysis able to evaluate the biomechanical effect of SFA on stent fatigue resistance. Computer simulations of the treatment of stenotic vessel by angioplasty and stenting and of the subsequent in vivo loading conditions (axial compression and bending) were carried out. Three different stenotic vessel models were defined, by keeping a constant stenosis rate and changing the plaque sharpness and number of stenoses. The fatigue behaviour was analysed comparing the amplitude and mean value distribution of the first principal strain in the whole stent for the different simulated conditions. Results showed that the maximum mean strain is similar in all the models, while the alternating strain is related to both plaque shape and loading conditions. In conclusion, this study confirms the requisite of replicating in vivo loading conditions. It also reveals the importance of taking into account the thickness variation of the vessel in the stenotic zone in the assessment of the stent fatigue resistance
Computational Study of Axial Fatigue for Peripheral Nitinol Stents
Full text linkDespite their success as primary treatment for vascular diseases, Nitinol peripheral stents are still affected by complications related to fatigue failure. Hip and knee movements during daily activities produce large
and cyclic deformations of the superficial femoral artery, that concomitant to the effects of pulsatile blood pressure, may cause fatigue failure in the stent. Fatigue failure typically occurs in cases of very extended lesions, which often require the use of two or more overlapping stents. In this study, finite element models were used to study the fatigue behavior of Nitinol stents when subjected to cyclic axial compression in different conditions. A specific commercial Nitinol stent was chosen for the analysis and subjected to cyclic axial compression typical of the femoral vascular region. Three different configurations were investigated: stent alone, stent deployed in a tube, and two overlapping stents deployed in a tube. Results confirm that stent oversizing has an influence in determining both the mean and amplitude strains induced in the stent and plays an important role in determining the fatigue response of Nitinol stents. In case of overlapping stents, numerical results suggest higher amplitude strains concentrate in the region close to the overlapping portion where the abrupt change in stiffness causes higher cyclic compression. These findings help to explain the high incidence of stent fractures observed in various clinical trials located close to the overlapping portion
Fatigue behaviour of Nitinol peripheral stent: finite element analyses of in vivo loading conditions
No full textFatigue resistance of Nitinol peripheral stents implanted into femoropopliteal arteries is a critical issue due to the particular biomechanical environment of this district. Hip and knee joint movements, combined with the cyclic loading due to the arterial blood pressure, expose the superficial femoral artery (SFA), and therefore the implanted stents, to quite large and cyclic deformations that may influence the fatigue resistance of the device. In this study, simulations of angioplasty, stenting and subsequent in vivo loading conditions (cyclic pressure, axial compression and bending) have been developed in a stenotic vessel models, using the commercial code ANSYS (Ansys Inc., Canonsburg, PA, USA). A finite element model of a stent, resembling the geometry of the Maris Plus peripheral stent (Medtronic-Invatec), has been reconstructed. A stenotic vessel model has been developed. The results, analyzed in terms of amplitudes (ε1a) and mean values (ε1m) of the first principal strain through the stent, showed that the cyclic pressure is the less critical loading condition: the maximum alternating strain in this case is one order lower than in others loading conditions. Moreover, this study reveals the importance of replicating a realistic vessel morphology, since plaque shape could affect fatigue resistance of the stent
Comparison of different in vitro testing conditions for peripheral Nitinol stents: a computational study
No full textFatigue resistance of Nitinol peripheral stents implanted into femoropopliteal arteries is a critical issue due to the unique biomechanical environment of this district. Potentially in vitro testing offers a valid method to compare the fatigue behavior in terms of fracture risk of different marketed stents. However, there is still a lack of validated methodologies for bench testing that mimic in a satisfactory manner the complex biomechanical environment present in the SFA. In the present study, the finite element method (FEM) was used to investigate the fatigue behavior of stents when subjected to different in vitro tests. For this purpose, two models of commercial Nitinol stents were developed and different loading conditions were simulated using finite element analyses. In order to reproduce two different test methods recently proposed in the literature, stents were subjected to cyclic axial compression, either alone in the free-expanded configuration or after deployment in a silicone tube. Results were analyzed in terms of amplitudes (ε1a) and mean values (ε1m) of the first principal strain through the stent either on a constant-life diagram or in the form of strain field. Results indicated that: i) the two testing conditions lead to quite different fatigue fracture risk and strain fields in the stent, explaining the conflicting findings reported in the literature; ii) different stent designs exhibit a variable ability to withstand in vitro loading; iii) the mechanical interaction with the arterial wall cannot be disregarded as it significantly influences the stent fatigue behavior
FATIGUE BEHAVIOR CHARACTERIZATION OF NITINOL FOR PERIPHERAL STENTS
No full textNitinol stents are nowadays widely used for the treatment of occlusions in peripheral arteries. However, the expansion of this indication has also highlighted some complications. In particular, the patient daily activities expose the peripheral arteries to large and cyclic deformations which may cause long-term failure of the device and consequently re-occlusion of the artery. Accordingly, the assessment of the stent fatigue rupture is of primary importance to assure the effectiveness of stenting procedure. However the fatigue behavior characterization of Nitinol for peripheral stent is a quite difficult problem because of the complexity of the in vivo solicitations the stent is subjected to and the strong nonlinearity in the material response. In this paper we approached the problem in two steps: (i) in the first step the study of the stent solicitations under realistic (physiological) conditions was performed through the use of numerical simulations which allowed sophisticated patient-specific models of the stenting procedure; (ii) in the second step, the previous results were used for the design of an experimental campaign and the following execution of the tests for the material mechanical characterization and fatigue life study. The tests were performed on Nitinol specimens derived from the same tubes used for producing a commercial peripheral stent and created following the same procedure employed for the device. As a consequence of the small dimension of the specimens, a preliminary design of the experimental test set-up was also required. The obtained results allowed a sufficiently accurate characterization of the stent material fatigue behavior in the range of interest
Computational Modelling of In Vitro Set-Ups for Peripheral Self-Expanding Nitinol Stents: The Importance of Stent–Wall Interaction in the Assessment of the Fatigue Resistance
No full textAlthough the number of fractured stents in the femoro-popliteal district was largely reduced using second-generation devices, remarkable differences still exist among various products. In vitro testing offers a valid tool to comparatively assess the risk of fatigue fracture of different devices. However, there are no standardized methodologies for bench testing under the complex biomechanical environment of the femoro-popliteal district. A computational approach was used in the present study to investigate the fatigue testing conditions adopted in the in vitro studies to increase the understanding of the Nitinol stents fatigue behaviour, by focusing on the role of the stent–wall interaction. The finite element method was used coupled to fatigue analysis to investigate the behaviour of commercial devices during in vitro tests. Two peripheral stents were chosen for the analyses for which conflicting results are found in literature. Stent models were subjected to axial compression and bending either alone in the fully-expanded configuration or after their deployment in a silicone tube resembling the presence of the artery. Results indicate that the two testing conditions investigated produce quite a different fatigue behaviour both in terms of constant-life diagram and strain distribution in the stents. In particular, the results highlight that oversizing influence the fatigue behaviour of the devices with an effect on both the mean and amplitude values of the strain induced in the stents. Comparison with a fatigue limit typical for Nitinol indicates good agreement with the experimental results and confirms the validity of the adopted methodology. Stent oversizing plays an important role in determining the fatigue response of the devices and cannot be disregarded to assess the fatigue performance of Nitinol stents during in vitro tests. Moreover, the different behaviour found for the two stent models suggests that a careful stent design can improve the fatigue performance of these devices
A multiscale model for the study of cardiac biomechanics in single-ventricle surgeries: A clinical case
Full text linkComplex congenital heart disease characterized by the underdevelopment of one ventricular chamber (single ventricle (SV) circulation) is normally treated with a three-stage surgical repair. This study aims at developing a multiscale computational framework able to couple a patient-specific three-dimensional finite-element model of the SV to a patient-specific lumped parameter (LP) model of thewhole circulation, in a closed-loop fashion. A sequential approach was carried out: (i) cardiocirculatory parameters were estimated by using a fully LP model; (ii) ventricular material parameters and unloaded geometry were identified by means of the stand-alone, three-dimensional model of the SV; and (iii) the three-dimensional model of SV was coupled to the LP model of the circulation, thus closing the loop and creating a multiscale model. Once the patient-specific multiscale model was set using pre-operative clinical data, the virtual surgery was performed, and the post-operative conditions were simulated. This approach allows the analysis of local information on ventricular function aswell as global parameters of the cardiovascular system. This methodology is generally applicable to patients suffering from SV disease for surgical planning at different stages of treatment. As an example, a clinical case from stage 1 to stage 2 is considered here
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