Note: Descriptions are shown in the official language in which they were submitted.
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SURGIC~L INSTRUMENT HAVING SELF-REGULATING
DIELECTRIC HEATING OF ITS CUTTING EDGE ;
AND ~THOD OF USING THE SAME
Background of the Invention
The control of 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 axe 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
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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.
Further, apparatus of this type frequently 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 element 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
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between 300C. and 1000C. And for reasons noted above, it
is desirable that electrothermal hemostatic surgical cutting
instruments include a mechanism by which power is selective-
ly delivered to those portions of the blade that are cooled
by 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 3,768,482
and 3,826,263) in which the 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 heat-
ing elements to obtain the desired resistances. And such
resistive heating elements may be subjected to variations in
resistance-during use, as tissue juices and proteins become
deposited upon the surface of the blade.
Summary 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 a dielectric means disposed
in the region along said cutting edge; and electrode means
disposed adjacent said dielectric means for establishing an
electric field through said dielectric means to dissipate
power in said dielectric means in response to an alternating
electrical signal appearing on said electrode means.
In accordance with another aspect of this inven-
tion there is provided the method of heating the cutting
edge of a dielectric means operating at an elevated tempera-
ture and having a cutting edge, the method comprising the
steps of: establishing an alternating electric field in the
region of the cutting edge of the dielectric means; and
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dissipating power in the region of the cutting edge to heat
the cutting edge by dielectric losses associated with the
applied alternating electric field.
By way of added explanation, the present inven-
tion provides a surgical cutting instrument in which the
cutting portion of the blade is brought to an elevated
temperature by dielectric heating of a scalpel constructed
of a non-conducting material. Dielectric heating depends
on the heat generated by dipole rotation in a dielectric
material caused by an alternating electric field.
All materials can be characterized from an
electromagnetic consideration with respect to two para-
meters, namely, the magnetic permeability ~ , and the di-
electric constant ~, Most dielectric materials are non-
magnetic and the permeability is equal to that of free space.
Therefore, the controlling parameter in such materials is
the dielectric constant, which may be very large relative
to free space. To incorporate both a lost current and a
charging current, the dielectric constant of a material is
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generally written in complex form ~ '' where ~' is the
real dielectric constant and ~'' is the loss factor. The di-
electric constant is also often written in relative form k =
k' - jk'' where k = ~/~O and eO is t:he constant of free space.
The power generated in a dielectric is given by
p = 0 55 (1o-12) E2 f k' tan ~
in watts/cm3, where E is the electric field in volts per
centimeter, f is the frequency in hertz, k' is the relative
dielectric constant, and tan ~ is the ratio of loss current to
charging current or k''/k'. The power generated in a dielectric
is therefore dependent upon the voltage applied to it, the fre-
quency, and the complex dielectric constant of the material.
In the present invention, the tissue-cutting edge of a
blade-shaped structure including a dielectric element is heated
by the application thereto of a high frequency electrical signal.
The electrodes are disposed on the surfaces of the dielectric
element in a manner which establishes a high frequency electric
field within the element in a region thereof near the tissue-
cutting edge.
Further, selective heating of those portions of the
cutting edge that are cooled by tissue contact in order to main-
tain cutting temperature sufficiently constant (i.e., temperature
self-regulation) may be accomplished by fabricating the element
of a dielectric material in which the loss factor k'' (i.e., the
product of the re:Lative dielectric and the tan ~ [ratio of loss
current to charging current, or k''/k']) increases with de-
creasing temperature. Since each local region of the dielectri-
cally heated material is directly affected by the high ~requency
29 electric field, each local region may have its operating
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temperatures regulated independently of the operating temperatures
of adjacent regions. Thus, even in the presence of unpredictable
and substantial variations in cooling of the various regions of
the heated edge resulting from the edge being manipulated to cut
tissues, the heated tissue-cutting edge can be maintained within
a suitably constant temperature range.
Ferroelectric materials are examples of dielectrics that
have this property near their Curie points. The Curie point of
a ferroelectric material is the temperature at which, from an
electro-magnetic standpoint, the real dielectric constant
experiences a sharp peak and the loss tangent experiences a
sharp increase with decreasing temperature. Figure 3 shows
these properties for the ferroelectric barium titanate. It can
be seen that there is approximately a 5 to 1 increase in k''
tk' x tan ~) as the temperature drops ~rom 170C. to 120C.
Therefore, if this material were used to heat the cutting edge
of a scalpel blade in accordance with the present invention, and
i a constant frequency and voltage were assumed, there would be
a 5 to 1 heating increase as the temperature dropped from 170C.
to 120C. To obtain self-regulation in the 300C. to 1000C.
range, as is desirable in surgical procedures, it is desirable
to have a material with a Curie point within this latter temper-
ature range. There are ferroelectric materials available with a
wide range of Curie points. Figure 4 shows the effect on the
real dielectric constant of the addition of lead titanate to
barium titanate. The Curie point is moved upward in temperature
as the percentage of lead titanate increases. Lead zirconate
titanate is an example of a commercially available material with
- a Curie point in the 400C. range.
The ferroelectric materials, in addition to having a
Curie point that dielectric materials in general do not possess,
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have large values of k'. This permits generating the desired
power in the small volume of material that i5 present in the
scalpel at voltages that are attainable with standard oscillators
and that are small enough to prevent breakdown in small diameter
coaxial transmission lines. The following tabulation illustrates
the difference in power generated within the volume that is
typically to be expected between the electrodes on a scalpel
blade. Two dielectrics are illustrated, one a ferroelectric
and one a more conventional dielectric such as glass.
Dielectric Constant, Frequency, Watts in
k' - jk'' Hertz Volts/cm 0.01 cm
4-j 0.01 4(107) 2(103) 1o~2
1700-j 34 4(107) 2(103) 30
Description of the Drawings
Figure 1 is a partial side ~iew of a surgical cutting
instrument according to one embodiment of the present invention;
Figure 2 is an end sectional view of one embodiment of
a blade-shaped portion of an instrument as shown in Figure l;
Figure 3 is a graph showing the temperature dependence
of dielectric constant and loss tangent of barium titanate
ceramic; and
Figure 4 is a graph showing dielectric constant as a
function of temperature, with the percent of lead titanate in
barium titanate as a variable.
Description of the Preferred Embodiment
Referring now to Figure 1, there is shown in cutaway
side view a surgical cutting instrument which has a blade-shaped
28 element 9 that is suitably attached to a handle 11. An electrode
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13 is disposed on one major face o~ the element 9 near the
periphery thereof and another similar electrode 15 (not shown)
is disposed on the opposite major face in approximate registration
with electrode 13 on the one major face. These electrodes 13, 15
may be connected, respectively, to the terminals of a source 17 of
radio frequency signal in such a manner that a radio frequency
electric field is established within the element 9 between the
electrodes 13, 15 in response to the radio frequency signal applied
thereto. This causes local heating near the peripheral edges of
the element 9 in the manner as previously described. And since ;'
the radio frequency electric ~ield established between electrodes
13 and 15 independently affects the local regions of the dielectric,
the operating temperatures of local regions may be regulated
independently of the oper~ting temperatures of adjacent regions.
With a material which has the desirable characteristics previously
discussed in connection with the graphs of Figures 3 and 4, and
at the selected operating temperatures, the entire cutting edge
can be maintained within a suitably constant temperature range
despite the irregular and unpredictable manner in which the
various regions of the cutting edge are used.
The sectional view of Figure 2 shows the arrangement of
electrodes 13 and 15 disposed on opposite faces of the element 9
in approximate pattern registration adjacent the tissue-cutting
edge of the element 9. An insulating material 21 such as silicon
dioxide may be deposited on the major surfaces of element 9 and
over the respective electrodes 13 and 15 to insulate the body of
a patient from electrical signals appearing on these electrodes.
The radio frequency signal source 19 may be adjustable
in signal amplitude or in frequency, or both, to adjust the
ambient operating temperature of the cutting edge in air.
