June 2007

 

REFRACTIVE SURGERY

 

Reducing HOAs to improve quality of vision


by David Laber EyeWorld Staff Writer

 

 

 

Ophthalmologist lays out his plan for targeting zero aberrations and choosing the best aspheric IOL

Pre-op wavefront map of a patient with minimal aberrations.

Post-op wavefront map of a patient with induced spherical aberration.

Source: Brian Boxer Wachler, M.D.

Ophthalmologists should be targeting zero higher order aberrations (HOAs), and numerous intraocular lenses (IOLs) are available to help obtain this goal.

Steven C. Schallhorn, M.D., former director of cornea and refractive surgery, Naval Medical Center, San Diego, recently presented, “Targeting emmetropia and zero aberrations: Choosing the optimal aspheric IOL and refractive procedure for visual quality success.” In this presentation, Dr. Schallhorn described his method to determine the correct target and then choose an IOL.

Gaining better understanding of HOAs

“I think it’s important to understand the target type and amount of higher order aberrations (HOAs) that we should be striving for in surgery,” Dr. Schallhorn said. “That is really central to this discussion.”

He said there are three ways to get a better understanding of how HOAs affect vision: visual modeling, studying the visual effect of inducing HOAs, and understanding the HOAs and visual performance of patients with excellent vision.

Visual modeling demonstrates that the best retinal image quality is obtained with the least amount of aberrations, Dr. Schallhorn said.

There can be a coupling effect between aberrations, but if an eye has no sphere or cylinder, we would want to have the least amount of HOA, especially spherical aberration.

In studying the visual effect of inducing HOAs, Dr. Schallhorn provided his data from 150 consecutive patients that underwent conventional LASIK several years ago.

“The patients were given a very detailed questionnaire on their quality vision, like night halos, before and after surgery. We compared their responses to their post-op spherical aberration as well as their pre-op to post-op change in spherical aberration. While the correlation is not strong, we find a positive relationship,” Dr. Schallhorn said. Patients with less spherical aberration tend to have fewer symptoms.

Finally, Dr. Schallhorn said he and Commander Dave Tanzer have studied patients with excellent vision—carrier-based U.S. Navy pilots. In the study, the pilots had 20/12 uncorrected vision on average.

From reviewing the pilots’ eyes, Dr. Schallhorn said there was no relationship between HOAs and their high contrast visual acuity. But he said there was a positive relationship on contrast acuity under photopic and mesopic conditions.

“In these pilots, the lower the level of higher order aberrations, the better their contrast sensitivity,” he said. “Even in this group with excellent vision, pilots with less higher order aberrations tended to have better visual acuity.”

As for spherical aberrations, the pilots had an average of 0.08 microns (6 mm pupil analysis).

“So does this mean we should target 0.08 microns of spherical aberrations?” Dr. Schallhorn asked. “The answer is no. Just like found with LASIK patients, just like what we found in visual modeling. Even in this group of aviators—super normal vision aviators—the less the spherical aberration, the better the quality of vision.”

Dr. Schallhorn also addressed corneal aberrations. He said the average corneal spherical aberration measured previously in 127 cataract patients was 0.27 microns.

Selecting the optimal IOL for zero aberrations

There are IOLs now that can counteract that natural, positive spherical aberration, such as the Tecnis IOL (Advanced Medical Optics, AMO, Santa Ana, Calif.), which was designed to counteract the corneal aberration on average.

Dr. Schallhorn did note, however, that in patients without corneal aberrations, they would be better served with a different lens such as the AO lens (Bausch & Lomb, Rochester, N.Y.).

He concluded by noting there are two take-home messages.

“One is that if you could measure the corneal aberrations with a topography unit, you could select the perfect lens that would minimize the spherical aberration,” he said. “Another easier method is to select a lens that encompasses the majority of patients.” Dr. Schallhorn added that using this approach, the Tecnis lens would be ideal because it encompasses the majority of a normal cataract population. In all cases, the IOL needs to be well centered in order to obtain the best vision.

Dr. Schallhorn also summed up his presentation by pointing to his conclusions about less aberrations equals better quality of vision.

“The ideal intraocular lens would negate the spherical aberration of the cornea,” he said.

Editors’ note: Dr. Schallhorn is a consultant for Advanced Medical Optics (AMO, (Santa Ana, Calif.).

Contact Information

Schallhorn: 858-695-3849, scschallhorn@yahoo.com

Reducing HOAs to improve quality of vision Reducing HOAs to improve quality of vision
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