Note: Descriptions are shown in the official language in which they were submitted.
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SURGICAL INSTRUMENT HAVING SELF-REGULATED
ELECTRICAL INDUCTION HEATING OF ITS CUTTING
EDGE AND METHOD OF USING T~IE SAME
Background of the Invention
The control o bleeding during surgery accounts for a
major portion of the total time involved in an operation. The
bleeding that occurs from the plethora of small blood vessels
that pervade all tissues whenever tissues are incised obscures
the surgeon's vision, reduces his precision, and often dictates
slow and elaborate procedures in surgical operations. It is
well known to heat the tissues to minimize bleeding from in-
cisions, and surgical scalpels which are designed to elevate
tissue temperatures and minimize bleeding are also well known. ~ `
One such scalpel transmits high frequency, high energy sparks
from a small electrode held in the surgeon's hand to the tissues,
where they are converted to heat. Typically, substantial elec-
trical currents pass through the patient's body to a large
electrode beneath the patient, which completes the electrical
circuit. Discharge of sparks and temperature conversion in the -
tissue are poorly controlled in distribution and intensity, and
erratic muscular contractions in the patient are produced so
that this apparatus cannot be used to perform precise surgery.
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Further, apparatus of this type frequency produce severe tissue
damage and debris in the form of charred and dead tissue, which
materially interfere with wound healing.
Another well-known surgical scalpel employs a blade with
a resistive heating elament which cuts the tissue and provides
simultaneous hemostasis. Although these resistive elements can
be readily brought to a suitably high and constant temperature
in air prior to contacting tissues, as soon as portions of the ~ ~
blade come in contact with tissues, they are rapidly cooled. - :
During surgery, non-predictable and continuously varying portions
of the blade contact the tissues as they are being cut. As the
blade cools, the tissue cutting and hemostasis become markedly
less effective and tissue tends to adhere to the blade. If
additional power is applied by conventional means to counteract
this cooling, this additional power is selectively delivered to
the uncooled portions of the blade, frequently resulting in
excessive temperatures which may result in tissue damage and
blade destruction. This results from the fact that in certain
known resistively heated scalpels, the heating is a function of
the current squared times the resistance (I R). In conventional
metallic blades of this type, the higher the temperature of any
blade portion, the greater its electrical resistance, and con-
sequently the greater the incremental heating resulting from
incremental power input.
It is generally recognized that to seal tissues and
effect hemostasis it is desirable to operate at a temperature
between 300C. and lOOO~C. And for reasons noted above, it is
desirable that electrothermal hemostatic surgical cutting
instruments include a mechanism by which power is selectively
delivered to those portions of the blade that are cooled by
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tissue contact so that the cutting edge may be maintained
at a substantially uniform operating temperature within the
desired optimal range. Recently, hemostatic scalpels have
been described (see, for example, U. S. Patents 3,768,~82
and 3,826,263) in which temperature-controlling mechanisms
include resistive heating elements disposed on the surface
of the scalpel blade. However, such instruments require
precision in fabricating the dimensions of the heating
elements to obtain the desired resistance. And s~ch resis-
tive heating elements may be subjected to variations inresist~nce during use, as tissue juices and proteins become
deposited upon the surface of the blade.
_ummary of the Invention -
In accordance with one aspect of this invention
there is provided a blade comprising:
a cutting means including a cutting edge having
electrically conductive means disposed in the region along
said edge, wherein the electrically conductive means has an
electrical parameter that varies as a function of temperature ``
to increase power dissipation on an applied electrical signal
in the regions of the cutting edge when said regions of said
cutting edge are selectively cooled; and
means disposed in electrically insulated and electro-
magnetically coupled relationship to the electrically conduc-
tive means for inducing current therein. -
In accordance with another aspect of this invention
there is provided a method of heating the tissue-cutting
edge of a hemostatic scalpel blade having electrically conduc-
tive means disposed proximate to a tissue-cutting edge of
said blade and having an electrical conductor means insulated
from, and electromagnetically coupled to, said electrically
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conductive means adjacent said tissue-cutting edge, the
method comprising: applying an alternating signal to the
electrical conductor means to induce current within said
electrically conductive means near the tissue-cutting edge
for heating said electrically conductive means; and
increasing power dissipation in the regions of said tissue-
cutting edge which are selectively cooled upon contact with
the tissue being cut responsive to an electrical parameter
of said electrically-conductive means that varies as a
function of temperature thereof.
In accordance with another aspect of this invention
there is provided a method of heating a cutting blade having
electrically conductive material in the region of a cutting
edge operating at an elevated temperature, the method com-
prising the steps of: supplying an alternating electrical
signal along an electrical conductor adjacent the cutting -
edge; electromagnetically coupling the alternating electrical
signal in the electrical conductor to the electrically conduc-
tive material in the region to said cutting edge for inducing
current therein to heat the cutting edge; and increasing
power dissipation in response to variations with temperature
of an electrical parameter of said electrically conductive
material in the regions of said cutting edge which are
selectively cooled.
By way of added explanation, in one aspect the
present invention provides a surgical cutting instrument in
which the cutting portion of the blade is brought to an
elevated temperature by heating of the internal structure of
the blade. Current is induced within the internal structure
of the blade, preferably in the region of the cutting edget
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in response to electromagnetic energy that is coupled
thereto from a conductor which is disposed on the surface
of the blade along the cutting edge to carry an applled
alternating signal. The thickness of the surface conductor
is not critical in determining the density of the induced
currents and the resultant blade temperatures. The average
temperature of the cutting edge may be adjusted by adjust-
ing the amplitude and/or frequency of the alternating
signal being carried in the surface conductor.
Further, the portions of the cutting edge that are
cooled by tissue contact may be selectively heated to maintain
cutting edge temperatures sufficiently constant by inducing the
local currents in a material which exhibits substantial changes
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in electrical parameters such as permeability or electrical
resistivity as a function of temperature. For applied radio fre-
quency signal, the circulating currents tend to be concentrated
near the surface of the blade material and to decrease exponential-
ly with depth. The skin depth is defined as the depth at which the
density of the induced current is 37~ of its surface value and it
varies inversely as the square root of magnetic permeability, in-
versely as the square root of frequency and directly as the square
root of the resistivity of the material. The induced currents are
responsible for the I R Joule heating of the blade material.
By way of example, ferromagnetic materials composed of
iron, nickel, and cobalt and their alloys exhibit large changes
in relative permeability as their temperature goes through a
transition point called the "Curie" point. In many iron-nickel
alloys this Curie point occurs in the temperature range of
interest. Above the Curie point, the relative permeability may
be near unity and at temperatures below the Curie point the
permeability may rapidly increase by factors of 100 to 1000 for
magnetic field strengths of the dimension that would be utilized
in this application. Thus, if prior to cutting, the scalpel is
operated at a temperature somewhat above the Curie point, then
as various portions of the blade are cooled by contact with the
tissues, the temperature of those and only those portions of
the blade will tend to drop below the Curie point, at which time
the permeability of the material in that region will increase by
100 to 1000 with resultant increases in the heating of the cooled
portions by factors of 10 to 30.
Description of the Drawings
Figure 1 is a side view of the surgical instrument of
the present invention;
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Figure 2 is an end sectional view of the cutting element
in the apparatus of Figure l; and
Figure 3 is a side view of another embodiment of the
surgical instrument of the present invention.
Description of the Preferred Embodiment
In accordance with one embodiment of the present i
invention, as shown in Figures 1 and 2, radio frequency current
is induced to flow in an electrically conductive material which
forms the scalpel blade 9 that is suitably attached to handle 10.
As shown in Figure 1, a current-carrying conductor 13 is placed
about the scalpel blade 9 and is insulated therefrom by a layer
11 of insulating material. Current will be induced in the blade
9 in response to the magnetic field associated with a radio fre-
quency electrical signal applied to conductor 13. The conductor
13 is placed along the edge of the blade, as shown in Figure 2,
in a single loop around it. A high frequency signal supplied by
source 19 through connections 15 and 17 to the conductor 13
induces circulating currents in the blade 9 which will heat it
to a temperature controlled by the applied power.
Self-regulation of the operating temperature is achieved
by making the blade 9 of a ferromagnetic material which has a
Curie point that is below the temperature of the cutting edge
prior to cutting but that is well within the acceptable oper-
ating temperature range. As tissue cutting is initiated, the ~ -
regions of the cutting edge which contact the tissue may be
cooled to the Curie point temperature or below, thus producing
in the cooled regions an increase in magnetic permeability which
produces a decrease in the skin depth of the induced currents
and thereby an increased current density. Power dissipation
and heating thus increase in the regions that are cooled by
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contact with tissue. For optimal self-regulation the thickness ;;;
of the blade should exceed twice the maximum skin depth in the
operative temperature range.
The following table indicates some values of power
dissipation in a scalpel blade 3 cm. long and 20 mils thick
made from a 50-50 iron-nickel alloy in which the conductor 13
disposed upon the surface of the blade is 40 mils wide and the
scalpel is energized with a current of about 5 amperes at 6
megahertz. This RF signal current may be maintained constant
where desired using conventional circuitry of well-known design.
Resistivity Relative Power, watts/cm
ohm-cm(10~ ) Permeability of Blade Length
Material 500C. 400C. 500C. 400C. 500C. 400C.
50-50 Fe-Ni 105 100 1 100 2.45 24.0
It is evident that there is nearly a tenfold increase in power
dissipation when and where the temperature decreases below the
Curie point. The Curie point temperatures, resistivities,
relative permeabilities and changes in permeability as a function
of temperature may be varied by altering the composition of the
material used in the blade 9 or by altering the percentages of ; -
elements that form the alloy material of blade 9.
The signal amplitude or the frequency, or both, of the !
high frequency signal source 19 may be adjustable in response
to a control signal 27 supplied by a thermally-responsive -~
element 29 to establish the ambient operating temperature of the
cutting edge in air.
In Figure 2, there is shown a sectional view of the
28 blade 9 including the conductor 13 disposed on opposite sides of
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the blade 9 near the cutting edge thereof. A layer of insulating
material 23 is shown disposed over the electrode 13 to insulate
the electrode and electrical signal appearing thereon from the
tissue being cut.
In another embodiment of the present invention, the
conductive material of blade 9 exhibits a negative temperature
coefficient of resistance to provide increased power dissipation
from the induced currents in the regions of the cutting edge
that are cooled upon contact with tissue being cut.
In another embodiment of the present invention, as shown
in Figure 3, the region adjacent to the entire cutting edge of
the blade 39 is energized by high frequency signal sources 49
and 50 which supply power via conductors 53, 54 and 55, 56 to
the multiple segments 57, 58, respectively, which are disposed
as contiguous regions along the length of the cutting edge. As
various segments are cooled by their contact with the tissues,
the resultant temperature change may be sensed in a conventional
manner (for example, by resistance changes in each of the
current-carrying conductors 53, 54 and 55, 56, or by thermo-
couple sensors, or the like), the power input to each segm~nt ~ -
may be increased by increasing the amplitude and/or frequency
of the applied high frequency signals from the respective
sources 49, 50 to increase the density of induced currents and
concomitant heating within the regions of the cutting edge of
the blade.
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