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FIELD OF THE INVENTION
This invention relates to the use of a laser as a medical instrument; more
particularly, it relates to laser debridement of wounds.
BACKGROUND OF THE INVENTION
There has been a longstanding need for a new surgical technique for
debridement of wounds, especially burn wounds, which will significantly
lessen the trauma and enhance healing. Laser surgery has held the hope for
fulfillment of the need but, for two decades, a successful laser technique
has eluded the investigators.
Laser beams are used in a variety of surgical procedures. Earlier
investigators have undertaken to utilize the laser for ablation of tissue
in treating surface lesions of the skin and dermatologists are using such
devices today. However, there has been negligible success in treating burn
wounds. While there are certain inherent advantages afforded by laser
surgery for serious burns, these cannot be availed of unless the time
required for the surgical procedure becomes at least comparable to that in
the conventional scalpel surgery presently used in serious burn cases and
unless the debridement leaves a tissue bed that will take a viable skin
graft.
A serious burn on the human body usually extends well below the superficial
skin surface. A significant thickness of tissue dies as a result of the
thermal assault and this necrotic material develops into a scar mass
called eschar. The interface between the eschar and the sublying viable
tissue is a fertile anaerobic region for growth and propagation of
infection. The current technique for medical treatment involves
debridement (i.e., removal) of the eschar by a surgical procedure called
"tangential excision", accomplished with a scalpel. After the debridement,
the exposed viable tissue is covered with a skin graft which provides
protection against fluid loss and contamination and initiates the healing
process. This surgical procedure is known to be highly traumatic.
Debridement by tangential excision for severe and extensive burn wounds
may require many hours for the procedure and there is an attendant very
high loss of blood during the debridement. Much effort has been expended
by investigators in seeking to develop a laser debridement technique to
replace the scalpel excision technique with its attendant trauma.
The Prior Art
Levine et al have reported the investigation of the use of a carbon dioxide
(CO.sub.2) laser for the excision of third degree burns with a view toward
minimizing blood loss and minimizing damage to the underlying tissue. See
"Use Of A Carbon Dioxide Laser For The Debridement Of Third Degree Burns",
Levine et al, Annals of Surgery, Volume 179, No. 2, pages 246 through 252,
February, 1974. This report describes an instrument comprising a CO.sub.2
laser with an articulated surgical arm which permits the laser beam to be
focused by a system of mirrors and an output lens in the handpiece held by
the operating surgeon. The CO.sub.2 laser is described as having a power
output which is variable from zero to forty watts with a beam having a
wavelength in the far infrared region at 10.6 microns. It is reported that
the laser was operated with a continuous wave output which enables use of
the laser as a scalpel in general surgical procedures. The report
describes an initial technique of "laser vaporization" in which the beam
was directly focused on the burned tissue. It was reported that this
method was found to be too slow and that injury to underlying tissue often
occurred. It also describes use of the laser as a scalpel, a technique
referred to as "laser excision". In this, the laser is first used to
incise a rim of tissue completely surrounding the burned area; then a flap
of tissue containing the burned skin was dissected free of the underlying
tissue, using the laser as a "photo knife" or scalpel. Laser debridement
of burns using the same type of CO.sub.2 laser and similar procedure is
described in "Laser Excision of Acute Third Degree Burns Followed By
Immediate Autograft Replacement: An Experimental Study In The Pig" by
Stellar et al, Journal of Trauma, Volume 13, No. 1, pages 45 through 53,
January, 1973. In the described procedure with the CO.sub.2 laser operated
in the continuous wave mode, the removal of tissue is described as "laser
vaporization" in which the laser is directly focused on the burned tissue,
converting it to smoke.
Also, in the prior art, it is known to use the ablative effects of a laser
beam in a dermatological procedure for removing thin lesions, such as port
wine scars and warts. In this technique, a CO.sub.2 laser is operated in a
pulsed mode with the beam applied perpendicularly to the treated surface.
This dermatological laser is operated with pulses of less than 0.1
microseconds duration at a repetition rate of about one pulse per second
with two or three joules per pulse. This dermatological laser is
infeasible for removing eschar over large areas because it is too slow; it
would, for example, take many hours to ablate the eschar from an entire
limb.
Carbon dioxide lasers are currently available which, typically, emit pulses
of infrared radiation at 10.6 microns wavelength in the regime of several
joules per pulse. These lasers have a characteristically short pulse
length, less than 0.1 microsecond, as an inherent feature of their design.
Such lasers are used for various purposes in the manufacturing industry
and are also used as surgical cutting instruments. Such lasers are of the
so-called TEA type and are available from Lumonics Company and Coherent
Radiation, Inc., for example.
Events Leading to the Invention Disclosed in this Patent
We made the invention set forth below in the course of our undertaking to
provide an improved technique for the debridement of burns which will
greatly reduce the time required for the procedure, impose significantly
less traumatic effect and enhance skin graft and healing of the wound. It
was our objective to utilize a laser to ablate the eschar of a burn wound
in such a manner that a high rate of removal would be realized without
significant injury to the viable tissue.
The above-mentioned dermatological CO.sub.2 laser is not capable of
achieving our objective because of the very low rate of ablation which can
be realized. The use of a CO.sub.2 laser of the TEA type was researched
with the objective of achieving a high rate of ablation by using a high
pulse repetition rate, say 100 pulses per second. The TEA laser delivers
an output energy of about three joules per pulse in a beam approximately
one centimeter in diameter. The fluence of deposited energy with this
beam, about four joules per square centimeter, is not enough to ablate the
surface layer of eschar. However, with an infrared transmitting lens, the
diameter of the beam impinging on the surface can be reduced sufficiently
so that the fluence, i.e. the joules per square centimeter, is high enough
to ablate the surface layer. It was found that the required fluence could
not be achieved with the laser without causing atmospheric breakdown which
was found to occur at a fluence of about ten joules per square centimeter.
The atmospheric breakdown and attendant electric discharge commences
within a small fraction of a microsecond after laser pulse initiation and
the resulting plasma shields the eschar from absorbing the remaining
energy from the laser pulse. Thus, the commercial TEA type of CO.sub.2
laser was found to be incapable of achieving our objective.
This led our investigation to the discovery that successful ablation can be
realized with a pulsed beam of correlated pulse duration and energy
fluence. With such correlation, a high pulse repetition rate can be used.
Further, it was realized that the laser beam energy is absorbed by only a
few wavelengths penetration of water-laden tissue and thus, the surgeon
could control the beam manually to ablate the eschar without significant
injury to the viable tissue.
SUMMARY OF THE INVENTION
We have discovered that the objectives of our invention are achieved by a
wound debridement technique in which a pulsed laser beam is caused to
impinge upon a layer of exposed tissue with individual pulses sufficiently
energetic to ablate the thin layer of tissue, each pulse having a time
duration short enough to cause the ablation but long enough so that it
does not cause atmospheric breakdown.
Further, in accordance with this invention, the pulsed laser beam is
produced by a CO.sub.2 laser having a wavelength in the far infrared
region. Further, with a CO.sub.2 laser, the individual pulses have an
energy of preferably about one joule per pulse or greater. Further, the
laser beam is produced with pulses having a duration between one
microsecond and ten microseconds. Further, the laser beam has a diameter
at the tissue so that a fluence of approximately ten joules per square
centimeter is realized. Further, the laser beam has a pulse repetition
rate in the range of approximately fifty to one hundred fifty pulses per
second.
A complete understanding of this invention may be obtained from the
detailed description that follows.
DESCRIPTION OF THE DRAWING
The single FIGURE of the drawing depicts apparatus useful for carrying out
the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
An illustrative embodiment of the invention will now be described utilizing
a pulsed CO.sub.2 laser for debridement by ablation of the eschar of a
burn wound. It will be appreciated, as the description proceeds, that the
invention may be realized in other embodiments and may be utilized in
various applications.
Referring now to the drawing, apparatus is illustrated for carrying out the
invention. The apparatus comprises a CO.sub.2 laser 10 and an optical
delivery system 12 in the form of an articulated surgical arm. The
delivery system 12 has its input coupled with the output of the laser 10
and includes a beam output head 14 which is adapted to be hand-held and
manually manipulated with complete freedom of movement, both angularly and
translationally. The laser beam 16 is emitted from the end of the head 14
and by manipulation thereof, the beam may be directed as desired. As
illustrated, the beam 16 is caused to impinge approximately
perpendicularly on the surface of the eschar 18 of a burn wound. The head
14 (or a foot pedal) is provided with a control switch, not shown, for
turning the beam 16 on and off. The surgical apparatus may also include
lighting on the head 14, and a means for controlling the pulse repetition
rate. Preferably, the apparatus is provided with means for clearing, from
the path of the laser beam, smoke or other debris resulting from the
ablation of eschar or the tissue. It is also desirable to blow away any
collected blood or other fluid in the area of impingement of the laser
beam. For this purpose, the beam output head 14 is fitted with an air
conduit 24 which is provided with an output nozzle 26 adjacent the laser
beam and directed across the surface of the wound. An air blower, not
shown, is connected with conduit 24. The nozzle is adapted to provide a
thin, laminar stream of air flow over the surface of the wound at the area
of beam impingement with a sufficiently high velocity to clear the air and
wound surface between laser pulses. For a pulse rate of about one hundred
pulses per second an air flow at the speed of about mach 0.1 or 0.2 is
desirable. In order to collect the smoke and debris entrained in the air
flow over the wound, a vacuum cleaner, not shown, is connected with a
conduit 32 extending to an intake opening 34 opposite the nozzle 26. Thus,
the beam path is cleared of smoke and other debris which would otherwise
adversely affect the transfer of energy from the laser beam to the wound.
The CO.sub.2 laser 10 emits pulses of infrared radiation at 10.6 microns
wavelength. The laser is capable of delivering individual pulses which may
be selectively preset to comprise energy in the range of several joules
per pulse. The laser may also be preset to produce pulses having a
duration between one and ten microseconds and to produce a pulse
repetition rate in the range of approximately fifty to one hundred fifty
pulses per second. The optical delivery system is provided with optical
focusing to vary the beam diameter at the surface of the tissue so that a
fluence of approximately ten joules per centimeter can be realized. The
infrared energy is uniformly distributed over the cross-section of the
beam within about ten percent variation. A laser with these capabilities
is available from Plasmatronics of Albuquerque, N. Mex.
In accordance with the invention, the method of wound debridement is as
follows. The CO.sub.2 laser 10 is operated to produce a pulsed beam with
individual pulses having an energy of about one joule per pulse or
greater. The beam is focused to produce a beam diameter at the tissue so
that a fluence of approximately ten joules per square centimeter is
realized. If the energy per pulse is increased, the beam size may be
increased to cover a larger area of tissue with the fluence being
maintained at approximately ten joules per centimeter. The laser is
operated with a pulse repetition rate in the range of approximately fifty
to one hundred fifty pulses per second with a pulse duration between one
microsecond and ten microseconds. If the pulse duration is much less than
one microsecond, atmospheric breakdown may occur and preclude ablation of
tissue. If the pulse duration is increased significantly beyond ten
microseconds, the result will be a very deleterious smoking and
cauterization, accompanied by heat penetration into the sublying living
tissue.
With the laser operating as set forth above, the surgeon manipulates the
beam head 14 to cause the beam to impinge on selected eschar to ablate the
eschar over the required area. Experiments using this method demonstrate
that eschar may be removed at a high rate without atmospheric breakdown.
Experiments were conducted on a fully anesthetized fifty pound piglet.
Four burns, each of one square inch, treated with this method permitted
remarkably successful skin grafts. The surgeon was able to learn the
technique in a matter of minutes even though the experimental optical
delivery system was rather crude. It was found that the surgeon had no
difficulty in controlling the beam in order to achieve satisfactory
ablation without significant damage to underlying viable tissue. It is
believed that the experimental work shows that the damaged layer is thin
enough so that the attendant subfusing of liquids is sufficient to
adequately "wash" the bed and thus increase the viability of the
subsequent skin graft. The damage level is believed to be less than a few
microns, an amount which has no clinical significance to subsequent skin
grafting.
As used herein, the term "ablate" is used to mean the blowing off of
tissues by the absorption of pulses of intense radiation such as that
produced by a laser. In the ablation process with short pulsed lasers a
large quantity of heat energy is absorbed almost instantaneously in a very
thin layer of the eschar which literally explodes. It is believed that
part of this is due to actual evaporation of surface tissue and part of it
arises from an enormous internal pressure developed by the vaporization of
water in the tissue which, in turn, literally explodes the material away
from the surface. The term "instantaneous" as just used means time periods
of one microsecond or so which are extremely short compared to the time it
takes for heat to diffuse into sublying tissue to the point where
significant damage could be caused.
Further, as used herein, the term "atmospheric breakdown" is used to mean a
phenomenon which occurs when the electric field component of a beam of
radiant energy becomes strong enough to cause actual electrical discharge
characterized by a flash of visible light and accompanied by a "cap pistol
sound". Atmospheric breakdown occurs when the intensity of a beam is in
the one hundred megawatt per square centimeter regime.
Although the description of this invention has been given with reference to
a particular embodiment, it is not to be construed in a limiting sense.
Many variations and modifications will now occur to those skilled in the
art. For a definition of the invention reference is made to the appended
claims.
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