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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to catheters, and more particularly, to
catheter guidewires and core and safety wires for use therewith.
Current techniques of introducing a catheter into the vascular system of a
patient include the followng steps: insertion of a sharp cannula through
the skin and into the vascular system, insertion of a spring guidewire
through the cannula and into the vascular system, and removal of the
cannula from the patient's body and insertion of the catheter into the
body by sliding said catheter over the guidewire. The guidewire is then
withdrawn, and the catheter is ready for further positioning and use.
It should be evident that the guidewire must be flexible and yet strong. It
must be flexible enough to negotiate the desired tortuous path of the
vascular system and do no damage with its leading tip portion and yet be
strong enough to resist doubling back, kinking or breaking during the
insertion and retraction procedures. It is accordingly desirable that the
guidewire have a flexible and yet guidable distal tip, and a relatively
stiff, strong elongated body portion. In addition to the foregoing, the
guidewire should have an ultra-smooth outer surface.
A guidewire, sometimes referred to as a spring guide, is constructed of a
finely wound spring with one or more wires running longitudinally within
the spring's central lumen. More detailed structure and function will be
explained later.
An ultra-smooth outer guidewire surface is desirable because rough surfaces
can traumatize the patient's tissues during movement of the guidewire. In
addition, a smooth guidewire surface facilitates cleaning, thus minimizing
the possibility of introducing potential toxic or pyrogenic material to
the patient's system.
While to the naked eye, the outer surface of a typical guidewire will
appear smooth, microscopic surface irregularities are oftentimes present.
Any biologically foreign material introduced into the bloodstream may
initiate blood clots (thrombosis). Such thrombogenicity is an undesirable
side effect of known angiographic guidewires, increasing the possibility
of generating blood clot emboli in the vascular system with the
angiographic procedure. Blood clot generation (thrombosis) may be
partially a function of the quantity of foreign material surface area. It
is well known that surface roughness may multiply the actual exposed
surface area of a material many times over a equivalent quantity of smooth
area. Thus surface smoothness in itself is a factor in thromboresistance.
An existing approach to a blood clot resistant guidewire surface has been
the chemical bonding of an anticoagulant material to the guidewire
surface. The present invention offers a novel alternative to a chemically
bonded anticoagulant.
One known type of guidewire is developed from a coiled flatwire which forms
the outer casing. As used here, a flatwire has an approximately
rectangular cross section. While the flatwire has several properties which
are favorable to its use as a guidewire outer casing such as increased
strength and resistance to fracture, it also has drawbacks. Due to
internal forces which are developed during the required winding, or
coiling operation, the flatwire tends to take a concave shape across its
periphery, becoming somewhat sharp at its edges. Some attempts have been
made to overcome this drawback of concavity with edging by coating or
grinding the wires after winding. However, neither coating nor grinding
has been found satisfactory because coatings often crack during guidewire
flexing and grinding still leaves microscopic surface irregularities.
Structural integrity with flexibility is another requisite for a successful
guidewire. A broken guidewire, with the possibility of leaving debris in
the patient's body cannot be tolerated. A commonly employed precaution
against leaving a broken guidewire tip in the vascular system of a
patient, is the provision of a thin wire termed a safety wire inside the
wound outer guidewire casing. The safety wire is customarily soldered both
to the proximal and distal ends of the guidewire to enable the removal of
a broken distal fragment should a break occur in the outer spring or
guidewire casing. Then, to provide some degree of rigidity to the
guidewire body, a core wire is frequently positioned coextensive with the
safety wire, inside the outer casing of the guidewire, and connected to
the proximal end but ending freely several centimeters short of the
guidewire distal tip. Such a structure as this has resulted in the problem
that there is a semi-abrupt change from rigidity to flexibility at the
body tip junction. Such a change of flexibility may produce a breakpoint
when the guidewire is subjected to repeated flexion.
One might postulate that by tapering the core wire down to a thin flexible
tip portion that the breakpoint could be eliminated. However, if this
portion is free and not connected distally, such a thin free tip portion
could protrude from the spring casing with excessive flexion and rough
handling of the guidewire. An interesting alternative would be to taper
down the core wire to achieve the desired flexibility and connect the tip
to the end of the guidewire thus combining the functions of the safety
wire and core wire into one safety-core wire. In practice one manufacturer
has done essentially this by grinding a taper on the core wire and
soldering a braid to the core wire tip which in turn is connected to the
tip of the guidewire. Another approach has been to grind a taper followed
by a reduced diameter tip portion which is subsequently rolled to a
flattened cross-sectional configuration. The first alternative is labor
intensive and adds an extra solder connection. The second method runs a
risk of weakening or damaging the safety-core wire from the secondary
operations of grinding and rolling.
The guidewire of the present invention overcomes the above drawbacks.
SUMMARY OF THE INVENTION
Briefly, the present invention relates to a guidewire having a wound outer
casing with an ultra-smooth surface, and an integral safety-core wire for
ensuring structural integrity of the guidewire without impairing guidewire
flexibility at the distal tip. A method of manufacturing the guidewire
forms another aspect of the present invention.
In one embodiment of the present invention, the ultra-smooth outer surface
of the wound guidewire is developed by coating the base flatwire with a
lubricating agent such as Teflon, prior to being spring-wound. In this
manner, flaking of the outer coating is minimized during bending of the
guidewire when in use. In another embodiment, the surface is smoothed by
first lightly grinding the surface of the spring-wound flatwire by
abrasion, and subsequently electropolishing the guidewire surface.
The inventive safety-core wire extends longitudinally through the guidewire
outer casing and is attached to the casing at the distal and proximal
ends. The distal tip of the safety wire is made ultra-flexible in one
direction by flattening a circular wire and immersing the wire into an
electro-etching solution. Withdrawing the safety wire from the etch at a
predetermined rate produces a uniformly tapered distal tip for the safety
wire. In this manner, the transition between the ultra-flexible flattened
distal tip and the relatively rigid circular body is smooth and uniform,
having a carefully controlled cross-sectional area. Therefore, the
possibility of breaking or kinking is minimized. That is, locations of
preferential bending are eliminated.
Practice of the present invention results in a smooth guidewire with an
integral safety-core wire, which is relatively rigid in the major portion
of its length, which has a flexible distal tip, and which is free of
preferential bending zones.
It is accordingly a broad object of the present invention to provide a
guidewire having an ultra-smooth exterior surface.
A further object of the present invention is to provide a wound wire
guidewire having an outer surface which is free from sharp edges.
Still a further object of the present invention is to provide a guidewire
which exposes a minimum of foreign body microscopic surface area to the
blood stream of the patient.
Another object of the present invention is to provide a guidewire having a
combined core and safety wire which optimizes strength and flexibility.
Yet another object of the present invention is to provide a guidewire
having a flexible distal tip, a relatively rigid body portion, and which
is free from zones of preferential bending.
Still another object of the present invention is to provide a guidewire
having a combined core and safety wire which has a smoothly tapered distal
tip.
Yet another object of the present invention is to provide a guidewire
assembly protected against distal fragments being lost in a patient's
body.
A further object of the present invention is to provide a method of
fabricating a guidewire such as that forming a part of the present
invention.
These and other objects of the present invention, as well as many of the
attendant advantages thereof, will become more readily apparent when
reference is made to the following description taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a and 1b show the general features of the inventive and conventional
guidewires respectively;
FIGS. 2a through 2c represent sequential steps in the manufacture of a
coiled flatwire guidewire according to the teachings of the present
invention;
FIGS. 3a and 3b illustrate the steps in an alternative method of
manufacturing a coiled flatwire guidewire according to the teachings of
the present invention;
FIGS. 4a through 4d illustrate the steps in the manufacture of a safety
wire in accordance with the teachings of the present invention; and
FIGS. 5a through 5f represent a safety wire formed in accordance with the
teachings of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
An elongated guidewire 10, constructed in accordance with the teachings of
the present invention, is shownin FIG. 1a. The guidewire 10 comprises an
elongated body with a proximal portion 14 and a distal tip portion 16. A
combined core and safety wire 18 extends from the proximal end 14 to the
distal end 16 of the guidewire 10. The body of the guidewire 10 takes the
form of a coil spring, and is developed from a wound wire, such as a
flatwire. One coil is indicated at 20. The safety core wire 18 is welded,
or in some other fashion, connected to the respective ends of the wound
body of the guidewire 10, as shown at 80 and 21.
FIG. 1b shows the corresponding features of a commonly used guidewire where
77 is the core wire and 78 is the safety wire. Area 79 is subject to
preferential flexion due to the ending of the core wire.
FIGS. 2a, 2b, and 2c illustrate the steps in an inventive process for
manufacturing a guidewire having an ultra-smooth outer surface.
In FIG. 2a, there is illustrated a flatwire 22, having a rectangular cross
section. As shown in FIG. 2a, the flatwire 22 has not yet been wound into
the spiral which defines the casing of guidewire 10. The unwound flatwire
22 is first coated with a lubricating agent 23, such as Teflon, and then
appears as illustrated in FIG. 2b. After the wire is coated with lubricant
23, a suitable winding, or coiling apparatus is employed to wind the
coated wire into a spiral. Typical dimensions for lengths "a" and "b" of
wire 22 are 0.012 by 0.006 inches, respectively. The longer dimension of
the flatwire, when coiled, defines the outer surface of the guidewire 10.
This is illustrated in FIG. 2c. In this figure it can be seen that the
outer surfaces of the respective coils of the flatwire take a concave
shape or at best present leading and trailing edges when flatwire is used.
This is due to internal stresses arising from the coiling of the flatwire,
and result in the formation of relatively sharp edges, 24 and 26. Coating
23 also assumes a concave shape and has edges 24' amd 26' on each coil,
corresponding to the edges 24 and 26 of the flatwire. The coating 23 is
applied in a liquid state that subsequently hardens. Said coating
naturally assumes a more rounded edge than the underlying raw metal.
Should coating 23 be of an open cell microporous plastic nature, such a
coating would be softer and less traumatic to blood vessel's intima and in
addition could be a reservoir for an anticoagulant fluid such as a heparin
solution or suspension that could slowly leach from the guidewire surface
providing thromboresistance. The angiographer could infuse the solution
into the plastic simply by pulling the guidewire through a compression die
located in an anticoagulant solution. Such a guidewire may have to be
inserted into the body through an arteriotomy insertion sheath so as not
to anticoagulate the arteriotomy site.
Were it not for the lubricating or anticoagulant reservoir coating 23, the
edges 24 and 26 on each coil would present sharp corners tending to
traumatize body tissue or blood vessels during insertion or removal of the
guidewire. The coating 23 absorbs some of the effects of the edges 24 and
26, with the corresponding edges 24' and 26' of the coating beng
relatively smooth.
With wire 22 coated prior to the coiling process, coiling the flatwire into
the body of the guidewire may be accomplished as is customary. Then, the
guidewire is sterilized prior to use. In this manner, even though the
guidewire undergoes twisting and bending motions during insertion, guiding
and extraction, the coating remains integral. With this inventive
guidewire, relative motion between adjacent coils is permitted without
stressing the coating on the respective coils. In the prior art guidewires
which are coated after winding, movement between adjacent coils introduces
severe stresses on the coating material with resultant breakage between
respective windings.
The steps involved in an alternative method of forming a catheter guidewire
having an ultra-smooth outer surface are illustrated in FIGS. 3a and 3b.
FIG. 3a illustrates both the first step of the alternative inventive
procedure as well as one method that has been attempted to achieve
macro-smoothness of a guidewire. In this method, an uncoated stainless
steel flatwire is wound into a spring and subsequently mechanically
ground. As can be seen, though the concave shape of the coils can be
virtually eliminated, minute surface irregularities still remain. In FIG.
3a, a coiled flatwire is illustrated at 30, surface irregularities are
shown at 32, and the inter-coil spaces designated at 34.
FIG. 3b illustrates a guidewire manufactured in accordance with the
teachings of the present invention. First, a flatwire is coiled to form a
spiral guidewire, and then is mechanically ground to take the
configuration of FIG. 3a. Finally, the roughened surface of the spiral is
subjected to an electropolishing operation to remove any remaining
roughness as well as providing edge smoothing. For example, 3 to 5 volts
in an 80 to 100% solution of phosphoric acid produces a polished, minimum
surface area surface. Increasing the voltage to between 10 and 20 volts
and reducing the concentration of phosphoric acid to 50-60% removes metal
much faster but has less of a polishing effect. As can be seen, the
electropolishing operation exaggerates the inter-coil spaces 34 of FIG.3a,
shown at 36 in FIG. 3b, but results in a surface 38 which is virtually
free from irregularities.
A safety-core wire suitable for use with guidewire 10 is shown in FIG. 4d
and denoted by the numeral 50 (18 in FIG. 1).
Safety-core wire 50 comprises an essentially flat expanded distal tip
portion 81, a uniform flexible portion 52, a circular body or shank
portion 54, and a smoothly tapering transition region 56. The result is an
extremely flexible distal, or leading tip 52 and 81, a transition region
56 gradually increasing in stiffness from the distal to the proximal
regions and a stiff shank 54 suitable for use as a core wire to add
stiffness to the associated guidewire casing. The safety-core wire is
welded to the guidewire casing at both its proximal and distal ends.
With reference now to FIGS. 4a through 4d, the steps involved in
manufacturing the inventive safety wire indicated at 18 in FIG. 1 and in
greater detail at 50 in FIG. 4d, will be described. FIG. 4a illustrates an
elongated wire 60 having, for example, a circular cross-section. One end
of wire 60, the distal end, is flattened by the application of high
pressures by means of forming dies. The flattened distal end is indicated
at 70 in FIG. 4b, and is illustrated in cross-section in FIG. 4c. The
application of pressures results in the development of relatively flat
opposed surfaces 71 and transition ramps 72. The distal region of the
flattened wire illustrated in FIG. 4b is then immersed into and then
slowly withdrawn from an electro-etching/polishing solution.
Alternatively, the wire may be partially immersed and then slowly immersed
further into an electro-etching/polishing solution. the important factor
is that for a given wire position the time spent in the solution is
proportional to the amount of metal that should be removed. Typically, the
safety-core is started with a short distal section (uniform flexible
portion 52) in the solution and the cross-sectioned area transition zone
is gradually immersed. If the safety core is withdrawn, the entire distal
section to be formed is inserted in the solution and the transition zone
is gradually withdrawn. The slow removal or immersion of the distal end of
the wire 70 from or into the etching bath, on the other hand, results in a
gradual tapering and cross-sectional area modification over the transition
zone 56 to the distal flexible region 52.
FIGS. 5b through 5f represent sections of the distal region of the
electropolished wire 50 as shown in FIG. 5a. FIG. 5b is a cross section of
the wire 50 taken along the line b--b of FIG. 5a; FIG. 5c is a cross
section taken along line c--c of FIG. 5a, and so forth. As can be seen the
wire 50 gradually tapers from a substantially circular cross section as
illustrated in FIG. 5b, to a smooth-surfaced generally elliptical section
as illustrated in FIG. 5e. FIG. 5f shows the distal diameter that is
enlarged over the cross section shown in adjacent FIG. 5e which is
accomplished by shielding the distal wire portion during the
electro-etch/polish process. This increased cross-sectional area
compensates for wire strength loss during the welding process that
connects the safety core to the casing spring at the guidewire tip.
Without the enlarged safety core, tip welding heat annealing would produce
a weak area in the 5e region.
In the manner described above, the most distal end of the core-safety wire
50 has a high degree of flexibility in one direction, allowing the
associated guidewire to easily follow the tortuous path of the human
vascular system. At the same time, the more proximal body region 54 of the
safety-core wire 50 is less flexible, and capable of adding the necessary
stiffness for propelling the guidewire through the vascular system. Most
important is the fact that the relatively rigid body of the safety-core
wire 50 is gradually transformed into a highly flexible distal region.
Because of the gradual transition, regions of preferential bending and
breaking are eliminated.
The safety-core wire illustrated in FIG. 4 and 5 may typically have a
cross-sectional diameter of from 15 to 18 mils in the region of section
b--b, diminishing to a minimum diameter of approximately 4 mils at section
5e. The ramp 72 of FIG. 4b is on the order of approximately 1.degree. with
respect to the center line of the wire, and extends over a distance of
approximately 1/2 inch. It has been found that a distance of from 3 to 6
inches between section b--b and the guidewire tip is sufficient lengthof
guidewire tip flexibility.
Above, there has been provided a description of the inventive
smooth-surfaced guidewire having a combined core and safety wire
associated therewith. The inventive guidewire has an ultra-smooth outer
surface, resulting from the application of a lubricating coating prior to
winding, or by first grinding and then electropolishing a wound metallic
base. The inventive safety-core wire comprises a main body having a
relatively uniform cross section, such as a circular cross section, and a
smoothly and uniformly tapering distal tip which is formed from first
flattening and then electropolishing techniques. It should be appreciated
that these embodiments of the present invention have been described for
purposes of illustration only, and are in no way intended to be limiting.
Rather, it is the intention that the present invention be limited only as
defined in the appended claims.
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