Author + information
- Dennis W. Kim, MD, PhD∗ ()
- ↵∗Reprint requests and correspondence:
Dr. Dennis W. Kim, Children’s Healthcare of Atlanta/Emory University, 2835 Brandywine Road, Suite 300, Atlanta, Georgia 30341.
In Goldilocks and the Three Bears, our young heroine was faced with 3 choices, 1 of which was “just right.” With intravascular stent therapy in pediatrics, interventionalists may be faced with many more choices, but rarely is the stent “just right.” For infants and young children, stenting does not occur in a static milieu. Rather, compromises are made, because the natural somatic growth of the patient can be expected to exceed the expansion properties of the stents inserted. Coronary or other small-diameter stents inserted into small vessels such as neonatal pulmonary veins or pulmonary arteries are not designed to be serially dilated to full adult size, which ideally should be many times larger than at the index stenting procedure. Once the limit of stent dilation is achieved, surgical transection and patching of the area of the embedded stent with nonliving tissue or grafts may be required to further augment the size of the stented vessel. Those stents that can be serially dilated to adult size are quite bulky for use in neonates and infants and can be difficult to precisely deliver on small-diameter balloon catheters. Larger diameter stent platforms also face challenges in delivery to the target zone because of the very small diameter vessels available for percutaneous access or because of the creation of significant hemodynamic instability from intracardiac passage of the stent and delivery systems, particularly to the pulmonary arteries or antegrade passage from the venous side to the aorta. The unfortunate current reality of stent treatment of vascular stenoses in infants and children is that these procedures are done with the sobering knowledge that future nonsurgical augmentation of these vessels may be limited because of the inherent properties of the implanted stents.
In this issue of JACC: Cardiovascular Interventions, Sathanandam et al. (1) present in situ feasibility data supporting the concept of intentional longitudinal stent fracture by overdilation of coronary, nitinol, and renal stents. This is an extension of this group’s previous benchtop investigation describing the characteristics of a variety of balloon-expandable and self-expanding stent platforms when purposefully overdilated to attempt longitudinal fracturing, or “unzipping” of these stents (2). A total of 50 stents ranging from 4 to 7 mm were implanted into 5 neonatal piglets with a mean weight of 3.4 kg. Accommodating for the variety of stent types used in pediatric interventions, implanted stents included pre-mounted coronary, biliary, self-expanding nitinol, and renal stents. Implantation sites included systemic arteries, pulmonary arteries, and systemic veins. Nominal diameters, stent shortening characteristics, and unzipping success were evaluated 3 months after stent implantation. Because this porcine model is one of very rapid growth, the median weight of 32 kg achieved at 3 months represents the 50th percentile of a 10-year-old boy. Evaluation of in-stent stenosis by angiography, pressure gradients, and fluoroscopic determination of stent geometry was performed. Histopathologic assessments were made of the vessel walls in the area of unzipping, with categorization of vessel wall injury and dissection patterns for the different stent types. Some technical considerations to maximize success in stent unzipping, including slow and deliberate inflation with noncompliant angioplasty balloons and avoidance of exceeding the diameter of the adjacent “nondiseased” vessel segments, are noted by the investigators. From a practical perspective, with some stent types, the use of ultra-high-pressure noncompliant balloon catheters is necessary to break the stent struts.
Although this report by Sathanandam et al. (1) represents an interesting in vivo proof-of-concept description of intentional stent disruption, several questions remain. The histopathologic descriptions include only small sections of the vessel wall, and it is not clear if and how dissections and wall injury encountered affect the entire length of the unzipped stented segment of the vessel wall. Nor is it understood how weakening of the vessel wall over a longer duration of time may affect the characteristics of continuing transcatheter interventions to further dilate these areas, whether by serial angioplasties or by repeat stenting with larger diameter stents. This understanding of potential pseudoaneurysm formation or vascular rupture is crucial in determining how further interventions to serially dilate or restent these areas may be approached, particularly in high-pressure arterial environments. But the largest issue may be that the “ideal” stent determined by the investigators for unzipping (VeriFLEX, Boston Scientific Corporation, Natick, Massachusetts) is now discontinued, making it obsolete for use in the future. The next-generation platform does not retain the same unzipping characteristics of the VeriFLEX stent. Most other stent platforms may be somewhat unpredictable in their abilities to fully unzip.
It seems that the ideal strategy for accommodating somatic growth after intravascular stenting would be to intentionally design stents with the capability to either predictably separate longitudinally or to degrade over time to allow further intervention without the constraints of a pre-existing circumferential stent. With today’s advent of commercially available bioabsorbable stent technologies, it is interesting to note that the concept of a partially bioabsorbable stent designed to accommodate further growth of the target vessel was reported more than a decade ago, with implantation into a small number of infants for the treatment of aortic coarctation (3–5). These stent platforms used 2 halves of a balloon-expandable stent platform that were longitudinally joined by bioabsorbable suture. Another study used a biodegradable magnesium platform (6). Unfortunately, these proofs of concept did not result in further significant evolution or commercialization. There are design challenges in creating fully bioabsorbable scaffolds in diameters larger than a few millimeters that can maintain radial force, prevent recoil, and be delivered via low-profile systems.
Admittedly, stent unzipping is a Neolithic approach to a modern problem. However, the fact remains that there are no commercially available stents in the United States that have been developed specifically for pediatric intravascular applications. Instead, coronary, biliary, or renal stents are repurposed for use in the pediatric vasculature. Rarely is stenting considered to be a single-procedure, lifelong therapy in young children; instead it is part of a stepwise strategy involving further catheter-based and surgical interventions. Market forces currently do not support the rigorous undertaking of creating a variety of pediatric-specific stent platforms that would be part of a lifelong strategy for vascular rehabilitation. Until such stent technologies are realized, developed, and commercialized, breaking stents placed in the past may be a necessary, albeit inelegant, solution for a vexing but predictable problem.
↵∗ Editorials published in JACC: Cardiovascular Interventions reflect the views of the authors and do not necessarily represent the views of JACC: Cardiovascular Interventions or the American College of Cardiology.
Dr. Kim has reported that he has no relationships relevant to the contents of this paper to disclose.
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