Preliminary testing and evaluation of the renata minima stent, an infant stent capable of achieving adult dimensions

Abstract

Objectives: This study sought to obtain in vivo data on a new stent and delivery system specifically designed for implantation in infants with the ability to be enlarged to adult dimensions.

Background: There are no endovascular stents designed for or approved for use in infants, nor is there a stent capable of being implanted at infant vessel diameters and achieving adult size while maintaining structural integrity. The Minima stent was designed to address these needs.

Methods: This study was performed in 6 piglets who underwent implantation of 22 Minima stents into the following locations: aorta (n = 11), branch pulmonary arteries (n = 6), and central veins (n = 5).

Results: Successful deployment occurred in 21/22 attempts. Two instances of post-deployment migration occurred. Stents were re-expanded at 1, 2, 3 and 5 months after implant. All stents regardless of location could be re-dilated to the intended diameter to keep pace with somatic growth (implant diameter 6.9 +/- 1.2 mm; final diameter 16.1 mm +/- 1.4 mm). Histopathology at 1 and 5 months demonstrated widely patent vessel lumens with stent apposition to vessel wall, early mild inflammatory response surrounding stent struts, typical vascular damage and healing response to acute dilation and a progressive smooth neointimal growth covering stent struts over time.

Conclusions: In this in vivo study of the Minima stent, there was high implant success, predictable re-dilatability to adult diameters and favorable histopathology. Further study is warranted.

Keywords: congenital heart disease; pediatric intervention; stent; vascular stenosis.

FIGURE 1

Renata minima stent and delivery system. Stent as it would be mounted on delivery balloon (a), expanded to 8 mm (b) and expanded to 22 mm (c). Schematic drawings of the delivery system viewed from above (d) with the stent fully covered by the outer catheter which is locked in place at the back of the control handle. When viewed from the side and unlocked (e), the outer catheter has been pulled back along the rigid hyoptube to uncover the stent and allow for angiography via a flush tubing attached to the outer catheter. (f)‐(h) Sequential photographs of the catheter tip as the outer protective catheter is withdrawn to uncover the stent

FIGURE 2

Minima stent foreshortening. Graphic representation of bench testing data illustrating stent length foreshortening (y‐axis) as a function of increasing stent diameter (x‐axis). Note the exponential increase in foreshortening at larger (> 14 mm) diameters

FIGURE 3

Implant and re‐dilation of LPA stent (Animal #5, stent #16). Pre‐implant pulmonary angiogram (a) and post‐LPA stent implant (b) using an 8 mm diameter balloon. Note the LPA upper lobe branch crossed by the stent (arrowhead). Pre‐re‐dilation angiogram (c) 2 months after implant demonstrating no significant in‐stent restenosis and following re‐dilation (d) with a balloon diameter of 10 mm. Final pre‐re‐dilation LPA angiogram (e) 5 months after implant and a final angiogram (f) after re‐dilation with an 18 mm balloon. Note persistency patency of the left upper lobe branch throughout the study period (arrow)

FIGURE 4

Graphic display of chronically re‐dilated stents. Stents were re‐dilated at 2 and 5 months (dashed lines), 3 and 5 months (dotted lines) and at 5 months only (solid line). All stents could be significantly enlarged irrespective of time interval between re‐dilations, during a period of rapid animal weight gain (4.7–99.6 kg) which mimicked expected growth from infancy to adulthood

FIGURE 5

Animal 6, Stent #21, implant and subsequent re‐dilation. Initial thoracic aorta stent implant showing pre‐implant vessel diameter (a), maximum balloon inflation during implant (b) and implant result (c). Note the implant balloon is slightly oversized for the native vessel and stent recoil observed. Re‐dilation was performed 5 months and 101 kg weight gain later (d)‐(g). There has been virtually no reduction in stent diameter despite significant somatic growth (d). Maximum balloon used for re‐dilation was 18 mm (e), oversized when compared to the surrounding native vessel. Follow‐up angiogram demonstrates a stent that has been over‐dilated resulting in some aortic wall irregularity consistent with dissection or contained pseudoaneurysm (f). After implantation of an IntraStent MaxLD (g) telescoped within the Minima™ stent there is improved appearance of the aortic wall which continues to be oversized in the area of the stents

FIGURE 6

Postmortem gross and radiographic appearance of three stented vessels 1 month after implantation. All vessels demonstrate wide and even stent expansion within the vessel lumens. (a) Infrarenal aortic stent implanted at 6 mm and re‐dilated to 8 mm prior to sacrifice. (b) Suprarenal aortic stent implanted at 7.5 mm and re‐dilated to 8.3 mm prior to sacrifice. (c) Right pulmonary artery stent implanted at 7.8 mm and not re‐dilated prior to sacrifice

FIGURE 7

Transverse histological sections from the 3 stented arteries corresponding to Figure 6. (a) Infrarenal aortic stent showing widely patent lumen partially filled with postmortem clot and struts with complete incorporation with organizing neointimal growth. (b) Focal areas of medial disruption beneath the stent struts with some red blood cell extravasation around the struts (arrows) as well as a focal area of medial disruption (c) with fibrin deposition (thick arrow). (d) Suprarenal aortic stent showing a widely patent lumen with stent widely expanded with struts well‐apposed to the vessel wall. Incorporation of the stent struts with smooth muscle neointima (e, arrow) as well as some areas of bare struts (f, red arrows) adjacent to an incorporated strut (black arrow). Histologic images of the proximal (g) and distal (h) RPA stent demonstrating stent struts in the distal section with complete incorporation while the proximal section showed mostly bare or partially incorporated struts consistent with non‐apposition of the struts at the ostia of the vessel

FIGURE 8

Chronic histologic images stent # 17, RPA stent re‐dilated 3 months prior to sacrifice. Uniformly well‐apposed stent struts with extensive covering of struts. (a) The double arrow delineates an area of healed medial disruption from earlier implant and/or re‐dilation. Higher power images (b, c) of the margins of the same area demonstrating a disrupted portion (b) of the media (arrow) on one end and organized neointimal growth (c) incorporating stent struts with a small focus of chronic inflammatory cells (arrow) on the other end

FIGURE 9

Chronic histologic images of re‐dilated aortic stent #22 and re‐dilated left pulmonary artery stent # 16. Low and higher power images (a‐c) of stent # 22 demonstrate a widely patent lumen and well apposed stent struts along the surface. (a) A large area of medial disruption the length of which is illustrated by the double arrow and associated with an area of adventitial hemorrhage (single arrow). Higher power magnification shows some stent struts incorporated with organized neointimal growth (b) while others (c) are adjacent to ruptured ends of the media (black arrow) associated with platelet thrombus covers the ruptured ends (yellow arrows). Low power image (d) of stent # 16 demonstrate a widely patent lumen and well apposed stent struts mostly covered by neointimal covering. (e) Higher power image of LPA stent struts incorporated with organized neointima with adjacent unorganized thrombus (double arrow) and (f) an area of medial disruption adjacent to a stent strut (arrow)