Method for laser surgery

A method of laser surgery, comprising the steps of selecting lasers whose output radiation has appropriate extinction lengths in the tissue to be ablated, coagulated, and/or shrunk, and directing radiation from those lasers coaxially and substantially simultaneously at the tissue.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to a method for performing laser surgery and, 
more particularly, to a method for simultaneously ablating, coagulating, 
and/or shrinking biological tissue. 
Directing coherent radiation from a laser at a target is a well known 
method for precisely cutting that target by ablating or vaporizing a 
portion of it. When the target is living biological tissue, the dynamic 
nature of the target poses special problems. For example, fluids such as 
blood may flow into the area of the cut, obscuring that area and absorbing 
part of the energy that otherwise would go into ablating the target. 
This problem can be mitigated by directing beams of coherent radiation of 
two or more wavelengths at the tissue, one beam to ablate the tissue and 
the other to perform some other action, such as coagulating small blood 
vessels to prevent inflow of blood. For example, Freiberg, in U.S. Pat. 
No. 5,139,494, which is incorporated by reference for our purposes as if 
fully set forth herein, advocates using radiation in a range of 
wavelengths between about 0.1 and about 0.3 microns, and between about 2.0 
and about 12.0 microns, for ablative cutting, and radiation in a range of 
wavelengths between about 0.3 microns and about 2.0 microns for 
coagulation. These beams of coherent radiation are directed coaxially at 
the tissue to be cut. Suitable means for combining laser beams coaxially 
are well known in the art. One such means is disclosed by Nakajima in U.S. 
Pat. No. 4,408,602. Another is disclosed by Jako in U.S. Pat. No. 
4,503,854. Both of these patents are incorporated by reference for all 
purposes as if fully set forth herein. 
Among the surgical procedures, to which laser surgery may be applied are 
skin resurfacing and hair implantation. In skin resurfacing, the upper 
layer of skin is ablated by a first laser beam while the underlying 
collagen is coagulated and shrunk by a second laser beam. In hair 
implantation, the accuracy of the drilling of holes for the implantation 
of new hair using a first laser beam is enhanced by the use of a second 
laser beam to coagulate small blood vessels and prevent inflow of blood. 
Both of these procedures are very delicate and require precise selection 
and control of the wavelengths, intensities and durations of the laser 
beams. 
There is thus a widely recognized need for, and it would be highly 
advantageous to have, a more precise method for using lasers to perform 
delicate surgical procedures such as skin resurfacing and hair 
implantation. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a method for surgical 
alteration of skin tissue by simultaneous ablation, coagulation, and 
shrinkage, comprising the steps of: (a) selecting a first coherent 
radiation source characterized by emitting a first coherent radiation 
having an extinction length in the skin tissue of between about 0.01 
millimeters and about 0.001 millimeters; (b) selecting a second coherent 
radiation source characterized by emitting a second coherent radiation 
having an extinction length in the skin tissue of between about 0.1 
millimeters and about 0.01 millimeters; (c) directing a first beam of the 
first coherent radiation at the skin tissue; and (d) directing a second 
beam of the second coherent radiation at the skin tissue, substantially 
coaxially and substantially simultaneously with the first beam. 
According to the present invention there is provided a method for surgical 
alteration of skin tissue by simultaneous ablation, coagulation, and 
shrinkage, comprising the steps of: (a) selecting a first coherent 
radiation source characterized by emitting a first coherent radiation 
having an extinction length in the skin tissue of between about 0.01 
millimeters and about 0.001 millimeters; (b) selecting a second coherent 
radiation source characterized by emitting a second coherent radiation 
having an extinction length in the skin tissue of between about one 
millimeter and about 0.1 millimeters; (c) directing a first beam of the 
first coherent radiation at the skin tissue; and (d) directing a second 
beam of the second coherent radiation at the skin tissue, substantially 
coaxially and substantially simultaneously with the first beam. 
The criteria for selecting the parameters for delicate laser surgery on 
skin tissue are the desired physical effects. The ablative laser beam 
should be strongly absorbed by the target tissue, so that the ablative 
effects of the laser beam are confined to the target tissue. Furthermore, 
the pulse duration should be shorter than the thermal relaxation time of 
the target tissue, to prevent thermal damage to adjacent tissue, while the 
pulse intensity should be sufficiently high to achieve the desired 
ablation. In skin resurfacing, the laser beam used to shrink the collagen 
should not be significantly absorbed in the overlying skin, but should be 
absorbed by the collagen. In hair implantation, the laser beam used should 
be absorbed only to an extent sufficient to coagulate the capillaries that 
are cut by the ablative laser beam. 
The present invention successfully addresses the shortcomings of the 
presently known procedures for skin resurfacing and hair implantation by 
providing an appropriate range of wavelengths, pulse durations, and pulse 
intensities for the laser beams used therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is of a method for delicate laser surgery. 
Specifically, the present invention can be used for precision skin 
resurfacing and hair implantation. 
The principles of precision laser surgery according to the present 
invention may be better understood with reference to the drawings and the 
accompanying description. 
Referring now to the drawings, FIG. 1 is a graph of the extinction length 
in water of infrared radiation of various wavelengths. Because skin tissue 
is 77% water by weight, water can be used as a proxy for skin tissue in 
selecting wavelengths for surgery. 
To minimize peripheral damage, the extinction length of coherent radiation 
used for ablative cutting should be as small as possible. According to 
FIG. 1, this length is between 0.01 millimeters and 0.001 millimeters. 
Note that the corresponding range of wavelengths is between about 2.5 
microns and about 3.2 microns. This range is substantially narrower than 
the 2-12 micron range recommended by Freiberg for ablative cutting. The 
2.94 micron radiation of an erbium YAG laser has an extinction length in 
this range. The thermal relaxation time of human skin tissue is 
approximately one millisecond. Thus, the laser pulse duration should be no 
longer than this, and preferably about 0.3 milliseconds. The energy 
density of each pulse preferably is between one Joule per square 
centimeter and 50 Joules per square centimeter. 
The extinction length of coherent radiation used for coagulation of small 
blood vessels should be somewhat longer than the extinction length of 
coherent radiation used for ablation, to spread the heating effect of the 
laser beam over a larger depth range than is used for ablation. The intent 
here is merely to coagulate the blood, not to vaporize it. Between 0.1 
millimeters and 0.01 millimeters is an appropriate extinction length for 
coagulation. The 10.6 micron radiation of a carbon dioxide laser has an 
extinction length in this range. 
The laser beam used for coagulation may be either continuous or pulsed, as 
long as the duration of the coagulation beam substantially overlaps the 
duration of the ablation beam, as shown in FIGS. 2A, 2B, and 2C. In the 
four plots shown in these Figures, time T is the abscissa and beam 
intensity E is the ordinate. FIG. 2A shows separate firing schedules for 
an erbium YAG ablation laser and a carbon dioxide coagulation laser in a 
preferred embodiment of the present invention in which the carbon dioxide 
laser is a continuous wave laser. The erbium YAG laser emits periodic 
pulses. The carbon dioxide laser fires continuously. The total laser 
output is the superposition of these two outputs, as shown in FIG. 2B. 
Preferably, the power level of the carbon dioxide laser is sufficiently 
high to coagulate the blood vessels cut by the erbium YAG laser in between 
pulses of the erbium YAG laser, but not sufficiently high to cause 
peripheral damage by unwanted ablation. The preferred power density for a 
continuous wave carbon dioxide laser is between one Watt per square 
centimeter and 10 Watts per square centimeter. 
FIG. 2C shows the combined output of the erbium YAG laser and the carbon 
dioxide laser in a preferred embodiment of the present invention in which 
both lasers are pulsed. Note that the duration of each carbon dioxide 
laser pulse overlaps, and extends substantially beyond, the duration of 
the corresponding erbium YAG laser pulse. Again, the object here is to 
coagulate the blood vessels cut by the erbium YAG laser without causing 
peripheral damage by unwanted ablation. The preferred carbon dioxide pulse 
duration is between one millisecond and 10 milliseconds, and the preferred 
power density is between one Watt per square centimeter and 100 Watts per 
square centimeter. 
The extinction length of coherent radiation used to shrink collagen 
preferably should match the thickness of the target collagen layer, which 
may be as thick as about one millimeter. Collagen thinner than about 0.1 
millimeters is shrunk by a laser appropriate for coagulation, for example 
a carbon dioxide laser. Thicker collagen is shrunk by a laser whose 
radiation has an extinction length of between about one millimeter and 0.1 
millimeters. The 2.12 micron radiation of a holmium YAG laser has an 
extinction length in this range. The shrinkage laser beam may be 
continuous or pulsed. Preferred pulse durations for a holmium YAG laser 
used to shrink collagen are between 0.3 milliseconds and one millisecond, 
and the preferred pulse energy density is about one Joule per square 
centimeter. 
The range of wavelengths useful for laser surgery, as shown in FIG. 1, is 
in the invisible infrared. In preferred embodiments of the present 
invention, a third, low power beam of visible coherent radiation is 
directed coaxially with the other two beams, so that the surgeon can see 
where the beams strike the patient. 
While the invention has been described with respect to a limited number of 
embodiments, it will be appreciated that many variations, modifications 
and other applications of the invention may be made.