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Early outcomes offer high hopes for unique accommodating IOL
The lens’ aberration-free optic seems to provide more depth of focus and be more forgiving of residual sphere and cylinder than previous models.
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U.S. surgeons who use premium implants have shown increasing interest in accommodating technologies, especially as improved lens designs yield better results. Currently, the only accommodating implant approved by the U.S. Food and Drug Administration, the Crystalens from Bausch + Lomb, has no competition in this category and has become increasingly popular among surgeons. The latest generation, Crystalens AO, offers surgeons a new optic design. In this article, we will review the optics of this lens and the clinical implications of this design change.
Technical specifications
The Crystalens AO is a silicone, biconvex IOL with a 1.4301 refractive index and a 5-mm optic. It has an A-constant of 119.1, an anterior chamber depth of 5.61 mm and is available in powers ranging from 10 D to 33 D in 0.5 D increments. It can be implanted through a 2.7-mm incision using Bausch + Lomb’s Crystalsert injector system.
Unlike the earlier four generations of the Crystalens, however, the AO has an aspheric optic that eliminates spherical aberration and has a uniform lens power from edge to edge. This familiar “AO” has been used successfully on a number of popular monofocal IOLs, including Bausch + Lomb’s SofPort AO, Akreos AO and Akreos MICS lenses.
Eliminating spherical aberration
 Crystalens in the eye. Image: Hovanesian JA |
An optic free of spherical aberration has several theoretical advantages over other aspheric lens designs. First, mild degrees of lens tilt or decentration, which can occur with any type of IOL, are less likely to be visually disturbing. The second and probably more important theoretical advantage relates to image quality and depth of field. Although the AO optic contributes no spherical aberration to the eye, the cornea of an older adult typically has a mild degree of positive spherical aberration, with an average asphericity value of +0.27 µm. With total spherical aberration of about +0.27 µm, one should expect a good balance of visual quality and depth of field.
Why is an eye with mildly positive spherical aberration likely to have better vision than an aberration-free eye? In pure physics, spherical aberration is considered an undesirable phenomenon because its presence causes light rays passing through the periphery of a lens to fall out of focus. However, in physiologic optics, a mild degree of positive spherical aberration can permit greater depth of field (yielding greater spectacle independence) without significantly sacrificing image quality.
Clinical results
In clinical practice, one would expect the Crystalens AO lens to be more forgiving of residual refractive error than its predecessor lenses with spherical optics, and the data from a post-market surveillance study of 37 surgeons supports this idea. In 45 patients who had the Crystalens AO implanted binocularly, 100% achieved 20/30 or better uncorrected distance and near vision when evaluated 3 months postoperatively. The entire 45-patient cohort achieved J3 or better uncorrected near vision at this same follow-up interval. Additional data collected at the 3-month mark indicate that 97% to 98% achieved 20/25 uncorrected distance and intermediate vision, and 93% achieved J2 uncorrected near vision.
Analysis of monocular outcomes was also promising. Monocular data collected on 91 eyes show that after 3 months, 93% attained uncorrected distance visual acuity of 20/40 or better, 69% were 20/25 or better, and 40% achieved uncorrected distance visual acuity of 20/20 or better.
Experience with this lens has shown that most patients can achieve reasonable spectacle independence if the first eye treated is targeted for plano to –0.25 D and the second eye for –0.25 to –0.5 D. Patients should be counseled that the full range of accommodation with Crystalens may take 6 to12 months to achieve, and that daily reading without spectacles will encourage the development of postoperative accommodation.
In summary, clinical experience thus far with the Crystalens AO has shown it to provide similar or better accommodation to prior generations of this lens. At the same time, its aberration-free optic should provide more depth of focus and be more forgiving of residual sphere and cylinder than previous models.
- 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. Dr. Hovanesian is a consultant for Bausch + Lomb.