U.S. patent application number 11/983648 was filed with the patent office on 2008-05-29 for method for electrosurgery with enhanced electric field and minimal tissue damage.
Invention is credited to Daniel V. Palanker, Alexander B. Vankov.
Application Number | 20080125774 11/983648 |
Document ID | / |
Family ID | 32908488 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080125774 |
Kind Code |
A1 |
Palanker; Daniel V. ; et
al. |
May 29, 2008 |
Method for electrosurgery with enhanced electric field and minimal
tissue damage
Abstract
The present invention is directed towards an electrosurgical
cutting system. The system comprises an electrically conductive
blade, having first and second blade surfaces. First and second
insulators are affixed to the first and second blade surfaces,
respectively. A blade edge, a region between the first and second
blade surfaces, has an edge radius of curvature, which preferably
is small. A source of pulsed electrical energy coupled to the
electrically conductive blade provides a substantially uniform and
highly enhanced electric field along a cutting portion of the blade
edge. The system can also be comprised of a wire electrode. Despite
the fact that its field is strongly enhanced around the apex, a
uniform vapor cavity is formed and then ionized using an
appropriately designed burst of pulses, preferably of alternating
polarity.
Inventors: |
Palanker; Daniel V.;
(Sunnyvale, CA) ; Vankov; Alexander B.; (Menlo
Park, CA) |
Correspondence
Address: |
MORRISON & FOERSTER LLP
755 PAGE MILL RD
PALO ALTO
CA
94304-1018
US
|
Family ID: |
32908488 |
Appl. No.: |
11/983648 |
Filed: |
November 9, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10779529 |
Feb 13, 2004 |
7357802 |
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11983648 |
|
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60447715 |
Feb 14, 2003 |
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Current U.S.
Class: |
606/45 |
Current CPC
Class: |
A61B 2018/144 20130101;
A61B 2018/1412 20130101; A61B 2018/1407 20130101; A61B 2018/00083
20130101; A61B 18/1402 20130101; H05H 2245/122 20130101; H05H 1/48
20130101 |
Class at
Publication: |
606/45 |
International
Class: |
A61B 18/14 20060101
A61B018/14 |
Goverment Interests
GOVERNMENT SPONSORSHIP
[0002] The present invention was made with support from the
National Institutes of Health, under contract number R01-EY-12888.
The government has certain rights in this invention.
Claims
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28: A method for cutting of biological tissue along a cutting zone
of an electrode immersed in a liquid medium, the method comprising:
a) delivering a burst of electrical pulses to said electrode to
form a vapor layer of said liquid medium surrounding the cutting
zone of the electrode, wherein a first vapor cavity forms in a high
electric field region of said electrode, and wherein said
electrical pulses do not ionize said first vapor cavity, and
wherein said burst of electrical pulses vaporizes said liquid
medium in regions of lower electric field wherein said vapor layer
is formed along the whole cutting zone before said first vapor
cavity collapses; and b) ionizing said vapor layer resulting in
plasma-mediated discharge into the biological tissue contacting
said vapor layer.
29: The method of claim 28, wherein a total duration of said burst
of pulses is less than 10 ms, whereby thermal damage to said tissue
is reduced.
30: The method of claim 29, wherein said total duration of said
burst of pulses is less than 1 ms.
31: The method of claim 30, wherein said total duration of said
burst of pulses is less than 0.1 ms.
32: The method of claim 28, wherein said pulses have a pulse
duration between 10 ns and 10 .mu.s.
33: The method of claim 28, wherein each pulse in said burst of
pulses has opposite polarity than the previous pulse.
34: The method of claim 28, further comprising repetitively
performing said delivering a burst of electrical pulses, wherein
any two time-adjacent bursts are separated by an interval greater
than 1 ms in duration.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to and claims priority from U.S.
provisional application 60/447,715, filed on Feb. 14, 2003, and
hereby incorporated by reference.
FIELD OF THE INVENTION
[0003] The present invention relates generally to an
electro-surgical device, and in particular, to the design of
efficient electro-surgical probes and waveforms for pulsed
plasma-mediated cutting, fragmentation, and evaporation of
biological tissue in fluid media.
BACKGROUND
[0004] Plasma-mediated cutting of soft biological tissue in
conductive liquid media with sub-microsecond pulses of high voltage
is described in the patent of Palanker [U.S. Pat. No. 6,135,998].
Dissection of tissue based on explosive vaporization by short
(under few microseconds) pulses of high voltage is described in the
patent of Lewis et al. [U.S. Pat. No. 6,352,535]. In these
applications an inlaid cylindrical electrode (i.e. a wire embedded
into a thick insulator and exposed at its end) is applied to
ionize, evaporate and fragment tissue in proximity of electrode
using dielectric breakdown or vaporization of water induced by a
high electric field. An inlaid cylindrical electrode cannot
penetrate into tissue and thus can only produce shallow cuts on its
surface. Due to the pulsed regime of application, this device
produces a series of perforations in tissue, which often do not
merge into a continuous cut. In addition, cavitation bubbles
accompanying each pulse create substantial collateral damage in
tissue during their growth and collapse phases [Effect of the Probe
Geometry on Dynamics of Cavitation, D. Palanker, A. Vankov, and J.
Miller, Laser-Tissue Interactions XIII, vol. 4617 SPIE (2002)]. The
size of such a damage zone typically far exceeds the size of the
electrode and the corresponding zone of initial energy deposition
[Effect of the Probe Geometry on Dynamics of Cavitation, D.
Palanker, A. Vankov, and J. Miller, Laser-Tissue Interactions XIII,
vol. 4617 SPIE (2002)]. Reduction in pulse energy helps to reduce
the mechanical damage, but also leads to decreased cutting
depth.
[0005] A second mechanism of electrosurgical ablation is
vaporization of tissue in the proximity of the probe by overheating
a conductive medium with either a continuous radio frequency
waveform or with sub-millisecond long bursts of pulses. This
mechanism is universally applicable to soft and hard biological
tissue ranging from membranes and retina to skin and cartilage. In
such regimes wire electrodes are typically used, although the use
of a device that could provide a uniform electric field along its
length would be preferable.
[0006] Without considering end effects, the electric field in a
conductive liquid at distance r from a cylindrical electrode with
potential U and radius r.sub.o much smaller than its length L
is:
E=U/(r ln(r.sub.o/L)), (1)
assuming that the return electrode is much larger and positioned at
infinity. The threshold electric field required for dielectric
breakdown in water is on the order of 10.sup.5-10.sup.6 V/cm
[Jones, H. M. & Kunhardt, E. E. Development of Pulsed
Dielectric Breakdown In Liquids. Journal of Physics D-Applied
Physics 28, 178-188 (1995); Jones, H. M. & Kunhardt, E. E.
Pulsed Dielectric Breakdown of Pressurized Water and Salt
Solutions. Journal of Applied Physics 77, 795-805 (1995)]. Such a
threshold electric field E.sub.th can be achieved with electric
pulses of several kV on a wire electrode with a diameter of several
tens of micrometers. The threshold voltage required for ionization
of a surface layer of water is:
U.sub.th=E.sub.thr.sub.o ln(L/r.sub.o). (2)
[0007] The corresponding threshold energy is:
F.sub.th=2.pi.E.sub.th.sup.2r.sub.o.sup.2L ln(L/r.sub.o). (3)
Evaporation of water in the proximity of an electrode begins when
the temperature is elevated above 100.degree. C. The threshold
voltage required for vaporization of a surface layer is:
U.sub.th=(c.rho..DELTA.T/(.tau..gamma.)).sup.1/2r.sup.o
ln(L/r.sub.o) (4)
where .tau. is a pulse duration, .gamma. is the electrical
conductivity of the liquid, .rho. is the liquid density, c is the
liquid heat capacity, and .DELTA.T is the temperature change. The
corresponding threshold energy is:
F.sub.th=2.pi.c.beta..DELTA.Tr.sub.o.sup.2L ln(L/r.sub.o). (5)
[0008] Lower threshold voltage and energy, as well as better
localization of energy deposition can be achieved by decreasing the
radius of electrode r.sub.o, as follows from equations 1-5.
However, this approach is limited by the mechanical strength of the
thin wire and its visibility. In addition, the problem of
non-uniform distribution of electric field along the electrode, and
particularly, enhancement at the apex remains.
[0009] This enhancement is illustrated in FIG. 1A, which shows the
electric field surrounding a wire electrode. The field is stronger
at the apex (i.e., at distance=0) and is weaker in its cylindrical
portion. Thus ionization and vaporization on such an electrode will
always begin and be dominant at locations of enhanced field
strength, leading to uneven cutting and excessive damage in front
of these singular points, as shown in FIG. 2.
[0010] One geometry that provides uniform enhancement of an
electric field is a ring electrode shown in FIG. 3. Its field is
uniform except for the points of deviation from perfectly round
shape, such as where the ring electrode contacts with a holder.
Fortunately, these regions of deviation can be kept away from
tissue during surgery. The threshold voltage on such an electrode
is set by the wire radius (equations 2 and 4) and thus is limited
by the mechanical strength of the wire. For example, a thin wire is
very weak and flexible and is thus inapplicable to manipulation of
tissue. In addition, wires thinner than 25 microns are barely seen
under a conventional surgical microscope, and this makes their use
even more difficult. An additional problem with the application of
thin wires is that erosion of thin wires greatly limits their
lifetime.
[0011] Below we describe probe geometry and pulse waveform
structures that provide solutions to these and other problems.
SUMMARY
[0012] What is desired is a penetrating electrode that can cut
tissue uniformly along an extensive cutting zone, rather than just
with its apex. As will be shown below, this objective can be
achieved through geometric tailoring of the electrode, careful
design of the electrical pulses applied to the electrode, or a
combination of these approaches.
[0013] Tissue can be cut uniformly along an extensive cutting zone
through the use of an electrosurgical cutting system that comprises
an electrically conductive blade, insulators, and a source of
pulsed electrical energy coupled to the blade. In particular, the
blade has a first blade surface, a second blade surface, and a
blade thickness. The blade thickness is the smallest local distance
between the first blade surface and the second blade surface. First
and second insulators are affixed to the first and second blade
surfaces, respectively. The first blade surface and the second
blade surface come together along a blade edge. Ideally, the blade
edge is perfectly sharp, but in practice the blade edge will be
somewhat rounded and it is this rounded region between the first
and second blade surfaces that will be called the blade edge. The
blade edge will have an edge radius of curvature, which ideally
will be small (thereby providing a sharp blade edge). In practice,
the entire blade edge is unlikely to be used for cutting, but only
a blade cutting portion, which is a predetermined length of the
blade that is used for cutting biological tissue. Unlike the ring
electrode discussed earlier, the use of a blade provides
substantial mechanical strength while the use of a blade edge with
a small edge radius of curvature can provide a substantially
uniform enhanced electric field along its cutting zone.
[0014] In preferred embodiments, biological tissue is cut with the
electrosurgical system with a sharp blade edge by manipulating the
blade in a biological medium such that the sharp blade edge is in
close proximity to the tissue to be cut. The approach then involves
applying at least one electrical pulse along the cutting zone of
the blade edge that contacts the region of biological tissue to be
cut. In preferred embodiments, multiple electrical pulses are
applied to the sharp blade edge. The electrical pulses are of
sufficient strength to generate electric breakdown in the tissue
that is in a close proximity to the sharp blade edge. The pulse
duration is sufficiently long for the generation of a streamer and
spark discharge but is sufficiently short to avoid the development
of a high current arc discharge. In this case, whether the current
was high would be with comparison to the current generated in the
biological medium prior to the development of the arc.
[0015] Tissue can also be cut uniformly along an extensive cutting
zone without the use of a blade as described above. In this
approach, biological tissue immersed in a liquid medium can be cut
uniformly along a cutting zone of an electrode (not necessarily in
the form of a blade) by first forming a uniform vapor cavity
surrounding the cutting zone of the electrode. This can be
accomplished through the tailoring of the electrical pulses applied
to the electrode. After forming the uniform vapor cavity, this
approach involves ionizing the vapor in the cavity. This results in
a plasma-mediated discharge into the biological tissue inside the
vapor cavity.
[0016] These two approaches can be combined to form very effective
methods for cutting biological tissue. In the combined approach for
cutting biological tissue, a burst of pulsed electrical energy is
applied to a blade having a blade edge with a relatively small edge
radius of curvature. The number of pulses and the energy of each
pulse is chosen such that liquid adjacent to the blade cutting
portion of the blade edge prior to application of the burst of
pulses is, at some time prior to completion of the burst of pulses,
vaporized along the entire blade cutting portion of the blade edge.
With the combined approach, nonuniformities in the electric field
along the blade edge are effectively smoothed out.
[0017] In the most preferred embodiments of these methods, the
electrical pulses have alternating polarity. Alternating the
polarity of the pulses greatly reduces the electroporation-related
tissue damage away from the immediate vicinity of the cut.
[0018] An electrosurgical cutting system as described above can be
readily fabricated. A blade of an electrically conductive material
is provided. The blade will have a first blade surface and an
opposing second blade surface. The first and second blade surfaces
join at a blade edge. In preferred embodiments, the first and
second blade surfaces in a predetermined cutting zone near the
blade edge are tapered to form a tapering region, which is the
region in which the first and second blade surfaces converge
towards each other. The blade is coated with a thin layer of
insulator to form a coated blade. The coated blade is immersed in a
conductive medium. A source of pulsed electrical energy is coupled
to the blade. Pulsed electrical energy is then applied to the blade
until the thin layer of insulator is removed from the vicinity of
the blade edge. Preferably the thin layer of insulator is removed
over the entire tapering region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1A illustrates the electric field along wire electrodes
of 10, 25 and 50 microns in diameter (410, 440, and 430
respectively) and 530 microns in length, and along the 5
.mu.m-thick edge of a disk electrode of 400 .mu.m in diameter
(420). The exposed zone of the disk electrode is 50 .mu.m from the
edge. The electrode potential is 600 V in all cases. FIG. 1B
illustrates the edge of the disk electrode used in FIG. 1A.
[0020] FIG. 2 shows the formation of a cavitation (vapor) cavity at
the apex of the wire electrode in saline several microseconds after
beginning of electrical pulse. This effect demonstrates that
electric field at the apex is much higher than in other parts of
the wire electrode.
[0021] FIG. 3 shows a 10 .mu.m-thick wire loop electrode in saline
with cavitation bubbles forming simultaneously along all its
length. This effect demonstrates the uniformity in distribution of
the electric field along its surface.
[0022] FIG. 4A shows an electrically conductive blade with
insulators adjacent to the blade surfaces. The blade surfaces join
at a blade edge. FIG. 4B shows a magnified view of the region
around the blade edge. The blade tapering angle and the edge radius
of curvature are shown.
[0023] FIG. 4C shows a blade having a circular planform. FIG. 4D
shows a blade with an elliptical planform. FIG. 4E shows a blade
having a planform of more general shape, with the heavier line
weight corresponding to the blade cutting portion.
[0024] FIG. 5A shows a blade electrode with insulated flat sides
and an exposed sharp edge and tapering region on a perimeter. FIG.
5B shows light emission by plasma forming on the exposed portion
after a 200 ns pulse of 3.4 kV in saline. FIG. 5C shows vapor
(cavitation) bubbles uniformly covering the exposed portion 5 .mu.s
after the pulse.
[0025] FIG. 6 shows a sequence of photographs demonstrating
formation of a uniform cavity along an electrode with a non-uniform
electric field using a sequence (burst) of pulses. For complete
coverage of the electrode the duration of the burst should not
exceed the lifetime of the first bubble
[0026] FIGS. 7A-C show a sequence illustrating the initiation of an
electric arc.
[0027] FIGS. 8A-C shown an electrode as a blade edge and biological
tissue immersed in saline. FIG. 8A shows the electrode before the
vapor cavity formation. FIG. 8B shows a vapor cavity forming over
the portion of the electrode not covered by the insulator. When the
electrical potential is high enough, an electric discharge occurs
between the electrode and the tissue as shown in FIG. 8C. As shown
in FIG. 8C, the discharge is concentrated in the region of smallest
separation (least resistance) between the electrode and the
tissue.
[0028] FIG. 9 shows that etching of a 15 .mu.m-thick Tungsten blade
by electric discharges at surgical settings leaves the blade edge
sharp as it shortens.
DETAILED DESCRIPTION
[0029] Referring now to the drawings, where similar elements are
numbered the same, FIG. 4A depicts an electrically conductive blade
100 having a first blade surface 110, a second blade surface 120,
and a blade edge 130. In practice, the blade edge 130 is somewhat
rounded, the edge radius of curvature 140 being shown in the
magnified view of FIG. 4B. A first insulator 210 is affixed to the
first blade surface 110. Similarly, a second insulator 220 is
affixed to the second blade surface 120. To complete an
electrosurgical cutting system, a source of pulsed electrical
energy 300 is coupled to the blade 100. The other terminal from the
source of pulsed electrical energy 300 is connected to a return
electrode (not shown) immersed in the medium in which the blade 100
is inserted.
[0030] At any position on the blade 100, the blade thickness is the
smallest distance between the first blade surface 110 and the
second blade surface 120. In preferred embodiments, in the region
adjacent the blade edge 130, the blade thickness is reduced
approximately linearly as the first 110 and second 120 blade
surfaces approach the blade edge 130. A blade tapering angle 150 is
the angle of convergence of the first 110 and second 120 blade
surfaces as the blade edge 130 is approached. In preferred
embodiments the blade tapering angle 150 is less than 45 degrees;
in more preferred embodiments the blade tapering angle 150 is less
than 30 degrees; and in the most preferred embodiments the blade
tapering angle 150 is less than 15 degrees.
[0031] Although in some embodiments the first 210 and second 220
insulators extend completely to the blade edge 130, in preferred
embodiments the first 210 and second 220 insulators terminate prior
to the blade edge 130. This leaves an exposed portion of the blade
100. As shown in FIGS. 4A and 4B, in the most preferred embodiments
the exposed portion of the blade 100 extends through all or most of
the tapering region. The exposed portion of the blade 100 between
the blade edge 130 and the first 210 and second 220 insulators does
not significantly reduce the electric field on the blade edge 130,
but it decreases electrical impedance and thus increases the energy
deposited into the biological tissue. Ending the first 210 and
second 220 insulators some distance from the blade edge 130 keeps
the insulators away from stresses induced by pulsed heating,
vaporization and ionization. The extra distance also provides for
some depth of metal for etching, which helps to increase the
productive lifetime of the blade 100.
[0032] FIGS. 4C-E show a variety of planform, or in-plane, shapes
that are useful in various embodiments of the blade 100. In a
canonical embodiment shown in FIG. 4C, the blade 100 takes the form
of a disk, and hence the blade 100 is sometimes denoted a disk
electrode. In such a blade 100, the first and second blade surfaces
each has a radius of curvature in a plane perpendicular to the
thickness, (sometimes known as the planar or in-plane radius of
curvature 160) that is constant at all points on the blade 100. In
another canonical embodiment shown in FIG. 4D, the blade 100 has an
elliptical planform and the planar radius of curvature 160 (shown
only schematically) varies considerably along the blade edge.
[0033] The planform shown in FIG. 4E is more general. In preferred
embodiments the planar radius of curvature 160 is much larger than
the edge radius of curvature, at least in the blade cutting portion
170. The blade cutting portion 170 is a predetermined length of the
blade 100 that is used for cutting biological tissue. In FIG. 4E,
the blade cutting portion 170 has been chosen to coincide with the
heavier line. In preferred embodiments the planar radius of
curvature 160 in the blade cutting portion 170 is at least 5, 10,
25, 50, 100, or even thousands of times greater than the edge
radius of curvature. Regions where the planar radius of curvature
160 is much greater than the edge radius of curvature are
considered to have a sharp blade edge. Having an extensive blade
cutting portion 170 with a sharp blade edge facilitates uniform (or
nearly uniform) enhancement of the electric field along the blade
edge of the blade cutting portion 170.
Electrode with Uniformly Enhanced Field for Dielectric Breakdown in
Liquid
[0034] The electric field around a sharp exposed blade edge is
similar to that on a ring electrode, but the radius of curvature is
not limited by mechanical strength anymore. The blade edge can be
sharpened much more because the mechanical strength for this
structure is provided by the blade. In addition, visibility of this
electrode is no longer a problem--the macroscopic blade can be
easily observed while its very sharp blade edge might not be well
resolved in a conventional surgical microscope. Thus the blade edge
of such an electrode can have an edge radius of curvature much
smaller than 10 microns. This will strongly reduce the threshold
voltage and energy, as well as the penetration depth of the field
into the tissue, which in turn leads to a cleaner cut with a
smaller zone of damaged tissue. The distribution of electric field
along a 5 .mu.m-thick blade edge on a disk electrode is shown in
FIG. 1.
[0035] The small radius of curvature and low threshold energy make
the interaction zone with tissue very shallow, thus fast cutting
can be achieved at sufficiently high pulse repetition rates.
Cutting tissue by small steps at high repetition rate results in a
very smooth action leaving very clean edges of the lesion. For
successful insertion of such thin electrode into tissue the layer
of insulator on its flat sides (first and second blade surfaces)
should be thin--comparable or thinner than the edge radius of
curvature.
[0036] Providing the blade edge is sharp with nearly uniform edge
radius of curvature, the electric field on the blade edge remains
uniform, or nearly uniform, even if the planar shape of the
electrode is not exactly round. The electric field remains uniform
as long as the planar radius of curvature of the blade remains much
larger than the edge radius of curvature of the blade edge and the
edge radius of curvature is uniform or nearly so. Thus a disk
electrode can be deformed into an ellipse or other shape of a
blade. Such a blade electrode will preserve a substantially uniform
distribution of electric field along the blade edge and can be used
for uniform dissection or ablation of tissue with any part on its
perimeter. Examples of uniform formation of vapor bubbles and
ionization along the blade edge of such an electrode are shown in
FIGS. 5A-C.
Optimal Tapering Angle and Material for the Blade Electrode
[0037] The field enhancement at the blade edge of the blade
electrode depends on the blade tapering angle. The lower the
tapering angle, the more effective is the enhancement of the
electric field. In addition, a lower blade tapering angle
facilitates access to tissue and penetration into it. The threshold
energy is reduced by a factor of 2 when the tapering angle changes
from 30.degree. to 0.degree.. Thus, for maximal enhancement of the
field, as well as for the most convenient access to tissue and
penetration into it, the blade tapering angle should preferably be
less than 45.degree., and more preferably less than 30.degree., and
most preferably less than 15.degree..
[0038] To reduce the rate of etching of the blade by hot plasma,
the blade electrode should be made of a material capable of
withstanding high temperatures. In addition, the material should be
hard enough to provide sufficient rigidity when used as a thin
blade. Additionally, to reduce the outflow of heat from the treated
area via the blade electrode, it should be made of a material with
low thermal conductivity. Materials fitting all these
characteristics are for example, Tungsten, and more preferably
Titanium since its thermal conductivity is 8 times lower.
Pulse Structure for Minimization of Electroporation
[0039] One of the mechanisms of tissue damage in electrosurgery is
electroporation. This is a direct effect of high electric fields on
the membranes of cells. Electroporation results in an increase of
the cell permeability and may lead to cell injury or death. To
reduce this effect a voltage-balanced or a charge-balanced pair of
pulses of opposite polarity instead of a single pulse of one
polarity can be applied. This change leads to significant reduction
in tissue damage. For example, application of a single pulse of 200
ns duration and 4 kV in amplitude produces electroporation-related
damage on the order of 260 .mu.m, while charge-balanced bi-phasic
pulses applied to the same electrode at the same amplitude produces
only 90 .mu.m of electroporation-related damage (measured on
chorioallantoic membrane of chicken embryo using Propidium Iodide
staining technique). In addition to its biological advantage,
alternating the polarity of the pulses also decreases the erosion
rate of the electrode.
[0040] In a preferred embodiment, a microblade of 0.2-0.6 mm in
length with insulated flat sides and exposed sharp edges serves as
an electrode using bi-phasic charge-balanced waveforms with pulse
duration varying from 0.1 to 5 us. Retinal dissection has been
performed with complete and partial vitrectomy on excised pig eyes
and in-vivo rabbit eyes. Results were analyzed clinically and
histologically. When no energy is applied the instrument can be
used as a vitreoretinal pick to elevate and expose membranes. A
train of charge-balanced pulses of alternating polarity can create
uniform cutting along the edge of the blade without generation of
visible gas in vitreous or fluid medium. Smooth cutting without
turbulent flow or other mechanical interference occurs when
operating at repetition rates around 100 Hz. Histology and
propidium iodide staining of live tissue demonstrate that the
collateral damage zone extends 40-80 um from the edge. With
different waveforms the blade electrode can also coagulate.
[0041] To reduce electroporation, a symmetric AC waveform,
(voltage-balanced rather that charge-balanced) can be applied,
which results in a damage zone less than 40 .mu.m.
Pulsed Waveform for Neutralization of "Hot Spots".
[0042] Uneven distribution of the electric field along the
electrode affects its performance not only in the regime of
dielectric breakdown in liquid, but also in the regime of
evaporation of water. This effect can be neutralized using
specially designed pulse waveforms. The energy should be delivered
in a burst of pulses in such a way that evaporation of the liquid,
leading to vapor bubble growth, first occurs in the areas of high
electric field. Providing that the electric field is not
sufficiently strong for ionization inside the vapor bubble, the
vapor bubble will isolate that part of electrode from the
conductive fluid. Hence, evaporation will begin in the surrounding
areas having a somewhat weaker electric field. This process should
continue until the last area of the electrode is covered by the
vapor cavity before the first bubble collapses exposing the
electrode in that area. This requirement sets the amplitude and
optimal duration of the pulse or burst of pulses. The size of
individual bubbles and the number of them can be set by choosing
the energy of each pulse in the burst and by number of pulses. An
example of such process producing uniform vapor cavity along an
electrode with a non-uniform electric field is shown in the
sequence of photos of FIG. 6.
[0043] In the example of FIG. 6, the wire diameter is 25 microns
and the wire length is 1 mm. A single burst of pulses is applied to
the wire, having a burst duration of 30 .mu.s, and containing
pulses (or minipulses) having a duration of 2.5 .mu.s separated by
a pulse interval of 2.5 .mu.s. The pulse voltage is 360 V.
[0044] The lifetime of an empty spherical cavity of radius Ro in
water (density .rho.=1000 kg/m.sup.3) and under atmospheric
pressure (Po=10.sup.5 N/m.sup.2) is t=0.91Ro(.rho./Po).sup.1/2.
That means an empty bubble with radius 100 .mu.m will collapse in
approximately 10 .mu.s. If the bubble is not empty, i.e. if the
vapor pressure inside is significant, the lifetime will increase.
No simple estimates for the cavity lifetime is known, but as a
first approximation P, which is a difference between the pressure
outside and inside the bubble, can replace Po. Thus if the vapor
pressure inside is 0.9 Po, then P=0.1 Po, and the lifetime t will
increase by a factor of 10.sup.1/2, approximately 3. As the vapor
pressure inside the cavity approaches atmospheric pressure the
lifetime of the bubble extends to infinity. The amount of vapor
inside the cavity depends on the dynamics of the cavity formation.
If the bubble is formed as a result of a very fast (as compared to
lifetime of the cavitation bubble, which is typically above 10
microseconds) explosion the cavity quickly becomes very cold and is
virtually empty. If the bubble is formed by slow (above 10
microseconds) heating and vaporization, the vapor pressure inside
will be higher and closer to ambient pressure. These theoretical
guidelines can be used to help design waveforms, but some
experimentation is likely to be necessary to determine the best
waveforms for any particular set of circumstances.
[0045] The duration of a burst of pulses is preferably less than 10
ms, and can be less than 1 ms or even less than 0.1 ms, to reduce
thermal damage to tissue being cut. The duration of pulses within a
burst is preferably between 10 ns and 10 .mu.s. Preferably,
adjacent pulses within a burst of pulses have opposite polarity to
reduce electroporation damage to tissue. Preferably, bursts are
repetitively applied to the electrode such that successive bursts
are separated by a burst interval of 1 ms or more.
[0046] After the vapor cavity covers the entire electrode, with the
proper level of the electric field, ionization of the vapor can
occur. FIGS. 7A-C illustrate the start of electric discharge in a
saline solution. In FIGS. 7A-C, the electrode is a metal anode,
glass serves as an insulator, the saline solution is the liquid
conductive medium, and a cathode is immersed in the saline
solution. FIG. 7A shows the early formation of a vapor cavity in
the saline solution. R.sub.1 is the resistance from an
equipotential through point A to an equipotential through point B.
R.sub.2 is the electrical resistance from the equipotential through
point B to the cathode. R.sub.2 is typically much larger than
R.sub.1, because not all of the anode is blocked by the vapor
cavity. Thus, only a small fraction of the anode potential U (i.e.,
U R.sub.1/(R.sub.1+R.sub.2)) is present across the vapor cavity. In
other words, the saline alongside the vapor cavity acts as a shunt
resistor and thus the voltage drop across a vapor cavity is small
until the vapor cavity completely covers the electrode.
[0047] FIG. 7B shows the vapor cavity at a later time in which it
has grown to completely encompass the anode. Therefore the entire
anode potential U is present across the vapor cavity, since current
flow is blocked by the vapor cavity. FIG. 7C shows ignition of an
electric discharge 500 inside the cavity. When the electrical
potential different from A to B exceeds the ionization threshold
for the vapor cavity, the gas in the vapor cavity ionizes and
current flows from the electrode, across the vapor cavity to the
conductive liquid medium. Preferably, the anode voltage U is
selected so that U is greater than the ionization threshold for the
complete vapor cavity of FIG. 7B, and U R.sub.1/(R.sub.1+R.sub.2)
is less than the ionization threshold of the partial vapor cavity
of FIG. 7A. Selection of the anode voltage according to this
condition ensures that the partial vapor cavity of FIG. 7A does not
break down until it has grown to completely cover the anode.
[0048] Ideally the bubbles formed during this process grow slowly,
on the order of tens of microseconds, so that the maximum velocity
associated with bubble growth is below about 10 m/s. Such slow
growing bubbles are not as mechanically damaging as cavitation
bubbles that have maximum velocities on the order of 100 m/s. In
addition, small bubbles are preferred to further minimize
mechanical damage at the boundary of the surgical cut.
[0049] In applications that involve the cutting of biological
tissue, ionization begins and the discharge is predominant in front
of tissue, i.e. in the areas where tissue is located closer to
electrode than the boundary of the vapor cavity in liquid.
Therefore, using this approach, the uniformity of the original
electric field is not critical because the tissue will only be
exposed to electric current after ionization of the vapor cavity,
which will occur substantially uniformly along the vapor cavity.
For minimization of electroporation-related damage a burst of
pulses should consist of pairs of symmetric bi-phasic or
charge-balanced pulses, as described above.
[0050] With high electric fields, when ionization of water begins
before vaporization, or when vapor cavity is ionized immediately
after its formation, the disconnect of electrode from liquid does
not occur and thus this process of sequential creation of multiple
vapor bubbles along the electrode will not work.
Combination of Sharp Edge with a Burst of Pulses.
[0051] A burst of pulses can be applied for vaporization of liquid
along a sharp edge of a disk or blade electrode. If a sharp edge is
produced along a blade that has a singular point (small planar
radius of curvature) at its apex then ordinarily, the advantage of
an enhanced electric field associated with the sharp blade edge is
tempered by the nonuniformity of the field caused by the apex.
However, by using the approach described above for vaporizing the
region along the electrode prior to ionizing the vapor bubble the
problem of the field non-uniformity can be fixed. The sharp blade
edge provides field enhancement that leads to a smaller damage zone
and lower threshold energy and is mechanically supported by the
thicker part of the insulated blade. The apex with an associated
strong field can be neutralized by application of a burst of pulses
of optimal duration.
[0052] FIGS. 8A-C show the use of a pulsed electric field to first
generate a vapor bubble around a sharp blade edge and then produce
an electric discharge from the blade to the targeted biological
tissue by ionization of the vapor. FIG. 8A shows the blade
electrode before the vapor cavity is formed. FIG. 8B shows a vapor
cavity forming over the portion of the blade electrode not covered
by the insulator. When the electrical potential is high enough, an
electric discharge occurs between the blade electrode and the
tissue as shown in FIG. 8C. As shown in FIG. 8C, the discharge is
concentrated in the region of smallest separation (least
resistance) between the electrode and the tissue.
Self-Sharpening of the Edge During "Controlled" Erosion of the
Blade Electrode
[0053] A thin electrode is rapidly etched during use, especially in
the evaporation mode. A sharp blade edge of a blade electrode also
is rapidly etched in use. Rounding the edge by etching, i.e.,
increasing the edge radius of curvature, leads to an increase in
the threshold voltage and pulse energy, which in turn, will
increase the extent of the collateral damage zone. To prevent this
effect a "controlled etching" leading to self-sharpening can be
implemented.
[0054] Etching is most efficient inside the zone of evaporation
(i.e., the vapor bubble); Therefore, the region of most efficient
etching can be determined by parameters of the driving waveform,
which determine the size of the vapor bubble. Self-sharpening can
be achieved by sizing the vapor bubble to include the tapering
region near the blade edge. In such a case, efficient etching
occurs over the entire tapering region, and the blade edge can be
maintained with an approximately constant edge radius of curvature.
Optimal width of the etching zone is determined by the thickness of
the blade and the desirable tapering angle. For a blade of
thickness D outside of the tapering region, blade tapering angle
.alpha., and edge radius of curvature r.sub.0, the tapering region
extends a distance r.sub.0+(D/2-r.sub.0)/tan(.alpha./2) inward from
the end of the blade edge. Ideally the vapor bubble should extend
at least this far inward from the end of the blade edge. Such a
self-sharpening regime keeps the electrode functional for a long
time despite the erosion. Alternatively, blade 100 can be slidably
mounted between insulators 210 and 220 such that erosion of blade
100 during operation can be compensated by extending a fresh
section of blade 100 from between insulators 210 and 220.
[0055] Technology for fabrication of such a blade can be simplified
by using the electrical discharge itself to remove the insulators
from the blade surfaces near the blade edge. Preferably, the blade
is milled to achieve an appropriate blade tapering angle either
before, or immediately after the blade surfaces are covered with
thin layers of insulators. The blade is immersed into a conductive
medium and electrical pulses are applied with waveform parameters
similar or identical to those appropriate for electrosurgery. The
electrical discharge at discontinuities will break and remove the
insulator from the active surfaces of the electrode, but in other
areas the insulator will remain intact. As the blade edge is etched
during use, the insulator in its proximity will be removed as well.
FIG. 9 shows the etching of a Tungsten blade by discharges at pulse
settings that would be appropriate for surgical cutting. The edge
remains sharp as the blade gets shorter.
* * * * *