New lens design may be more forgiving of residual refractive error

Equal power from center to edge reduces theoretical defocus from tilt, decentration and residual refractive error.

John A. Hovanesian

A recently released IOL offers the potential to be more forgiving of residual refractive error after lens exchange.

The Crystalens AO (Bausch + Lomb) merges an aberration-free aspheric design with an accommodating lens. According to John A. Hovanesian, MD, FACS, the combination offers surgeons the potential to more accurately predict postoperative vision while allowing for some variability from surgically induced error.

“Healing is what influences the end result. Whatever implant you use, there’s usually some mild residual refractive error,” Dr. Hovanesian said. “What I’ve seen with the AO is that we’re getting somewhere up to what we had with the HD, but we seem to probably have a little bit easier time predicting the refractive result.”

While the Crystalens HD model delivered on its promise of better near vision than the Crystalens 5.0, “surgeons who had residual refractive error after surgery were finding that patients were not quite as pleased with the results and not getting the spectacle independence they wanted, so we had to do enhancements more often with that lens than probably was ideal,” Dr. Hovanesian said.

The AO model offers a uniform power from center to edge, which theoretically allows the natural corneal shape to affect a greater depth of field. The aberration-free design also means the optical system may be less prone to image degradation from lens tilt, decentration and residual refractive error.

Design considerations

According to Dr. Hovanesian, a typical aspheric IOL designed with negative spherical aberration counteracts the patient’s naturally occurring positive corneal spherical aberration, so the total system of the lens and cornea working together will eliminate spherical aberration to provide good visual quality at one well-focused point. However, leaving a modest amount of positive spherical aberration within the optical system allows greater depth of field than zero spherical aberration.

“The Bausch + Lomb approach has been to allow the lens, the IOL, to offer no spherical aberration, let the cornea continue to provide some minor positive spherical aberration, and then you have a system with a little bit of positive spherical aberration,” Dr. Hovanesian said.

According to Dr. Hovanesian, studies have shown that reducing spherical aberration yields consistent contrast sensitivity. But, he added, cataract surgery remains an inexact science, with patients often drifting to over- or undercorrection and experiencing some cylinder after surgery. When the refractive correction is not exactly on target, “you actually lose image quality more quickly than you do if there’s a little positive asphericity in the system,” he said.

“All the premium lenses, multifocals and accommodating, are very sensitive to defocus,” Dr. Hovanesian said.

The AO lens may also be more forgiving of decentration or tilt after implantation. The AO technology added to the Crystalens platform was recently released to market, so there is limited data to support whether this will bear out with clinical experience.

Previous experience with other AO lenses — the SofPort AO, for instance — suggests that patients implanted with the Crystalens AO will be less sensitive to postoperative refractive error such as cylinder, decentration and tilt. As a result, Dr. Hovanesian said, it appears possible that it will be easier to hit the desired refractive target after a single procedure.

“I believe we’re going to be doing a lot fewer enhancements with the AO than with the HD,” he said.

Early clinical results

According to Dr. Hovanesian, the Crystalens AO may provide best results when the first eye is targeted from plano to –0.25 D while the second eye is targeted from –0.25 D to –0.5 D. Although there is not much data on the lens, a pre-release study of 300 eyes operated by 37 surgeons showed promising results.

At 1 month after implantation, 73% of patients achieved 20/20 distance vision and J1 acuity, which increased to 80% at 3 months. The percentage of patients with 20/20 intermediate vision and J1 was 93% at both time points, and the percentage of patients with 20/20 at near and J1 was 62% at both time points.

However, the percentage of patients who achieved 20/30 or better at distance and J3 was 98% at 1 month and 100% at 3 months, while the percentage achieving 20/30 or better intermediate or near and J3 was 100% after 1 month, which did not change at 3 months.

In the study, distance-corrected near visual acuities 1 month postop were 34% achieving J1, 65% achieving J2 and 91% achieving J3. These compare favorably with data from 1,207 patients analyzed 1 month after receiving a Crystalens 5.0 lens: 29%, 60% and 79% achieved J1, J2 and J3, respectively. – by Bryan Bechtel

• John A. Hovanesian, MD, FACS, can be reached at Harvard Eye Associates, 24401 Calle De La Louisa, Suite 300, Laguna Hills, CA 92653; 949-951-2020; fax: 949-380-7856; e-mail: drhovanesian@harvardeye.com.