Conventional and Wavefront LASIK
Let's take a look at the differences between the more
advanced wavefront guided excimer lasers and conventional
excimer lasers. Lets start with conventional excimer laser
basics, then I’ll explain wavefront-guided excimer
laser. Finally, we’ll compare the two and we’ll look
at the outcomes using as much objectivity as possible.
Maybe you’re thinking that all excimer lasers are
the same. The short answer is really a resounding “no”!
And once again, if you desire the very best possible
vision,
then you need to consider wavefront as an option if you’re
choosing any excimer laser procedure! But is it really
better than conventional LASIK? Lets take a look.
Just to briefly review,
the excimer laser is used to complete the laser ablation in
refractive surgical procedures such as LASIK, Epi-LASIK,
IntraLASIK, and
PRK. The excimer laser is a “cold” laser
that literally vaporizes corneal tissue as it precisely
reshapes the cornea, thereby permanently changing the
contour of the cornea so as to enhance one’s vision.
Conventional Excimer LaserWith conventional
excimer
lasers, the laser ablation is based purely on the patient’s
refractive error, that is, the pre-operative refractive
error. Using an example, if a patient presents for a LASIK evaluation, they generally are asked to bring their
glasses or
contact lenses and this prescription is recorded.
The patient then undergoes a typical refraction (you know
the routine… which is better, one or two?, three or four?,
etc. as you look through the phoropter) and the prescription
is re-determined and, hopefully, validated by the present
contact lens or glasses prescription. This procedure
is generally repeated again after dilation, which helps to
eliminate over-corrections in the myopic patient.
Then, when the patient returns at a later date for the
excimer laser assisted procedure (be it LASIK, Epi-LASIK,
IntraLASIK, etc.), the laser technician plugs in the numbers
from the refraction into the excimer laser and the treatment
proceeds based on these numbers and these numbers only.
When the surgeon steps on the pedal to begin the laser
ablation process, the complex algorithms of the excimer
laser take over to complete the laser ablation, but again,
are based on the numbers entered into the machine from a
simple refraction.
This treatment is based on lower-order aberrations (myopia,
hyperopia, and
astigmatism), the correction of which
certainly has the greatest impact on visual acuity.
However, conventional excimer laser treatment ignores
higher-order aberrations, which represent about 17% of the
optical error of the human eye. Are the higher-order
aberrations all that important then? With conventional
laser treatment, will the patient have the
vision he or she
hoped for? Could it be better? Even if higher-order
aberrations are analyzed, can they be treated? Enter wavefront-guided excimer lasers.
Wavefront-Guided
Lasers
Wavefront-guided LASIK is a promising newer technology
that provides an advanced method for measuring the optical
distortions of a given eye and taking these distortions into
account when the laser ablation procedure is completed.
This technology goes far beyond simple myopia
(nearsightedness), hyperopia (farsightedness), and
astigmatism determinations that have been used for centuries
in correcting vision. So how does it work?
Instead of just measuring a patient’s refraction and
plugging this information into the excimer laser, a
wavefront-analyzer may be utilized to determine a patient’s
unique ocular “fingerprint” and this same data helps guide
the laser ablation. To gather the wavefront profile, flat
waves of light are passed into the eye using a
computer-generated wavefront. These light waves are altered
as they are reflected from the
retina, the eye’s natural
lens, and the cornea, and finally, the light waves exit the
eye.
The wavefront analyzer then captures the reflected and
uniquely distorted waves of light that return to the
analyzer. The distorted light waves that exit the eye are
compared to the perfectly flat light waves that would have
been reflected if the eye had no optical distortions. As
such, the entire eye as a visual system is analyzed using
wavefront analysis.
Finally, the computerized algorithms generate a
3-dimensional map that represents the eye’s unique visual
distortions, the latter of which includes both lower and
higher-order aberrations. This map is then linked to the
excimer laser and is utilized to guide the laser treatment
in a highly customized way. Thus, the term “Custom LASIK”
is sometimes used, which truly should represent wavefront-guided
(WFG) LASIK. VISX Wavescan
Wavefront System (Courtesy of VISX)
Wavefront technology is obviously an
adjunct to excimer lasers that is being utilized to enhance
patients’ treatment, thereby, at least theoretically,
improving outcomes. Patients who undergo WFG LASIK
procedures are expected to have better vision. They
may have sharper, crisper, better quality vision, with a
reduction in halos and glare and better nighttime vision.
But do they, actually? Do the scientific studies
support this contention?
WFG vs. Conventional LASIK: Review
of Studies
On Monday, May 3, 2004, Steve
Schallhorn, M.D., the Director of Cornea and Refractive
Surgery at the Navy Medical Center in San Diego, reported
results obtained in their studies of conventional vs. WFG LASIK in military personnel. In brief, Dr. Schallhorn
found that “custom” or WFG LASIK yielded superior quality of
vision compared to conventional LASIK[i].
“While conventional LASIK is good, we’re finding that
wavefront-guided LASIK yields sharper and higher quality
vision, and higher patient satisfaction compared to
conventional LASIK. The improvement will likely be most
apparent when driving at night, “ said Dr. Schallhorn.
“We are finding that
custom LASIK, without the use of glasses, is giving many
patients the ability to see as well or better than they
could see with glasses before surgery. refractive surgery
{such as LASIK} plays a vital role in the military,” he
said. “By reducing dependence on glasses and contact lenses
in active duty personnel, it enhances combat readiness and
improves performance. It also expands the applicant pool to
talented young men and women for a variety of Navy
programs.”
Dr. Schallhorn added,
“Most significant is the improved quality of vision with the
wavefront-guided procedure, fewer problems with halos and
glare, better night vision, and higher patient satisfaction
based on a detailed patient questionnaire.”
Schallhorn’s findings were based on follow-up
results the Navy Medical Center obtained in 908 patients who
underwent conventional LASIK vs. 34 patients using WFG
LASIK. Eighty-eight percent of the conventional LASIK
patients achieved 20/20 or better vs. 97% of the WFG LASIK
group when tested six months post-operatively. Furthermore,
69% of the conventional LASIK patients had 20/16 or better
vs. 85% of the WFG LASIK patients. Dr. Schallhorn also
noted that 30% of the conventional LASIK patients reported
an increase in night driving halo symptoms compared to none
of the patients in the WFG LASIK group. However, most of
the halo symptoms abated in the conventional LASIK group by
the time they reached the three-month post-op time period.
According to the results of a prospective, nonrandomized,
comparative clinical study conducted at the Hadassah
University Hospital in Jerusalem, WFG LASIK results in
significantly improved contrast sensitivity compared with
standard LASIK at the one-month post-op interval[ii].
This was a very interesting study because both patient
groups were treated with the same excimer laser, namely the
WaveLight-Allegretto scanning spot laser. However,
only the WFG LASIK group was treated with the wavefront
analyzer connected, thus further minimizing any differences
between groups. Twenty-four eyes were treated with
conventional LASIK and 22 eyes were treated with standard
(conventional) LASIK.
Uncorrected visual acuity was very similar between
the groups: 72% of the eyes in the WFG LASIK group achieved
20/20 or better vs. 70% of the eyes in the standard LASIK
group. One month after LASIK, 88% of the contrast
sensitivity measurements improved in the WFG LASIK group
whereas only 40% of the contrast sensitivity measurements
improved in the standard LASIK group. The investigators
conclude, “The ability of WFG LASIK to correct optical
aberrations results in significantly improved contrast
sensitivity compared with standard LASIK one month after
surgery.”Marcelo Netto, M.D., William Dupps, M.D., Ph.D.,
and Steven Wilson, M.D. completed an extensive literature
review in an attempt to collate the vast amount of data on
conventional vs. WFG LASIK procedures[iii].
The group noted that more than 400 reports exist that
investigate wavefront applications in refractive surgery,
but they state that, “studies comparing the outcomes of
wavefront-guided treatment with conventional treatment are
few in number.” They state further in their review
that, “available studies do not overwhelmingly demonstrate
superior visual results attributable to a wavefront-guided
approach.”
The group concludes with the following: “While wavefront-guided refractive surgery provides excellent results, evidence is
limited that it outperforms conventional laser in situ
keratimileusis {LASIK} that incorporates broad ablation
zones, smoothing to the periphery (of the cornea),
eye-trackers, and other technological refinements. However,
it is evident that wavefront-customized ablation holds a
promising future and merits ongoing investigation.”
I have to agree with
the above, that to date, there is a relative paucity of
clinical studies that make head-to-head comparisons between
conventional LASIK and WFG LASIK. However, I would submit
that, at present, it appears that WFG LASIK does result in
somewhat better visual acuity outcomes and contrast
sensitivity improvement, especially in the early post-op
period.
In a recent publication
of Ocular Surgery News, world-renowned ophthalmologist,
optics expert, and refractive surgeon Jack T. Holladay,
M.D., FACS, of Houston, Texas, was interviewed on the
subject of modern excimer laser improvements and wavefront
technology[iv].
“There’s no question that wavefront-guided treatments are
better than the results we had before with conventional
ablation, but it has very little to do with the wavefront
measurements on the patients,” he said. Dr. Holladay
explained that, at the same time that some laser companies
made advancements in their excimer laser systems in terms of
wavefront technology, they also modified their laser systems
with respect to specific optical concepts and what Dr.
Holladay calls the radial compensation function, a term
which he developed to address the issue of the need for
alterations in laser energy (fluence) as the laser treats
the peripheral cornea.Prolate vs. Oblate Corneas and
the Effect on Vision
To fully appreciate Dr. Holladay’s extraordinary insight
on conventional vs. wavefront-guided laser vision
correction, I’ll need to give a brief explanation of some
basic concepts of corneal shape and its affect on vision.
First of all, according to Dr. Holladay, 99.9% of the
population normally has prolate shaped corneas. This
means that the cornea simply has a steeper curvature in the
center and a flatter curvature in the periphery.
However, the normal (unoperated) cornea also has a type of
higher order aberration (HOA) known as spherical aberration,
and in this case it is positive spherical aberration.
This occurs because the peripheral cornea brings light to a
different focal point than the central cornea. In the
younger eye, this positive spherical aberration in the
cornea is offset by an equal amount of negative spherical
aberration in the natural lens of the eye. In fact,
the eye has its best optical performance at age 19. As
we age, the lens of the eye begins to thicken and change
shape, even without the development of
cataract, and this
adds more positive spherical aberration to the eye. By
age 40, the lens of the eye has no spherical aberration and
by age 60 it has a substantial degree of positive spherical
aberration, which, when added to the positive spherical
aberration of the cornea, results in an overall degradation
of visual quality. “That is why people see so poorly
at age 60,” Dr. Holladay said. “Although their daytime
vision may be good through a small pupil, when their pupil
dilates and they have a black background, like driving at
night, they see halos around headlights.”
The Radial Compensation Function and Excimer Laser
Systems
Dr. Holladay went on to explain that the early versions
of most excimer laser systems did not address the spherical
aberration of the cornea. In fact, to treat myopia
(nearsightedness), the excimer laser must flatten the
central cornea and leave the peripheral cornea relatively
steep, which actually results in what is known as an oblate
cornea. This condition, in fact, adds to the positive
spherical aberration of the cornea, making spherical
aberration of most eyes worse!
This occurred in part because early excimer laser systems
were calibrated on flat surfaces rather than spheres and the
cornea is naturally more spherical, that is, dome-shaped.
When the excimer laser energy hits the peripheral cornea, it
removes less tissue because it is hitting the cornea at an
oblique angle. In the central cornea, where the
excimer laser hits the cornea perpendicularly, it removes
tissue as intended and as expected based on the
calibrations. Plus, when the laser energy strikes the
peripheral cornea obliquely, as it does in the peripheral
cornea, the energy is spread out over a larger shape (an
oval rather than a circle), which results in less energy (fluence)
being delivered. Less energy delivered means less
ablation than intended. This is all part of what Dr.
Holladay calls the radial compensation function.
Manufacturers of excimer laser systems did not appreciate
the effects of the radial compensation function in early
systems, and thus, the effective optical zone (area of
treatment) tended to decrease as the magnitude of intended
treatment increased. The greater the treatment in the
myopic eye, the greater was the discrepancy between the
intended amount of correction in the corneal periphery and
the actual correction achieved.
Wavefront Excimer Lasers and the Radial Compensation
Function
After Dr. Holladay delivered his keynote presentation on
the radial compensation function at the refractive surgery
Interest Group Subspecialty Day in 1999, several excimer
laser system manufacturers modified their laser ablation
profiles accordingly. Subsequently, the big three
excimer laser system manufacturers in the U.S., namely VISX,
Bausch and Lomb, and Alcon, all introduced their wavefront-guided
excimer laser systems. Each of these companies
reported improvements in their visual acuity outcomes over
their older conventional excimer lasers.
Dr. Holladay explained
that, in fact, there were improvements in visual acuity
after the introduction of wavefront technology. “But, as I
explained in my Barraquer Lecture last year (at the American
Academy of Ophthalmology’s annual convention), the three
factors that resulted in the biggest improvement for
wavefront had little to do with wavefront.” Dr. Holladay
stated that the foremost reason that the wavefront-guided
machines improved results is because they also addressed the
radial compensation function and included this in their
laser ablation profiles. They also enlarged the optical
zone in patients that were being treated for
astigmatism.
Finally, the ablation profiles created a smoother central
cornea by avoiding an older and fairly common condition
called “central islands”, the latter condition being one in
which a small, elevated island remained in the center of the
cornea following an excimer laser procedure.
“All three of those
things were included in the new wavefront treatment
protocols, so there’s no question that the wavefront-guided
treatments were better, but it had little to do with the
wavefront measurements on the patients,” Dr. Holladay said.
Which Type Of Excimer Laser For Your
Procedure: Conventional or Wavefront-Guided?
Taking into account Dr. Holladay’s
recommendations, it appears that one could have excellent
results with a conventional laser system, that is, without
wavefront-guided treatment. Wavefront-guided laser
vision correction should definitely be utilized for any
individual who has previously undergone a refractive procedure such as
PRK, LASIK, etc., and has aberrations of
the cornea that result in troublesome visual disturbances.
This is where WFG procedures would make a tremendous
difference. But if you have “virgin” eyes, then a
conventional laser system would be fine as long as that
machine was of very recent vintage and addressed the radial
compensation function. I believe that all of the
following laser manufacturers have addressed this with their
newest machines:
Alcon
AMO/VISX
Bausch and Lomb
Carl Zeiss Meditec
LaserSight
Nidek
Schwind
Wavelight
ConclusionMy final advice: If you choose an
excimer laser procedure, such as LASIK, Epi-LASIK,
IntraLASIK, or PRK, just be certain that your eye
surgeon is
using the latest version of one of the above excimer laser
machines, even if your procedure is completed with a
conventional (non-WFG) laser. If your procedure is
truly “custom”, which means wavefront-guided (WFG), you will
simplify this decision process because all of the WFG
machines are of the latest vintage and would utilize the
algorithms that take into account the radial compensation
function that Dr. Holladay so brilliantly conceived.
If I were having any one of these procedures (e.g., LASIK,
Epi-LASIK, IntraLASIK, or PRK) today, I would be certain
that my surgeon was using a WFG
excimer laser. It will
probably cost you a few hundred dollars more per eye, but
once again, it’s your eyes!
[i]
New “Custom” LASIK Individualizes Vision Correction
Study Shows Better Quality Results of Wavefront-guided LASIK. Program and abstracts from the American
Society of Cataract and refractive surgery 2004
Symposium on Cataract, IOL, and refractive surgery;
May 3, 2005; San Diego, CA.
[ii]
Kaiserman, I, et al., Contrast sensitivity after
wave front-guided LASIK. Ophthalmology, Mar. 2004.
Vol 111, Issue 3, pp 454-457.
[iii]
Netto, MV, Dupps, W., Wilson, S. Wavefront-guided
Ablation: Evidence for Efficacy Compared to
Traditional Ablation. Amer. J. Ophthalmology.
Published online Oct. 2005,
www.ophsource.org.
[iv]
Modern excimer improvements driven by more than
wavefront. Ocular Surgery News. Oct. 1, 2005.
Vol. 23, No. 19, pp. 5-6.
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