Long and short of optimal stent design

Abstract

The ideal stent must fulfil a broad range of technical requirements. Stents must be securely crimped onto the delivery balloon and, in this form, must have a low profile and be sufficiently flexible to facilitate deliverability to the lesion site without distortion or displacement. Following expansion, stents must exert sufficient radial force on the vessel wall to overcome lesion resistance and elastic recoil. To achieve an optimal lumen diameter, the lesion must be uniformly and adequately scaffolded, with minimal tissue prolapse between struts but without compromising side-branch access. Furthermore, the deployed stent must conform to the vessel curvature to minimise vessel distortion, particularly at the stent edges. Radio-opacity is also important to guide safe positioning, adequate deployment and postdilataion and to permit assessment of optimal stent expansion. Equally though, the stent lumen must also be sufficiently visible to allow radiographic assessment of flow dynamics and restenosis. Efforts to optimise one characteristic of stent design may have detrimental effects on another. Thus, currently available stents all reflect a compromise between competing desirable features and have subtle differences in their performance characteristics. Striving to achieve stents with optimal deliverability, conformability and radial strength led to a reduction in longitudinal strength. The importance of this parameter was highlighted by complications occurring in the real-world setting where percutaneous coronary intervention is often undertaken in challenging anatomy. This review focuses on aspects of stent design relevant to longitudinal strength.

Keywords: bench testing; drug eluting stents; longitudinal deformation; percutaneous coronary interventions.

Figure 1

Technical requirements of coronary stents. The ideal coronary stent requires compromise in desirable characteristics. Improvement in one aspect of stent performance may have deleterious effects on another.

Figure 2

Design of coronary stents. Depicted are Vision scans of the 3.0 mm diameter examples of six stent designs and listed are the stent names and design characteristics. The Vision and Multi-Link 8 have in-phase sinusoidal hoops linked by three bridges that join peaks and troughs and are aligned with the long axis of the stent. Each connector has a U-shaped loop to increase flexibility. The Biomatrix Flex has out-of-phase sinusoidal hoops with peaks linked by two S-shaped connectors. The Element design has sinusoidal hoops with offset peaks linked by two straight bridges per hoop. The Promus Premier has the same design as the Element except that the proximal three hoops are linked by four connectors, in contrast to two connectors in the Element. The red arrows indicate the connectors. The Integrity design has a single sinusoidal component that winds helically from one end of the stent to the other, with 
two or three welds between adjacent hoops. CoCr, cobalt chromium; CoNi, cobalt nickel; PtCr, platinum chromium. Reprinted with permission from Ormiston JA et al.

Figure 3

Angiographic evidence of longitudinal deformation. Angiographic evidence of compression of the distal end of a stent placed in the left anterior descending artery following removal of a tightly trapped ‘buddy’ wire. StentViz (GE Healthcare) images illustrating the crumpled distal stent edge.

Figure 4

Longitudinal integrity assessed using bench testing (A, upper panel). Stent compression under a fixed load of 0.5 N. The least compressible stent was the six-connector Cypher, and the most easily compressed were the two-connector Promus Element and Driver stents (B, lower panel). Stent elongation testing showed similar, but reciprocal, findings to the compression test results. Reproduced with permission from Ormiston et al.