Review
Impact of stent strut design in metallic stents and biodegradable scaffolds

https://doi.org/10.1016/j.ijcard.2014.09.143Get rights and content

Highlights

  • We review the evidences of the impact of strut thickness specifically in Bare and Drug Eluting Stent.

  • We provide a comprehensive review of the impact of stent strut design in terms of stent performance and clinical results.

  • We describe the characteristics of materials used in bioresorbable technology and the implication for scaffold design and strut thickness.

Abstract

Advances in the understanding of healing mechanisms after stent implantation have led to the recognition of stent strut thickness as an essential factor affecting re-endothelialization and overall long term vessel healing response after Percutaneous Coronary Interventions (PCI). Emergence of Drug-eluting stents (DESs) with anti-proliferative coating has contributed to reducing the incidence of restenosis and Target Lesion Revascularization (TVR), while progress and innovations in stent materials have in the meantime facilitated the design of newer platforms with more conformability and thinner struts, producing lesser injury and improving integration into the vessel wall.

Recent advances in biodegradable metal and polymer materials now also allow for the design of fully biodegradable platforms, which are aimed at scaffolding the vessel only temporarily to prevent recoil and constrictive remodeling of the vessel during the initial period required, and are then progressively resorbed thereby avoiding the drawback of leaving an unnecessary implant permanently in the vessel.

The aim of this article is to review recent evolution in stent material and stent strut design while understanding their impact on PCI outcomes. The article describes the different metallic alloys and biodegradable material properties and how these have impacted the evolution of stent strut thickness and ultimately outcomes in patients.

Introduction

Since the emergence of the first coronary stents nearly 30 years ago, stent designs have constantly evolved through the use of new material and adaptation in stent architecture.

The impact of strut thickness on stent design has been recognized early on: in pre-clinical studies, bare stents with thicker stent struts were shown to develop more inflammation and restenosis than thin strut designs [1], [2]. Furthermore, randomized studies in patients showed that a reduction of strut thickness of coronary stents is associated with improved follow-up angiographic and clinical results [3], [4].

The first generation DESs were shown to be superior to BMS but were limited by recurrent problems of late target lesion revascularization (late catch-up) and Stent Thrombosis (ST). The second generation DESs with more conformable designs, more hemo- and biocompatible polymers and improved kinetics of drug release have proven superior to previous DES technology in terms of both safety and efficacy.

Evolution towards new platforms and thinner struts has been driven by clinicians' desire for more consistently deliverable stents that cause less injury and restenosis. PCI practice has considerably evolved as a result of the emergence of new devices, allowing for more complex lesions and procedures to be performed, with improved clinical results. Nowadays, emerging fully biodegradable stent technologies available in clinical practice are constantly challenging existing beliefs derived from studies with permanent metallic platforms.

The aim of this article is to review recent advancements, both in metal and biodegradable stent materials and understand their influence on stent design. To understand how strut design may impact stent performance and clinical outcomes, the article reviews the evidence from the evolution and development of stent design from bare stents to the latest DESs and fully biodegradable technologies.

Section snippets

Strut design in the bare metal stent (BMS) era

Several preclinical studies have reported early on the effect of stent strut geometry on the degree of vessel injury and restenosis [1], [2], [5], [6]. In particular, strut thickness and stent flexibility have been recognized to impact degree of injury, risk of rupture of the elastic laminae and overall inflammation, with a larger strut design generally leading to higher inflammation and restenosis than the thinner strut and open cell design [1], [2], [5], [6], [7].

Thicker stent struts have

Strut design in the DES era

The impact of strut design in DES is less evident compared with BMS since the addition of a drug coating acts as a barrier against neointimal proliferation which retards the natural healing process [11].

Biodegradable coating

Pathology and IVUS studies suggested that the polymer in the first generation DES might trigger a delayed hypersensitivity reaction resulting in very-late stent thrombosis [24], [34], [35]. These evidences encouraged the idea that a strategy of “leaving no polymer” could be better than leaving a durable polymer “behind”.

In biodegradable coated stents, such as the Biolimus-eluting BioMatrix stent (Biosensors International, Morges, Switzerland), the polymer coating is progressively absorbed,

Impact of strut thickness with DES

The impact of strut thickness is less evident in DES as each DES differs not only in terms of platform design but also in polymer formulation, drug type and concentration, release kinetics, polymer degradation, etc.., all of which contribute to the in vivo response.

Comparison of the first generation thick DES with thinner bare stents showed that the effect of drug coating prevails over strut thickness [11], [32], [39], [40]. Therefore, the impact of strut thickness can be hard to delineate in

Fully biodegradable scaffolds

Although conformable platforms with thinner strut and more biocompatible coating have resulted in improved outcomes, permanent metal implants still have some inherent drawbacks, including the risk of late-catch-up, hypersensitivity reaction to the polymer, allergic reaction to the metal, stent fracture or late stent thrombosis. Furthermore, the presence of a metallic stent implanted in a diseased vessel can preclude further interventions or by-pass surgery. For these reasons, numerous companies

Biodegradable polymers

Polymers are macromolecules built from large volumes of repeating small monomer units. The polymer's mechanical characteristics such as strength, stiffness and degradation rate are linked to the number of monomer units (molecular weight) and their arrangement. Polymers with longer chains (higher molecular weight) are usually stronger and have longer absorption times. Similarly, dense linear arrangement of the monomers (high linearity/crystallinity) results in higher strength and slower

Biodegradable polymer based scaffolds

A number of scaffolds based on PLLA, PDLLA or PLGA biodegradable polymers have been developed in recent years. The Igaki-Tamai stent (Kyoto Medical Planning Co., Kyoto, Japan) was the first of this type of PLA based biodegradable scaffolds [46], [61]. It has a PLLA based architecture with a strut thickness of 170 μm and an absorption time of approximately 24–36 months. Encouraging initial results with this scaffold [62] has raised the interest of the clinical community and industry for such

Magnesium based biodegradable alloys

In addition to scaffolds based on biodegradable polymer materials, biodegradable metal stents based on Iron or Magnesium biodegradable alloys, have also been recently investigated. Data about the clinical performance of bioabsorbable metallic stents is emerging [71], [72], [73].

In contrast to conventional metal stents, which are made resistant to corrosion, biodegradable metals use a controlled corrosion by body fluids as a mechanism of biodegradation. Pure magnesium degrades rapidly in body

Impact of strut thickness with bioresorbable technologies?

The impact of strut thickness has been evidenced in the number of studies for metallic stents. Can these results be extended to biodegradable scaffolds?

The neointimal thickness has been shown to be large in biodegradable scaffolds, in a similar way that large metallic stent strut does produce larger neointimal area than the thinner strut design [68], [74], [75], [76]. However, the healing response with biodegradables is markedly different to that of the metal stents; remodeling continues as the

Conclusion

Thinner struts produce less arterial injury, faster re-endothelialization and reduce the risk of restenosis and thrombosis. Because of its fundamental impact on vessel response, reducing strut dimension while maintaining radial support remains the focus for manufacturers looking for ways to improve stent deliverability and performances.

Conflict of interest

None of the authors has a direct conflict of interest in this connection with this article. Dr. Joner and Virmani are employees of CV Path.

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    Dr. Joner and Virmani are employees of CV Path. Dr Wong is a founder and Chief Scientific Officer of Innoheart Plc.

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