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Modern excimer improvements due to more than wavefront technology, optics expert says
A better understanding of the radial compensation function and other optical concepts has helped to improve LASIK outcomes.
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The latest generation of excimer laser systems for refractive surgery has greatly improved the accuracy of refractive outcomes over earlier versions, but much of that improvement is due to optical concepts other than wavefront-guided correction, according to an optics expert.
“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,” said Jack T. Holladay, MD, MSEE, FACS.
According to Dr. Holladay, modern excimer laser systems that take into account what he calls the radial compensation function and other optical concepts can produce excellent refractive results even without customized wavefront-guided treatment. He said it was the incorporation of these other optical concepts in modern laser systems at the same time that wavefront-guided treatment was introduced — and not the wavefront-guided protocols themselves — that has resulted in the improved results with the latest-generation technologies.
In this installment of the Quality of Vision series in Ocular Surgery News, Dr. Holladay explains that surgeons can achieve excellent outcomes without customized treatment in the great majority of their refractive surgery patients. (For other articles in this series, go to www.osnsupersite.com and key in "Holladay + QOV" in the search box.) Surgeons can then reserve wavefront-guided treatments for those who can benefit most from the customization — most often, patients with unsatisfactory results from previous refractive surgery, he said.
“The optics of excimer laser surgery is extremely important for ophthalmologists to understand if we are to move forward in terms of the quality of vision of our patients,” Dr. Holladay said.
Preserving the prolate
The cornea has a prolate shape in “99.9% of people,” Dr. Holladay said. He explained that this means the cornea has a steeper curvature in the center than in the periphery. Although the normal cornea is prolate, the normal cornea still has positive spherical aberration, which is balanced in a young person by an equal amount of negative spherical aberration in the crystalline lens, he noted.
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(For a thorough discussion of spherical aberration and the human visual system, see the third installment in the Quality of Vision series, “Aspheric treatments reduce spherical aberration after cataract, refractive surgery,” June 15 issue, page 1.)
“For young people, up to age 20, that coupling of +0.27 µm spherical aberration in the cornea and –0.27 µm in the crystalline lens balances out to where young people have little ocular spherical aberration, and the best performance of the human eye is at age 19,” Dr. Holladay said.
As people age, however, the crystalline lens becomes more positive in spherical aberration, he said, so that by age 40 years there is no spherical aberration in the crystalline lens, and by 60 years there is about +0.27 µm. Consequently, the spherical aberration in the entire eye goes from zero at age 20 to about +0.54 µm by age 60.
“That’s 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.”
Radial compensation function
Because of this change, refractive surgical treatments must be designed not only to correct sphere and cylinder, but also to address the eye’s spherical aberration, Dr. Holladay said. But early versions of refractive laser systems not only did not take spherical aberration into account, they often made it worse, resulting in an oblate cornea, he said.
Early excimer systems, for several reasons, tended to remove less tissue than intended peripherally, resulting in greater positive spherical aberration in the cornea. Those early systems ablated less efficiently in the periphery of the cornea because they were not calibrated on spheres, Dr. Holladay said.
“They were calibrated on flat surfaces, but the cornea is a dome. The only time the laser hits the cornea perpendicularly is at vertex normal — the apex of the cornea — and from then on out the effectiveness of the laser diminishes because it’s hitting the surface obliquely,” he said.
The oblique incidence of the laser on the periphery of the cornea causes a decrease in the efficacy of ablation for several reasons, he said. First, the corneal surface reflects more and transmits less light when the light strikes it obliquely. Second, whether delivered in a broad beam or a flying spot, the energy is spread out over a larger area (an oval rather than a circle), so the effective fluence delivered to the cornea is diminished. Third, the reduction in tissue removal is greater than the reduction in fluence.
“When you cut the fluence by 25%, you remove a lot less tissue,” he said. “You may remove only 50% of what you intended. So the result is, the reduction in tissue removal is greater than would have been predicted by just the difference in reflectance, transmittance and delivered fluence to the cornea. It’s all part of what I call the radial compensation function.”
Because manufacturers did not anticipate the effect of this radial compensation function, the effective optical zone of early excimer systems tended to decrease with greater amounts of refractive correction, he said. The greater the treatment, the greater the discrepancy between the intended amount of correction in the periphery and the actual correction achieved.
“Spherical aberration is the most significant aberration that we have to deal with after sphere and cylinder. But not only did we not reduce it, we actually made it worse with standard treatments because there was no radial compensation function in standard treatments with lasers used in the United States,” he said.
Improvement introduced
Dr. Holladay noted that after he first described the radial compensation function in a keynote speech at the Refractive Surgery Interest Group Subspecialty Day in 1999, several excimer laser system manufacturers outside the United States modified their ablation profiles.
“Laser manufacturers like Schwind, Meditec, WaveLight, LaserSight, Nidek, all empirically, through an iterative process, refined their standard treatments to compensate for this peripheral reduction in laser energy,” he said.
During the same period of time, the concept of wavefront-guided customized refractive surgery was introduced and generated a lot of interest among the ophthalmic community.
Dr. Holladay noted that when the three main U.S. ophthalmic excimer laser manufacturers, Visx, Bausch & Lomb and Alcon, introduced their wavefront-guided systems, they reported improvements in visual acuity outcomes over their older, conventional excimer refractive systems.
“And in fact we did get better outcomes when wavefront came along,” he said. “But as I explained in my Barraquer Lecture last year, the three factors that resulted in the biggest improvement for wavefront had little to do with wavefront.”
Delivering the Barraquer Lecture at the American Academy of Ophthalmology meeting last year, Dr. Holladay outlined the three reasons he believes that modern wavefront-guided lasers achieve better refractive results than conventional treatments.
One reason is that the U.S. manufacturers have now taken the radial compensation function into account and included it in their ablation profiles.
The second is that the manufacturers have been required by the Food and Drug Administration to specify the minor axis of astigmatism as the size of the optical zone in astigmatic treatments, which are oval.
Dr. Holladay explained: “With the previous standard systems, if you wanted to do a plano -4.00 X90 treatment, when you specified a 6-mm optical zone on the laser, you’d actually get a 4.5-mm-by-6-mm optical zone. But the 4.5-mm axis was the one that was critical. A few years ago I pointed out to the FDA that the smaller dimension was the one that was critical, and they began to require subsequent software to consider the smallest dimension as the effective optical zone. So previously when you did a plano -4 X90 with a standard treatment you got a 4.5-by-6-mm optical zone. But when you do it with wavefront now you get a 6-by-7.5-mm optical zone, and that’s not a fair comparison between the two types of treatment.”
The third reason is that the manufacturers have learned how to create a smoother central cornea and avoid the central islands that older systems sometimes induced, he said.
“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.
Dr. Holladay said the introduction of the WaveLight Allegretto laser to the U.S. market has highlighted his point. In U.S. clinical trials for approval of the WaveLight conventional ablation protocol, he said, the laser achieved results similar to the custom platforms of other manufacturers because the company had implemented all three of these new developments into their standard software.
“WaveLight was the first laser company in the U.S. to include the radial compensation function in its conventional protocol. Plus they specified the minor axis of the optical zone and already had good refinement of the central cornea. So the FDA studies with the WaveLight standard treatment were equivalent to the outcomes that we got with wavefront treatments with the other lasers.”
What to do
Wavefront-guided treatment is still valuable in certain patients, Dr. Holladay noted, and ideally it could be reserved for them while conventional treatments are used for the great majority of refractive surgery patients.
“In people who have significant higher-order aberrations in the cornea, it’s beneficial to do wavefront-guided treatment,” he said. “This is normally people who have had poor outcomes from previous laser treatment. The wavefront-guided treatment can provide improvement in people who have moderately aberrated eyes.”
He cautioned that treatment on the cornea of an aberration in the lens can actually make the problem worse, he said.
“If you have trefoil in the crystalline lens, which is usually where it is, if you try to treat that on the cornea, it only works for a star, one point in space,” he said. “You shouldn’t be correcting lenticular aberrations above spherical aberrations in the cornea. If they’re there, sometimes lensectomy would be a better operation.”
Even astigmatic correction on the cornea can be problematic if some of the astigmatism is in the lens, he said. This is why many experts consider a large amount of lenticular astigmatism to be a contraindication to corneal refractive surgery, he said.
For Your Information:Reference:
- Jack T. Holladay, MD, MSEE, FACS, can be reached at the Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401; 713-668-7337; 713-668-7336; e-mail: docholladay@docholladay.com; Web site: www.docholladay.com.
- Holladay JT. Barraquer Lecture: Optical quality after refractive surgery: Corneal vs. phakic IOLs. Paper presented at: American Academy of Ophthalmology and European Society of Ophthalmology Joint Meeting; October 26, 2004; New Orleans, La.
- Tim Donald, OSN Copy Chief, is writing the QOV series.