The use of implantable stents in the vasculature and other body conduits has become commonplace since first proposed by Dotter in the 1960's. These devices were required to have a small, compacted diameter for insertion into the intended body conduit and transport, typically via a catheter, to a desired site for deployment, at which site they were expanded to a larger diameter as necessary to fit interferably with the luminal surface of the body conduit. They developed into balloon expandable stents that were expanded by plastically deforming the device with an inflatable balloon on which the expandable stent was mounted in the compacted state, the balloon being attached to the distal end of the catheter and inflated via the catheter. Self-expanding stents subsequently evolved, these devices being forcibly compacted to a small diameter and restrained at that diameter by a sleeve or other means. Following delivery to a desired site for deployment, they are released from the restraint and spring open to meet the luminal surface of the body conduit. These devices are typically made from nitinol metal alloys and typically rely on the superelastic and biocompatible character of this metal. Nitinol stents that rely on the shape memory attributes of that material are also known.
The evolution of implantable stents included the use of a tubular covering fitted to the stent, either to the outer or the luminal surface (or both surfaces) of the stent. These covered stents have generally come to be referred to as stent-grafts. The coverings were generally of a polymeric biocompatible material such as polyethylene terephthalate (PET) or polytetrafluoroethylene (PTFE). See, for example, U.S. Pat. No. 4,776,337 to Palmaz. This patent also describes that the covering may be optionally provided with perforations if desired for particular applications. Because of the open area provided by the perforations, such devices having perforated coverings may be considered to be a sort of hybrid stent and stent-graft, as are devices that include stent frame having metallic stent elements and polymeric elements connecting, covering or other otherwise being attached to the stent elements. The presence of the polymeric elements reduces the otherwise open space between the adjacent metallic stent elements, either very slightly or very substantially depending on the intended application and mechanical design.
Generally, a fully covered stent-graft can be considered to have a surface area (hereinafter Amax) equal to the circumference of the expanded stent multiplied by the length of the stent. For a conventional, open frame stent (as opposed to a stent-graft), the surface area represented by all of the stent elements is only a small portion of the maximum surface area Amax. The actual surface area covered by the stent, meaning the area covered by all components of the stent (including connecting elements) in their deployed state, is Astent. The porosity index, or P.I., describes the open area (the portion of the maximum surface area not covered by all components of the stent assembly) as a percentage of maximum surface area, wherein:P.I.=(1−(Astent/Amax))×100%.
The open area may be a continuous single space, such as the space between windings of a single helically wound stent element. Likewise the open area may be represented by the space between multiple individual annular or ring-shaped stent elements. The open area may also be represented by the total area of multiple apertures provided by either a single stent element (e.g., as shown by FIGS. 1B and 2B of U.S. Pat. No. 4,776,337) or by multiple stent elements providing multiple apertures. If multiple apertures are provided they may be of equal or unequal sizes. The use of a perforated graft covering or of polymeric elements in addition to metallic stent elements may also reduce the open area.
Stents having a porosity index of greater than 50% are considered to be substantially open stents.
In addition to the porosity index, the size of any aperture providing the open area must be considered if it is intended to cover only a portion of a stent area for a specific stent application. For multiple apertures, often the consideration must be for the largest size of any individual aperture, particularly if the apertures are to provide for a “filtering” effect whereby they control or limit the passage of biologic materials from the luminal wall into the flow space of the body conduit.
Various stent devices combining metallic stent elements with polymeric connecting elements are known; see, for example U.S. Pat. No. 5,507,767 to Maeda et al. Another is a stent provided with a flexible knitted sleeve having small open apertures in the fashion of chain link fencing, from InspireMD Ltd. (4 Derech Hashalom St., Tel Aviv 67892 Israel). Perforated stent-grafts are also known; see, for example WO00/42949.