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.

ByTim Donald, ELS

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.

Jack T. Holladay

“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.

Dr. Holladay explained that surgeons can achieve excellent outcomes without customized treatment in the great majority of their refractive surgery patients. They 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.

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.

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.

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.”

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 modified their ablation profiles.

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.

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.”

For Your Information:
Jack T. Holladay, MD, MSEE, FACS, can be reached at the Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401 U.S.A.; +1-713-668-7337; e-mail: docholladay@docholladay.com; Web site: www.docholladay.com.

Reference:
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, ELS, is OSN Copy Chief.