Number Of Struts Research Articles

Manipulating failure mechanism of rapid prototyped scaffolds by changing nodal connectivity and geometry of the pores

The performance of cellular solids in biomedical applications relies strongly on a detailed understanding of the effects of pore topology on mechanical properties. This study aims at characterizing the failure mechanism of scaffolds based on nodal connectivity (number of struts that meet in joints) and geometry of the pores. Plastic models of scaffolds having the same relative density but different cubic and trigonal unit cells were designed and then fabricated via three dimensional (3-D) printing. Unit cells were repeated in different arrangements in 3-D space. An in-situ imaging technique was utilized to study the progressive deformation of the scaffold models. Different nodal connectivities resulted in a wide range of compressive behaviors in scaffold models, from elastic-plastic to fully brittle. The Maxwell necessary criterion for rigidity was used to explain mechanical behavior of the scaffolds. Nodal connectivity of 4 satisfied Maxwell's criterion for rigidity in the examined structures. In a stress–strain curve of scaffolds with cubic unit cells and nodal connectivities of 3 and 4, pore deformation was observed after yielding. On the other hand, scaffolds with trigonal unit cells and nodal connectivities of 4 and 6, exhibited brittle behavior in the absence of pore deformation. These results highlight the role of nodal connectivity on failure mechanism and subsequently mechanical performance of scaffolds. This study reveals that appropriate pore geometry can provide sufficient condition for rigidity when Maxwell's necessary condition is satisfied. In addition, this study demonstrates that Maxwell's criterion can be used in pre-designing of pore geometries for scaffolds with distinct nodal connectivities.

Integrated optimization of actuator placement and vibration control for piezoelectric adaptive trusses

In this paper, the integrated optimization of actuator placement and vibration control for piezoelectric adaptive truss is studied. Based on the dynamic finite element (FE) model and a linear-quadratic-Gaussian (LQG) model of vibration control for an adaptive truss, an integrated optimization model is built in which an improved quadratic performance index is adopted as the objective function and the mode closed-loop damping ratio, modal controllability and actuator number are selected as the constraints. A layered optimization strategy is implemented to address this optimization problem with discrete-continuous design variable. To prevent the optimization process from converging to the local optimal solution, the genetic algorithm (GA) for outside-layer optimization is extended with an improved penalty function. Numerical examples and the vibration control experiments for piezoelectric adaptive truss were used to validate the efficiency of the proposed method. The following conclusions were drawn from the results. (1) The improved penalty function can orient the optimization process to the global optimal solution. (2) Although the number of struts on the truss is large, the optimization computation time is short because of the high efficiency of the proposed method. (3) In the experiment, the quadratic performance index, modal response and sensor signals for the present paper's optimal actuator placement scheme are better than those described in the literature, but the placement requires more energy, which is consistent with the numerical results.

On the role of equivalent strut models in the seismic assessment of infilled RC buildings

The participation of masonry infill panel to the overall seismic resistance of a framed building has a significant variation according to the specific mechanical characteristics of the infill, the geometrical distribution within the building and the local interaction among the panel and the surrounding primary RC elements. Especially in the case of structure designed only for vertical loads, essence of the infill can be decisive under an unexpected earthquake, providing an additional contribution to the strength and to the stiffness. On the other side, this beneficial role is often accompanied by the modification of the global collapse mechanisms, with the appearance of brittle failure modes.In the present paper, an existing RC framed building for which a good level of knowledge was available, including a wide experimental database, was chosen as a case study. A reference frame was considered for performing nonlinear static analyses aimed at investigating some significant aspects about the modelling of the infill and the relapse induced by the related computational choices on the structural response. In particular, it is faced the sensitivity analysis about specific parameters involved in the definition of the equivalent strut models: the width bW of the strut; the constitutive Force–Displacement law of the panel; the number of struts adopted to simulate the panel.

Automatic stent strut detection in intravascular optical coherence tomographic pullback runs

We developed and evaluated an automatic stent strut detection method in intravascular optical coherence tomography (IVOCT) pullback runs. Providing very high resolution images, IVOCT has been rapidly accepted as a coronary imaging modality for the optimization of the stenting procedure and its follow-up evaluation based on stent strut analysis. However, given the large number of struts visible in a pullback run, quantitative three-dimensional analysis is only feasible when the strut detection is performed automatically. The presented method first detects the candidate pixels using both a global intensity histogram and the intensity profile of each A-line. Gaussian smoothing is applied followed by specified Prewitt compass filters to detect the trailing shadow of each strut. Next, the candidate pixels are clustered using the shadow information. In the final step, several filters are applied to remove the false positives such as the guide wire. Our new method requires neither a priori knowledge of the strut status nor the lumen/vessel contours. In total, 10 IVOCT pullback runs from a 1-year follow-up study were used for validation purposes. 18,311 struts were divided into three strut status categories (malapposition, apposition or covered) and classified based on the image quality (high, medium or low). The inter-observer agreement is 95 %. The sensitivity was defined as the ratio of the number of true positives and the total number of struts in the expert defined result. The proposed approach demonstrated an average sensitivity of 94 %. For malapposed, apposed and covered stent struts, the sensitivity of the method is respectively 91, 93 and 94 %, which shows the robustness towards different situations. The presented method can detect struts automatically regardless of the strut status or the image quality, and thus can be used for quantitative measurement, 3D reconstruction and visualization of the stents in IVOCT pullback runs.

LATERAL BUCKLING OF THE STRUTS IN BEAM STRING STRUCTURES CONSIDERING THE LAYOUT OF STRINGS

The struts in a beam string structure (BSS) may buckle laterally under compression. The lateral buckling of the struts is determined not only by the rotational stiffness of the beam–strut joints and the length and bending stiffness of the struts, but also by the rise and lateral stiffness of the beam, the number of struts, and the layout of strings. In this paper, the multi-strut BSS with several types of layout of strings is studied. An analytical method for estimating the lateral buckling load of the struts in BSS is proposed. Parametric studies are carried out to investigate the variation of the lateral buckling of the struts in the BSS for different string layouts. In the end, the validity of the proposed method is examined by means of numerical simulations using the geometrically nonlinear finite element method.

Agreement and reproducibility of gray‐scale intravascular ultrasound and optical coherence tomography for the analysis of the bioresorbable vascular scaffold

To report the agreement between gray-scale intravascular ultrasound (GS-IVUS) and optical coherence tomography (OCT) in assessing the bioresorbable vascular scaffolds (BVS) structures and their respective reproducibility. BVS are composed of an erodible polymer. Ultrasound and light signals backscattered from polymeric material differs from metallic stents using GS-IVUS and OCT. Forty-five patients included in the ABSORB trial were treated with a 3.0 × 18 mm BVS and imaged with GS-IVUS 20 MHz and OCT post-implantation. Qualitative (ISA, side-branch struts, protrusion, and dissections) and quantitative (number of struts, lumen, and scaffold area) measurements were assessed by two investigators. The agreement and the inter- and intraobserver reproducibility were investigated using the kappa (κ) and the interclass correlation coefficient (ICC). GS-IVUS and OCT agreement was predominantly poor at a lesion, frame, and strut level analysis (κ and ICC <0.4) for qualitative measurements. GS-IVUS demonstrated a reduced ability to detect cross-sections with ISA (4.5% vs. 20.6%), side-branch (SB) struts (6.3% vs. 7.8%), protrusions (3.2% vs. 9.6%), and dissections (0.2% vs. 9.0%) compared with OCT. GS-IVUS reproducibility was poor-moderate (κ and ICC <0.6) except for ISA and SB-struts (κ and ICC between 0.2 and 0.75). OCT showed an excellent reproducibility (κ and ICC > 0.75) except for the assessment of tissue protrusion (κ and ICC between 0.47 and 0.94). GS-IVUS reproducibility was poor-moderate (ICC ≤ 0.5) in assessing the number of struts but excellent with OCT (ICC > 0.85). The reproducibility to assess lumen and scaffold areas was excellent using both techniques (ICC > 0.85). GS-IVUS has a poor capacity to detect qualitative findings post-BVS implantation and its reproducibility is low compared with OCT. The use of GS-IVUS should be limited when assessing lumen and scaffold areas.

Interaction Deep Excavation – Adjacent Structure: Numerical Two and Three Dimensional Modeling

Urban development often requires the construction of deep excavations near to buildings or other structures. We have to study complex material structure interactions where we should take into consideration several particularities. In this paper, we perform a numerical modeling with the finite element method, using PLAXIS software, of the interaction deep excavation-diaphragm wall-soil-structure in the case of non linear soil behavior. We focus our study on a comparison of the results given respectively by two and three dimensional modelings. This allows us to give some recommendations concerning the validity of twodimensional study. We perform a parametric study according to the initial loading on the structure and the struts number.

The Strength Evaluation and Reliability Analysis of Periodic Symmetric Struts Support

In this paper, strength evaluations and reliability analysis are conducted for different types of PSSS(Periodically Symmetric Struts Supports) based on the FEA(Finite Element Analysis). The numerical models are established at first, and the PMA(Prestressed Modal Analysis) is conducted. The nodal stress value of all of the gauss points in elements are extracted out and the stress distributions are evaluated for each type of PSSS. Then using nonlinear least squares method, curve fitting is carried out, and the stress probability distribution function is obtained. The results show that although using different number of struts, the stress distribution function obeys the exponential distribution. By using nonlinear least squares method again for the distribution parameters a and b of different exponential functions, the relationship between number of struts and distribution function is obtained, and the mathematical models of the stress probability distribution functions for different supports are established. Finally, the new stress distribution model is introduced by considering the DSSI(Damaged Stress-Strength Interference), and the reliability evaluation for different types of periodically symmetric struts supports is carried out.

Optical coherence evaluation of everolimus-eluting stents 8 months after implantation

ObjectiveThe aim of this study was to evaluate detailed vessel response after everolimus-eluting stents (EES) implantation in human de novo coronary lesions by optical coherence tomography (OCT).Design, setting and patientsBetween...

In vivo evaluation of stent strut distribution patterns in the bioabsorbable everolimus-eluting device: an OCT ad hoc analysis of the revision 1.0 and revision 1.1 stent design in the ABSORB clinical trial

The ABSORB Cohort A clinical study has shown the feasibility and safety of the fully bioabsorbable everolimus-eluting structure (BVS, revision 1.0). However, the study also demonstrated somewhat higher acute and late recoil with the BVS structure compared to metallic drug eluting stents. Based on these clinical observations, modifications to the stent design (BVS, revision 1.1) were introduced for the ABSORB Cohort B study in order to decrease recoil. The aim was to compare in vivo the strut distribution between the BVS revision 1.0 (Cohort A), and BVS revision 1.1 (Cohort B) designs.OCT analysis was performed by two independent analysts in four patients from each cohort of the ABSORB study. Strut distribution was assessed in cross-section, and longitudinally in a frameby-frame analysis. Variables recorded included inter-strut angle, maximum inter-strut angle and number of frames with < or =3 struts. The inter-observer correlation coefficient was also assessed. For both designs, on a patient level there was no significant difference in the number of analysed struts corrected for the length of the scaffold (p=0.78). Likewise, on a frame by frame analysis mean stent area, number of struts per frame, mean maximum inter-strut angle, and mean inter-strut angle were similar for both groups. However, in both structures there was a cyclical variation in the maximum number of struts per frame. The frequency of this variation was significantly higher in Cohort B. The inter-observer correlation coefficient for strut counts, inter-strut angle and maximum inter-strut angle was 0.91, 0.87 and 0.74 respectively.This ad hoc analysis confirms that the revision 1.1 BVS design has a different longitudinal strut distribution to the revision 1.0 BVS design, indicating that the new design has a reduced maximum circular unsupported cross sectional area.

An optical coherence tomography study of a biodegradable vs. durable polymer-coated limus-eluting stent: a LEADERS trial sub-study

Incomplete endothelialization has been found to be associated with late stent thrombosis, a rare but devastating phenomenon, more frequent after drug-eluting stent implantation. Optical coherence tomography (OCT) has 10 times greater resolution than intravascular ultrasound and thus appears to be a valuable modality for the assessment of stent strut coverage. The LEADERS trial was a multi-centre, randomized comparison of a biolimus-eluting stent (BES) with biodegradable polymer with a sirolimus-eluting stent (SES) using a durable polymer. This study sought to evaluate tissue coverage and apposition of stents using OCT in a group of patients from the randomized LEADERS trial. Fifty-six consecutive patients underwent OCT during angiographic follow-up at 9 months. OCT images were acquired using a non-occlusive technique at a pullback speed of 3 mm/s. Data were analysed using a Bayesian hierarchical random-effects model, which accounted for the correlation of lesion characteristics within patients and implicitly assigned analytical weights to each lesion depending on the number of struts observed per lesion. Primary outcome was the difference in percentage of uncovered struts between BESs and SESs. Twenty patients were included in the analysis in the BES group (29 lesions with 4592 struts) and 26 patients in the SES group (35 lesions with 6476 struts). A total of 83 struts were uncovered in the BES group and 407 out of 6476 struts were uncovered in the SES group [weighted difference -1.4%, 95% confidence interval (CI) -3.7 to 0.0, P = 0.04]. Results were similar after adjustment for pre-procedure lesion length, reference vessel diameter, number of implanted study stents, and presence of stent overlap. There were three lesions in the BES group and 15 lesions in the SES group that had > or =5% of all struts uncovered (difference -33.1%, 95% CI -61.7 to -10.3, P < 0.01). Strut coverage at an average follow-up of 9 months appears to be more complete in patients allocated to BESs when compared with SESs. The impact of this difference on clinical outcome and, in particular, on the risk of late stent thrombosis is yet to be determined.

Topology design of tensegrity structures via mixed integer programming

This paper presents a numerical method for finding a tensegrity structure based on the ground structure method. We first solve a mixed integer programming (MIP) problem which maximizes the number of struts over the self-equilibrium condition of axial forces and the discontinuity condition of struts. Subsequently we solve the minimization problem of the number of cables in order to remove redundant self-equilibrium modes, which is also formulated as an MIP. It is regarded to be advantageous that our method does not require any connectivity information of cables and struts to be known in advance, while the obtained tensegrity structure is guaranteed to satisfy the discontinuity condition of struts rigorously.

Nonlinear simulations on the interaction of disorder and defects in open cell structures

Abstract Various types of defects in regular and disordered Simple Cubic open cell structures with a relative density of 12.5% are investigated by the Finite Element Method with respect to their influence on the nonlinear mechanical behavior. The three-dimensional models are based on beam elements and account for the material distribution and the deformation constraints in the vertices. Cylindrical samples are modeled consisting of some 2600 base cells with four different lattice orientations. Disordered modifications thereof are created by randomly shifting the position of the vertices within a spherical domain. Defects are introduced by removing a fixed number of struts in three different manners, i.e., randomly distributed, as small clusters, and as big clusters. The nonlinear overall mechanical behavior is predicted for uniaxial compression of the samples with various orientations of the material’s principal axes. Elastic–plastic strut material, large deformations, and localization of deformation is considered. The defect sensitivity is studied with respect to material orientation, structural disorder, and defect type.

Shorten the Intermediate Turbine Duct Length by Applying an Integrated Concept

The demand of further increased bypass ratio of aero engines will lead to low pressure turbines with larger diameters, which rotate at lower speed. Therefore, it is necessary to guide the flow leaving the high pressure turbine to the low pressure turbine at larger diameters minimizing the losses and providing an adequate flow at the low pressure (LP)-turbine inlet. Due to costs and weight, this intermediate turbine duct has to be as short as possible. This would lead to an aggressive (high diffusion) s-shaped duct geometry. It is possible to shorten the duct simply by reducing the length but the risk of separation is rising and losses increase. Another approach to shorten the duct and thus the engine length is to apply a so called integrated concept. These are novel concepts where the struts, mounted in the transition duct, replace the usually following LP-vane row. This configuration should replace the first LP-vane row from a front bearing engine architecture where the vane needs a big area to hold bearing services. That means the rotor is located directly downstream of the strut. This means that the struts have to provide the downstream blade row with undisturbed inflow with suitable flow angle and Mach number. Therefore, the (lifting) strut has a distinct three-dimensional design in the more downstream part, while in the more upstream part, it has to be cylindrical to be able to lead through supply lines. In spite of the longer chord compared with the base design, this struts have a thickness to chord ratio of 18%. To apply this concept, a compromise must be found between the number of struts (weight), vibration, noise, and occurring flow disturbances due to the secondary flows and losses. The struts and the outer duct wall have been designed by Industria de Turbopropulsores. The inner duct was kept the same as for the base line configuration (designed by Motoren und Turbinen Union). The aim of the design was to have similar duct outflow conditions (exit flow angle and radial mass flow distribution) as the base design with which it is compared in this paper. This base design consists of a single transonic high pressure (HP)-turbine stage, an aggressive s-shaped intermediate turbine duct, and a LP-vane row. Both designs used the same HP-turbine and were run in the continuously operating Transonic Test Turbine Facility at Graz University of Technology under the same engine representative inlet conditions. The flow field upstream and downstream the LP-vane and the strut, respectively, has been investigated by means of five hole probes. A rough estimation of the overall duct loss is given as well as the upper and lower weight reduction limit for the integrated concept.

Dihedral ‘star’ tensegrity structures

This paper presents conditions for self-equilibrium and super stability of dihedral ‘star’ tensegrity structures, based on their dihedral symmetry. It is demonstrated that the structures are super stable if and only if they have an odd number of struts, and the struts are as close as possible to each other. Numerical investigations show that their prestress stability is sensitive to the geometry realisation.

Relation of Drug-Eluting Stent Strut Distribution to Stent Thrombosis in Coronary Arteries

The distribution of stent struts is critical to drug deposition and, therefore, may affect the amount of neointima and the risk of thrombosis after drug-eluting stent (DES) implantation. The aim of our study was to evaluate stent strut distribution in the setting of a drug-eluting stent thrombosis (ST). We retrospectively analyzed postprocedural intravascular ultrasound (IVUS) images of 13 patients who subsequently developed ST (14 DES thrombotic lesions) and a control group of 27 patients (30 DES lesions) matched for stent type and presence of chronic renal failure. In addition to standard IVUS measurements, visible struts were counted and maximum interstrut angle was measured at 1-mm intervals. Early ST was defined as < or =30 days after DES deployment and late ST as >30 days after DES deployment. Compared with DES controls, the ST group had a larger maximum interstrut angle (60.8 +/- 8.3 degrees vs 55.7 +/- 4.8 degrees , p = 0.014) and a similar number of stent struts (8.4 +/- 0.6 vs 8.7 +/- 0.6, p = NS). Maximum interstrut angle tended to be larger in late ST than in early ST (66.1 +/- 10.8 degrees vs 57.8 +/- 5.0 degrees , p = 0.071). The incidence of maximum interstrut angles > or =90 degrees and > or =120 degrees observed continuously for > or =2 mm of stent length was higher in the ST group (p = 0.009 and p = 0.096, respectively). In conclusion, DES-treated lesions leading to ST had larger maximum interstrut gaps distributed circumferentially and longitudinally, but a similar number of struts at the time of DES implantation compared with DES controls.

Reproducibility of quantitative optical coherence tomography for stent analysis

To assess the inter- and intra- observer reproducibility for strut count, strut apposition and strut tissue coverage measurements with optical coherence tomography (OCT). Ten drug-eluting stents (244 frames, 1712 struts) imaged with OCT nine months after implantation were analysed by two independent analysts. One of the analysts repeated the analysis of five stents (120 frames, 795 struts) one week later. Offline analysis was performed with the proprietary LightLab Imaging software. The number of struts was counted and lumen and stent area contours were traced. Tissue coverage thickness was measured at 360 degrees of vessel circumference and in front of every individual strut. The number of malapposed struts was determined. There was good agreement for strut number count (Kendall's Tau-b 0.90 for inter- and 0.94 for intra- observer variability). The relative difference for lumen area, stent area and tissue coverage measurements was around 1%. There was complete inter- and intra- observer agreement for malapposed struts classification (4 out of 1708 struts, Kappa=1). In a Corelab setting, the inter- and intra- observer reproducibility for strut count, strut apposition and strut tissue coverage measurements with OCT is excellent. This emphasises the value of OCT as a tool for the clinical long-term assessment of stents.

Development of Computer-assisted Diagnosis Using Digital Radiography for the Evaluation of Dental Implant Osseointegration

To develop an osseointegration analyzing system for dental implants, a new analyzing system which can assess the level of osseointegration between an implant and trabecular bone was constructed using digital radiography with morphological filter and node-strut analysis. For assessment of this system, a grayscale test chart that simulates six levels of an osseointegration was created. In addition, digital implant images were made in which the trabecular pattern around the implant was varied over a total of five levels. Implant osseointegration was evaluated on the basis of seven parameters related to the number of nodes (Nd) and terminuses (Tm) of the skeleton bound to the implant (Im) and the skeletal length. The seven parameters were as follows: the number of struts connecting the Im with the Nd and Tm (N.ImNd, N.ImTm), the total number of N.ImNd and N.ImTm (N.Im), the strut length connecting the Im with the Nd and Tm (ImNd, ImTm), and the ratios of the struts connecting the Im with the Nd and Tm (ImNd/TSL, ImTm/ TSL), where TSL is the total strut length.Strong correlations (R2= 0.971–1.0) between the theoretical values from the test charts and the measured values were demonstrated. N.ImNd showed the strongest correlation, R2= 0.948, from the digital implant images, followed by N.Im and ImNd, with correlations of R2= 0.86 and R2= 0.84, respectively. This new system for evaluating implant osseointegration by applying morphological processing and node-strut analysis could be useful for computer-assisted diagnosis of digital dental implant images.

Percolation theory relates corticocancellous architecture to mechanical function in vertebrae of inbred mouse strains

Complex corticocancellous skeletal sites such as the vertebra or proximal femur are connected networks of bone capable of transferring mechanical loads. Characterizing these structures as networks may allow us to quantify the load transferring behavior of the emergent system as a function of the connected cortical and trabecular components. By defining the relationship between certain physical bone traits and mechanical load transfer pathways, a clearer picture of the genetic determinants of skeletal fragility can be developed. We tested the hypothesis that the measures provided by network percolation theory will reveal that different combinations of cortical, trabecular, and compositional traits lead to significantly different load transfer pathways within the vertebral bodies among inbred mouse strains. Gross morphologic, micro-architectural, and compositional traits of L5 vertebrae from 15 week old A/J (A), C57BL6/J (B6), and C3H/HeJ (C3H) inbred mice ( n = 10/strain) were determined using micro-computed tomography. Measures included total cross-sectional area, bone volume fraction, trabecular number, thickness, spacing, cortical area, and tissue mineral density. Two-dimensional coronal sections were converted to network graphs with the cortical shell considered as one highly connected node. Percolation parameters including correlation length (average number of connected nodes between superior and inferior surfaces), chemical length (minimum number of connected nodes between surfaces), and backbone mass (strut number) were measured. Analysis of the topology of the connected bone networks showed that A and B6 mice transfer load through trabecular pathways in the middle of the vertebral body in addition to the cortical shell. C3H mice transfer load primarily through the highly mineralized cortical shell. Thus, the measures provided by percolation theory provide a quantitative approach to study how different combinations of cortical and trabecular traits lead to mechanically functional structures. The data further emphasize the interdependent nature of these physical bone traits suggesting similar genetic variants may affect both trabecular and cortical bone. Therefore, developing a network approach to study corticocancellous architecture during growth should further our understanding of the biological basis of skeletal fragility and, thus, provide novel engineering approaches to studying the genetic basis of fracture risk.

Analysis of wall shear stress in stented coronary artery using 3D computational fluid dynamics modeling

In this study, an investigation on the effects of stent geometry on blood flow in a stented human coronary artery 2D and 3D computational fluid dynamics (CFD) models with different stent geometries is reported. Blood velocity profiles and shear stress values were computed in three different sites, including stented arterial segment, pre-stent, and post-stent regions. Blood flow was assumed as a fully developed incompressible Newtonian flow. Rigid boundary conditions were assumed for all models. The governing Navier–Stokes equations were solved using commercial software. The arterial wall shear stress distribution was investigated in three major regions and critical sites were located. It is concluded that shear stress is influenced by three stent design parameters, i.e., strut spacing, strut profile, and number of struts. To achieve the most secure shear stress value, the optimum stent geometry can be obtained with respect to the mentioned parameters for a specific arterial segment. Different stents may be used for different arteries and arterial branches due to the dependency of the shear stress value to the geometry of the artery. It is shown that analyses of wall shear stress profile between stent struts, and in pre-stent and post-stent regions are essential in stent design. Additionally, it is shown that by application of a flow divider, the wall shear stress in stented segment increases markedly.