STENT AND METHOD FOR MANUFACTURING STENT

A stent having a tubular shape in a self-expanding state, including: a first wire group including a plurality of wires; and a second wire group including a plurality of wires intersecting with the wires of the first wire group at a plurality of locations, wherein the wires of the first wire group and the wires of the second wire group intersect with each other to form a plurality of cells having a substantially rhombic shape in a state of being self-expanding in a tubular shape, and a cross-sectional shape of each of the wires is a rectangular shape thick in a radial direction of the tubular shape.

TECHNICAL FIELD

The present invention relates to a technical field of a stent, and particularly to a structure of a self-expanding stent suitable for surgery of aortic dissection.

BACKGROUND ART

Conventionally, for aortic diseases, a thoracotomy surgery has been performed most commonly. The thoracotomy surgery is a surgery of literally performing thoracotomy and replacing a disease site with an artificial blood vessel, but a physical burden on a patient is high, and a death ratio is also high. Even if the surgery is successful, it is a reality that complications and the like are developed and a patient is bedridden in many cases.

Thereafter, it is also a reality that “stent graft surgery” is performed via the aorta by a catheter inserted from a groin without thoracotomy (see Non Patent Literature 1). Since thoracotomy is not required in the “stent graft surgery”, there is an advantage that surgery time is short and a physical burden and an economic burden on a patient can be reduced.

CITATION LIST

Patent Literature

Non Patent Literature

Non Patent Literature 1: The Japanese Association for Thoracic Surgery, Website on Thoracic Aortic Aneurysms http://www.jpats.org/modules/general/index.php?content_id=16

SUMMARY OF INVENTION

Technical Problem

However, meanwhile, in a case of a disease such as arch aorta dissection, which is common in Japanese, it is extremely difficult to perform a surgery using this stent graft at present because there is a branch blood vessel in the arch aorta. Since the stent graft has a “membrane” structure on a surface of the stent, if the stent graft is directly placed on the arch aorta, a blood flow to the branch blood vessel is inhibited. Therefore, a method for partially opening (fenestration) the membrane only at a position corresponding to the branch blood vessel has also been proposed. However, it is extremely difficult to accurately position the branch blood vessel and the opening when the stent graft having the opening is disposed at a disease site with a catheter, and complications due to inconsistency are often developed.

On the other hand, there is a demand for using a general stent having no membrane structure (see Patent Literature 1), but the general stent has poor flexibility against bending in a direction perpendicular to an axial direction, and is usually not suitable for use in the arch aorta. For example, in a case of an all-linked stent illustrated inFIG.19, when the stent is bent, a hollow portion is crushed and a blood flow cannot be ensured as illustrated inFIG.20. In a case of a partially linked stent illustrated inFIG.21, although the stent exhibits flexibility against bending to a certain degree in a specific direction (seeFIG.22), there is a problem that the stent kinks when being bent at a certain curvature or more (seeFIG.23). In addition, there is a problem in handleability, for example, when the stent is contracted and housed in a catheter, a break of a link is likely to be caught by the catheter, and it is difficult to house the stent in the catheter.

Therefore, the present invention has been made in order to solve these problems, and an object of the present invention is to provide a stent that does not include such a membrane as in a stent graft, has sufficient kink resistance, and has excellent flexibility, and a method for manufacturing the stent.

Solution to Problem

That is, the stent according to the present invention is a stent having a tubular shape in a self-expanding state, the stent including: a first wire group including a plurality of wires; and a second wire group including a plurality of wires intersecting with the wires of the first wire group at a plurality of locations, in which the wires of the first wire group and the wires of the second wire group intersect with each other to form a plurality of cells having a substantially rhombic shape in a state of being self-expanding in a tubular shape, and a cross-sectional shape of each of the wires is a rectangular shape thick in a radial direction of the tubular shape.

A method for manufacturing a stent according to the present invention is a method for manufacturing a stent, including: a step of forming, from a tubular body made of a shape memory alloy, by laser processing, a mesh tubular body in which meandering structures in which wires meander in a circumferential direction at an equal pitch are arranged in an axial direction, and apexes of arc portions of the adjacent meandering structures are connected to each other to be integrated; a step of enlarging the mesh tubular body in diameter to a predetermined diameter; and a step of causing the mesh tubular body to memorize the shape thereof in a state where the mesh tubular body is enlarged in diameter, in which the wires intersect with each other to form a plurality of cells having a substantially rhombic shape in a state where the mesh tubular body is enlarged in diameter to the predetermined diameter, and a cross-sectional shape of each of the wires is a rectangular shape thick in the radial direction.

Advantageous Effects of Invention

The present invention can provide a stent that does not include such a membrane as in the stent graft, has sufficient kink resistance, and has excellent flexibility, and a method for manufacturing the stent.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a stent and a method for manufacturing the stent according to an embodiment of the present invention will be described with reference to the attached drawings. Note that, for easy understanding of the drawings, the size and dimensions of each part are exaggerated in some parts, and do not necessarily coincide with those of an actual product.

FIG.1illustrates a schematic front view of a stent according to an embodiment of the present invention in a self-expanding state.FIG.2illustrates an enlarged photograph illustrating a wire intersecting portion in a range C ofFIG.1.FIG.3illustrates an enlarged schematic diagram of a cell S inFIG.1.FIG.4illustrates a cross-sectional view taken along line D-D inFIG.2. Hereinafter, the configuration of the stent100in a self-expanding state will be described based on these drawings. Note that the self-expanding state refers to a state in which a stent expands to a diameter at the time of use set in specifications as a stent.

The stent100illustrated inFIG.1is made of metal. The metal constituting the stent100is, for example, a nickel-titanium alloy, a cobalt alloy, tantalum, or stainless steel, and is preferably a shape memory alloy that can be deformed from a contracted state to a self-expanding state by body temperature or the like.

The stent100of the present embodiment has a length and a diameter suitable for use in the arch aorta, for example, has a total length of 80 mm or more or 100 mm or more and 200 mm or less or 250 mm or less, and a diameter of 15 mm or more or 20 mm or more and 30 mm or less or 40 mm or less in a self-expanding state.

In a self-expanding state as illustrated inFIG.1, the stent100has a first wire group100aincluding six wires101,102,103,104,105, and106that are parallel to one direction (solid arrow A inFIG.1) oblique to an axial direction and extend in a spiral shape coaxially (on an axis O). In addition, the stent100has a second wire group100bincluding six wires111,112,113,114,115, and116that are parallel to an oblique direction (dotted arrow B inFIG.1) symmetrical with the first wire group100aagainst the axis O and extend in a spiral shape coaxially with the first wire group100a(on the axis O).

Specifically, the six wires101to106of the first wire group100ahave spiral shapes whose phases are shifted by 60° from each other. The six wires111to116of the second wire group100balso have spiral shapes whose phases are shifted by 60° from each other. The first wire group100aand the second wire group100bhave spiral shapes in different directions, and therefore intersect with each other at a plurality of locations, and form a mesh extending in a tubular shape in the axis O direction as a whole. In the stent100constituted in this manner, for example, as illustrated by a hatched portion inFIG.1, the first wire group100aand the second wire group100bintersect with each other to form a plurality of cells S having a substantially rhombic shape (including a square shape).

The intersecting portions between the wires101to106of the first wire group and the wires111to116of the second wire group are integrally formed without overlapping with each other in a radial direction of the stent100. For example, as illustrated inFIG.2in which a range C ofFIG.1is enlarged, an intersecting portion120has a shape in which apexes of two arc portions α and β of the wire102of the first wire group100aand the wire112of the second wire group100bfacing each other in the axis O direction are merged to be integrated. The same applies to other intersecting portions.

In addition, in the stent100of the present embodiment, a first intersection angle γ which is an angle opened in the axis O direction at the intersecting portion120in a self-expanding state is equal to or less than a second intersection angle σ which is an angle opened in a circumferential direction (that is, an opening degree of each of the arc portions α and β). A relationship between the first intersection angle γ and the second intersection angle σ is preferably σ/γ≥1.22, and more preferably σ/γ≥1.36. A basis for these numerical values will be described in Examples described later. Note that an upper limit of σ/γ is preferably σ/γ≤2.0 (for example, γ=60° and σ=120°) such that the stent100can ensure a sufficient length in the axis O direction in a self-expanding state.

That is, such a relationship between the first intersection angle γ and the second intersection angle σ is such that each cell S formed by the first wire group100aand the second wire group100bhas a square shape or a rhombus shape long in the circumferential direction in the self-expanding state of the stent100. When the cell S has a rhombus shape, as illustrated inFIG.3obtained by enlarging the cell S portion ofFIG.1, the first intersection angle γ is an acute internal angle, and the second intersection angle σ is an obtuse internal angle. In addition, a long diagonal line dl of the cell S extends in the circumferential direction of the stent100, and a short diagonal line ds extends in the axis O direction of the stent100. That is, when a value of σ/γ increases, the long diagonal line dl becomes longer, and the shape becomes a rhombus shape longer in the circumferential direction. Note that each apex of the cell S is a central position of the intersecting portion120, that is, an apex of the two arc portions α and β.

In addition, as illustrated inFIG.4which is a cross-sectional view taken along line D-D inFIG.2, a cross-sectional shape perpendicular to an extending direction of the wires101to106of the first wire group100aand the wires111to116of the second wire group100bis a rectangular shape thick in the radial direction of the stent100.

Specifically, when the thickness of each wire in the radial direction is represented by t and the length of the short width orthogonal to the thickness (that is, line width) is represented by w, a relationship between the thickness t and the line width w is preferably w/t<0.9. The relationship is more preferably w/t<0.81. A basis for these numerical values will be described in Examples described later. Note that a lower limit of w/t is preferably w/t≥0.5 in consideration of strength and the like at the time of processing for manufacturing the stent100and using the stent100.

As described above, in the stent100of the present embodiment, the wires of the first wire group100aand the second wire group100bhave a rectangular shape thick in the radial direction, and the cell S formed by the first wire group100aand the second wire group100bhas a square shape or a rhombus shape long in the circumferential direction.

(Method of Manufacturing Stent)

FIG.5illustrates a photograph of a tube made of a shape memory alloy before processing.FIG.6illustrates a photograph of a stent made of a shape memory alloy after tube processing.FIG.7(a)illustrates an overall view of the stent before self-expansion.FIG.7(b)illustrates an enlarged view of a range E ofFIG.7(a). Hereinafter, a method for manufacturing the stent100will be described based on these drawings.

The stent100of the embodiment of the present invention is first formed from a tube (tubular body)200made of a shape memory alloy as illustrated inFIG.5. Specifically, laser processing (cutting processing) is performed on the tube200so as to cut out an unnecessary portion, and thereafter, surface treatment such as chemical treatment or electrolytic polishing is performed in order to remove burrs and edges, whereby a mesh tubular body as illustrated inFIG.6is formed. That is, this mesh tubular body corresponds to the stent100in a contracted state. In addition, the stent100is enlarged in diameter to a predetermined diameter (for example, a maximum diameter that satisfies the relationship between the first intersection angle γ and the second intersection angle σ described above), is subjected to heat treatment or the like in a state where the stent100is enlarged in diameter to memorize the shape thereof, and is finished as the self-expanding stent100.

As illustrated inFIG.7(a), the stent100before self-expansion, that is, in a contracted state has a structure in which meandering structures201,202,203,204,205. . . extending in a meandering manner in the circumferential direction at an equal pitch are formed by wires, and the meandering structures201,202,203,204,205. . . are connected to each other in a line in the axis O direction. Specifically, as illustrated inFIG.7(b), each of the meandering structures201,202,203,204,205. . . is constituted by a linear portion extending in the axis O direction and a semi-circular arc portion folded back in the circumferential direction, and apexes of semi-circular arc portions of adjacent meandering structures are connected to each other to be integrated. In the stent100in the contracted state constituted as described above, the diamond-shaped cell S portion in the self-expanding state forms a substantially oval space long in the axis O direction in the contracted state.

(Action and Function of Stent)

FIGS.8to12are photographs illustrating a state of the stent100when the stent100of the embodiment of the present invention is placed in an arch aorta model400imitating the arch aorta of a human. Hereinafter, an action and a function of the stent100will be described using the arch aorta model400based on these drawings.

FIG.8illustrates a photograph illustrating a state before the stent100is exposed from a catheter300. As illustrated inFIG.8, similarly to a human body, the arch aorta model400has branch blood vessels of a brachiocephalic artery411, a left common carotid artery412, and a left subclavian artery413on an upper side (head side of the human body). The stent100is housed in the catheter300in a contracted state. When being actually used in the human body, the catheter300is inserted from the femoral aorta or the like through the abdominal aorta and the thoracic aorta to the arch aorta. Then, as illustrated inFIG.8, a distal end portion of the catheter300is curved along the curved shape of the arch aorta model400, and the stent100in a contracted state housed in the catheter300also has a shape along the curved shape.

FIG.9illustrates a photograph illustrating a state in which the stent100starts to be exposed from the catheter300.FIG.10illustrates a photograph illustrating a state in which the stent100ends to be exposed from the catheter300. As illustrated inFIG.9, the stent100is exposed from a distal end side of the catheter300. The stent100self-expands from a portion exposed from the catheter300by superelasticity of the shape memory alloy. Then, as illustrated inFIG.10, when the stent100is entirely exposed from the catheter300, the entire stent100is in an expanding state.

FIG.11illustrates a photograph illustrating a state in which the stent100is completely placed in the arch aorta (model)400.FIG.12illustrates an enlarged view of a main part ofFIG.11. In addition, as illustrated inFIGS.11and12, when the entire stent100is completely expanded, the catheter300is removed from the blood vessel, and only the stent100is placed over the entire region of the arch aorta (model)400.

As described above, in the present embodiment, when the catheter300housing the stent100is disposed in the arch aorta model400and the stent100is sequentially delivered, the stent100expands (self-expands) to a state in which the shape thereof is memorized by temperature (body temperature in the body). Therefore, the stent100sticks to an inner peripheral wall of the arch aorta model400so as to sufficiently push and spread the inner peripheral wall and follows the curved shape flexibly.

The kink resistance of the stent100of the present embodiment will be described in detail below.

FIG.13illustrates a schematic front view when the stent according to the embodiment of the present invention is curved in a self-expanding state.FIG.14illustrates a bottom view of a range F ofFIG.13as viewed from a stress direction (lower side inFIG.13).FIG.15illustrates a cross-sectional view comparing a wire cross section of the present embodiment with a wire cross section of a comparative example.FIG.16illustrates an enlarged front view comparing the wire of the present embodiment with the wire of the comparative example in a range G ofFIG.13.

As illustrated inFIG.13, when the stent100of the present embodiment is curved, for example, along the arch aorta, stress concentrates on a central portion in the axis O direction, which is an apex of the curve, that is, the most load is applied to the central portion. That is, a stress s in a direction perpendicular to the axis O direction is received on an inner side of the curve (a center side of a curved arc) of the central portion as indicated by the hollow arrow inFIG.13. Meanwhile, in the stent100, a portion of each wire between intersecting portions, surrounded by a circle inFIG.13is a fragile portion f1or f2that is likely to act as a starting point of kink.

As illustrated inFIG.14in which the range F ofFIG.13corresponding to the stress concentration portion is enlarged, in the stent100, a cell S1(hatched portion) corresponding to the stress concentration portion particularly receives a stress. The fragile portion f1in the cell S1is a central position of each side of the rhombus.

As illustrated inFIG.15(a), a cross section perpendicular to the extending direction of the wire corresponding to each fragile portion f1is thick in the radial direction of the stent100. Meanwhile, a wire cross section of a comparative example illustrated inFIG.15(b)has a width w′ longer than a thickness t′. Since the stress s acts in the radial direction of the stent100in the range F ofFIG.13corresponding to the stress concentration portion, the wire cross section of the present embodiment having the thickness t in the radial direction has higher strength with respect to the stress s than that in the comparative example and is excellent in kink resistance.

In addition, in the curved stent100illustrated inFIG.13, a stress is relatively concentrated also on a side surface of a central portion in the axis O direction. As illustrated inFIG.16(a), since a wire extends obliquely with respect to the axis O direction, the stress s acts on the fragile portion f2in the side surface portion obliquely with respect to the extending direction of the wire.

As described above, in the stent100of the present embodiment, the first intersection angle γ which is an angle opened in the axis O direction is equal to or less than the second intersection angle σ which is an angle opened in the circumferential direction. Meanwhile, in a comparative example illustrated inFIG.16(b), the line width w of a wire is the same as that of the wire of the present embodiment, but the first intersection angle γ′ is larger than the second intersection angle σ′.

In this case, a width ws of the wire of the present embodiment (hereinafter, a stress direction width ws) in a stress direction (circumferential direction) inFIG.16(a)is longer than a stress direction width ws' of the comparative example inFIG.16(b). That is, the wire of the present embodiment has a more sufficient cross-sectional area in the stress direction. Therefore, even in the side surface portion of the stent100in which the stress s acts in the radial direction in the stress concentration portion, a large cross-sectional area in the stress direction can be ensured, the strength against the stress s is higher than that in the comparative example, and the kink resistance is excellent.

As described above, the stent100of the present embodiment achieves the flexible stent100having excellent kink resistance without including such a membrane as in the stent graft. Absence of the membrane makes it difficult to inhibit a blood flow to a branch blood vessel. In particular, in the stent100of the present embodiment, since each wire has a narrow line width w with respect to the thickness t, it is further difficult to inhibit the blood flow. Therefore, it is possible to reduce difficulty for a physician to perform precise stent positioning required to align an opening formed in the stent graft with the branch blood vessel.

Furthermore, since the intersecting portion (for example,120) is integrally formed without overlapping in the radial direction of the stent100, the thickness of the intersecting portion (for example,120) is reduced by half as compared with an overlapping type stent, and unevenness is not generated in the portion. Therefore, insertion into the catheter300having a smaller diameter is possible, and the catheter300can be operated more smoothly at the time of stent placement with small frictional resistance. Furthermore, a blood vessel wall is less affected (stimulated). In addition, as compared with a wire braided stent, an expansion force of the stent itself can be ensured strongly, and the stent is also resistant to crush due to a pressing force caused by a blood flow that can flow into a detachment side.

Note that, in the stent100of the above embodiment, a metal tube is subjected to laser processing to be formed into a shape in which the wires101to106of the first wire group and the wires111to116of the second wire group are integrated, but the intersecting portion of the spiral wires may be fixed by heat or the like. In addition, the stent100may be made of a spring material. In a state where the stent100is enlarged in diameter, annealing is performed such that the spring material is in a free state, and the stent100is contracted when the stent is inserted into a catheter. Then, by exposing the stent100from the catheter, the stent100is enlarged in diameter by a reaction force of the spring. Also in this case, by forming the cross sections of the wires101to106of the first wire group and the wires111to116of the second wire group into a rectangular shape thick in the radial direction and setting the first intersection angle γ to be equal to or less than the second intersection angle σ, the first wires101to106and the second wires111to116are enlarged in diameter into a tubular shape, kink resistance to curving can be ensured while an internal space of the stent100is ensured, and flexibility can be ensured as a whole.

EXAMPLES

Hereinafter, results of a kink resistance test of the stent according to the present embodiment will be described with reference to Tables 1 to 5 andFIGS.17and18.

Table 1 presents design specifications of a base material and a stent in each of Examples. Table 2 presents final specifications of a stent after forming in each of Examples. Table 3 presents results of a kink resistance test in each of Examples.

Specifically, Table 1 presents an outer diameter and a thickness of a base material which is a tube made of a shape memory alloy, and the outer diameter and the thickness of the base material in each of Examples are 3 mm and 0.3 mm, respectively. In addition, as design specifications of a stent, a line width, a width of a link portion (a length of a wire intersecting portion in an axial direction), the number of crowns, the number of cells, a slit length (a length of a substantially oval space in an axis O direction), presence or absence of a marker, and a total length (a length in the axis O direction) are described. Note that the marker is formed by welding a material having high radiation opacity, such as tantalum, to both ends of a stent in order to enhance visibility of the stent at the time of X-ray photography. The number of cells is the number of cells between markers (M to M) at both ends of a stent.

Table 2 presents specifications of a stent in a self-expanding state immediately before the kink resistance test is performed, and describes an expanded maximum diameter, a total length (a length in the axis O direction), a shortening ratio (total length in a self-expanding state/total length in a contracted state), a polishing ratio by electrolytic polishing, a thickness t, a line width w, a wire cross-sectional area, a cell width (short diagonal line of a cell), a cell height (long diagonal line of a cell), a cell area, a first intersection angle γ, a second intersection angle σ, and 80% [N/cm] (a pressure value per cm when a stent is contracted to 80% with a maximum expansion state as 100%) in each of Examples.

TABLE 3Results of kink resistance testExampleR75R70R65R60R55R50R45R401○○○○○○○○2○○○○○○○x3○○○○○○○x4○○○○○○x5○○○○○○○x6○○○○○○○○7○○○○○○○○8○○○○○○○○9○○○○○○○○10○○○○○○x

Table 3 presents results of the kink resistance test, and presents whether or not kink occurred when stents in Examples were fitted into molds having different curvatures. A case where no kink occurred is indicated by ∘, and a case where kink occurred is indicated by x.

For example,FIG.17illustrates a state in which a stent of Example 4 is fitted into a mold of R50 (curvature radius: 50 mm), and when the stent is curved with a smooth curve as illustrated inFIG.17, it is determined that no kink has occurred. Meanwhile,FIG.18illustrates a state in which the stent of Example 4 is fitted into a mold of R45 (curvature radius: 45 mm), and when bending occurs at a central portion of the stent in the axis O direction as illustrated inFIG.18, it is determined that kink has occurred.

Table 3 indicates that the stents in Examples 1 and 6 to 9 have the best kink resistance, the stents in Examples 2, 3, and 5 have the second-best kink resistance, and the stents in Examples 4 and 10 have poor kink resistance. Since the curvature of the arch aorta is usually R45 or more, Examples other than Examples 4 and 10 are suitable for use in the arch aorta.

Table 4 presents results of calculating w/t of each of Examples based on the thickness t and the line width w in Table 2, and sorting w/t in descending order together with the results of the kink resistance test in Table 3. As presented in Table 4, it can be seen that when w/t is 0.87 or less, kink resistance up to R45 is exhibited, and when w/t is 0.81 or less, kink resistance up to R40 is reliably exhibited. Therefore, in a relationship between the wire thickness t and the line width w in a stent, w/t<0.90 is preferable, and w/t≤0.81 is more preferable.

Table 5 presents results of calculating σ/γ of each of Examples based on the first intersection angle γ and the second intersection angle σ in Table 2, and sorting σ/γ in ascending order together with the results of the kink resistance test in Table 3. As presented in Table 5, when σ/γ is 1.22 or more, kink resistance up to R40 is exhibited. Note that, in Example 4 in which σ/γ is 1.21, kink occurs at R45, and therefore σ/γ is preferably 1.22 or more such that kink resistance is more reliably exhibited. Therefore, in a relationship between the first intersection angle γ and the second intersection angle σ in a stent, σ/γ≥1.22 is preferable, and σ/γ≥1.36 is more preferable.

The description of the embodiment and Examples of the present invention is completed here, but aspects of the present invention are not limited to the embodiment and Examples.

REFERENCE SIGNS LIST