U.S. patent application number 17/497192 was filed with the patent office on 2022-01-27 for electrosurgical snare.
The applicant listed for this patent is Creo Medical Limited. Invention is credited to Mohammed Sabih CHAUDHRY, Craig GULLIFORD, Christopher Paul HANCOCK, Steven MORRIS, Brian SAUNDERS, Sandra May Bernadette SWAIN, Malcolm WHITE.
Application Number | 20220022958 17/497192 |
Document ID | / |
Family ID | 1000005895114 |
Filed Date | 2022-01-27 |
United States Patent
Application |
20220022958 |
Kind Code |
A1 |
HANCOCK; Christopher Paul ;
et al. |
January 27, 2022 |
ELECTROSURGICAL SNARE
Abstract
The disclosure relates to three enhancements for a surgical
snare: an electrosurgical snare in which the loop of snare wire
extends from an energy transfer surface which can act both as a
physical reaction surface for mechanical cutting using the snare
and as a region for emitting electromagnetic energy; a surgical
snare having a snare wire having a first end connected to a movable
boss that is slidably mounted on a coaxial cable; and a surgical
snare having an end cap with a distally facing reaction surface and
a pair of channels for guiding a snare wire, where the distally
facing reaction surface is arranged to contact the retractable loop
when fully retracted
Inventors: |
HANCOCK; Christopher Paul;
(Bath, GB) ; WHITE; Malcolm; (Chepstow, GB)
; MORRIS; Steven; (Chepstow, GB) ; GULLIFORD;
Craig; (Chepstow, GB) ; SWAIN; Sandra May
Bernadette; (Chepstow, GB) ; CHAUDHRY; Mohammed
Sabih; (Chepstow, GB) ; SAUNDERS; Brian;
(Rickmansworth, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Creo Medical Limited |
Chepstow |
|
GB |
|
|
Family ID: |
1000005895114 |
Appl. No.: |
17/497192 |
Filed: |
October 8, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15745356 |
Jan 16, 2018 |
11172985 |
|
|
PCT/EP2016/070990 |
Sep 6, 2016 |
|
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17497192 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 17/221 20130101;
A61B 2018/00077 20130101; A61B 2018/00214 20130101; A61B 2018/00083
20130101; A61B 2018/00589 20130101; A61B 18/04 20130101; A61B
2018/00601 20130101; A61B 18/1815 20130101; A61B 2017/2212
20130101; A61B 2018/141 20130101; A61B 17/32056 20130101; A61B
2018/00494 20130101; A61B 2018/183 20130101 |
International
Class: |
A61B 18/18 20060101
A61B018/18; A61B 17/3205 20060101 A61B017/3205 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 7, 2015 |
GB |
1515828.0 |
Claims
1. A surgical snare comprising: a distal head assembly; and a snare
wire slidably mounted in the distal head assembly, wherein the
distal head assembly comprises an end cap having: a distally facing
reaction surface, and a pair of channels, each of the pair of
channels extending axially between an outlet on the distally facing
reaction surface and an inlet on a proximal surface of the end cap,
wherein the snare wire is disposed within the pair of channels to
form a retractable loop beyond the distally facing conductive
surface, and wherein the distally facing reaction surface is
arranged to contact the retractable loop when fully retracted.
2. A surgical snare according to claim 1, wherein the distally
facing reaction surface includes a groove for receiving the
retractable loop.
3. A surgical snare according to claim 1, wherein the distally
facing reaction surface is rounded.
4. A surgical snare according to claim 3, wherein the distally
facing reaction surface is a dome, wherein the outlets of the pair
of channels are located on the dome.
Description
CROSS-REFERENCE OF RELATED APPLICATIONS
[0001] This application is a Divisional application of U.S. patent
application Ser. No. 15/745,356, filed on Jan. 16, 2018, which is a
National Stage Entry of International Application
PCT/EP2016/070990, filed on Sep. 6, 2016, which claims priority to
British Patent Application No. 1515828.0 filed on Sep. 7, 2015. The
disclosures of the priority applications are incorporated in their
entirety herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to a surgical snare, e.g. for use in a
polypectomy procedure. In particular, the invention may relate to
medical snares suitable for insertion down the instrument channel
of an endoscope (or any other type of scoping device used in the
gastrointestinal (GI) tract or elsewhere in the human or animal
body, such as the nasal cavity), and which may include a means for
introducing electromagnetic energy into biological tissue.
BACKGROUND TO THE INVENTION
[0003] Polyps in the GI tract can be removed using a medical snare
in an endoscopic procedure, e.g. using a colonoscope. In the case
of pedunculated polyps, the snare is passed over the polyp and
tightened around the polyp's neck (or stem), which is then cut and
the polyp removed. The cutting process may be performed or enhanced
by passing a radiofrequency (RF) current through the biological
tissue. The current may also facilitate cauterisation.
[0004] Sessile polyps can be removed in a similar manner. It is
preferable to "plump up" such polyps before removal by injecting
saline or sodium hyaluronate, under the polyp to raise it away from
the surrounding colon wall. This may help to reduce the risk of
bowel perforation.
[0005] WO 2015/004420 discloses an electrosurgical snare in which
an electrode was extendable into the loop of the snare.
SUMMARY OF THE INVENTION
[0006] The disclosure herein provides three enhancements for a
surgical snare instrument. The first enhancement concerns how
electromagnetic energy (particularly microwave energy) is delivered
to tissue, both when tissue is encircled by a loop of snare wire in
an extended configuration and when tissue is located radially
outwardly from the loop of snare wire in a retracted configuration.
Thus, the snare may be operable in two positions: an open position
(corresponding to the extended configuration) and a closed position
(corresponding to the retracted configuration). In the open
position, the snare may be used to ensnare tissue for excision. In
the closed position, the snare may be used as a general purpose
haemostat.
[0007] The second enhancement relates to means for actuating (i.e.
extending and retracting) the snare wire.
[0008] The third enhancement relates to the geometry and structure
of the distal head assembly from which the loop of snare wire
extends.
[0009] At its most general, a first aspect of the invention
provides an electrosurgical snare in which the loop of snare wire
extends from an energy transfer surface which can act both as a
physical reaction surface for mechanical cutting using the snare
and as a region for emitting electromagnetic (e.g. microwave or RF)
energy.
[0010] According to the first aspect of the invention, there is
provided a surgical snare comprising: a coaxial cable having an
inner conductor, an outer conductor and a dielectric material
separating the inner conductor from the outer conductor; a distal
head assembly disposed at a distal end of the coaxial cable; and a
snare wire mounted in the distal head assembly, wherein the distal
head assembly comprises an end cap having: a distally facing energy
transfer structure that is connected to the inner conductor, and a
pair of channels, each of the pair of channels extending axially
between an outlet on the distally facing energy transfer surface
and an inlet on a proximal surface of the end cap, wherein the
snare wire is disposed within the pair of channels to form a
retractable loop beyond the distally facing energy transfer
surface. The coaxial cable may be arranged to deliver
electromagnetic energy to the distal head assembly. The distally
facing energy transfer structure may be configured to transmit the
electromagnetic energy conveyed to the distal head assembly by the
coaxial cable into biological tissue at the distal head
assembly.
[0011] The coaxial cable may be arranged (e.g. appropriately
dimensioned) to convey microwave electromagnetic energy, wherein
the energy transfer structure may be configured as an antenna for
radiating microwave electromagnetic energy. The antenna can be
formed of an electrically conductive material, or a microwave
ceramic or similarly low-loss dielectric that enables the effective
propagation of microwave energy.
[0012] The coaxial cable may be arranged to convey radiofrequency
(RF) electromagnetic energy. The RF energy may be conveyed by the
same coaxial cable as the microwave energy. The RF energy and
microwave energy may be conveyed separately or simultaneously. If
the energy transfer structure is to transmit RF energy, it may
comprise an electrically conductive material electrically connected
to the inner conductor. For example, the energy transfer structure
may comprise an electrically conductive surface formed on the end
cap.
[0013] The snare wire may comprise an electrically conductive
material electrically connected to the outer conductor and
preferably electrically insulated from the inner conductor and the
energy transfer structure. The energy transfer structure may act as
an active electrode and the snare wire may act as a return
electrode. In order to isolate the electrically conductive surface
(i.e. active electrode) from the snare wire (i.e. return
electrode), insulating material can be provided inside of the
channels to prevent shorting between the inner and outer
conductors.
[0014] If the device is configured to use microwave electromagnetic
energy only, it may not be necessary for the snare wire and
conductive surface are insulated. For example an H-field loop may
be used to ensure efficient propagation of the microwave
energy.
[0015] The configuration of the snare wire and distally facing
energy transfer structure in combination may act to ensure that the
delivered energy goes into tissue encircled by the retractable
loop. In use, the electromagnetic energy may be used to coagulate
tissue that is grasped by the retractable loop and/or to assist in
the cutting operation. When the retractable loop is retracted, the
energy can be delivered outwardly and away from the distal end of
the head assembly. In the retracted state, the loop may have a
diameter of between 5 mm and 0.5 mm In this manner, the device can
be used to "spot" coagulate the area around a polyp stalk to stem
blood flow before beginning a polypectomy procedure. The device may
be used in this retracted configuration to coagulate vessels in the
bowel or around an area where the polyp stalk is to be removed.
Alternatively or additionally, the device may be used in the
retracted configuration to mark out a region around a sessile polyp
or tumour.
[0016] The snare wire may be slidably mounted in the distal head
assembly, whereby the loop is retractable towards the energy
transfer structure. The retractable loop may be arranged to contact
the energy transfer structure when fully retracted. The energy
transfer structure may therefore act as a reaction surface for a
physical force applied by the snare wire.
[0017] The end cap may comprise an electrically conductive body
electrically connected to the inner conductor. In other words, the
end cap may comprise a single solid conductive mass that provides
both the proximal surface and a distally facing conductive surface
that is the energy transfer structure. The pair of channels may be
holes formed (e.g. bored or drilled) through the electrically
conductive body. The channels may be parallel to each other and
aligned with the axis of the device (e.g. the axis of the coaxial
cable). The holes may be arranged symmetrically with respect to the
axis. However it will be understood that the arrangement of the
holes may vary, e.g. according to the specific application of the
device. The holes may have an insulating layer on their inner
surfaces to electrically insulate the snare wire from the
electrically conductive body. Alternatively or additionally the
snare wire itself may have an insulating cover along the portions
which pass through the channels during normal operation. The end
cap may be coated with an insulating and/or non-stick layer of
material to prevent coagulated tissue sticking to the radiator.
This insulating material may be, for example, a layer of Parylene
C, PTFE, Teflon, or a material with similar properties. It may also
be preferable that the loop of the snare wire is coated with a thin
layer of insulating and/or non-stick material to a thickness of,
for example, 10 .mu.m or less.
[0018] As discussed above, the distally facing energy transfer
structure may provide a reaction surface for contacting the
retractable loop when fully retracted. In other words, the area
encircled by the loop may be reduced to zero as it is retracted.
The reaction surface may be a portion of the distally facing energy
transfer structure that extends between the outlets of the pair of
channels. The reaction surface may be flat. However, preferably the
reaction surface is curved to fit against the snare wire as it is
retracted. The reaction surface may have a range of radii of
curvature, e.g. from 1 mm to 10 mm For example, the reaction
surface may resemble a portion of a conical or cylindrical surface.
The reaction surface may include or comprise a recess on the energy
transfer structure.
[0019] The reaction surface may include a cutting feature, e.g.
sharpened edge or blade, to facilitate cutting of the biological
tissue captured by the snare wire. The cutting feature may be
provided inside the recess discussed above so that it does not
protrude from the reaction surface. This configuration reduces the
risk of perforation or unwanted tissue damage if the device is
pushed against the wall of the bowel, oesophagus or other
organ.
[0020] If the energy transfer structure includes an electrically
conductive surface, the reaction surface may comprise a strip of
insulating material across the distally facing conductive surface
to avoid creating an electrical connection between the distally
facing conductive surface and the snare wire. The strip may be
formed separately from the end cap and attached, e.g. bonded later.
For example, the end cap may have a recess formed across it for
receiving the strip. The reaction surface may be a groove in the
distally facing conductive surface. For example, the strip of
insulating material may be formed in a concave manner to cooperate
with the cross-section profile of the snare wire. The strip may be
a thin microstrip line or the like.
[0021] The distally facing conductive surface may be rounded, e.g.
in a hemispherical or dome-like manner. This shape may assist in
delivery of the electromagnetic energy and may also provide a
smooth surface to prevent accidental snagging on tissue. The
distally facing conductive surface may be a dome, wherein the
outlets of the pair of channels are located on the dome. In other
words, the retractable loop extends out from the radiating surface
of the instrument rather than having a separate radiating element
that is insertable into the area encircled by the loop.
[0022] In order to focus the electromagnetic energy into the area
encircled by the retractable loop, and to prevent the
electromagnetic energy from entering healthy tissue surrounding the
instrument, the end cap may have insulating cover portions on its
side surfaces that are aligned with the plane of the retractable
loop. In other words, portions of the end cap that lie above and
below the retractable loop do not present an outward conductive
surface.
[0023] The snare wire may be connected to the outer conductor of
the coaxial cable at a proximal end of the distal head assembly. In
one example, a joint a joint that connects one end of the snare
wire to the outer conductor also serves as a fixed anchor point for
the snare wire. Thus, the distal head assembly may include a fixed
boss mounted on the coaxial cable and electrically connected to the
outer conductor, wherein the snare wire is electrically connected
to the fixed boss. The fixed boss may be a conductive (e.g. metal)
ring clamped onto the outer conductor at the proximal end of the
distal head assembly. The snare wire may be soldered to the fixed
boss. Alternatively the snare wire can be secured to the cap using
an interference fit or a threaded connection in one of the
channels.
[0024] A first end of the snare wire may be connected to a push rod
that is axially slidable relative to the coaxial cable, and a
second end of the snare wire may be attached to the fixed boss.
Movement of the first end forwards and backwards along the coaxial
cable causes the retractable loop to extend and retract. In order
to maintain alignment of the retractable loop, the first end of the
snare wire may be connected to a movable boss that is slidably
mounted on the coaxial cable. The moveable boss may be a sleeve
that slides over the coaxial cable. This configuration may help to
prevent uncontrolled movement of the snare wire loop by restricting
the snare wire to a plane generally parallel with the plane of the
loop.
[0025] Alternatively, the second end may also be movable e.g.
simultaneously with the first end. For example, the second end may
be connected to the push rod, e.g. via the moveable boss. Or the
first and second ends of the snare wire may be joined to each other
to form a common wire which is movable. For example, the common
wire may be connected to the movable boss or push rod which is
axially slidable relative to the coaxial cable.
[0026] The distal head assembly may include an impedance
transformer portion (also referred to herein as `transformer
portion`) mounted between a distal end of the coaxial cable and the
end cap, the transformer portion being arranged to match the
impedance of the coaxial cable to the impedance of the end cap.
This is useful if the impedance of the end cap is not the same as
the impedance of the coaxial cable. The transformer portion may be
arranged to act as a quarter wave impedance transformer.
[0027] The transformer portion may include a length of electrically
conductive material extending axially between a distal end of the
inner conductor and the proximal surface of the end cap, and a pair
of passages that extend axially on opposing sides of the length of
electrically conductive material, wherein the snare wire passes
through the pair of passages. Preferably the passages are lined
with an insulator, thereby isolating the snare wire from the inner
conductor. These passages help to prevent the wire buckling or
moving in an uncontrolled fashion. The axial length of this
structure may be chosen in conjunction with its impedance to
provide the required impedance match.
[0028] The surgical snare may have a sleeve (e.g. an electrically
insulating sheath) arranged to enclose side surfaces of the distal
head assembly. In other words, the sleeve may enclose the coaxial
cable, push rod, transformer portion and parts of the snare wire
other than the retractable loop.
[0029] In an embodiment, a distal end of the sleeve may be attached
(e.g. bonded) to a proximal peripheral edge of the end cap or
reaction surface. The snare wire may thus be movable relative to
both the end cap and the insulating sheath to extend and retract
the retractable loop. In embodiment the snare wire may be fixed
relative to the instrument channel of the endoscope through which
the surgical snare is introduced. The surgical snare is therefore
operable by moving the insulating sheath.
[0030] Alternatively, the sleeve may be slidable relative to the
distal end assembly so as to enclose the loop of the snare wire. In
one embodiment, the retractable loop may be fixed relative to the
end cap, and the diameter of the loop may be reduced (i.e. the loop
may be retracted) by sliding the sleeve over it.
[0031] The sleeve may have an internal longitudinal partition which
separates an internal volume of the sleeve into a first
longitudinal cavity for carrying the coaxial cable and a second
longitudinal cavity for carrying a push rod that is connected to
the snare wire. The push rod may be a tube or sheath mounted around
the co-axial cable and slidable relative to it.
[0032] The manner in which the snare is actuated in the first
aspect above may be a second aspect of the invention. According to
the second aspect, there is provided a surgical snare comprising a
coaxial cable having an inner conductor, an outer conductor and a
dielectric material separating the inner conductor from the outer
conductor; a distal head assembly disposed at a distal end of the
coaxial cable, the distal head assembly having an end cap that is
electrically connected to the inner conductor; and a snare wire
slidably mounted in the distal head assembly to form a retractable
loop beyond the end cap, wherein a first end of the snare wire is
connected to a movable boss that is slidably mounted on the coaxial
cable.
[0033] As discussed above, the snare wire may comprise an
electrically conductive portion that is electrically connected to
the outer conductor. This connection may be made at an opposite end
of the snare wire to where the snare wire is connected to the
moveable boss. However the snare wire may be electrically connected
to the outer conductor at any suitable point, for example via the
moveable boss. As with the first aspect, the snare wire may be
electrically insulated from the inner conductor if the device is to
be configured for use with RF electromagnetic energy. Providing a
movable boss on the coaxial cable assists in maintaining a secure
spatial relationship between the snare wire and the coaxial cable,
which can prevent the snare wire from twisting in use.
[0034] Features of the first aspect mentioned above may also be
provided in the second aspect. For example, the distal head
assembly may include a fixed boss mounted on the coaxial cable, and
wherein a second end of the snare wire is attached to the fixed
boss. The fixed boss may be electrically connected to the outer
conductor.
[0035] However, in an alternative arrangement, a second end of the
snare wire may also be attached to the movable boss. This means
both sides of the snare wire move when the movable boss slides
along the coaxial cable. This can assist in shortening the length
of the instrument, since the movable boss only needs to traverse
half the distance along the coaxial cable to achieve the same size
loop as an arrangement in which only one end of the snare wire is
attached to the movable boss. This alternative may also provide a
more evenly distributed cutting force at the end cap (i.e. at the
reaction surface).
[0036] In a further alternative arrangement, a second end of the
snare wire may be joined with the first end of the snare wire
between the fixed boss and the moveable boss. In this arrangement
the second end may pass through the fixed boss before connecting to
the first end. Again, this can assist in shortening the length of
the instrument and provide the other advantages discussed
above.
[0037] The movable boss may be operated using a push rod or the
like. In an embodiment, the push rod is a sleeve mounted around and
slidable relative to the coaxial cable. This configuration may
provide the user with more control over the movement of the snare
because the coaxial cable is less susceptible to bending or
twisting than a separate thin rod.
[0038] As discussed above with relation to the first and second
aspects of the invention, the loop may be retracted into an almost
or completely retracted position in which it abuts or is very close
to the reaction surface. When the loop is in the almost or
completely retracted configuration, the device is useable in an
alternative mode in which energy is delivered away from the end cap
and into tissue which the device is near or abuts. Such a mode can
be used to apply electromagnetic energy to points of tissue not
encircled by the loop i.e. the device may be used as a point
applicator. For example, before a polyp is removed, it is desirable
to inhibit blood flow in the area around the stem. The device may
be used in this alternative mode to apply electromagnetic energy to
the bleeding tissue so as to aid coagulation in this region. The
device may also be used to stop any residual bleeding following the
removal of the polyp. In this situation, the loop will be pulled
into the reaction surface and the device will be used as a point
applicator in order to aid coagulation with the distal end of the
snare-wire functioning as a microwave energy radiating antenna.
[0039] Thus, the coaxial cable can be connected (e.g. at its
proximal end) to a suitable generator to receive microwave energy.
The retractable loop may be movable between an extended
configuration for delivering the microwave energy to tissue
encircled by the snare wire and a retracted configuration for
delivering microwave energy outwardly from a distal exposed portion
of the snare wire, i.e. a portion of the snare wire that is not
inside the end cap when retracted. The snare wire may be fully
retracted, i.e. in contact with the distally facing conductive
surface, when the retractable loop is in the retracted
configuration. Alternatively, there may be a small gap between the
snare wire and the distally facing conductive surface when the
retractable loop is in the retracted configuration.
[0040] The geometry of the end cap may be a third aspect of the
invention. This aspect may be used in both electrosurgical snares,
where electromagnetic energy is supplied, and in "cold" snares,
where only mechanical cutting is performed. According to the third
aspect of the invention, there is provided a surgical snare
comprising: a distal head assembly; and a snare wire slidably
mounted in the distal head assembly, wherein the distal head
assembly comprises an end cap having: a distally facing reaction
surface, and a pair of channels, each of the pair of channels
extending between an outlet on the distally facing reaction surface
and an inlet on a proximal surface of the end cap, wherein the
snare wire is disposed within the pair of channels to form a
retractable loop beyond the distally facing conductive surface, and
wherein the distally facing reaction surface is arranged to contact
the retractable loop when fully retracted. The pair of channels may
extend parallel to each other. They may extend in an axial
direction through the end cap. As discussed above with respect to
the first aspect, it may be desirable to include a small blade on
or in the end cap to cut through the tissue, e.g. following
application of microwave energy if available. Ideally, the blade
should not protrude from the end cap, otherwise this presents a
risk of damage to the wall of the colon or perforation due to the
device being pushed against the wall of the bowel (or another
organ).
[0041] Features of the first and second aspects mentioned above may
also be provided in the third aspect. For example, the distally
facing reaction surface may include a groove for receiving the
retractable loop, and the distally facing reaction surface may be
rounded, i.e. convex in the distal direction.
[0042] The surgical snare described herein may be used in a
polypectomy procedure. The retractable loop can be passed around
the stem of polyp, which is then cut from the gut wall by the
application of electrical and/or mechanical energy. Advantageously,
the distally facing conductive surface forms a part of the boundary
of the retractable loop, thereby reducing the chance of snagging
the conductive dome on any tissue.
[0043] This device could also be used as a general purpose
microwave haemostat when the loop is fully retracted. In this
configuration, the microwave radiation will be emitted from the end
cap and full retracted loop.
[0044] Herein, "microwave energy" may be used broadly to indicate
an electromagnetic energy in a frequency range of 400 MHz to 100
GHz, but preferably in a range of 1 GHz to 60 GHz, more preferably
2.45 GHz to 30 GHz or 5 GHz to 30 GHz. The invention may be used at
a single specific frequency, such as any one or more of: 915 MHz,
2.45 GHz, 3.3 GHz, 5.8 GHz, 10 GHz, 14.5 GHz and 24 GHz.
[0045] Herein, radiofrequency (RF) may mean a stable fixed
frequency in the range 10 kHz to 300 MHz. The RF energy should have
a frequency high enough to prevent the energy from causing nerve
stimulation and low enough to prevent the energy from causing
tissue blanching or unnecessary thermal margin or damage to the
tissue structure. Preferred spot frequencies for the RF energy
include any one or more of: 100 kHz, 250 kHz, 400 kHz, 500 kHz, 1
MHz, 5 MHz.
[0046] The surgical snare of the invention may be configured for
insertion down an instrument channel of an endoscope, gastro scope,
etc., or may be arranged for use in laparoscopic surgery or in
natural orifice translumenal endoscopic surgery (NOTES), transanal
endoscopic microsurgery (TEMS), or trans-anal submucosal endoscopic
resection (TASER) procedures or in a general open procedure. The
diameter of the instrument channel in the endoscope may be 2.2 mm,
2.8 mm, 3.2 mm or larger. The maximum width of the structures
discussed herein may thus be set to be lower than one or more of
these dimensions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] Embodiments of the invention are described in detail below
with reference to the accompanying drawings, in which:
[0048] FIGS. 1A and 1B show respectively a front-on and side-on
schematic view of a conductive cap for a surgical snare that is an
embodiment of the invention;
[0049] FIGS. 2A and 2B show respectively a front-on and side-on
schematic view of a truncated conductive cap for a surgical snare
that is another embodiment of the invention;
[0050] FIGS. 3A and 3B show respectively a front-on and side-on
schematic view of a truncated conductive cap for a surgical snare
with insulating portions that is another embodiment of the
invention;
[0051] FIG. 4 shows a cross-sectional top-down view of a surgical
snare which is another embodiment of the invention;
[0052] FIG. 5 shows a cross-sectional top-down view of a surgical
snare used without an energy supply;
[0053] FIG. 6A shows a cross-sectional top-down view of a surgical
snare which is another embodiment of the invention;
[0054] FIG. 6B shows a side-on view of a spring vane connector used
in the surgical snare of FIG. 6A;
[0055] FIG. 7 shows a perspective view of a model of the surgical
snare of FIG. 4 used to simulate the microwave delivery performance
of the invention;
[0056] FIG. 8 shows a side view of simulated power loss density
into a polyp stem from the model surgical snare shown in FIG.
7;
[0057] FIG. 9 shows a top view of simulated power loss density into
a polyp stem from the model surgical snare shown in FIG. 7;
[0058] FIG. 10 is a graph showing return loss (impedance match)
into liver for the model surgical snare shown in FIG. 7;
[0059] FIGS. 11A and 11B show respectively a top-down and end-on
schematic view of the end of a surgical snare that is an embodiment
of the invention, when the snare wire is retracted;
[0060] FIG. 12 shows a perspective view of a model of the surgical
snare of FIGS. 11A and 11B used to simulate the microwave delivery
performance of the invention; and
[0061] FIG. 13 is a graph showing return loss (impedance match)
into liver for the model surgical snare shown in FIGS. 11A and
11B.
DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES
[0062] FIG. 1A shows a front-on view of a distal end cap 100 for
use on a surgical snare. As explained below, the distal end cap is
suitable for use with both electrosurgical snares, in which RF or
microwave energy is delivered to assist the cutting operation, and
purely mechanical snares (sometimes referred to as "cold" snares),
in which no additional energy is supplied. In this embodiment, the
end cap 100 is formed from a single piece of electrically
conductive material, but the invention is not limited to this
arrangement. For example, the end cap 100 may be formed of a
microwave ceramic or another suitable dielectric that is able to
transmit microwave electromagnetic energy. In this example, the end
cap 100 has a round proximal face which smoothly curves in the
distal direction to form a tip 107, which resembles a dome. In this
example the tip 107 had a diameter of 2.4 mm. The tip 107 has two
channels 101 passing through it, which act as guides for two ends
of a loop of wire which forms the snare. Each channel has an inlet
at the proximal surface and an outlet at the distal surface of the
tip 107. In this example, the channels 101 each have a diameter of
0.7 mm. The channels 101 are both lined with an electrical
insulator 102 such that the interior of each channel 101 is
electrically isolated from the tip 107. In practice this means that
the snare wire passing through the channels 101 is electrically
insulated from the electrically conductive material of the end cap
100.
[0063] In this example, the channels 101 have a circular
cross-section. The shape of the cross-section of the channels may
be the same shape as the cross-section of the snare wire. This
shape may be non-circular, e.g. triangular, rectangular, etc.
[0064] In an embodiment, the snare wire may be fixed relative to
the distal end cap 100. In other words a fixed length of snare wire
may extend in a loop beyond the distally facing surface of the end
cap. In such an embodiment, the loop may be retracted (i.e. the
area encircled by the loop may be reduced) by sliding a sleeve over
the end cap and loop.
[0065] In another embodiment, the snare wire may be slidably
mounted in the distal end cap 100. The cross-sectional area of the
snare wire may be less than the cross-sectional area of each
channel so that there is enough play to permit the snare wire to
slide through the channel.
[0066] A groove 103 may be formed between the two channels 101 on
the front (distal) surface of the tip 107. The groove 103 may be
shaped to receive the snare wire as it is pulled against the tip
107. Groove 107 may be less than 1 mm deep to 10 mm deep. The
groove 103 may therefore represent a reaction surface against which
a mechanically cutting force is applied to tissue (e.g. a polyp
stem) that is disposed within the loop of the snare. In some
embodiments, the groove 103 may be provided with a blade or other
sharp surface to facilitate or improve the cutting action. The
groove 103 may have a layer of electrically insulating material
disposed therein to maintain electrical isolation between the snare
wire and tip 107 even when the loop is fully closed. When the loop
is fully closed, it may form a continuous surface, i.e. one without
a gap between the loop and the groove 103, and act as a general
purpose microwave coagulator or haemostat.
[0067] FIG. 1B shows the end cap 100 in a side-on view. Here it can
be seen that tip 107 presents a distally facing convex surface,
whereas the groove 103 is a distally facing concave indentation. It
may be desirable to make the concave ends sharp or rounded. In the
former, the wire will prevent it cutting the bowel wall in the
manner discussed above.
[0068] The end cap 100 also may have a recess 106 extending in a
distal direction from the proximal surface. The recess 106 is
shaped to receive a signal feed (e.g. a portion of an inner
conductor of a coaxial cable that protrudes beyond the coaxial
cable's outer conductor and dielectric material). This is discussed
below in more detail with reference to FIG. 6. In this embodiment,
the inner conductor recess 106 is situated generally midway between
the channels 101, but the invention is not limited to this
configuration.
[0069] An annular recess 104 is formed around the periphery of the
proximal surface. The annular recess 104 is arranged to receive and
be attached (e.g. bonded) to the distal edge of a sleeve (not
shown). This is discussed below in more detail with reference to
FIG. 6.
[0070] FIG. 2A shows a front-on view of another embodiment of a
distal end cap 200 for a surgical snare. The end cap 200 shares a
number of features with the end cap 100 shown in FIG. 1A, and so
the same reference numerals are used to label corresponding parts.
The end cap 200 has a tip 207, which is electrically conductive
and, as with cap 100, the tip 207 is curved to partially form a
dome. In contrast with the tip 107 of cap 100 shown in FIG. 1A,
however, the tip 207 is truncated so as to form flat surfaces at
the top 208 and bottom 209 of the tip 207. The tip 207 of the cap
200 then has a smaller profile than the tip 107 of the first cap
100. In this example, the cap 200 had a thickness of 1.4 mm. This
allows any undesirable loss of energy into the wall of the bowel to
be minimised, as the contact between the cap 200 and the wall of
the bowel can be reduced. FIG. 2B shows a side-on view of the cap
200, which illustrates the truncation of the tip 207.
[0071] FIG. 3A shows a front-on view of another embodiment of a
distal end cap 300 for use in a surgical snare. Again, this end cap
300 shares a number of features with the end caps 100, 200 shown in
FIGS. 1A, 1B, 2A and 2B, and so the same reference numerals are
used for corresponding features.
[0072] In this example, the end cap 300 has a tip 307 formed of two
portions: a conductive portion 306, which has the same form as the
truncated tip 207 of the cap 200; and an insulating portion 305,
which is attached to the flat upper and lower surfaces of the
conductive portion 306. The outer profile of the insulating portion
305 is shaped to form a dome at the distal end of the cap 300
similar to the dome in FIGS. 1A and 1B. FIG. 3B shows a side-on
view of the cap 300, which illustrates the dome formed of the
conductive portion 306 and insulating portion 305.
[0073] The end caps discussed may be made of different materials
depending on the specific application required. For example, it may
be important that the end cap be sufficiently biocompatible (i.e.
have a known host response in a particular situation). Therefore
the end cap may be made of platinum, platinum iridium, gold,
tantalum or a mixture thereof. Where the end cap is made of a
metal, the device may be used in fluoroscopic procedures as the end
cap is then opaque to x-rays. In order to prevent tissue sticking,
as discussed above the end cap may have an outer coating (not
shown) of Teflon, PTFE or Parylene C.
[0074] FIG. 4 shows a top-down cross-sectional view of a surgical
snare 400 that is another embodiment of the invention. In this
example, the surgical snare may be dimensioned for endoscopic use.
For example, the largest width (i.e. the diameter of the distal end
cap) of the device is less than 2.6 mm, and may be around 1.4 mm,
in order to make it suitable for passing through the instrument
channel of an endoscope or any other type of surgical scope.
[0075] The surgical snare 400 comprises a coaxial cable 411 and a
distal head assembly 419 connected to the distal end of the coaxial
cable 411. The coaxial cable has an inner conductor 406, an outer
conductor 412, and a dielectric 405 separating the inner conductor
406 from the outer conductor 412. The coaxial cable 411 may
typically have an impedance of around 50 ohms. For example, it may
be a Sucoform.RTM. 47 or Sucoform.RTM. 86 cable from Huber &
Suhner.
[0076] The outer conductor 412 terminates within a fixed boss 404
at the proximal end of the distal head assembly 419. The fixed boss
404 comprises an electrically conductive element that is
electrically connected to the outer conductor 412. The fixed boss
may be an electrically conductive ring element that is clamped or
otherwise secured to the outer conductor 412 of the coaxial cable
411.
[0077] A movable boss 402 is slidably mounted on the coaxial cable
411 proximally to the fixed boss 404. In this embodiment, the
movable boss is a ring that fits around the outer conductor 412.
The outer conductor 412 may have a lubricious coating or may be
encased in a suitable sheath (not shown) to reduce friction or
prevent the braid of the outer jacket of the co-axial cable
becoming troublesome. The ring may have an outer diameter of 2.4 mm
and an inner diameter of 2.2 mm so as to fit around the coaxial
cable and within the instrument channel of an endoscope, in some
examples the ring may have an outer diameter of 1.4 mm. The outer
diameter of the ring is generally dependent on the dimensions of
the instrument channel of the endoscope the device is to be used
in. The movable boss 402 has a push rod 401 attached to it. The
push rod 401 may extend through the instrument channel of the
endoscope, whereby the movable boss 402 can be moved axially
relative to the coaxial cable, e.g. to vary the distance between
the movable boss 402 and the fixed boss 404. This mechanism is used
to extend and retract the snare, as explained below.
[0078] The distal head assembly 419 comprises a distal end cap 408
connected to the coaxial cable 411 by a transformer portion 409 to
match the impedance of the cable (the characteristic impedance) to
that of the tissue load. The distal end cap 408 may be any of the
caps discussed with reference to FIGS. 1A and 1B or FIGS. 2A and 2B
or FIGS. 3A and 3B. In other words the distal end cap 408 comprises
an electrically conductive body or a low loss dielectric, e.g. a
microwave ceramic, having a pair of channels 413, 414 extending
there through from a proximal surface to a curved (dome-like or
hemispherical) distal surface. The pair of channels 413, 414 are
preferably aligned with each other in the axial direction, and are
preferably arranged symmetrically with respect to the axis of the
device. The pair of channels 413, 414 are arranged to convey a
snare wire 403 as discussed below. If the distal end cap 408
comprises an electrically conductive body, the inside surface of
the pair of channels 413, 414 has a layer of insulating material
formed thereon to electrically insulate the snare wire 403 from the
electrically conductive body.
[0079] The transformer portion 409 comprises a length of
electrically conductive material which provides an electrical
connection between the inner conductor 406 of the coaxial cable 411
and the electrically conductive body of the distal end cap 408. In
this embodiment, the length of electrically conductive material has
a cuboidal shape with a recess formed in a proximal face thereof
for receiving an exposed length of the inner conductor 406.
However, the invention is not be limited to this geometry. The
physical length of the electrically conductive material may be such
that it has an electrical length equal to an odd multiple of a
quarter wavelength at the frequency of choice. A distal face of the
length of electrically conductive material may abut the
electrically conductive body of the distal end cap to provide the
electrical connection. Alternatively the electrically conductive
material may be integral with the electrically conductive body of
the distal cap, thereby forming a single electrically conductive
body.
[0080] A pair of axially extending insulated passages 410, 415 are
located on opposing sides of the transformer portion 409. The pair
of insulating passages convey the snare wire 403 to the distal end
cap 408 as discussed in more detail below.
[0081] In this embodiment, the transformer portion 409 and pair of
axially extending insulated passages 410, 415 are enclosed in a
protective insulating sheath 417, which has a distal end secured
(e.g. bonded) to a proximal portion 407 of the distal end cap 408
and a proximal end secured (e.g. bonded) to the fixed boss 404. The
insulating sheath 417 may be made from polytetrafluoroethylene
(PTFE) or polyether ether ketone (PEEK) or the like. These
materials may also be used to coat the end cap to prevent tissue
sticking. Other materials such as Parylene N, C or D may also be
used.
[0082] As mentioned above, the outer conductor 412 of the coaxial
cable 411 terminates within the fixed boss 404. However, the
dielectric material 405 and the inner conductor 406 protrude beyond
the distal termination of the outer conductor 412 and extend
axially inside the insulating sheath 417. The dielectric material
405 terminates at the distal face of the transformer portion 409,
while the inner conductor 406 protrudes further beyond the distal
termination of the dielectric material and extends into the recess
formed in the proximal face of the transformer portion 409. In this
example, the inner conductor 406 is soldered into a 0.35 mm
diameter hole in the length of electrically conductive
material.
[0083] A snare wire 403 has a first end fixed to the movable boss
402. The snare wire 403 extends from the movable boss 402 towards
and through the fixed boss 404 to enter the distal head assembly
419. The snare wire 403 extends through the first insulating
passage 410 into the first channel 413 to exit the distal end cap
408. The snare wire 403 forms a loop (not shown), preferably a
nibless loop, around a region beyond the distal end cap 408 and
then returns into the distal end cap 408 via the second channel
414. The snare wire 403 extends through the second channel 414 into
and through the second insulating passage 415 until it reaches the
fixed boss 404. The snare wire 403 has a second end that is
connected both physically and electrically. In this arrangement the
snare wire is connected by a soldered joint 416, however the
connection could be through crimping, welding, or another means
that ensures a physical and electrical connection to the fixed boss
404 at the proximal end of the second insulating passage 415. Since
the fixed boss 404 (or a portion of it) is electrically connected
to the outer conductor 412 of the coaxial cable 411, the snare wire
is also electrically connected to the outer conductor 412 of the
coaxial cable 411. The insulating material of the insulating
passages 410, 415 and the channels 413, 414 prevent the snare wire
403 from contacting portions of the device that are electrically
connected to the inner conductor 406 of the coaxial cable.
[0084] The snare wire 403 is made of any suitable electrically
conductive material such as nickel titanium (also known as
nitinol), and in this embodiment has a diameter of 0.3 mm. In some
applications, the snare wire 403 is made of nitinol which has shape
memory properties. In other examples, the snare wire 403 may be
made of platinum, a platinum and iridium alloy, or gold-plated
tungsten. The snare wire 403 can be plated, for example with gold
or silver, to reduce the resistance of the core of the snare wire
in order to assist effective propagation of the microwave signals.
The snare wire 403 with a diameter of 0.3 mm, when present in the
insulated passages 410, 415, forms a transmission line with an
impedance of around 36 ohms.
[0085] In use, when the movable boss 402 is slid towards the fixed
boss 404, the snare wire 403 passes through the fixed boss 404 and
the length of the snare wire 403 which protrudes from the end cap
408 is increased. This has the effect of increasing the radius of
the snare loop. Likewise, sliding the movable boss 402 away from
the fixed boss 404 reduces the amount of snare wire 403 which
protrudes from the end cap 408, thereby reducing the radius of the
snare loop.
[0086] If the snare wire 403 is electrically connected to the fixed
boss 404 both at the solder joint 416 and as it enters the distal
head assembly, a pair of parallel transmission lines exist, each
having an impedance of around 72 ohms. Using this fact, the length
of the insulating guides 415 and 410 can be chosen to provide a
quarter-wave transformer.
[0087] In some examples, the snare wire 403 is not soldered to the
fixed boss 404 at any point, instead the fixed boss 404 has
channels through it with a sufficiently tight diameter (e.g. 0.3
mm) that the snare wire 403 will be in electrical contact with it,
without any solder. In examples such as this, the snare wire 403
may extend as two strands, each strand optionally passing through
the ring 402, which can be attached to a common push rod.
[0088] In this example, the length of electrically conductive
material in the transformer portion 409 may be 0.8 mm thick, 1.6 mm
wide, and 12.5 mm long. The bulk of the transformer portion 409 may
be made of any suitable material, e.g. metal or plastic so long as
an electrically conductive path is formed from the inner conductor
406 to the end cap 408. The transformer portion 409 should also be
fairly rigid as it acts as a structural member of the device to
resist compression or buckling. It may be flexible to an extent, so
as to facilitate passing the device down an endoscopic channel. The
insulated passages 410, 415 may be formed wholly or partially
within the length of electrically conductive material. For example,
each of the side edges of the length of electrically conductive
material may have a semi-cylindrical recess formed therein. The
insulated passage 410, 415 may thus sit flush with the length of
electrically conductive material. The insulated passages 410, 415
may have a diameter of 0.7 mm.
[0089] The transformer portion 409 functions as a quarter-wave
transformer for microwave energy transmitted through the coaxial
cable 411. It does this by having a length which is substantially
one quarter or an odd multiple thereof of the wavelength of the
microwave radiation to be transmitted into the tissue.
[0090] Microwave energy (e.g. having a frequency of 5.8 GHz) may be
delivered to the surgical snare 400 from a suitable electrosurgical
generator (not shown) connected to a proximal end of the coaxial
cable 411 (e.g. outside the endoscope). The exposed conductive part
of the distal end cap 408 functions as a microwave antenna
(preferably a radiating monopole antenna) to radiate microwave
energy supplied to it from the coaxial cable 411.
[0091] In use, the snare loop would encircle a polyp stem, the
operator then reduces the radius of the snare loop by moving the
push rod 401 away from the fixed boss 404. The polyp stem is then
brought into contact with the conducting portion 107, 207, 306 of
the cap 408 and preferably the cutting groove 103 of the cap 408.
In this configuration, the microwave energy supplied to the
surgical snare 400 can enter the polyp stem, where it will promote
coagulation and therefore assist in the removal of the polyp stem
or prevent bleeding which would otherwise occur if mechanical
action only was employed.
[0092] The total length of the surgical snare 400 from movable boss
402 to the end of the cap 408 was approximately 17.2 mm.
[0093] FIG. 5 shows a cross-sectional view of a surgical snare 500.
The surgical snare 500 comprises a sleeve 508, which is connected
to a cap 505 via a joint 501. The cap 505 illustrated in FIG. 5 is
the cap 100 shown in FIGS. 1A and 1B.
[0094] As with the surgical snare shown in FIG. 4, a push rod 507
extends from the operator end of the endoscope to the surgical
snare 500 through the instrument channel of the endoscope. The push
rod 507 in this embodiment however is directly connected to the
snare wire 503. The snare wire 503 extends inside the sleeve 508,
and through the joint 501. The portion 510 of the snare wire 503
passing first through the joint 501 is freely moveable within the
joint 501. The snare wire 503 then extends through a channel 509 of
the cap 505 until it extends freely from the cap 505. The snare
wire 503 then forms the snare loop 512, by passing into a second
channel 504 of the cap 505. A portion 502 of the snare wire 503 is
secured within the second channel 504 via a weld (this could also
be a crimp or glue bond). In other examples of the device other
fixing means can be used; for example a mechanical clamp or forming
a taper in the channel 504. Therefore, when the push rod 507 is
moved towards the joint 501, the amount of snare wire 503 available
to form the snare loop 512 is increased, thereby increasing the
radius of the snare loop 512. Therefore, in use, a polyp stem or
similar tissue can be encircled by the snare loop 512. The operator
then retracts the pull rod 507, which closes the snare loop 512
until the tissue is adjacent to the cutting groove 103 in the cap
505. The sharp edges of the cutting groove 103 then act as a
reaction surface, enabling the tissue to be cut away from the
surrounding bowel wall.
[0095] This embodiment is known as a "cold snare" in that no
microwave energy is provided to the surgical snare, and it acts by
mechanical action alone to remove tissue. Whilst not shown in FIG.
5, it is possible to use the moveable boss as discussed above in
such devices. In one embodiment, both ends of the retractable loop
can be attached to the movable boss. This arrangement can prevent
twisting of the loop during extension and retraction. In another
embodiment, one end of the retractable loop is attached to the
movable boss and the other is fixed, e.g. in the end cap. The
moveable boss can be located behind the joint 501. It is also
possible in this embodiment to use a snare wire 503 which is
attached at both ends to the push rod 507 i.e. two strands of snare
wire 503 attach to the push rod 507, this mechanism can be used in
conjunction with the moveable boss described above.
[0096] FIG. 6A shows a top-down cross-sectional surgical snare 600
which is another embodiment of the invention. In this embodiment,
the surgical snare 600 comprises an insulating sleeve 611
surrounding a coaxial cable 610. The coaxial cable 610 has an outer
conductor 601, an inner conductor 607, and a dielectric 612
separating the inner and outer conductors. The outer conductor 601
terminates after passing through an earth ring 602, and before a
joint 603. The dielectric 612 and inner conductor 607 extend beyond
the termination of the outer conductor 601, terminating adjacent to
a joint 603. The inner conductor 607 then extends into a distal end
cap 606. The end cap 606 in this embodiment is that shown in FIGS.
1A and 1B, such that the inner conductor 607 extends into the inner
conductor recess 106 of the cap 606. The inner conductor 607 is
therefore electrically connected to the conductive tip 107 of the
cap 606. FIG. 6B shows a spring vane connection between the outer
conductor 601 and earth ring 602. Here the earth ring 602 is
connected via spring vanes 623 to the outer conductor 601. These
spring vanes 623 are preferably made of an electrically conductive
material, to aid in ensuring a good electrical contact between the
earth ring 602 and outer conductor 601.
[0097] The earth ring 602 is connected (e.g. by soldering,
crimping, or welding) to the outer conductor 601, as well as to a
first end 614 of a snare wire 615 to fix this portion 614 of the
snare wire 615 in place. As discussed above, spring vanes or the
like may be used to ensure good electrical contact is made.
Therefore the snare wire 615 is electrically connected to the outer
conductor 601 of the coaxial cable 610. A push rod 609 is again
present, and again extends from the operator end of the endoscope
to the surgical snare 500 through the instrument channel of the
endoscope. The push rod 609 connects directly to a second end of
the snare wire 615. A portion 608 of the snare wire 615 extends
through the earth ring 602 to the push rod 609. In contrast to the
first end 614 of the snare wire 615, this portion 608 is free to
move within the earth ring 602. The snare wire 615 then extends
through a first channel 613 of the cap 606. The snare wire 615 then
extends freely from the cap 606 so as to form a snare loop 604 by
extending through a second channel 605 of the cap 606.
[0098] Therefore, in use, the push rod 609 can be moved forwards or
backwards as discussed with relation to FIG. 5 to increase or
decrease the radius of the snare loop 604. In contrast to the
embodiment of FIG. 5 however, the surgical snare 600 may also
utilize microwave energy in addition to mechanical action.
Microwave energy may be provided via the coaxial cable 610 such
that the inner conductor 607 and conductive tip 107 of the cap 606
may radiate microwave energy into biological tissue. The conductive
tip 107 preferably functions as a monopole antenna so as to radiate
the microwave energy supplied by the coaxial cable 610.
[0099] The insulating sleeve 611 may be a multi-lumen tube arranged
to convey the push rod 609 or snare wire in a first longitudinal
passageway 621 which is separated from a second longitudinal
passageway 622 for conveying the coaxial cable 610 by a suitable
partition 620.
[0100] FIG. 7 depicts a representative model 700 of a surgical
snare as shown in FIG. 4 with the snare loop, coaxial cable, and
insulation sleeve omitted for clarity. It was modelled using CST
MICROWAVE STUDIO.RTM., and the performance simulated as various
modifications were made to the structure to improve the return loss
(impedance match into tissue load model) and power density in the
tissue. Where appropriate reference numerals indicate the
corresponding features from FIG. 4.
[0101] FIG. 8 is a cross-sectional side-view of the model surgical
snare 700 shown in FIG. 7 (with an snare loop in place beyond the
distal end thereof) showing power loss density into a polyp stem
801. The polyp stem 801 was modelled as a cylinder with a diameter
of 5 mm, and height of 2 mm from a tissue base which is 1 mm in
thickness. The snare loop is approximately 4 mm wide and 5 mm long.
The cross-section has been taken along the middle of the surgical
snare 700. The snare loop is wrapped around and cuts into the polyp
stem 801. The polyp stem 801 is connected to the gut wall 802, and
both were modelled as liver tissue i.e. with a high blood content.
The dielectric properties of liver used in the simulation were as
follows:
TABLE-US-00001 Conductivity Relative Loss Wavelength Penetration
[S/m] permittivity tangent [m] Depth [m] Liver 4.6417 38.13 0.37727
0.0082302 0.0071829
[0102] The average specific heat capacity of blood is 3617
J/kg.degree. C. (range 3300 J/kg.degree. C. to 3900 J/kg.degree.
C.) and the average density of blood is 1050 Kg/m.sup.3 (range 1025
Kg/m.sup.3 to 1060 Kg/m.sup.3). Therefore, the average specific
heat capacity of blood is around 3.6 J/(gK), and that the density
of tissue is about 1050 Kg/m.sup.3=1.05 g/cm.sup.3, so that the
volumetric heat capacity of the tissue is about 3.6
J/(gK).times.1.05 g/cm.sup.3=3.78 J/(Kcm.sup.3). The polyp stem 801
within the snare loop has a power absorption ranging from around
83.3-123 dBm/m.sup.3 (0.213-1995 W/cm.sup.3) for the modelled 1 W
input power. In FIG. 8 the region 804 closest to the end cap
indicates a power absorption of 112 dBm/m.sup.3 to 118 dBm/m.sup.3
(158-630 W/cm.sup.3, which corresponds to a temperature increase of
41.8 K/s to 167 K/s. Region 806 represents a power absorption of
around a tenth of the region 804, and so indicates a temperature
increase of 4.2 K/s to 16.7 K/s. Regions 808, present both at the
end cap and at a distal portion of the loop, represent a power
absorption of around third of the region 806, and therefore
indicate a temperature increase of 1.4 K/s to 5.6 K/s.
[0103] FIG. 9 is a top-down cross-sectional view of the scene
depicted in FIG. 8 and shows power loss density in the plane of the
loop. It can be seen that the delivered power is concentrated both
at the reaction surface and on the inside edge of the distal region
of the snare loop. This means that energy is supplied from opposing
directions as the snare loop closes around the captured tissue. The
power loss into the rear of the polyp stalk (i.e. the part furthest
from the distal head assembly) is up to 109 dBm/m.sup.3, this power
loss aids the overall heating of the polyp stalk snared within the
loop.
[0104] FIG. 10 is a graph showing the return loss of the surgical
snare 700. The graph represents the S.sub.11, parameter and
therefore the power reflected at the input port. This describes how
much of the power is not utilized in the system. As can be seen,
there is a dip at 5.8 GHz of around -12.8 dB which indicates that
around 5% of the power is reflected. The frequency of the dip can
be tuned by adjusting the length of the electrically conductive
material in the transformer portion 409. The length for this graph
was 12.5 mm.
[0105] FIGS. 11A and 11B show a top-down and end-on view
respectively of part of a snare 1101, corresponding to the first or
second embodiments, in an alternative configuration. In this
configuration, the snare loop 1102 is retracted to a near fully
retracted position i.e. the snare loop 1102 is very close to the
end cap, such that it encircles a very small area in comparison to
the other, non-retracted, configuration. FIG. 11A illustrates the
electromagnetic field 1103 radiating outward from the snare loop
1102. In this configuration the snare loop 1102 can be energised
(i.e. fed electromagnetic energy as discussed above) to coagulate
the vessels in the bowel or around the area where the stalk is
being removed. In this configuration, it may also be used as a
general purpose haemostat to aid coagulation. It may also be used
to mark out the region around a sessile tumour before excision, and
to stem bleeding in the GI tract and elsewhere.
[0106] FIG. 12 is a side cross-sectional view of a model snare 1203
in the configuration shown in in FIGS. 11A and 11B and shows power
loss density in the plane of the snare loop 1202. The snare loop
1202 intersects a small portion of the simulated polyp 1201,
simulating the situation in which the snare loop 1202 is used as a
point applicator of microwave energy. It can be seen that the
delivered power is concentrated around the snare loop 1202 and
radiates outwardly into the polyp stem 1202. In this configuration,
there is a slight increase in the power absorbed into the local
tissue 1204.
[0107] FIG. 13 is a graph showing the return loss of the surgical
snare into a polyp stem (which is modelled with the dielectric
properties of liver). The graph represents the S.sub.11 parameter
and therefore the power reflected at the input port. This describes
how much of the power is not utilized in the system. At 5.8 GHz the
S.sub.11 parameter is -3.6 dB, which indicates that around 44% of
the power is reflected.
[0108] When the loop is fully retracted into the reaction surface
(cap), a radiating dome or cylinder will be formed and the device
may also be used as a general purpose haemostat.
* * * * *