Distal stent graft-induced new entry (dSINE) is being increasingly reported after TEVAR for chronic type B aortic dissection (cTBAD). In addition to aortic wall fragility, stent-graft oversizing due to the discrepancy between the sizes of proximal and distal landing zones and the less compliant dissection flap to the expansion of the stent-graft carry a higher risk of a new intimal tear. To address these risk factors, a dissection-specific stent-graft (DSSG) was designed to include features such as: no proximal barbs, a customized longer body length with substantial taper, reduced radial force in the second and third distal stents that are sited internally to avoid any contact with the dissection membrane, and removal of the final stent, leaving 30 mm of unsupported Dacron fabric ("endovascular elephant trunk") (Figure 1). The combination of patient-specific anatomy and the implanted endograft brings about changes in hemodynamic of the stented and non-stented regions that can have consequences on flow patterns and shear stress distributions. As such, wall shear stress (WSS) is of particular interest, and computational fluid dynamics (CFD) can provide information on WSS. As a continuation of previous work in that we reported mid-term clinical effectiveness of DSSG with low rates of dSINE,1 the present study focuses on identifying hemodynamic properties (WSS) of DSSG compared with those of standard thoracic stent-graft (STSG) by means of CFD simulations.

Post-TEVAR computed tomographic angiography (CTA) images of sixteen patients with cTBAD who underwent TEVAR using DSSG were used to reconstruct three-dimensional models of patient-specific aortas, together with those of three cTBAD patients who underwent TEVAR using STSG and subsequently developed dSINE during follow up. The resulting aortic geometries were then discretized into finite element meshes for CFD simulations. Blood flow, modeled as Newtonian fluid, simulations were performed with rigid wall assumptions utilizing SimVascular's incompressible Navier-Stokes solver. With the intent of probing the DSSG versus STSG configurations in respect to their hemodynamic properties, inflow and outflow conditions were standardized; in-flow boundary conditions were prescribed using normal physiologic pulsatile circulation, and outlet boundary conditions consisted of Windkessel models with previously published values. The primary measurement from CFD analysis was the magnitude of the WSS (peak WSS, time averaged WSS (TAWSS)). The WSS values were determined: in the DSSG cases, at: 1) the reduced radial force segment where two distal stents are sited internally (RRF region), 2) the endovascular elephant trunk segment (EET region), 3) the non-stented segment immediately distal to the end of the stent graft (NS-DSSG region); in the STSG patients, at, 1) the distal end of the STSG (DE-STSG region), 2) the non-stented segment immediately distal to the end of the STSG (NS-STSG region).

In the STSG group, at 1-month follow-up, the WSS values at the NS-STSG region (non-stented segment, in the region of the future dSINE) were significantly elevated compare with those at the DE-STSG region (distal end of the STSG segment) (mean peak WSS, 40.34 dyne/cm2 vs. 15.4 dyne/cm2; mean TAWSS, 10.55 dyne/cm2 vs. 4.21 dyne/cm2) (Figure 2, lower left panel). In the DSSG group, however, at 1-month follow-up, such significant WSS gradient differences noticed in the STSG group were not observed between the EET region and the NS-DSSG region (mean peak WSS, 41. 9 dyne/cm2 vs. 42.9 dyne/cm2; mean TAWSS, 12.0 dyne/cm2 vs. 12.2 dyne/cm2) (Figure 2, lower right panel). Instead, a significant WSS gradient difference was observed between the RRF and EET regions (mean peak WSS, 20.77 dyne/cm2 vs. 41.9 dyne/cm2; mean TAWSS, 5.95 dyne/cm2 vs. 12.0 dyne/cm2) (Figure 2, upper right panel). After 12 months follow-up, unchanged WSS gradient patterns were found for both the DSSG and STSG groups. Noticeably, comparison of the EET volume over the follow up duration revealed a significant expansion of EET volume: 13.06 cm3 at 12-month vs. 8.96 cm3 at 1-month (p = 0.02).

This study showed differences in the occurrence site of high WSS gradient, which may lead to a new intimal tear, between STSG and DSSG: non-stented segment in the STSG cases versus stented segment in the DSSG cases. This difference, having a high WSS zone in the stented segment, may protect the intima from damage, thereby reducing the incidence of dSINE.

1. Burdess A, D'Oria M, Mani K, Tegler G, Lindström D, Mogensen J, Kölbel T, Wanhainen A. Early Experience With a Novel Dissection-Specific Stent-Graft to Prevent Distal Stent-Graft-Induced New Entry Tears After Thoracic Endovascular Repair of Chronic Type B Aortic Dissections. Ann Vasc Surg. 2021 Nov 14:S0890-5096(21)00879-7.