Variable stiffness catheter

A catheter composed of an outer coaxial tube or relatively high flexibility and three tandemly disposed inner coaxial tube segments that vary in stiffness with the stiffest being located at the proximal end of the catheter and the least stiff ending proximal of the proximal end of the catheter. The catheter thus has four regions of different stiffness/flexibility.

DESCRIPTION 
1. Technical Field 
This invention is in the general field of surgical devices and relates 
specifically to an improved catheter tube structure that may be used with 
a guide wire to access target sites through tortuous, small diameter 
vessels with less likelihood of kinking or other malfunction. 
2. Background 
Catheters are hollow tubes that are inserted through the vasculature or 
other internal body passageways to access a particular internal body site 
for various diagnostic or therapeutic purposes. Angiography catheters are 
used to deliver radiopaque agents to a target site to enable radiographic 
visualization of the site. In the treatment of localized diseases such as 
solid tumors, they are used to administer chemotherapeutic agents or 
vasoocclusive agents. Catheters are similarly used to deliver 
vasoocclusive devices (e.g., coils) to sites of aneurysm. Inflatable 
catheters often referred to as balloon catheters, are used to dilate 
vessels. 
For insertion through tortuous, small vessels such as those found in the 
peripheral vasculature or organs such as the brain and liver, catheters 
are commonly used in combination with a flexible torqueable guide wire. In 
this procedure, the guide wire is advanced through the vessel and the 
catheter is threaded over the guide wire. At tortuous sites in the vessel, 
the assembly is advanced by alternately guiding the wire through the site 
and then threading the catheter over the advanced segment of the wire. In 
order to be useful in such applications, the catheter must meet demanding 
physical requirements so that it does not become locked against the guide 
wire or become kinked as it is passed through particularly tortuous 
segments of the vessel. In this regard, commonly owned U.S. Pat. No. 
4,739,768 describes a catheter structure specifically designed to overcome 
problems associated with accessing tortuous, small vessels. 
The specific catheter embodiment shown in U.S. Pat. No. 4,739,768 consists 
of a coaxial assembly of two tubes, one of which is relatively long and 
stiff and defines a proximal portion of the catheter and the other of 
which is relatively short and flexible and defines the distal end of the 
catheter. The flexible distal end allows the catheter to be advanced 
axially over sharper and/or more frequent wire bends with less likelihood 
of malfunction. The patent mentions (col. 5) that for longer tortuous 
paths the catheter may include one or more intermediate segments having 
flexibilities intermediate those of the proximal and distal portions of 
the catheter and which, together with the distal portion, constitute 10% 
to 40% of the catheter length. The stated purpose of such intermediate 
sections is to provide greater column strength than the distal portion of 
the two-section embodiment and greater flexibility than the proximal 
section of that embodiment. The patent does not provide any specific 
examples of such multi-segment catheters or indicate any other purposes of 
a multi-segment structure. 
The present invention relates to a catheter which has four segments of 
different flexibility. This novel structure improves on the performance 
and durability of the two-segment structure depicted in U.S. Pat. No. 
4,739,768. 
DISCLOSURE OF THE INVENTION 
The invention is a catheter for use in combination with a guide wire for 
placement within a tortuous, small vessel, said catheter comprising an 
elongate tubular body having proximal and distal ends and a lumen 
extending between said ends for receiving the guide wire, said body 
comprising: 
(a) an outer coaxial tube extending continuously between said ends, having 
a wall thickness of 0.05 to 0.13 mm and being made of a polymer having a 
flexural modulus of about 100,000 to 250,000 Kpa and 
(b) proximal, intermediate, and distal inner coaxial polymeric tube 
segments positioned contiguously in tandem within the outer tube from said 
proximal end to a site proximal said distal end, the proximal segment 
having a wall thickness of 0.08 to 0.18 mm and being made of a polymer 
having a flexural modulus of about 1,500,000 to 1,800,000 kpa, the 
intermediate segment being less stiff than the proximal segment and the 
distal segment being less stiff than the intermediate segment but stiffer 
than the portion of the outer tube extending from said site to said distal 
end.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a general view showing a catheter assembly, generally designated 
11, that includes the inventive catheter 12 in combination with a guide 
wire 13. The details of the catheter construction that distinguish it from 
prior structures are not shown in FIG. 1. The assembly includes a standard 
fitting 14 through which the guide wire is received and to which the 
proximal end 15 of the catheter is removably attached. As depicted, the 
catheter is a continuous tubular body that extends from proximal end 15 to 
distal end 16 and through which the guide wire extends. The distal end of 
the guide wire extends outwardly of the distal end 16 of the catheter. The 
distal region of the catheter typically carries one or more radiopaque 
bands 17 so that the location of the distal region of the catheter within 
the vessel may be visualized radiographically. 
Details of the structure of catheter 12 are depicted in FIG. 2. It is 
composed of an outer tube 18 and three inner coaxial tubular segments 19, 
20, 21. As shown, the three inner coaxial tubular segments are disposed in 
tandem within the outer tube and are contiguous to each other (i.e., their 
respective ends abut each other). The outer tube 18 extends continuously 
over the entire length of the catheter, which typically will be over 50 to 
210 cm, more usually 80 to 150 cm. The outer diameter of tube 18 (as 
measured at proximal end 15) will normally be 0.75 to 2.00 mm, preferably 
0.85 to 1.30 mm. As seen in FIG. 2, the outer tube may neck down at its 
distal end and its outer diameter at the distal end may be slightly 
smaller than at its proximal end. The outer tube will normally have a wall 
thickness of about 0.08 to 0.16 mm, preferably about 0.10 to 0.13 mm. It 
is made from a polymer having a flexural modulus (as measured by ASTM 
D-790) of about 100,000 to 250,000 kpa, such as low density polyethylene. 
The proximal inner tubular segment 19 extends from the proximal end 15 of 
the catheter to junction 22. This distance will normally be 10 to 70 cm, 
more usually 40 to 60 cm, and preferably about 50 cm. Its wall thickness 
is about 0.08 to 0.18 mm, preferably about 0.10 to 0.13 mm, and it is made 
of a polymer having a flexural modulus of about 1,500,000 to 1,800,000 kpa 
such as polypropylene. The portion of the catheter from proximal end 15 to 
junction 22 is thus the stiffest portion of the catheter. The inner 
diameter of segment 19 will normally be 0.45 to 0.75 mm. 
Intermediate inner tubular segment 20 extends from the distal end of 
segment 19 (junction 22) to junction 23. That distance will normally be 30 
to 100 cm, more normally 70 to 90 cm, preferably about 80 cm. This segment 
is less stiff than segment 19. Accordingly, its wall thickness is less 
than segment 19 and/or it is made of a polymer with a lower flexural 
modulus than the polymer forming segment 19. In a preferred embodiment, it 
is made of the same polymer and has a smaller wall thickness, normally 
0.05 to 0.13 mm, more usually 0.05 to 0.08 mm. In the preferred 
embodiments segments 19 and 20 may be made of a continuous length of 
tubing having an appropriately tapered outer diameter. 
The third distal segment 21 extends from the distal end of segment 20 
(junction 23) to a site 24 proximal of the distal end of the catheter. The 
distance from junction 23 to site 24 will usually be 5 to 20 cm, more 
usually 7 to 15 cm, preferably about 10 cm. 
Corresponding, the distance from site 24 to the distal end 16 of the 
catheter will usually be 5 to 20 cm, more usually 7 to 15 cm, and 
preferably about 10 cm. The distance from junction 22 to the distal end of 
the catheter will be greater than about 50% of the entire length of 
catheter 12, more usually greater than about 60% of the entire catheter 
length. Segment 21 is less stiff than segment 20 and provides a transition 
in flexibility between segment 20 and the portion of outer tube 18 that 
extends beyond site 24. It follows that the wall thickness of segment 21 
is less than that of segment 20 and/or it is made from a polymer having a 
lower flexural modulus than the polymer forming segment 20. In this 
regard, it is preferable that segment 21 be made of a polymer having a 
significantly lower flexural modulus than the polymer forming segment 20 
but higher than that of the polymer from which the outer tube 18 is made. 
The distal segment 21, for instance, may be linear, low density 
polyethylene. Typically, the flexural modulus of the polymer forming 
segment 21 will be 150,000 to 350,000 kpa, more usually 200,000 to 300,000 
kpa. The wall thickness of segment 21 will normally be 0.05 to 0.10 mm, 
preferably 0.06 to 0.09 mm. The inner diameters of segments 20 and 21 are 
preferably substantially the same as that of segment 19. 
Although the joints 22 and 23 are depicted as butt joints in the drawings, 
these joints may be overlap joints. 
The invention catheter thus has four segments of different 
flexibility/stiffness and becomes increasingly flexible from 
segment-to-segment distally. The axial flexibility/stiffness gradient of 
the invention catheter is thus more gradual than in the two-segment 
embodiment of U.S. Pat. No. 4,739,768 and the change in flexibility 
stiffness between segments is not as great as in said two-segment 
embodiment. In particular, the inclusion of segment 21 allows the distal 
end of the catheter to be tracked around sharp bends with less likelihood 
of kinking occurring at the transition between the outer tube and the 
distal end of the inner coaxial tubing. This difference is shown in FIGS. 
3A and 3B. 
FIG. 3A represents the prior art structure in which there was a greater 
change in flexibility/stiffness at the transition between the coaxial tube 
portion of the catheter and the single tube distal end. When the region 
was tracked through a sharp bend, kinking could occur at the transition. 
Such kinking hinders the insertion (tracking) procedure and may lead to 
structural failure (delamination, separation) at the transition. 
FIG. 3B represents the invention structure in which the transition in 
flexibility/stiffness between the coaxial tube portion of the catheter and 
the single tube distal end is lessened. Such structure improves the 
trackability of the catheter through sharp bends (less likelihood of 
kinking) and reduces the likelihood of fatigue stress failure, 
delamination, or other structural failure at that transition. 
Aside from its improved performance and durability, the invention catheter 
operates in the same manner as the catheter described in U.S. Pat. No. 
4,739,768. Similarly, it may be constructed using the same basic 
techniques as are set forth in U.S. Pat. No. 4,739,768. In view of this, 
detailed descriptions of the manufacture and operation of the invention 
catheter are not required herein. 
Modifications of the above-described embodiments of the catheter that are 
obvious to those of skill in the fields of catheter design and 
manufacture, materials science, and related fields are intended to be 
within the scope of the following claims.