Experts debate value of customised
Sean Henahan in San Francisco
IS customised wavefront guided surgery the next important step in
laser refractive surgery or an overhyped technology with major technical
limitations? Two well-known ophthalmic surgeons assessed the current
status of custom ablation in a debate held during the annual meeting
of the American Society of Cataract and Refractive Surgery.
Custom ablation can improve quality of vision
"Wavefront sensing has revolutionised our thinking on ophthalmic
optics. Customised wavefront-guided ablation is gradually improving
our outcomes. The improvements in vision provided by the technology
are not as dramatic as we first thought. Rather, this approach offers
subtle improvement, with the main benefit being the correction of
spherical aberration. As we learn to better use the technology,
the improvements will increase. I would predict that within three
years, most laser refractive surgery will be done using customised
ablation," declared Scott MacRae MD.
He stressed that clinical trials provide ample support for the idea
that custom ablation represents a significant improvement over conventional
excimer laser refractive surgery. The reported uncorrected visual
acuity results for conventional treatment show that 40% to 80% of
patients achieve 20/20. In recent clinical trials, customised ablation
provided by equivalent systems provided 20/20 results in the 90%
or better range.
"There is no doubt that custom ablation has raised the performance
bar for refractive surgery. We are learning a lot. We initially
thought everyone was going to get super vision. There was a lot
of euphoria about the technology. We really didn't understand who
would benefit from customised ablation and who would not,"
Clinical experience has helped to elucidate who is likely to benefit
from the customised approach. Patients with high amounts of pre-operative
higher order aberrations stand to benefit most from customised ablation.
Citing his own results in the early trials with the Zyoptix system
he noted that patients with lower amounts of higher order aberrations
could end up with higher levels following wavefront-guided surgery.
"If you have a lot of higher order aberrations pre-operatively,
you are going to benefit significantly when treated with customised
ablation. This is the group we want to focus on," he stressed.
has become clear that patients with moderate to high degrees of
myopia, from -3D to -12 D who undergo conventional LASIK procedures
will get an increased amount of positive spherical aberration. This
is another patient group that can benefit from customised ablation,
The optical treatment zone also plays a role in the development
of spherical aberration. He noted that in a Zyoptix trial involving
patients with moderate myopia, those with a 6 mm treatment zone
saw an increased level of spherical aberration, while those with
a 7 mm treatment zone did not.
Patients with larger mesopic pupils also stand to benefit from the
custom approach. Other potential beneficiaries include myopic or
hyperopic LASIK patients presenting for retreatment, post-LASIK
PKP patients and those with irregular astigmatism, he asserted.
Custom ablation fundamentally limited
Wavefront guided custom ablation has not achieved its promise. This
can be attributed to fundamental limitations of the equipment being
used as well as the anatomy involved, responded Jack T. Holladay
MD, who presented the con side of the debate.
He enumerated several problems with the customised ablation technique.
One problem is that the lasers used to perform the surgery do not
compensate for the oblique incidence on the cornea, which results
in an oblate shape.
"The problems with the lasers are very simple. They haven't
been calibrated in a way that was based on a cornea shaped surface.
They were calibrated on a flat surface in which every photon of
excimer energy hits a perpendicular surface. This is not what occurs
on the cornea. We and other groups have shown that the amount of
energy that is applied in the periphery drops down to 28% less than
it should be. That is why we end up with a cornea that is oblate,
creating a blurred image on the retina."
Dr Holladay said that increasing the treatment zone to 7 mm or 8
mm instead of 6 mm is not going to solve the problems inherent in
custom ablation. The problem is that an increase of this size removes
twice as much corneal tissue. He contested the notion that this
would reduce induced spherical aberration, noting that the wavefront
analyser is set for a 6 mm aperture, so it simply doesn't measure
the aberrations of a larger pupil.
Another problem, he continued, has to do with the fact that corneal-based
wavefront correction does not take the effect of lenticular aberrations
into consideration. Correcting aberrations of the entire eye by
ablating the surface of the cornea assumes that the wavefront path
after it leaves the cornea goes through the same media and same
aberrations that it did before. This is not the case, and the result
is that the surgeon may have to retreat three or four times to get
near the desired result.
Additionally, correcting lenticular aberrations on the cornea creates
an extreme on-axis visual system:
"This means that as soon as you get slightly off axis, even
a couple of microns, it is like having a pair of keratoconus glasses.
It is perfect when you look down the barrel, but when you look one
micron off you can't see a thing. That is what's going to happen
if we begin to try and correct aberrations in the crystalline lens
because they are not lined up for off axis images."
The flap itself is another problem. He cited studies by Dr MacRae
and others showing that the creation of the flap itself induces
unpredictable amounts of astigmatism and higher order aberrations.
Cutting a flap causes irreversible damage to the associated collagen
fibres, resulting in an immediate worsening of RMS values.
Dr Holladay was also critical of the current eyetrackers upon which
customised ablation systems depend. Even the best tracker systems
today have a 5 to 10 millisecond delay. This means the laser spot
is 5 to 10 milliseconds behind where it is meant to be. This also
assumes that the patient is fixating perfectly, which is rarely
"The laser trackers are not perfect. The point is, you cannot
put the perfect treatment on the patient if you have a system that
is tracking because there is always a delay. It must have a fixation
where the patient can't move. Even if you have a tracker you can
still get a big decentred ablation zone if the patient is not looking
directly at the light."
Dr Holladay also criticised the way the custom ablation visual outcomes
are reported. He asserted that while these results are typically
reported in conventional Snellen acuity terms, this fails to convey
the effect of the surgery on contrast sensitivity, a much more useful
measure of visual quality. Contrast sensitivity typically decreases
following custom ablation, even if the drop is not as much as with
conventional refractive surgery.
He called on researchers conducting clinical studies with custom
ablation to be more forthcoming with data about the post-operative
quality of vision. In addition to contrast sensitivity comparisons,
wavefront derived metrics such as point spread function can provide
much useful information.