Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
2000-02-15
2001-10-09
O'Connor, Cary E. (Department: 3731)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
Reexamination Certificate
active
06299635
ABSTRACT:
FIELD OF THE INVENTION
The following invention relates to surgical stents of a generally cylindrical configuration which can be surgically implanted into a body lumen, such as an artery, and radially expanded. More specifically, this invention relates to radially expandable surgical stents having a high radial strength for implantation in body lumens which experience radial loads.
BACKGROUND OF THE INVENTION
Surgical stents have long been known which can be surgically implanted into a body lumen, such as an artery, to reinforce, support, repair or otherwise enhance the performance of the lumen. For instance, in cardiovascular surgery it is often desirable to place a stent in the coronary artery at a location where the artery is damaged or is susceptible to collapse. The stent, once in place, reinforces that portion of the artery allowing normal blood flow to occur through the artery. One form of stent which is particularly desirable for implantation in arteries and other body lumens is a cylindrical stent which can be radially expanded from a first smaller diameter to a second larger diameter. Such radially expandable stents can be inserted into the artery by being located on a catheter and fed internally through the arterial pathways of the patient until the unexpanded stent is located where desired. The catheter is fitted with a balloon or other expansion mechanism which exerts a radial pressure outward on the stent causing the stent to expand radially to a larger diameter. Such expandable stents exhibit sufficient rigidity after being expanded that they will remain expanded after the catheter has been removed.
Radially expandable stents come in a variety of different configurations to provide optimal performance to various different particular circumstances. For instance, the patents to Lau (U.S. Pat. Nos. 5,514,154, 5,421,955, and 5,242,399), Baracci (U.S. Pat. No. 5,531,741), Gaterud (U.S. Pat. No. 5,522,882), Gianturco (U.S. Pat. Nos. 5,507,771 and 5,314,444), Termin (U.S. Pat. No. 5,496,277), Lane (U.S. Pat. No. 5,494,029), Maeda (U.S. Pat. No. 5,507,767), Marin (U.S. Pat. No. 5,443,477), Khosravi (U.S. Pat. No. 5,441,515), Jessen (U.S. Pat. No. 5,425,739), Hickle (U.S. Pat. No. 5,139,480), Schatz (U.S. Pat. No. 5,195,984), Fordenbacher (U.S. Pat. No. 5,549,662) and Wiktor (U.S. Pat. No. 5,133,732), each include some form of radially expandable stent for implantation into a body lumen.
Each of these prior art stents suffer from a variety of drawbacks which make them less than ideal. For instance, many of these stents are formed from stainless steel or other materials which have a relatively low yield strength. Hence, if the body lumen is subjected to radial loads and related radial stresses, the stents are susceptible to collapse or other permanent deformation in an undesirable manner. If such stents are provided with segments of greater thickness to enhance their strength, they become too thick to be effectively collapsed for insertion and later expansion within the body lumen.
One material for forming higher strength radially expandable surgical stents is a shape memory Nickel-Titanium alloy. Shape memory Nickel-Titanium alloys and other shape memory alloys are unique in that they have two distinct solid phases. A high yield strength austenite phase (195-690 MPa) and a lower yield strength martensite phase (70-140 MPa). The material can be selectively transformed between the austenite phase and the martensite phase by altering a temperature of the shape memory Nickel-Titanium alloy. For instance, it is known to form the Nickel-Titanium alloy so that the stent is in the martensite phase when chilled to a temperature below body temperature and to be in the austenite phase when the stent is at body temperature.
Additionally, when such shape memory alloys are stressed beyond their yield strength while in the martensite phase, not to exceed certain maximum amounts of strain, the alloy has a “memory” of its shape before its yield strength in the martensite phase was exceeded so that when the alloy is heated and transformed into its austenite phase it returns to the shape it exhibited before it was plastically deformed in the martensite phase. In radially expandable surgical stents, this shape memory has been used to collapse the stent to a small diameter when in its martensite phase and then heat the stent up to body temperature and transform the stent into its austenite phase where it radially expands back to its original expanded diameter and exhibits a desired strength and size for supporting walls of the body lumen in which it is implanted. Hence, the relatively high yield strength of the shape memory alloy stent in its austenite phase provides beneficial characteristics for supporting the body lumen while the martensite phase for the shape memory alloy stent is utilized to allow the stent to be easily radially contracted and deformed during implantation of the stent.
While such shape memory Nickel-Titanium alloy stents are generally effective, known shape memory Nickel-Titanium stents have exhibited certain deficiencies. For instance, when such prior art shape memory Nickel-Titanium stents are radially expanded they tend to contract axially, enhancing the difficulty experienced by a surgeon in precisely implanting the stent where desired. Additionally, the limited degree of collapsibility of known prior art shape memory Nickel-Titanium stents has enhanced the difficulty of their implantation in many body lumens. Accordingly, a need exists for shape memory Nickel-Titanium alloy stents which have a configuration which beneficially overcomes the drawbacks of known prior art shape memory Nickel-Titanium alloy stents.
SUMMARY OF THE INVENTION
This invention provides a radially expandable stent formed of shape memory Nickel-Titanium alloy which exhibits little or no contraction along an entire axial length thereof when the stent is expanded radially. The stent includes a series of struts which act as circumferential segments circumscribing the cylindrical contour of the stent. Each strut is aligned with a separate plane perpendicular to a central axis of the cylindrical contour of the stent and parallel to other planes to which adjacent struts are aligned. The stent can have various different numbers of struts joined together to form the stent. However, at least two end struts are provided including a front end strut and a rear end strut which define ends of the cylindrical contour of the stent. Intermediate struts are also typically provided between the two end struts.
Each of these struts exhibits a wave-like shape as they circumscribe the cylindrical contour of the stent. Thus, each strut has a series of bends which have troughs and crests alternating along the length of each strut. Substantially linear legs extend between each bend. Each trough defines a portion of the strut which is most distant from adjacent struts and each crest defines a portion of the strut closest to adjacent struts. An amplitude of each strut, defined by the distance between the bottom of each trough and the top of each crest is modified when the stent is radially expanded so that the amplitude is decreased.
The end struts are attached to adjacent intermediate struts by tie bars which act as axial segments connecting the two adjacent struts together. Tie bars can also connect adjacent intermediate struts to each other. Each tie bar attaches to the struts adjacent thereto through a first end of the tie bar and a second end of the tie bar. Both the first end and the second end are located within troughs of the struts. Thus, the tie bars span a gap between adjacent struts at a maximum width portion of the gap. Not all of the gaps are necessarily spanned by tie bar axial elements. Rather, separate intermediate circumferential segments can be attached to each other through links which connect to the intermediate segments at locations spaced away from the troughs thereof.
To further enhance the collapsibility and expandability of the stent as well as an overall strength of the stent, the legs of the st
Cook Incorporated
Ness Anton P.
O'Connor Cary E.
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