Expandable endovascular stent

• US 20050080479A1
• Filed: 09/29/2003
• Published: 04/14/2005
• Est. Priority Date: 09/29/2003
• Status: Abandoned Application

• Current Assignee: Hui Sun, James Q. Feng
• Original Assignee: Hui Sun, James Q. Feng
• Inventors: Feng, James Q., Sun, Hui
• Application Number:
  US10/671,691
• Publication Number:
  US 20050080479A1
• Time in Patent Office: Days
• US Class Current: 623/1.150
• CPC Class Codes: A61F 2/91 made from perforated sheet material or tubes, e.g. perforated by laser cuts or etched holes
• CPC Class Codes: A61F 2/915 with bands having a meander structure, adjacent bands being connected to each other
• CPC Class Codes: A61F 2002/91525 within the whole structure different bands showing different meander characteristics, e.g. frequency or amplitude
• CPC Class Codes: A61F 2002/91541 Adjacent bands are arranged out of phase
• CPC Class Codes: A61F 2002/91558 connected peak to peak
• CPC Class Codes: A61F 2002/91583 by a bridge, whereby at least one of its ends is connected along the length of a strut between two consecutive apices within a band
• CPC Class Codes: A61F 2230/0013 Horseshoe-shaped, e.g. crescent-shaped, C-shaped, U-shaped

Abstract

Disclosed herein is a tubular endovascular stent comprising a plurality of annular segments connected by one or more bridging elements. Each annular segment takes forms of periodic wavelets with a plurality of alternating symmetric peaks and valleys, preferably consisting of circular arc segments of large radii connected tangentially with straight segments to minimize stress concentration when the stent undergoes radial deformation, transverse to the longitudinal axis of the stent. The points of connection between the bridging elements and adjacent annular segments are so designed that deformations of the bridging elements remain negligible as the stent deforms radially, namely, the longitudinal dimension of the stent does not vary during the radial expansion or contraction of the stent. Hence, the radial strength and the longitudinal flexibility of the stent made according to the principles disclosed by the present invention can be independently controlled by the design parameters for the annular segments and bridging elements, without compromising the longitudinal dimensional stability of the stent. Since stress concentration and deformation in the stent can lead to restenosis, stent made from the invention disclosed here can reduce the probability of restenosis.

Claims

1. An expandable endovascular stent for implanting in a body vessel comprising a single walled tubular body of a biocompatible material having a plurality of annular segments, said annular segments being transverse to the longitudinal axis and having periodic wavelets with a plurality of alternating symmetric peaks and valleys consisting of an arc segment and a straight segment, the said arc segment having a constant or nearly constant curvature, the straight segment being tangentially connected to the arc segment.

2. An expandable endovascular stent according to claim 1 further comprises bridge elements.

3. An expandable endovascular stent according to claim 2 wherein the said bridging elements are connected to the annular segments with the connection points located at or near the stress-neutral points, the said stress-neutral point being located midway or close to midway between the symmetric peaks and valleys of the wavelets of the annular segments.

4. An expandable endovascular stent according to claim 1 wherein the stent longitudinal dimension is substantially the same in the expanded state, the compressed state and any other states between.

5. An expandable endovascular stent according to claim 1 wherein the said annular segment is designed according to the folowing formula I for stress index X:

$\begin{array}{cc}\mathrm{\chi <br><br>}=\frac{{\mathrm{\sigma <br><br>}}_{\max }}{Q}=\frac{D<br><br>\text{\hspace{1em}<br><br>}<br><br>E<br><br>\text{\hspace{1em}<br><br>}<br><br>I}{w_s<br><br>t<br><br>\text{\hspace{1em}<br><br>}<br><br>m<br><br>\text{\hspace{1em}<br><br>}<br><br>R^3<br><br>f_0<br><br>\left(k\right)}<br><br>\left(1+\frac{6<br><br>R<br><br>\left(1+k\right)}{w_s}\right)& \left(I\right)\end{array}$

which is defined as the ratio of the maximum segmental stress (in circumferential direction) in the strut at the peaks and valleys (Φ

=π

/2, cf. FIG. 3) τ

_max and expansion ratio Q, where

${\mathrm{\sigma <br><br>}}_{\max }=\frac{F}{w_s<br><br>t}<br><br>\left(1+\frac{6<br><br>R<br><br>\left(1+k\right)}{w_s}\right),\mathrm{and}<br><br>\text{\hspace{1em}<br><br>}<br><br>F=L<br><br>\text{\hspace{1em}<br><br>}<br><br>p<br><br>\text{\hspace{1em}<br><br>}<br><br>D/2$

denoting the net segmental force in the strut in the hoop (circumferential) direction of the stent as the result of a radial pressure p in a tubular body vessel of diameter D according to the Laplace equation, where L is the length of bridges, which may become approximately the height of the struts L=2(H+R) (elements 30 or 31 shown in FIG. 1), w_s the segment width, t the segment thickness, k=H/R the ratio of the half length of straight segment H and the radius of the arc curvature R, and the expansion ratio Q is given by

$Q=\frac{4<br><br>m<br><br>\text{\hspace{1em}<br><br>}<br><br>d}{D<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}}=\frac{m<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>R^3}{D<br><br>\text{\hspace{1em}<br><br>}<br><br>E<br><br>\text{\hspace{1em}<br><br>}<br><br>I}<br><br>f_0<br><br>\left(k\right)$

with d denoting the deflection of the half wavelets of annual segment

$d=\frac{m<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>R^3}{4<br><br>\text{\hspace{1em}<br><br>}<br><br>E<br><br>\text{\hspace{1em}<br><br>}<br><br>I}<br><br>f_0<br><br>\left(k\right),$

m being the number of the wavelets, El the bending stiffness of the annual segment, and ƒ

₀(k) the geometric factor given by

$f_0<br><br>\left(k\right)=1+\frac{8<br><br>k}{\mathrm{\pi <br><br>}}+2<br><br>k^2+\frac{4<br><br>k^3}{3<br><br>\mathrm{\pi <br><br>}}.$

6. An expandable endovascular stent according to claim 1 wherein said stent is expandable by a balloon catheter.

7. An expandable endovascular stent according to claim 1 where in said stent is made of a bio-compatible material capable of elastic and plastic deformation.

8. An expandable endovascular stent according to claim 7 wherein said bio-compatible material is stainless steel.

9. An expandable endovascular stent according to claim 7 wherein said bio-compatible material is gold.

10. An expandable endovascular stent according to claim 7 wherein said bio-compatible material is a nickel titanium alloy

11. An expandable endovascular stent according to claim 10 wherein said nickel titanium alloy is nitinol.

12. An expandable endovascular stent according to claim 1 wherein said stent is coated with a substance that prevents blood coagulation.

13. An expandable endovascular stent according to claim 2 wherein said annular segments are connected by bridge elements to form stents having close cells.

14. An expandable endovascular stent according to claim 2 wherein said annular segments are connected by bridge elements to form stents having open cells.

15. An expandable endovascular stent according to claim 1 wherein said arc segment has an arc angle more than 180 degrees when the stent is in the compressed state.

16. An expandable endovascular stent according to claim 1 wherein the curvatures of the arc segments are varied.