FEL surpasses traditional laser procedures in the surgical
world of otolaryngology
In the surgical world of otolaryngology, moving from continuous lasers to
pulsed lasers can require a quick eye and even quicker movements of the
hand - 30 movements a second to be exact. Complicating the matter is the
fact that any unintended or misplaced laser incision can have disastrous
results.
"The larynx is a difficult area to access and a difficult area for
healing but a phenomenally 
important organ in terms
of communication. The smallest bit of scar tissue can create voice problems,"
says Robert Ossoff, Guy M. Maness Professor and chairman of the Department
of Otolaryngology in the School of Medicine.
Lasers are used to help access the larynx for surgery. However, with the
laser there is the potential for thermal damage to the surrounding tissue.
This may result in delayed healing or excess scar tissue formation. Since
the first laser was used in the larynx nearly 30 years ago, otolaryngologists
have been in search of improved lasers.
A team from the Department of Otolaryngology is exploring the use of the
free-electron laser for laryngeal surgery. The fact that FEL energy comes
in pulses of light 30 times a second is good news. When the laser's beam
hits a target in the larynx, the energy does not pour in continuously and
overheat the tissue around the target site like some lasers. The pulsing
efficiently delivers the energy in a quick burst and then gives a short
time for the laser to move to a new site. The quick movement does, however,
pose dexterity problems for surgeons.
"My hand-eye coordination as a surgeon is not fast enough to achieve
maximum benefit from the way the FEL is delivered - in other words - taking
advantage of the unique characteristics of the FEL in terms of achieving
minimal thermal damage to the tissue," says Ossoff.
"Lasers are not new to laryngeal surgery," comments Gaelyn
Garrett, assistant professor of otolaryngology. "The advantage
of a laser in the airway is the ease of use in a tight space. Working with
several instruments through a narrow tube with a diameter of less than an
inch is, to say the least, cumbersome. The laser gives you a hands-free
way of making incisions.
"Looking at our specialty 10 years ago, people thought the laser was
going to be the ultimate surgical tool. But we have found that because of
heat damage with the carbon dioxide (CO2) laser you may actually end up
creating damage to the normal portions of the vocal cords, and a patient
may end up with permanent voice problems," Garrett says.
The wavelength of the CO2 laser causes vaporization of water and may damage
normal tissue around the surgical site. The FEL, offering a wavelength that
is absorbed by water and protein, lessens this problem.
One area of research for Garrett is subglottic stenosis, a narrowing of
soft tissue in the trachea caused by scarring due to prolonged intubation
or traumatic injury to the airway.
"It's almost like having a donut within the trachea," she says.
"Many of these patients end up having to have a tracheotomy performed
to bypass the area of stenosis and improve breathing. We are trying to avoid
the need for a tracheotomy for these patients."
The process of relieving this condition is done endoscopically through a
laryngoscope inserted between the vocal cords. Using the operating microscope
and a laser the surgeon notes the areas of narrowing, makes radial spoke-like
incisions in the scarred tissue, and then dilates the area. Often, more
than one procedure is required to maintain an adequate airway.
With the FEL, Garrett says, it is possible that these patients will not
have to have another procedure or at least fewer procedures than are required
now.
Restoring healthy voices
The FEL will also be studied for use on benign vocal cord lesions. Polyps,
nodules and cysts can develop on the vibrating surface of the vocal cord
as a result of chronic irritation from vocal overuse or abuse. Excision
of these lesions should result in the least amount of scarring and optimum
preservation of vocal cord vibration and function. The FEL coupled with
a new surgical techniques tool should provide high-precision cutting and
minimal collateral scarring.
In his initial experiments in the use of the FEL for bone surgery, Lou Reinisch,
assistant professor of otolaryngology, has demonstrated the enormous precision
with which the FEL can cut, but he has noted that manually controlled cutting
has to be done slowly to avoid thermal damage problems.
In a spirit of collaboration between scientists and physicians for which
the FEL Center has become famous, Reinisch worked with Marcus Mendenhall,
associate research professor of physics and FEL technical liaison, to find
a solution to the thermal damage problem. Reinisch and Mendenhall have developed
a tool they dubbed CAST for Computer-Assisted Surgical Techniques.
"With CAST, the surgeon controls the laser beam through a computer.
It is similar to the antilock braking system in automobiles. When a driver
pushes on the brake pedal, the driver signals a computer system to apply
the brakes, and how hard to apply the brakes. However, the computer decides
which wheel should get the braking signal and for how long. The driver is,
in effect, controlling the brakes through a computer. In a very similar
fashion, the surgeon decides where to make the incision and the length of
the exposure time. Also with CAST, the computer reproduces the surgeon's
incision pattern with a series of step sizes to minimize lateral thermal
damage," Reinisch says.
CAST uses a computer that controls two motors. These motors deflect mirrors
to move the laser beam horizontally and vertically. There is also a high-resolution
video camera that surveys the surgical field and displays the real-time
image on the computer screen. To move the laser beam, physicians learn to
master a "joystick" reminiscent of computer games.
"Having the computer position the laser has two advantages," Reinisch
says. "Computer control can make a straight-line incision - even with
great care surgeons can't make a very accurate straight-line incision -
and the motion of the laser is reproducible.
"For research it is important to be able to make the same incision
many times and always move the laser beam at the same rate. The computer
can cut a particular line or shape while jumping around 30 times a second
to different locations. Not only is the pattern reproducible, the jumping
allows tissue to cool before the tissue next to the ablation site is irradiated."
Guiding light
Using computers to assist in the delivery of the laser also affords other
potential advantages and controls. Acoustic feedback can monitor the depth
and penetration of the laser cut.
For example, surgery on the sinuses is tricky because the surgical field
is viewed through an optical fiber bundle. When using the laser in the sinuses,
it is common for the optical fibers to become coated with blood and it becomes
difficult to see, Reinisch says.
"Instead of trying to see what the laser is cutting through, we listen.
A microphone can be placed on the patient's face and the computer prevents
the laser from damaging the soft tissue behind the bone, the soft tissue
of the eye or the brain.
"With the microphone the laser can tell if the laser is hitting bone
- a hard popping sound - or soft tissue, which is more of a thud. The computer
determines when the laser has penetrated the bone and avoids that spot again
as it makes the incision. The computer automatically stops the laser when
it hears the sound at the end of the bone," Reinisch says.
Another example is that of tissue movement: when tissue moves with respiration
and heart beat, the straight-line laser incisions are no longer straight.
With CAST, a tracking system to anticipate and compensate for motion during
the surgery is to be developed.
"To attack this problem we put a marker on the tissue near the place
the incision will be made and have the computer watch the marker to see
how the tissue moves through three breathing cycles. After three breathing
cycles, the computer predicts the tissue motion before the next laser pulse.
The computer directs the laser beam to compensate for that motion. Throughout
the laser cutting process the computer continues to make such adjustments,"
Reinisch says.
Jeff Davidson, a pathology professor, is also working in collaboration with
the team from otolaryngology. He is focusing on cellular effects of the
FEL as they relate to wound healing. "It is important to have someone
like Jeff on our team," remarks Ossoff.
The ultimate implementation of the FEL for surgery involves a tri-level
research approach. Each technique is investigated at the molecular and cellular
levels in addition to the surgical trials on the living organism.
"The laser still holds promise for otolaryngologists. I see the shortcomings
of the current technology, and we are working on ways to overcome these
problems. With the FEL we will move beyond the current technology,"
Garrett concludes.
-Ellen Bourne
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This document created November 18, 1996