Influence of shape-memory stent grafts on local aortic compliance

Abstract

The effect of repair techniques on the biomechanics of the aorta is poorly understood, resulting in significant levels of postoperative complications for patients worldwide. This study presents a computational analysis of the influence of Nitinol-based devices on the biomechanical performance of a healthy patient-specific human aorta. Simulations reveal that Nitinol stent-grafts stretch the artery wall so that collagen is stretched to a straightened high-stiffness configuration. The high-compliance regime (HCR) associated with low diastolic lumen pressure is eliminated, and the artery operates in a low-compliance regime (LCR) throughout the entire cardiac cycle. The slope of the lumen pressure-area curve for the LCR post-implantation is almost identical to that of the native vessel during systole. This negligible change from the native LCR slope occurs because the stent-graft increases its diameter from the crimped configuration during deployment so that it reaches a low-stiffness unloading plateau. The effective radial stiffness of the implant along this unloading plateau is negligible compared to the stiffness of the artery wall. Provided the Nitinol device unloads sufficiently during deployment to the unloading plateau, the degree of oversizing has a negligible effect on the pressure-area response of the vessel, as each device exerts approximately the same radial force, the slope of which is negligible compared to the LCR slope of the native artery. We show that 10% oversizing based on the observed diastolic diameter in the mid descending thoracic aorta results in a complete loss of contact between the device and the wall during systole, which could lead to an endoleak and stent migration. 20% oversizing reaches the Dacron enforced area limit (DEAL) during the pulse pressure and results in an effective zero-compliance in the later portion of systole.

Keywords: Aortic compliance; Collagen; Finite element; Stent-graft.

Fig. 1

a By removing the stitches that attach the NiTi stent rings to the graft material, we can observe that the diameter of the stent expands by a further 24%, to the true stress-free reference configuration. In vivo, the DEAL prevents the implant from returning to this reference configuration. b A finite element model of the stent graft (SG) is created and crimp and deploy steps are simulated. c The RF-D relationship of the SG model is recorded, and the effective stent membrane (ESM) model is created and subjected to the same displacement boundary conditions. Calibration of the parameter values of the ESM constitutive model results in an identical RF-D response throughout the crimp and deployment steps to that of the full stent-graft model. The reference configuration (point Z) is the configuration of the stent once it has been removed from the Dacron graft. Both the SG and ESM are then crimped to the internal diameter of the delivery sheath. During the deployment step, the implant expands until the Dacron enforced area limit (DEAL) is reached, after which the implant has essentially a zero-compliance due to the stiffness of the Dacron (point Y). The experimental test results of (Zhou et al. 2019) are superimposed for comparison (EXP)

Fig. 2

Overview of the key features of the MRI/FEA framework that is critical to the following study. a 1. A reference configuration is constructed with an area ${\mathbf{A}}_{\mathbf{r}}$. 2. An equilibrium zero-pressure configuration is computed, whereby the cross-sectional area of the vessel reduces to ${\mathbf{A}}_{\mathbf{e}}$, such that the internal tensile circumferential stress due to elastin pre-stretch and SMC contractility is in equilibrium with the compressive circumferential stress generated in the matrix. 3.) The lumen pressure is increased from zero up to the peak systolic value (117 mmHg). The pressure increases through a sub-physiological pressure regime (SPPR). At the diastolic pressure, ${\mathbf{P}}_{\mathbf{d}}$, (73 mmHg) the computed lumen area is denoted ${\mathbf{A}}_{\mathbf{d}}$. The pressure is then increased through the physiological pressure range (PPR) up to the peak systolic value, ${\mathbf{P}}_{\mathbf{s}}$ (117 mmHg), at which point the computed area is denoted ${\mathbf{A}}_{\mathbf{s}}$. b The model is shown to accurately capture the spatially varying MRI data simply by calibrating the spatially varying volume fractions of the constituents (${\mathbf{V}}_{\mathbf{c}\mathbf{o}\mathbf{l}}^{\mathbf{f}}$, ${\mathbf{V}}_{\mathbf{e}\mathbf{l}\mathbf{a}}^{\mathbf{f}}$, ${\mathbf{V}}_{\mathbf{s}\mathbf{m}\mathbf{c}}^{\mathbf{f}}$), and the transition strain of the collagen. c In this study, we investigate the effects of stenting on the biomechanics of the healthy (Young) aorta, in addition to an artificially aged (Old) aorta. By reducing the volume fraction of elastin by 10% and reducing the collagen transition strain by 0.15 in accordance with the experimental studies of (Hosoda et al. and Vande Geest and Vorp. 2004), we obtain the ‘Old’ aorta which exhibits a stiffer and less nonlinear response across the physiological pressure range compared to the young aorta

Fig. 3

a Flow chart for solution procedure. (1.) The unloaded reference configuration of the artery. (2.) The contractile elements within the arterial wall result in a reduction in area and a new equilibrium configuration. (3.) Diastolic pressure of 73 mmHg is applied to the internal lumen surface. (4.) The SG/ESM is crimped to the inner sheath diameter of the delivery device. (5.) The SG/ESM is deployed into the artery, which remains under the diastolic pressure load, and a new equilibrium area is reached. (6.) The system is pressurised with a pulse pressure to bring the total applied lumen pressure to 117 mmHg. b Circumferential stress versus strain plot for the ESM model shows that the device come in contact with the arterial wall at (1) along the ‘unloading plateau’, which results in an increase in the diastolic equilibrium area (2). By applying the pulse pressure, the NiTi behaviour follows the high compliance stiffness of the unloading plateau, which is significantly less than the LCR stiffness of the aorta. c Both models (ESM and SG) predict that the lumen area is ~ 53% higher than the untreated artery at diastolic pressure, and ~ 24% higher than the untreated artery at peak systolic pressure. As the effective compliance of the implant is negligible compared to the high stiffness of the low compliance regime (LCR) of the aorta, a similar slope is observed in the pressure–area relationship between the untreated LCR and the area increase due to the pressure pulse post-implantation

Fig. 4

Available treatment options for aortic disease. a, b Endovascular aortic repair (EVAR); c, d open surgical repair (OSR). a A schematic of the EVAR procedure where a stent-graft is deployed intravascularly to the disease site (Figueroa and Zarins 2011). b Postoperative CT scan showing proximal aortic and distal aortic stents (Sultan and Hynes 2014). c A schematic of the OSR procedure where the healthy aorta proximal and distal to the diseased site are clamped, the diseased portion is cut open and a synthetic graft is sutured into the native vessels proximally and distally (Figueroa and Zarins 2011). d Perioperative image of OSR of infrarenal aorta with Dacron graft (Baila et al. 2016)

Fig. 5

a Finite element mesh generation directly from MRI dataset. Heterogeneous wall thickness (t) is implemented based on histology data from Concannon et al., (2019). b-d Comparison of SG and ESM models in a subject-specific aorta. b Deployment of the SG results in stress concentrations in the regions where the struts directly contact the artery wall. c Both the SG and ESM models show a similar prediction of the implant-induced compliance alteration on the pressure–area relationship. d The ESM simulation exhibits more uniform distribution of stress in the peri-implant artery wall; however, the level of stress is similar to that of the SG model

Fig. 6

a The effects of ESM oversizing on the pressure–area relationship in Plane 6 of the human aorta. In each case, the 20% (red), 40% (green) and 60%(blue) oversizing results in any area gain due to pulse pressure to follow the secondary stiffness slope of the aorta. b In the case of 20% oversizing, the DEAL is indicated by the dashed red line, below which the pressure–area response follows the secondary stiffness of the aorta. Once the DEAL is reached, no further area gain is incurred for any further increase in pressure (as indicated by the change in slope of solid red line at ~ 93 mmHg). c Finite element contour plot of max. Principal logarithmic strain (LE), highlighting similar strain levels at the deployment site for each degree of oversizing. This can be explained by considering the device effective circumferential stress–strain history during crimping and deployment d. In each case, the diastolic equilibrium configuration lies on the unloading plateau where the stiffness of the Nitinol is negligible compared to that of the secondary stiffness of the native aorta. With the application of the pulse pressure, each degree of oversizing remains on the unloading plateau and therefore exerts approximately the same radial force on the arterial wall

Fig. 7

20% oversized stent-graft in the a proximal aorta and b distal aorta. Pre-operative pressure–area relationship is indicated by open circles and dashed line (fitted). Dotted line indicates the DIAL when the stent-graft is fully expanded, and no further area increase can be incurred for a given increase in pressure

Fig. 8

a Healthy (Young) pressure–area relationship for Plane 6 as indicated by blue open circles, and FE fit (dashed black line) compared to Old (solid black line). The Old aortic properties were achieved by reducing both the elastin volume fraction within the wall and the transition strain. b The effect of implant oversizing on the pressure–area relationship in an Old aorta. 20, 40 and 60% oversizing are indicated by the solid red, green and blue lines, respectively. The DIAL for each percentage oversizing is indicated by the dashed red, green and blue lines, respectively. c Finite element mesh of subject-specific aorta comparing healthy (Young) and pathologically stiffened (Old) properties in terms of max. Principal logarithmic strain (LE)