Aspheric treatments reduce spherical aberration after cataract, refractive surgery

Implanting an aspheric IOL or incorporating a modified laser treatment into corneal refractive surgery helps reduce naturally occurring spherical aberration.

Applying aspheric treatments to cataract and refractive surgery procedures can significantly reduce spherical aberration and the consequent effects of glare and haloes, improving postoperative visual results, said Jack T. Holladay, MD, MSEE, FACS.

Jack T. Holladay

“Aspheric treatments in the form of aspheric modified prolate IOLs or modified laser ablations reduce the spherical aberration in the eye,” Dr. Holladay told Ocular Surgery News. He said that spherical aberration is caused by spherical surfaces in the eye that cause rays of light in the periphery to “bend too strongly” compared to paraxial or central rays.

IOLs with spherical surfaces, laser ablations that are calibrated on flat surfaces (and consequently undercorrect the corneal periphery) and changes in the crystalline lens over time all contribute to spherical aberration in the human eye and diminish visual performance. A surface that is prolate – steeper in the center and flatter in the periphery – can reduce spherical aberration and “bring rays into a perfect point of focus,” Dr. Holladay said.

For the middle-aged eye

“Surgeons can reduce spherical aberration by using an excimer laser that maintains or increases the prolate shape of the natural cornea, or with modified prolate IOLs, both phakic and aphakic,” Dr. Holladay said.

He said correcting these higher-order aberrations returns the eye closer to the physiologic optics of the young human eye.

“By maintaining or making the cornea more prolate than it was preoperatively, we can minimize the spherical aberration of the entire eye in the middle-aged patient,” Dr. Holladay said. He said that reducing spherical aberration by making the cornea more prolate is the “next step” in improving surgical outcomes for patients over 40 years old.

“With IOL surgery, we have been able to consistently achieve our target refraction to within 0.25 D due to improved formulas, more precise axiometers and keratometers. We’ve also eliminated most of the cylinder through perioperative techniques, like limbal relaxations or secondary corneal refractive surgery,” Dr. Holladay said. “We’ve now moved to the point of correcting the next aberration in the human eye – spherical aberration.”

Asphericity quotient

Introducing an oblate ellipsoid, steeper in the periphery and flatter in the center, increases the amount of spherical aberration in the eye, Dr. Holladay said. Image: Holladay JT

Dr. Holladay said the key to reducing spherical aberration in the eye is to preserve or increase the natural prolate shape of the cornea during refractive surgery.

“The cornea is a three-dimensional prolate ellipsoid, like a bullet or a tulip,” Dr. Holladay said. “It’s steeper in the center and flatter in the periphery, in order to reduce or eliminate the spherical aberration of the entire eye.”

Introducing an oblate ellipsoid, steeper in the periphery and flatter in the center, increases the amount of spherical aberration in the eye, he said.

The curvature of an ellipsoid – whether prolate or oblate – can be expressed through an asphericity quotient called the Q-value, Dr. Holladay said. The Q-value for a sphere is 0, while the Q-value for a prolate ellipsoid is negative and for an oblate ellipsoid positive.

“The average Q-value for a normal human cornea is about –0.26. If the cornea were a perfect ellipsoid, with no spherical aberration, the Q-value would be approximately –0.52,” Dr. Holladay explained. “So, while the cornea still has positive spherical aberration, it is only about half as much as a sphere (Q-value = 0), because it is halfway between a sphere and a perfect ellipsoid, at Q = –0.52.”

Presbyopic shift

Relating the Q-value of a cornea to the Zernike calculations that are used in wavefront-guided refractive surgery, Dr. Holladay said a cornea with a Q-value of –0.26 translates to 25 µm of positive spherical aberration over a 6-mm optical zone. In a young person (20 years old or younger) the 25 µm of spherical aberration in the cornea is “cancelled out” by 25 µm of negative spherical aberration in the crystalline lens, he said.

“The spherical aberration of the entire eye is almost zero in the youthful eye,” he noted.

Studies by Adrian Glasser, PhD, an associate professor of optometry at the University of Houston, and Pablo Artal, PhD, at the Laboratorio de Óptica Universidad de Murcia, in Spain, have shown that the crystalline lens increases in positive spherical aberration over time, but the cornea remains constant (when corneal disease is not present).

“By age 40, the crystalline lens changes from a negative spherical aberration to a spherical aberration of about 0, resulting in about +25 µm of spherical aberration,” Dr. Holladay said. By age 60 years, the total eye has about 50 µm of spherical aberration: 25 µm from the cornea and 25 µm from the lens, he said.

Aspheric IOLs

Dr. Holladay said that Sverker Norrby, PhD, and Patricia A. Piers, MSc, IOL scientists at Pharmacia, originated the idea of implanting an aspheric modified prolate anterior surface IOL with –25 µm of spherical aberration, designed to mimic the negative spherical aberration of a young crystalline lens. The lens, originally developed by Pharmacia, is the Tecnis IOL currently available from Advanced Medical Optics.

“The idea of the aspheric modified prolate lens is that it eliminates spherical aberration in most elderly patients, correcting disabling visual effects like glare and halo. It brings patients back to the visual performance of a 20-year-old eye, rather than a 60-year-old eye without a cataract,” Dr. Holladay explained.

He said that in clinical trials of the Tecnis IOL, patients’ contrast sensitivity subjectively improved by 40%.

“The improvement in contrast sensitivity and reduction in glare and halo improved the reaction time of elderly drivers by 0.5 seconds in nighttime driving simulations,” Dr. Holladay said. (By way of comparison, he noted that the third rear taillight on a vehicle, mandated by the National Highway Traffic Safety Administration, improves reaction time by 0.3 seconds.)

“The Food and Drug Administration labeled the Tecnis IOL as actually improving the performance of night driving in the cataract age group, which improves their safety as well as others on the road,” Dr. Holladay said.

Other aspheric IOLs that correct for positive spherical aberration in the eye are also now available. These include the Alcon AcrySof SN60WF, with a Q-value of –12.5 µm, and the Bausch & Lomb SofPort AO with a Q-value of –2.5 µm, he said.

Dr. Holladay said the AcrySof lens corrects approximately 50% of the average corneal spherical aberration, and the SofPort lens corrects about 10% of the average corneal spherical aberration.

He said a trend toward aspheric lenses has begun, which he believes will continue to “drive the design” of both IOLs and phakic IOLs.

“Within the next 3 years, the majority of IOLs implanted by surgeons will be some version of the aspheric modified prolate IOL,” Dr. Holladay said.

Laser ablation

Similar attention to spherical aberration is needed in excimer laser algorithms for refractive surgery, Dr. Holladay said. He said standard corneal refractive surgical laser treatments transform the patient’s prolate cornea into an oblate cornea, inducing spherical aberration.

“Standard laser treatments, for both surface and stromal bed ablation, have been calibrated on flat surfaces, and they do not take into account that the cornea is a dome, where the effect of the energy is reduced proportional to the oblique incidence of the energy,” Dr. Holladay explained. As a result, such treatments put an inadequate amount of energy into the corneal periphery, he said.

“Consequently, standard corneal refractive surgery, like LASIK, LASEK, PRK, and epi-LASIK, induce spherical aberration,” Dr. Holladay said. In fact, he said, standard laser treatments can induce spherical aberration to a Q-value of up to 100 µm to 150 µm.

“The creation of an oblate corneal surface is the main reason why spherical aberration occurs after corneal laser surgery, causing reduced visual performance and the appearance of glare and haloes,” Dr. Holladay said. He noted that even when surgical parameters are “excellent” with no complications (such as damaged flaps or decentered ablations) spherical aberration can still cause “disabling” nighttime glare and haloes.

Modified laser treatments

With the development of wavefront-guided laser surgery, some laser manufacturers have incorporated a “radial compensation function” into their ablation profiles to compensate for undertreatment in the periphery by standard laser algorithms, Dr. Holladay said.

“Companies have implemented the new prolate function into their wavefront-guided systems to alter the laser energy power in the corneal periphery and reduce induced spherical aberration. However, they did not incorporate this modification into their standard laser treatments,” he said.

The WaveLight Allegretto Wave excimer laser system is the only laser system approved in the United States that has implemented the peripheral energy compensation into its standard platform, he said.

(WaveLight refers to this as a wavefront-optimized profile, distinguishing it from wavefront-guided treatment. U.S. clinical trials of the company’s wavefront-guided protocol are ongoing.)

In the Food and Drug Administration clinical trials of the WaveLight Allegretto Wave system, Dr. Holladay said, the Allegretto’s standard laser treatment “achieved the same performance outcomes as wavefront-guided treatments from other excimer laser systems.”

“The Allegretto system achieved the same results as wavefront-guided treatments because it had the same radial compensation modification incorporated into its system that the wavefront-guided treatments had,” Dr. Holladay said. He said he believes this modification — again, a compensation for spherical aberration in the eye — is the “major reason” for the improved results of wavefront-guided laser surgery as compared to standard treatments.

In recent years, Dr. Holladay said, he has encouraged laser manufacturers to implement radial compensation in their standard ablation profiles.

“We want laser companies to implement the radial compensation function to improve their results with the standard treatment, rather than just offering it in their wavefront-guided treatments,” Dr. Holladay said. “[The modification] is easy to do and would eliminate a lot of the spherical aberration from standard treatments to achieve the same results as wavefront-guided treatments in individuals with normal eyes.”

If the radial compensation modification is incorporated into standard ablation profiles, Dr. Holladay said, surgeons can compare their surgical outcomes to wavefront-guided outcomes to see if the customized surgery “offers any real benefit” over the standard surgery in routine cases.

“Perhaps, we will find that standard excimer laser surgery is sufficient for most patients, and wavefront-guided surgery should be reserved for patients with severe corneal irregularities,” he said.

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; 713-668-7337; 713-668-7336; e-mail:docholladay@docholladay.com; Web: www.docholladay.com.

• Alcon can be reached at 6201 South Freeway, Fort Worth, TX 76134; 817-293-0450; fax: 817-568-6142.
• Advanced Medical Optics can be reached at 1700 E. St. Andrew Place, Santa Ana, CA 92799; 800-449-3060; fax: 866-872-5635; Web site: amo-inc.com.
• Bausch & Lomb can be reached at 1400 N. Goodman St., Rochester, NY 14609; 585-338-5212; fax: 585-338-0898; Web site: bausch.com.
• WaveLight Inc., U.S. distributor and a wholly owned subsidiary of WaveLight Laser Technologie AG, can be reached at 46040 Center Oak Plaza, Sterling, VA 20166; 517-434-8500; www.wavelight-laser.net.

• Nicole Nader is an OSN Staff Writer who covers all aspects of ophthalmology, specializing in QOV, pediatrics/strabismus and neuro-ophthalmology.