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Medical implant such a stent

Medical implant such a stent description/claims

This invention relates to a medical implant (such as a stent) with a matrix of elongate struts arranged about a central longitudinal axis (long axis), the struts being joined at nodes, each said strut having a thickness between a luminal surface and an abluminal surface, and opposed lengthwise flank surfaces, one each side of the strut.

Generally, stents fall into two classes, namely, balloon-expandable stents and self-expanding stents. Typically, a balloon-expandable stent is made of stainless steel, from a stainless steel tube, by laser cutting, or perhaps by chemical etching. In one class of self-expanding stents, raw tube stock of nickel-titanium shape memory alloy is cut with a laser, to form a matrix of struts, and then the so-cut stent precursor is heat treated to provide it with a “memory” of a radially expanded disposition. The stainless steel balloon expandable stent is loaded onto a cylindrical balloon for delivery to a stenting site, and is then deployed by inflation of the balloon to cause plastic deformation of the nodes and struts of the matrix. Typically, a self-expanding nickel-titanium shape memory shape alloy stent is compressed into a confining cylindrical sheath, and deployed at the stenting site by proximal withdrawal of the sheath, progressively to release the stent, starting at its distal end.

Fatigue performance of the metal of the struts and nodes can be important, for example when the stent is flexed by a pressure pulse corresponding to each beat of the heart of the patient in which the stent is installed. In any event, the ability of the stent to flex during delivery on a catheter through a tortuous bodily lumen, and the ability of the stent to maintain patent the lumen in which it is deployed, puts severe demands on the mechanical properties of the nodes, and the struts.

Many stents are formed from tube stock by laser cutting the tube to form the strut matrix that characterises the stent. Typically, the cutting beam of the laser is directed vertically downwardly, towards a table, and the workpiece is mounted on the table, beneath the beam, for rotation about its longitudinal axis relative to the beam, and for axial movement perpendicular to the beam and perpendicular to the longitudinal axis, but with the beam in all cutting dispositions of the stent being arranged on a line which passes through the central longitudinal axis of the stent, that is to say, the longitudinal axis at the center of the tube stock. This in consequence results in struts that have one opposed pair of arcuate luminal (radially inside) and abluminal (radially outside) surfaces, and one opposed pair of flank surfaces, cut with the laser, with a line of action that can be projected through the central longitudinal axis of the stent. See FIG. 1 of the accompanying drawings.

Although the above description describes cutting with a laser it will be apparent that other cutting techniques are possible, such as by jets of energy (electron beam for example) or jets of fluid (water for example) as well as other cutting techniques such as chemical or electrical etching techniques. Whereas the advantages of the invention are evident most readily in metal stents, they are also available in implants other than stents (filters, for example) and materials other than metal (shape memory polymers, for example).

The basic function of a stent is to urge bodily tissue radially outwardly from the stented lumen, and prevent its ingress into the lumen. Stent performance therefore varies with the radial strength of the stent matrix, and thus in general with the pattern of the stent matrix and in particular with the fineness of the stent mesh. It is an object of the present invention to manage these parameters more effectively. A further object is to refine the fatigue resistance (or load-bearing potential) of the stent by improvements in the design of the stent matrix.

These technical effects can be achieved by the technical features that characterize the present invention, namely: 1. the flank surfaces of the struts are formed by a jet cutter; and 2. at least a portion of at least one of the flank surfaces has a plane that does not intercept said rotational axis. We call this “off-axis” cutting.

As will be brought out more clearly below, with reference to the drawing, off-axis cutting opens up possibilities to achieve, for any given number of struts around a circumference of the stent, a higher radial strength, or smaller apertures in the stent matrix.

Incidental benefits could include the provision of sharp edges to the struts, that would allow, for example, a cutting function of such a strut. One could modulate strut width along the length of each strut. By modulating the cross-sectional dimensions of the strut, along the length of the strut, one can compensate for varying levels of stress suffered by the material within each strut, so that stress levels vary less, within the strut matrix, from one location to another. A consequence is that fatigue performance can be enhanced, by removal of the peaks of stress that would otherwise place a limit on the fatigue performance.

Conventionally, the line of action of the laser or jet beam cutter that cuts the stent matrix from a tubular workpiece is perpendicular to the long axis of the workpiece, as well as passing through the long axis. A practical reason for this conventional practice is that the workpiece is more or less one dimensional, being long, but with a small cross-section, so it is easy to rotate under the laser cutting beam around its long axis, and relatively easy to translate (as with a microscope stage) so as to deliver an axial length to an “off-axis” cut surface. It is not so easy, however, to arrange to tilt or turn the workpiece length relative to the laser beam. It is within the scope of the present invention, nevertheless, to orient the line of action of the beam and the line of the said long axis at varying angles, or at an angle other than 90°. Either the laser is mounted for tilting or turning movement relative to the workpiece, or the workpiece is so mounted in a jig that its length can be tilted or turned relative to the line of action of the laser cutting beam. By presenting the beam not perpendicular to the long axis of the workpiece, flank surfaces can be created that further refine the mechanical performance of the implant, relative to the performance of implants with conventionally cut struts.

• Provisional Patent
• Utility Patent

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