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Customised wavefront-guided ablation: exciting
technology but beware the hype
Dermot McGrath in Bordeaux
WAVEFRONT-GUIDED customised ablation is an exciting addition to
the modern refractive practice, but surgeons should be aware of
its limitations and not overstate its potential for fear of a backlash
if patient expectations are not met, Terrence P O' Brien, MD warned
delegates here at the Cornea 2003 meeting.
"As we contemplate the new wave in ophthalmology we would do
well to remember the past and the way advances in technology generated
unrealistic expectations about what the majority of patients could
expect from refractive treatment.
"Over the last two years, all we have been hearing in the United
States is about this new wave that was coming that would suddenly
take care of all the problems of conventional refractive surgery
and reduce the aberrations that our current procedures are creating.
It is exciting technology but we have to keep it in its proper perspective
and know its limitations," he stressed.
Dr O Brien said that wavefront technology was to be welcomed for
the additional information it provided about the refractive state
of the eye, particularly the higher order aberrations that were
often induced by conventional LASIK treatments.
He explained that wavefront technology provides a detailed 'fingerprint'
of the eye. This gives a very accurate picture of the refractive
state along with higher order aberration information that can be
used to first learn why an individual patient might be unhappy with
their prior refractive surgery and then to plan a treatment to help
reduce those aberrations.
However, Dr O' Brien warned that all the talk of "super vision"
could plant unrealistic expectations in the minds of patients and
the profession needed to be careful how it marketed the benefits
of custom ablation to the general public.
"Patients today are very well informed and are reading about
this technology on the Internet. I think we have to remember as
refractive surgeons the formula for success - patient satisfaction
equals the clinical outcome minus the expectation of the patient.
So even if we are getting better outcomes with wavefront guided
systems, if their expectations are for super-normal vision, the
overall satisfaction level will go down," he said.
Limits of visual correction
He noted that factors such as the eye's photoreceptor spacing and
pupil diameter meant that there were limits on what could be achieved
by customised ablation. This means there is a physical limitation
somewhere near 20/10 to 20/8. While there is little the surgeon
can do about pupil diameter and photoreceptor spacing, tackling
the higher order aberrations could allow the surgeon to take the
normal vision curve and move it up closer to the retinal limits
by eliminating these ocular aberrations, producing better results.
Dr O' Brien said that the aims of custom cornea treatment should
be clearly defined and explained to patients in advance, so they
understand exactly what they can expect from the procedure.
Refractive surgeons today would like patients to have uncorrected
vision of at least 20/20 or better and improve on the best corrected
vision from the preoperative state, reduce or eliminate the induced
aberrations and reduce problems of vision especially under mesopic
or scotopic conditions.
The greatest gain with customised treatment over conventional LASIK,
in his view, lay in improving or maintaining contrast sensitivity,
especially under difficult conditions, not necessarily an improvement
in visual acuity.
"We want to try to preserve or improve upon contrast sensitivity,
which our current conventional procedures often limit. This is the
methodology we used in the U.S. FDA investigational trials with
the Visx CustomVue system. We obtain the accurate refraction state
and also the higher order aberrations. For the clinical trials,
we then put this information back into the laser and created a PreVue
lens so that the patient could assess the quality of vision prior
to undergoing the treatment."
Dr O' Brien said that advances in laser technology with beam shaping
and delivery meant that surgeons could "create just about any
shape in the cornea" and take care of defocus and higher order
aberrations in one procedure.
He noted that the VISX Star S4 system he uses combines the benefits
of the larger diameter excimer beam for rapid correction of the
defocus and then employs variable spot scanning to correct for the
higher order aberrations. He compared it to using a paint roller
sponge for the lower order aberrations and then a fine brush to
take care of the higher order ones.
Advances in eye tracking
He cited advances in eye tracking devices as critical to achieving
successful custom cornea treatments, by ensuring proper centration
of the advanced ablation profile over the entrance pupil.
"We have seen advances in eye tracking that allow these sophisticated
treatments to be placed over the entrance pupil, so that even with
micro-saccadic eye movements, the higher-order treatment can be
targeted to the right location. This is critical as we move into
the customised type of treatment, to ensure that the ablation is
placed in the proper location. It is a bit like a prime tailor,
removing a little extra here, backing off a bit there, in order
to achieve a result. This requires the precision of a small spot
laser guided by highly accurate eye tracking system."
Reflecting on the clinical trial data for the VISX CustomVue, Dr
O' Brien said that the results had demonstrated that for the first
time surgeons are close to being able to allow patients a 100% chance
of achieving 20/20 uncorrected vision.
"The gratifying thing was the improvement in quality as well
as quantity of vision. Most of the patients were highly satisfied
with the outcome and very few complained of problems of glare or
halos or difficulties with night vision. The results also showed
that even in dim light there was a preservation of contrast sensitivity
compared to the pre-operative state and sometimes even an improvement,"
he said.
Safety good with customised approach
The safety data was also gratifying, said Dr O' Brien, as over two-thirds
of patients experienced a gain in BCVA and there were very few eyes
losing more than a line of BCVA. Adverse advents were minimal in
the study and they did not lead to any loss of vision.
"It's exciting that with a single step, with non-nomogram adjusted
optical customisation, we can achieve very good results. Today we
have better measurements of the refractive state with wavefront
analysis and improved methods of beam shaping and delivery, but
we must always realise that all of these manipulations are taking
place in a biological system, the cornea. We really need to understand
the working of the cornea more fully and control wound healing at
the gene level before we can make significant advances," he
concluded.
Terrence
P. O'Brien, MD, Director, Refractive Eye Surgery
Wilmer Eye Institute
Johns Hopkins University School of Medicine
Baltimore, Maryland, USA
tobrien@jhmi.edu
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