As an alternative to vascular surgery, balloon angioplasty has been a common method for unblocking narrowed or occluded blood vessels. In this procedure, an angioplasty balloon is inflated within a stenosed vessel in order to dilate the vessel to provide an enlarged lumen. Although balloon angioplasty has been successful in restoring flow in stenotic or occluded vessels, these vessels often restenose due to elastic recoil of the diseased tissue. Subintimal dissection is also caused by balloon induced stresses and results in geometric irregularities at the inner wall leading to flow disturbances and decreased flow.
Consequently, intraluminal stenting has been used with increasing frequency to improve the success rate of transluminal balloon angioplasty. These tubular stents are introduced via catheter, expanded to a preset diameter and left in situ to resist elastic recoil and to hold dissections against the vessel wall. There are essentially three types of conventional stents all of which are metallic. The three types are balloon expandable, self-expandable, and memory metals (i.e., nitinol). The balloon-expandable stents deform plastically beyond the elastic limit of the material and are relatively rigid at their expanded diameter. The balloon expandable stents are mounted over a deflated angioplasty balloon and then positioned within a vessel. The balloon is then inflated transmitting outward radial forces across the tubular stent that plastically deforms into a final larger diameter against the vessel wall. The balloon is then deflated and removed from the vessel. Self-expandable stents rely on the potential energy stored in a reduced diameter to spring back to some new, larger diameter when released. Self-expandable stents tend to be more compliant than balloon-expandable stents. Self-expandable stents are compressed into a smaller diameter and then inserted into a sheath. The sheath is then inserted into a vessel and removed at the desired location to expose the stent. The compressed stent springs open against the vessel wall exerting a constant outward force thereby fixing the stent in place. Memory metal or nitinol stents assume a final enlarged diameter from an initial reduced diameter in the presence of temperature changes. Nitinol stents, along with resorbable polymeric stents, are not as widely used as balloon- and self-expandable stents. Memory metal stents respond to temperature changes by changing from a reduced diameter to a final expanded configuration at the stenotic site.
Although patency rates have improved when stenting is used in conjunction with balloon angioplasty, thrombosis and neointimal hyperplasia within the region of the stent continue to compromise the potential utility of these devices. Stent surface thrombogenicity and processes regulating neointimal hyperplasia are considered to be major contributors to the long-term problems associated with conventional stenting.
Ultimately, endothelialization of the stent surface properly represents the best chance for successful use of a stent since the endothelial layer has the potential to inhibit low-flow thrombosis and to moderate factors involved in maintaining luminal patency. The course of events leading to endothelialization of any metallic stent surface begins with thrombus formation at the stent surface. The thrombogenicity of the stent surface is dependent on surface characteristics such as the electronegative potential of the metal and surface roughness. Thrombus that initially forms is eventually replaced by fibromuscular tissue, fibrocytes and collagen. Endothelialization is allowed to proceed across the newly formed tissue from the endothelial cells exposed between the stent latticework and from the ends of the stent. The extent to which endothelialization occurs depends upon the number of cells to survive the trauma of stent deployment as well as the flow conditions set up by the introduction of the stent. Minimization of mechanically induced trauma to the endothelial lining of the vessel certainly becomes desirable. Accordingly, a stent design achieving a low ratio of metal surface area to open surface area therefore becomes desirable to reduce thrombogenicity while maximizing the potential for endothelialization.
Another factor to be considered is that blood flow is altered by the presence of a stent. Troughs created along the stented segment of the vessel create turbulence, boundary layer separation, and regions of potentially low flow and low shear. These kinds of flow conditions have been implicated as a mechanism for atherogenesis. Accordingly, a stent having an open structural design appears to be desirable.
Next, most conventional stents undergo longitudinal shortening with an increase in diameter. In the presence of arterial smooth muscle contraction/relaxation and pulsatile flow, length changes likely accompany diameter changes. Endothelium may be sloughed as a result and an additional inflammatory reaction may ensue due to relative motion at the stent-tissue interface. The foreshortening of conventional stents within the target location also creates problems in deployment accuracy and potentiates further damage to the wall of the vein at the target location. Accordingly, reducing or eliminating longitudinal shortening of the stent during expansion also becomes a desirable goal.
It is well known that the endothelial layer, formed by the cells lining the inner wall of a vessel, is a dynamic layer that is able to produce, secrete, and modulate factors involved in maintaining patency of the vessel lumen. These endothelial properties are thought to be the reason venous conduits have significantly higher patency rates than synthetic grafts when used in arterial reconstructions. combining the properties of endothelium and stents would therefore be desirable to create a better endoprosthesis. Specifically, a stent lined with an endothelial layer would be less thrombogenic. Additionally, the vessel wall, which is likely to be injured by the angioplasty, would be largely shielded from blood-borne components such as platelets which are known to be potent instigators of neointimal hyperplasia. Lastly, the stent itself would still retain its ability to counteract the elastic recoil of the vessel wall following angioplasty.
Accordingly, it would be highly desirable to have a stent that reduces surface contact with the vessel wall, that inhibits longitudinal shortening during expansion and that supports a venous lining to provide an inner endothelial layer.