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Nonaortic Stents and Stent Grafts
Endovascular therapy has changed the landscape of vascular surgical practice. A major thrust into catheter-based therapy was initiated in 1999 with the US Food and Drug Administration (FDA) approval of endografts for aneurysmorrhaphy. As this important technique became ubiquitous, surgeons began to utilize stents in other vascular beds both for initial therapy and as secondary treatment after failed surgical grafts. Endovascular therapy with stents is now utilized throughout the vascular system, including arteries and veins spanning from the intracranial circulation to the tibial vessels.
As the use of endovascular stents has undergone more critical investigation, it has become evident that various vascular beds react differently to stent placement. Furthermore, angioplasty and stenting alter the biology of the treated vessel, a finding that has implications for both short-term and long-term results. The nature of the lesion, as well as the vascular bed to be treated, dictate specific nuances that guide the surgeon in choosing an appropriate stent.
This chapter focuses on factors that relate to the choice of stents and stent grafts for endovascular intervention in the nonaortic vascular beds. The indications for stent use in conjunction with balloon angioplasty as well as the interaction between vessel and stent are reviewed. The multiple factors that influence the optimal stent choice in a given circumstance are considered, including device characteristics such as cell design, deployment precision, treatment length, deliverability, and adjunctive stent designs that may affect therapy.
Historical Background
Isolated dilation of vessels using dilation catheters was first introduced in 1964 by Dotter and Judkins This technique was refined in the 1970s by Gruentzig et al., who used smaller catheters with attached balloons that could be delivered through the vascular tree from a remote location. Balloon angioplasty became a popular technique in the 1980s but remained inferior to surgical reconstruction because of the high acute occlusion rate and intermediate restenosis rate. Acute technical failure occurred as a result of elastic recoil, vasospasm, plaque rupture, or dissection. Recurrent stenosis, caused by an intense hyperplastic response, was commonly seen within the first 2 years after intervention. In the initial series of coronary angioplasty, interval restenosis occurred in 30% to 50% of treated lesions. Likewise, angiographic failure of isolated angioplasty in the renal, iliac, and femoropopliteal arteries occurred in up to 26%, , 32%, and 50%, , of cases, respectively.
Stenting was introduced with the goal of improving results of angioplasty by ensuring an adequate vessel lumen, maintaining flow, and reducing embolic load. Achieving optimal luminal diameter lessens the impact of in-stent neointimal formation. However, paradoxically, the stent itself has inherent properties that alter the normal vascular intimal development and can lead to maladaptive remodeling through direct intimal damage related to the procedure and the interaction between the arterial wall and the stent itself.
Stent–Vessel Interaction
Vessel Injury
The degree of intimal response to stent placement has been linked directly to the extent of vessel injury. Sullivan et al. used an experimental stent with beveled struts to demonstrate this negative remodeling effect in vivo , utilizing a swine model. This experimental stent was designed to violate the internal elastic lamina. In comparison with Palmaz stents deployed in control animals, the experimental stent was associated with significantly greater neointimal formation. Furthermore, the extent of vessel injury demonstrated a linear effect on the absolute neointimal formation.
In addition, an inflammatory response to stent placement has been demonstrated histologically. In an early autopsy series, Farb et al. demonstrated inflammatory cell infiltration in the area of the vessel directly adjacent to the stent struts. The absolute number of inflammatory cells present was significantly increased when stent struts violated the internal lamina and penetrated into the lipid core of the plaque. Subsequent analysis demonstrated that inflammatory mediators were present more than 6 months after implantation. Another study showed that, in addition to the effect of the stent itself, bacterial contaminants introduced to the vessel wall during stent delivery also play a role in neointimal formation.
Fluid Dynamics
Endovascular stents have been demonstrated to alter the fluid dynamics of the stented vessel segment. The most widely studied impact of stent placement on flow and negative remodeling is the creation of areas of low (<5 dyne/cm 2 ) wall shear stress (WSS). Alteration in WSS occurs as a result of both a change in luminal diameter of the treated vessel and the presence of the stent itself. In a computational flow model, LaDisa et al. , demonstrated several characteristics of stent placement that create a low shear stress environment. The factor that led to the most significant increase in the proportion of vessel wall exposed to a low shear environment was overdistention of the stented segment. Although some degree of stent oversizing is necessary for appropriate apposition of the stent to the vessel wall, LaDisa’s group demonstrated a 13-fold increase in the total native vessel exposed to low WSS with 20% stent oversizing compared with that seen with 10% oversizing. The low WSS can subsequently lead to greater neointimal hyperplasia and recurrent stenosis. This finding is validated clinically, as other studies have shown that along with other factors such as burden of calcified plaque, oversizing is associated with a higher incidence of in-stent restenosis. Most surgeons aim for a 10% oversizing when stenting infrainguinal occlusive lesions because of this evidence.
Strut Characteristics
The tolerances for stent construction are very strict, and minimal alteration of the strut height can have a significant impact on shear stress and neointimal formation. Intuitively, the area of the vessel wall adjacent to the stent struts has the greatest potential for negative remodeling. Eddy currents created as blood flows over the stent struts lead to regions of low shear. This effect results in a proportional change in shear stress with alterations of strut thickness. The formation of neointima in such areas of low shear stress has been reproduced in several models, and the thickness of the resultant neointima correlates with strut coverage, configuration, and thickness. Sprague et al. demonstrated that positive remodeling, as measured by endothelial cell migration, is hampered in low WSS environments. In normal shear models, migration of endothelial cells increased 2.5-fold within 1 week after implantation of steel struts onto the endothelial surface. However, in stented models with low shear, this migration was delayed up to several months. The clinical effect of these findings was demonstrated by the angiographic restenosis rates of two nearly identical coronary stents that differed only by stent heights of 50 μm and 140 μm. The stent with the lower-profile design had less restenosis.
Stent Composition
The material composition of the stent also plays a role in neointimal formation. The most common bare metal (BM) stent components are stainless steel, nitinol (nickel and titanium alloy), cobalt chromium alloy, and tantalum. Although the actual mechanism of vessel injury from stent components is unclear, corrosive products from alloys have been found within sections of vessel wall. In addition, hypersensitivity of some patients to certain metals has been observed. Palmaz et al. demonstrated that galvanic currents are created within stented arteries and lead to corrosion and subsequent vessel injury.
Stent Types and Characteristics
Each stent has intrinsic properties that determine whether or not it is suitable for any given lesion. On the basis of the biologic interaction between vessel and stent, an ideal stent would be easy to deliver through a small-caliber sheath, be readily visible on fluoroscopy, conform to the vessel upon deployment, prevent acute procedural failure, provide long-term resistance to negative remodeling, and be fracture-resistant.
Generally, stents are divided into two groups according to their construction and mode of deployment – self-expanding (SE) versus balloon-expandable (BE) ( Table 69.1 ).
TABLE 69.1
Comparison of Balloon-Expandable and Self-Expanding Stents Based on Intrinsic Characteristics and Clinical Use
| Characteristic | Balloon Expandable | Self-Expanding |
|---|---|---|
| Radial force | High | Low |
| Flexibility | Low | High |
| Requires delivery sheath | Yes | No |
| Radiopacity | High | Variable |
| Oversizing recommended | No | Yes |
| Treats lesions with variable diameter | No | Yes |
| Resistant to external compression/bending | No | Yes |
Important Characteristics
In a comparison of SE and BE stents, the major characteristics that determine suitability are radial force, flexibility, and precision of deployment. Both SE and BE stents can be covered with polytetrafluoroethylene or polyester, and such stent grafts combine the advantages of a stent with those of a graft.
Radial force is defined as the force required to produce a 50% reduction in the luminal diameter of the stent. The radial force of the stent maintains its apposition to the vessel wall and tacks down intimal flaps that may obstruct flow. It also provides the support to resist immediate vessel recoil and acute occlusion. This outward force may lead to a better technical result than that of balloon angioplasty alone. As the stent becomes incorporated into the vessel, the radial force resists deformation and negative remodeling to maintain luminal diameter over time.
Ultimately, radial force is a product of both stent design and composition. The original Palmaz stent has a stainless steel slotted/diamond design. The slotted configuration allows the stent to maintain a low profile for loading on the balloon. Once expanded, the slots become diamonds, resisting further conformational change and providing a high radial force. The Wallstent (Boston Scientific, Natick, MA, USA) also has a diamond configuration, but is designed to change lengths with its diameter. Therefore, its radial force is related to both its design and the degree of endothelialization within the artery. As the stent becomes more securely anchored, it resists shortening and leads to increased radial force. Various BE and SE stents are shown in Figure 69.1 , demonstrating the variability in stent design.
Conversely, nitinol stents rely on the inherent nature of their metallic composition to provide resistance to deformation. The nitinol alloy assumes a predetermined configuration at a desired temperature. At low temperatures, the alloy exists in the martensite state (metallurgic property of shape-memory alloys in which the crystalline structure of the alloy is elongated or asymmetric at cooler temperatures), which is flexible and aids both mounting on the catheter shaft and deliverability. At higher temperatures, the alloy adopts a crystalline austenite state (metallurgic property of shape memory alloys in which the crystalline structure is symmetric) which makes the stent rigid, thereby providing more radial force.
Flexibility is determined by the same properties that govern radial force. BE stents require a force to change conformation. Thus, these stents are less able to maneuver through tortuous vessels. BE stents are susceptible to deformation in mobile arteries because the torque required to maneuver them may enact a conformational change in the stent. In contrast, the nitinol stent, while in the martensite state, can be easily deformed and, therefore, is more suitable for arterial anatomy, which requires significant maneuverability to reach the target lesion. Interestingly, nitinol may change states not only in the setting of temperature change, but also in response to external compression. These qualities afford nitinol stents improved flexibility in mobile arteries after implantation. Such is not the case for BE stents ( Fig. 69.2 ).
Radiopacity is a consideration important in stent deployment. The material used in stent construction is a major determinant of radiopacity. Stainless steel is more visible than nitinol; thus BE stents are, generally, more visible on radiographic imaging than SE stents. Improved visibility leading to increased accuracy of deployment must be considered in the choice of the appropriate stent for a given indication ( Fig. 69.3 ).
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