| Endovascular stent graft as the main body of minimally invasive surgery,the level of its performance will have a direct impact on clinical outcomes.The short-term clinical results of endovascular exclusion for the treatment of vascular diseases are ideal,but the long-term clinical results are not satisfactory,long-term postoperative complications such as displacement,leakage and stent graft-induced new entry(SINE)still need to pay close attention.There are many factors causing postoperative complications.The biomechanical properties of vascular stent-graft itself,especially radial compliance and longitudinal flexibility,are important factors in inducing postoperative complications.Literatures have reported that rigid anchoring does not reduce the incidence of displacement and leakage of stent grafts.Therefore stent grafts that are radially compliant with the host vessel are required to reduce the potential for displacement and leakage.On the other hand,stent grafts matched the radial compliance with the host vessels can reduce their internal blood flow disorders and reduce the occurrence of other vascular diseases.Poor longitudinal flexibility,poor adherence with the host vascular,is also one of the reasons leading to stent grafts displacement and leakage.At the same time,poor longitudinal compliance is also an important cause of SINE.Based on the above,it can be seen that the radial compliance and longitudinal flexibility of the stent grafts are two important biomechanical properties that affect the results of endovascular exclusion.At present,the research on radial compliance and longitudinal flexibility of stent graft is very limited.Clinically used stent grafts vary in structure and material,and structural factors that affect radial compliance and longitudinal flexibility are not clearly defined.In order to solve the above problems,a series of stent-graft prototypes with different material types and structural designs were designed and prepared in this dissertation.The dynamic compliance of the stent grafts was studied.At the same time,the structure-activity relationship between the material and the structure of the stent graft and the radial compliance and the longitudinal flexibility is discussed in depth.First of all,it is known that the weft yarn performance of tubular fabric has the greatest influence on its radial compliance through literature review and previous exploration.In order to explore the effect of the structure and properties of the fabric on the performance of the stent graft,PET,PTT and PU yarns were selected as the weft yarn of the fabric respectively.Plain weave and 3/1 twill weave were used to form a series of seamless tubular fabric with uniform wall structure and material difference by using weaving technique.The radial compliance of PET-wefted tubular fabric decreased after heat-setting,the radial compliance relationship shown as: tubular fabric with PU weft yarns > tubular fabric with PTT weft yarns > tubular fabric with PET weft yarns.In this research,a series of structurally differentiated metal stents were designed and prepared.The structural parameters studied were stent apex angles(30 °,45 ° and 55 °),distance between stents(5 mm,15 mm and 20 mm),stent configuration(Z-type and M-type)and the relative position of the stent apex.To explore the effect of graft on the performance of a stent graft,two of the most differentiated grafts(spandex thin-walled tube and tubular fabric with PET weft yarn)were incorporated with the same metal stent(named Nitinol-PU and Nitinol-PET)to test their stent graft performance.The diameter difference spandex thin-wall tube is at least 15 times than that of the graft with PET weft yarn under the test pressure,spandex thin-wall tube’s radial compliance greatly higher than that of graft with PET weft yarn.The radial compliance of the stented segment of Nitinol-PU was significantly lower than that of the unstented segment,and radial compliance was unevenly distributed along the axial direction,but the overall numerical value was higher.However,the radial compliance of the stented segment of Nitinol-PET was not significantly different from that of the unstented segment.The radial compliance is evenly distributed in the axial direction,but the overall numerical value is low.Therefore,the overall level of radial compliance of stent-graft and the uniformity of the axial distribution require a two-phase trade-off.The structurally differentiated metal stent designed and prepared in this research is respectively combined with the compliant polarizing grafts.The radial compliance of multiple representative test points on the stent graft is evaluated to characterize the effect of metal stent structure on the radial compliance of stent grafts.The results show that the increase of the stent apex angle only slightly increases the radial compliance of the center point(FM point)of the lower edge of the stent section,but has no effect on the unstented section.By increasing the distance between the stents,the radial compliance of the unstented section can be increased.But the length of the unstented section can not be increased indefinitely,and the radial compliance of the unstented section can be increased to the maximum level of the graft radial compliance.The relative positions of the metal stents have no effect on the radial compliance of their unstented zones.Considering the uniformity of radial compliance,the radial compliance of the M-shaped metal stent is more evenly distributed in the circumferential direction than that of the corresponding Z-shaped metal stent.Self-woven fabric grafts different in materials and structures and self-made metal stent were combined,compliance relationship shown as follow: stent graft with PU weft yarns > stent graft with PTT weft yarns > stent graft with PET weft yarns.It is fully and systematically verified that the radial compliance of fabric graft has a significant effect on the overall radial compliance and the distribution over the stent graft.Self-woven fabric showed lower radial compliance than the spandex thin-walled tube,the difference between the test points is not obvious.Further,in view of the blank of radial compliance after dynamic fatigue of stent-graft,the radial compliance and other structural properties were evaluated.The wall thicknesses,warp and weft density and diameter of 20 self-made stent graft specimens were tested at 4 dynamic fatigue time points.Results showed that as the dynamic fatigue test time passed,the thickness,warp and weft density of the stent graft decreased,while the diameter of the stent graft showed an increasing trend.After a certain period of dynamic fatigue test,the radial compliance of the stent graft decreased compared with that before the dynamic fatigue.The numeral relationship of the radial compliance of stent grafts with different types of weft yarn material remained the same,and the radial compliance numeral relationship between the test points on the same specimen also did not change.In order to explore the longitudinal flexibility of stent-graft,combined with the clinical situation of stent-graft induced new entry,we evaluated the spring-back force and bending force of the five commonly used commercial TEVAR stent graft(C-TAG,TAG,Valiant,Hercules and Zenith TX2 2PT),which indirectly reflects the longitudinal flexibility of stent graft.The spring-back force and bending force test device used is independently developed,designed and built in Donghua Univerisy,continuous and multi-angle test of spring-back force and bending force is realized(National Invention Patent Application No.201710535082.3),which ensures the stability of the testing process.Due to the differences in the structure,specifications and graft materials of the 5 stent grafts,the results of spring-back force and bending force are also different.The spring-back force and bending force of all the stent grafts increased with the increase of bending angle.For the same stent graft,the spring-back force and bending force under long arm are lower than the force when the arm is short.Most of the clinical situation is of short arm,which confirms the speculation that spring-back force of stent graft played a major role in the proximal SINE.And the critical curving angle(CCA)was defined at which angle spring-back force appears to predict the risk of stent graft implantation.At long arm,C-TAG reduces the risk of clinical use by increasing the CCA by 1.62 times or more than Valiant,Hercules and Zenith TX2 2PT due to its telescopic design.Thus CCA can be used as an important parameter for clinical stent graft selectionFinally,the spring-back force and bending force of the three groups of stent grafts with different metal stent structures were evaluated respectively.The metal stent is the most important factor to determine the spring-back force and bending force of stent graft.The structural parameters studied including the distance between stents,the angle of metal stent apex and the configuration of metal stent(Z-and M-type).Increasing the distance between the stent,the angle of the metal stent apex and the metal stent structure of the Z-shaped configuration are beneficial to improve the longitudinal flexibility of the stent graft.To sum up,we designed and prepared a series of grafts with gradient radial compliance and structurally differentiated metal stents,and composite them.We systematically analyzed the effect of the structure and properties of the graft on the radial compliance of the stent graft,the effect of metal stent structure on radial compliance of stent grafts,radial compliance of stent graft after dynamic fatigue.Spring-back force and bending force dynamic test device was independently designed and developed to achieve the objective stable,continuous and multi-angle evaluation of stent graft longitudinal flexibility.The longitudinal flexibility of the commercial stent graft,and the relationship between the stent-graft structure and its longitudinal flexibility were explored.Those work lays the foundation for further research and development of stent grafts with excellent performance. |
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