Laser surgery method

A laser surgery method is disclosed for use in efficient ablation of tissue with little or no thermal damage to adjacent tissues. The wavelength of the surgical laser is tuned to an absorption peak of a proteinaceous material or functional groups contained therein, the amides for example. A suitable power level is chosen to either vaporize or liquify the targeted tissue.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the use of laser radiation as a 
therapeutic tool in medicine and surgery, and more particularly to the use 
of an infrared laser in the precision surgical ablation or cutting of 
tissue under conditions where minimization of damage to adjacent 
non-targeted tissues is required. 
Laser technology is currently used in clinical medical practice in a 
variety of applications, including as a surgical tool for the therapeutic 
ablation of human tissues, both internal and external. In some 
applications, the precision obtainable by a narrowly and accurately 
focused beam of laser radiation is superior to other more traditional 
surgical techniques. However, the use of lasers in certain areas, such as 
in the eye or brain, carries also the risk of thermal damage being done to 
sensitive tissues adjacent to the areas where tissue incision or removal 
is desired. 
Although prior art laser surgery techniques have recognized the problems of 
thermal damage to healthy tissues during laser surgery, none of the 
proposed solutions have been entirely satisfactory. A principal deficiency 
is that prior art laser surgery techniques have not employed the optimum 
non-photochemical wavelengths of laser radiation which produce ablation 
without thermal damage. The infrared (IR) region has been preferred over 
ultraviolet (UV) in many surgical applications because the IR wavelengths 
are non-photochemical in their effect on tissue and because laser 
radiation at some UV wavelengths has been reported to cause cell mutation. 
For example, CO.sub.2 lasers are in common surgical use and have a nominal 
operating wavelength of 10.6 microns. Unfortunately, experience has 
demonstrated that thermal damage to healthy adjacent tissue is a 
predictable consequence of the use of a CO.sub.2 laser to ablate tissue. 
Although selection of an appropriate pulse structure and duty cycle can 
improve the effectiveness of CO.sub.2 lasers in some applications, the 
damaged areas cannot be eliminated. 
Other investigators and practitioners have used Er:YAG or other solid state 
lasers in the infrared range, often tuned to a wave length which is known 
to correspond to an energy absorption band of water, at 2.94 microns for 
example. The theory behind methods which use such laser wavelengths is 
that because human tissue is approximately eighty percent (80%) water, the 
interaction of laser energy in water will also characterize the response 
of human tissues to infrared radiation of the same wavelength. The O--H 
stretch mode of water, which corresponds to 2.94 microns, is the most 
efficient absorber of IR radiation. This theory (and these prior art 
methods), however, fail to take into account and properly compensate for 
the method of energy transfer into a biomaterial, in this case human or 
animal tissue, which universally includes one or more proteins and related 
structures which both confine and are affected by water vaporization. 
Thus, the use of a laser surgery method which maximizes transfer of energy 
into the water component of human tissue by targeting the O--H stretch or 
other vibrational modes of water may produce rapid and effective ablation 
of tissue. The problem of heating of water and consequent thermal and 
dynamic effects on the structure of adjacent tissues remains. Accordingly, 
those experienced in the art have reported that ablation of tissue using a 
laser tuned to 2.94 microns is achieved by an explosive mechanism 
involving rapid heating, vaporization, and subsequent high-pressure 
expansion of irradiated tissue. It is believed, then, that this mechanism 
can cause thermal damage to collateral tissues from the hot gases 
produced, and tearing of those same tissues by pressures exerted by both 
the expanding gases and liquified or vaporized tissue from the target 
area. Similar thermal and mechanical effects have been reported at shorter 
wavelengths and at 10.6 microns. This ablation mechanism is particularly 
undesirable in situations, such as in delicate eye surgery, where 
precision liquification and extraction of tissue is preferred over 
explosive vaporization. 
What is needed, then, is a laser surgery method which can efficiently and 
precisely ablate a variety of human tissue types with controllable, and is 
some cases, little or no discernable thermal or mechanical damage to 
non-targeted adjacent tissues. Such a method is presently lacking in the 
prior art. 
SUMMARY OF THE INVENTION 
In the method of the present invention, a pulsed infrared laser is tuned to 
a wavelength which corresponds to a vibrational mode of protein in human 
tissue, and specifically to one or more of the specific wavelengths known 
to target different amide bands found in all proteinaceous material. A 
preferred laser power level, pulse structure, and beam size is selected 
and applied to the targeted tissue using a conventional focusing lens 
and/or laser catheter system. The targeted tissue is then vaporized, 
leaving relatively intact and undamaged the non-targeted tissues 
immediately adjacent to the area of ablation. The laser wavelength can be 
adjusted to allow for a procedure in which tissue liquification and 
removal is preferable to vaporization, or to enhance other clinical 
effects such as increased hemostasis, if desired. 
An object of the present invention, then, is to provide a laser surgery 
method of ablating tissue which can efficiently remove or liquify targeted 
tissue without causing significant damage to adjacent tissues. 
Another object of the present invention is to provide a method of laser 
surgery which controls the partitioning of energy between protein and 
water and does not focus solely on an energy absorption peak of water. 
A further object of the present invention is to provide for a laser surgery 
technique which is effective in ablating a wide variety of tissues using a 
single infrared laser wavelength.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In all human and animal tissues, certain functional groups are universally 
found in all proteinaceous materials regardless of the tissue type or 
location. One such functional group is the organic amides, whose 
characteristic molecular structure is R.sub.1 CONR.sub.2, where R.sub.1 
and R.sub.2 are amino acid side chains, CH.sub.3 for example. Experimental 
evidence in the prior art indicates that these proteinaceous amides (and 
other bio-polymers) exhibit a series of discrete vibrational modes of peak 
infrared energy absorption. It is believed that each such mode corresponds 
to a stretching (symmetric or asymmetric) or bending (in plane or out of 
plane) vibration of a specific set of bands in the protein. These amide 
vibrational modes and associated infrared radiation absorption peaks have 
been identified and assigned conventional reference names, as set forth in 
Table 1 below. 
TABLE 1 
______________________________________ 
Frequency Range of Vibrational Modes in Biopolymers 
Frequency Range 
Band (cm-1) (microns) Assignment 
______________________________________ 
Amide A 3300 3.0 N--H 
Amide B 3100 3.3 N--H (Fermi resonance) 
Amide I 1670 6.0 C.dbd.O, N--H, C--N 
Amide II 1560 6.4 C--N, N--H 
Amide III 
1300 7.7 C--N, N--H 
Amide IV 625 16.0 O.dbd.C--N 
Amide V 725 13.8 N--H 
Amide VI 600 16.7 C.dbd.O 
Amide VII 
200 50.0 C--N 
______________________________________ 
These energy absorption peaks have been confirmed experimentally using 
neural and ocular tissue subjected to radiation from a Fourier transform 
infrared spectrometer. FIG. 1 shows the relative IR absorption of sheep 
brain tissue as a function of laser radiation wave number, where wave 
number is the inverse of the wavelength. Specific absorption peaks are 
observed at the Amide I, II, and III bands. The energy absorption peak of 
Amide I (6.0 microns) is very close to a bending mode of pure water (6.1 
microns), as seen on FIGS. 1, 3 and 4. FIG. 2 shows similar results using 
four different types of ocular tissue. 
Accordingly, and looking now at FIGS. 9 and 10, to carry out the method of 
the present invention, a laser 10 (FIG. 9), in this case an FEL, is 
positioned so as to be focused on the tissue 30 to be ablated through a 
conventional fluoride lens 20, although any laser beam focusing or 
catheter system can be used. Tissue 30 is positioned at a distance 
approximately 10 to 20 cm from lens 20. Laser 10 is then tuned to 
correspond to one or more of the wavelengths of peak absorption of an 
amide band. The diameter of the focused laser beam is then adjusted 
according to the specific clinical application. A preferred laser pulse 
structure and power level is then selected and programmed into laser 10 in 
a conventional manner. As used herein, ablation can include cutting, 
liquification, or vaporization of tissue. 
In a first embodiment of the method of the present invention, the 
wavelength of laser 10 is adjusted to 6.45 microns, corresponding to the 
energy absorption peak of the Amide II vibration mode. Tissue 30, in this 
embodiment brain tissue, is then subjected to 100 macropulses at a pulse 
frequency of 4 HZ, with a power density of approximately 10 millijoules 
per macropulse. For purposes of the method of the present invention, using 
an FEL, a macropulse consists of a train of approximately 10.sup.4 
micropulses, with each micropulse having an average duration of 
approximately one picosecond and with approximately 350 picoseconds 
between each micropulse, such that each macropulse has a duration of 
approximately 6 microseconds. 
Looking now at FIG. 6, the results of using the laser energy method of the 
present invention to ablate neural tissue is shown. The cone shaped area 
50 of ablated tissue is defined by relatively smooth vertical walls 55, 
approximately 2.3 mm deep. Using a beam diameter of approximately 20 
microns, the diameter of the opening at ablated area 50 is approximately 
1.5 millimeters. FIG. 6 shows an absence of perceptible coagulation 
necrosis. FIG. 5 shows the results from using a laser surgery method on 
similar tissue, but with the laser tuned to 2.5 microns, a wavelength 
commonly used in prior art methods. After exposing the neural tissue to 
100 macropulses at a duty cycle of 4 HZ, with a power density of 
approximately 20 mJ per macropulse, considerable coagulation necrosis 
(indicated by the loss of cellular markings) is found from the surface to 
beyond the trough of the incision. 
In another application of this first embodiment, as seen on FIG. 8, laser 
ablation of corneal tissue using the method of the present invention is 
shown. After the FEL is tuned to 6.45 microns, one macropulse of 
radiation, having a power density of approximately 20 mJ, is directed to 
the surface of the tissue. The quality of incisions was superior and the 
amount of tissue denaturation was reduced at a wavelength of 6.45 
micrometers compared to other wavelengths. This type of ablation would be 
clinically useful in corneal refractive surgery, removal of superficial 
tumors of the cornea and conjunctiva, and could also improve the optical 
quality of corneal grafts. In contrast, FIG. 7 shows the results achieved 
by laser surgery ablation using a prior art wavelength of 3.0 microns, 
after a one macropulse (36 mJ power density) exposure. It is estimated 
that, in this first embodiment, at least twenty percent (20%) of the 
energy transferred to the tissue to be ablated is absorbed by protein in 
the tissue. 
In a second embodiment of the method of the present invention, tuning laser 
10 to a wavelength other than 6.45 microns (the amide II absorption peak) 
can be of clinical benefit as well, while still minimizing collateral 
tissue damage. For example, tuning to the absorption peak represented by 
the Amide III vibrational mode (approximately 7.7 microns) can allow the 
practitioner to liquify without vaporizing the tissue. Such liquified 
tissue is then available for conventional suction removal. This can be 
advantageous in certain delicate eye surgeries, for example. In this 
second embodiment, after the laser is tuned to 7.7 microns, the beam will 
be delivered to intra-ocular structures such as cataractous lens nucleus, 
or intraocular tumors, via waveguides or fiberoptics inserted surgically 
into the eye and lens. Following liquification, diseased tissue will be 
removed with standard intraocular aspiration techniques. In this second 
embodiment, substantially all (greater than 99%) of the energy transferred 
to the tissue is absorbed by protein in the tissue rather than water. 
Also, tuning laser 10 to the absorption peak represented by the Amide I 
band, which is near an absorption peak of water, should produce a somewhat 
enhanced thermal effect from water vaporization. Such limited thermal 
effect can be helpful in achieving good hemostasis where needed. 
In yet other applications, laser 10 can be tuned proximate to but slightly 
away from a wavelength corresponding to an amide band, such that the IR 
wavelength falls on a "wing" of an absorption peak rather than directly on 
the peak itself (see FIG. 4). With such slight "de-tuning", a variation in 
the ratio of energy being transferred to protein in the tissue as compared 
to water in the tissue can be effected. In such applications, as long as 
the de-tuned or "wing" wavelength does not fall on an absorption peak of 
water, the improved clinical results of the present invention can still be 
achieved. 
Other wavelengths corresponding to energy absorption peaks of different 
proteinaceous structures, or functional groups in proteins, even those 
other than the amide functional groups, may also be of benefit in 
particular clinical applications. 
Also, the laser surgery method described and claimed herein is not limited 
to use with a free electron laser. For example, a more conventional solid 
state laser or optical parametric oscillator can be manufactured and tuned 
to a fixed wavelength, 6.45 microns for example, and thereby be useful as 
a tool for tissue ablation in a variety of applications. It would also be 
within the scope of the present invention to simultaneously irradiate 
tissue with multiple laser beams of different wavelengths, each of which 
could be targeted to a different proteinaceous material absorption peak, 
for example. 
Thus, although there have been described particular embodiments of the 
present invention of a new and useful laser surgery method, it is not 
intended that such references be construed as limitations upon the scope 
of this invention except as set forth in the following claims. Further, 
although there have been described certain dimensions used in the 
preferred embodiment, it is not intended that such dimensions be construed 
as limitations upon the scope of this invention except as set forth in the 
following claims.