Nitinol Stents in the Femoropopliteal Artery: A Mechanical Perspective on Material, Design, and Performance

Abstract

Endovascular stenting has matured into a commonly used treatment for peripheral arterial disease (PAD) due to its minimally invasive nature and associated reductions in short-term morbidity and mortality. The mechanical properties of the superelastic Nitinol alloy have played a major role in the explosion of peripheral artery stenting, with modern stents demonstrating reasonable resilience and durability. Yet in the superficial femoral and popliteal arteries, even the newest generation Nitinol stents continue to demonstrate clinical outcomes that leave significant room for improvement. Restenosis and progression of native arterial disease often lead to recurrence of symptoms and reinterventions that increase morbidity and health care expenditures. One of the main factors thought to be associated with stent failure in the femoropopliteal artery (FPA) is the unique and highly dynamic mechanical environment of the lower limb. Clinical and experimental data demonstrate that the FPA undergoes significant deformations with limb flexion. It is hypothesized that the inability of many existing stent designs to conform to these deformations likely plays a role in reconstruction failure, as repetitive movements of the leg and thigh combine with mechanical mismatch between the artery and the stent and result in mechanical damage to both the artery and the stent. In this review we will identify challenges and provide a mechanical perspective of FPA stenting, and then discuss current research directions with promise to provide a better understanding of Nitinol, specific features of stent design, and improved characterization of the biomechanical environment of the FPA to facilitate development of better stents for patients with PAD.

Keywords: Design; Femoropopliteal artery; Nitinol; Peripheral arterial disease; Stent.

Figures

FIGURE 1

Deformations of the FPA with limb flexion in a human cadaver model. Inserts demonstrate severe deformations of the artery at the adductor hiatus (a) and in the popliteal segment below the knee (b). Intra-arterial markers used to quantify FPA deformations with limb flexion are presented in the insert (c).

FIGURE 2

Irreversible deformation of the balloon-expandable iCAST stent in the FPA.

FIGURE 3

Thermal phase transitions of Nitinol. Left: Temperature hysteresis of austenite and martensite transitions, Right: effect of elemental composition on Ms transition temperature.

FIGURE 4

Effect of temperature on mechanical properties of Nitinol. Insert on the right schematically represents the R-phase illustrating unit cell elongation along the [111] direction. More detailed crystallographic description of the R-phase can be found elsewhere.,

FIGURE 5

Deformation behavior of Nitinol alloy in its superelastic form with corresponding microstructure.

FIGURE 6

Design patterns for some commonly used PAD stents.

FIGURE 7

Biomechanical performance of self-expanding Nitinol stents.

FIGURE 8

Schematics of typical test setups used in mechanical characterization of stents.