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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.

Pro:
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," he noted.

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.

It 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, he noted.

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.

Con:
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 case.

"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.

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