Aberration-correcting intraocular lenses

Over the past 5 years, aberrations of the human eye have been intensely investigated. In the normal eye, the average positive spherical aberration of the cornea is partly compensated for by a negative spherical aberration of the crystalline lens. While the cornea is a relatively stable optical system throughout a lifetime, aberrations of the lens change from the young to the older eye beginning at approximately 40 years of age.1,2 Wavefront sensing was introduced to optimize corneal laser corrections beyond the traditional spherocylindrical excimer corrections with the goal of improving the visual outcome after refractive corneal surgery. Current results of wavefront-guided excimer laser surgery suggest that with this technology, higher-order aberrations can be decreased.3 This may result in better optical and visual quality than occurs after standard “noncustomized” laser corrections. However, given that lens aberrations will change over a lifetime, we have to understand that these optimized results will also change. Cataract removal with intraocular lens (IOL) implantation is the most successful procedure in modern medicine worldwide; increasingly, this procedure is being used for refractive purposes. The concept of customized corneal procedures may be transferred to lens surgery, which means that after lens removal, customized IOLs could compensate for residual ocular aberrations. The average cornea has a positive spherical aberration, which could be reduced or eliminated by implanting an IOL with a negative spherical aberration. These IOLs have been termed “aspheric IOLs.”4 The concept of an anterior-surface-modified prolate IOL has been patented (International Publication Number WO 01/89424 A1), and initial results were recently published.5,6 In this issue, Mester and coauthors (pages 652–660) compare their results with an aspheric foldable silicone IOL (Tecnis, Pharmacia) with those with a standard foldable silicone IOL (SI-40, AMO). Low-contrast visual acuity and contrast sensitivity were superior with the aspheric IOL. Wavefront measurements revealed low spherical aberrations in the eyes with the Tecnis IOL, while there was a larger amount of positive spherical aberration in the eyes with the SI-40. The authors conclude that spherical aberrations of the eye can be eliminated by modifying the anterior surface of the IOL. Studies by Holladay et al.,5 Packer and coauthors,6 and Mester and coauthors suggest that aspherical IOL designs may affect the way we correct the optical system after lens removal in the future. However, several questions must be answered: Will the measured difference in quality of vision after aspheric IOL implantation depend on different IOL optic and haptic materials and designs? How will IOL position, especially decentration and tilt, affect the outcome? How important are spherical aberrations for the visual outcome? How much of the preexisting error should we change? Will there be negative consequences such as loss of depth of focus? Will the early effects be influenced by long-term posterior capsule opacification? How important are pupil and capsulorhexis sizes under different illumination levels in these eyes? How will the retina receptors diminish the positive impact of the “new” system? In the beginning of IOL surgery and before the advent of IOL power calculation formulas, IOLs with the same power were implanted in all eyes, resulting in large postoperative refractive errors. Currently, a standard aspheric IOL is implanted in all eyes, regardless of the magnitude of the corneal spherical aberration. Will it be necessary to customize the IOL to each cornea? Will the measured differences in low-contrast visual acuity, contrast sensitivity, and wavefront measurements really affect a patient's daily life, particularly in nighttime conditions? Will refractive lens exchange with implantation of an aspheric IOL be a treatment for eyes with excessive spherical aberration? Will an even more sophisticated aspheric IOL be required in future cataract procedures for today's corneal refractive patients? In this issue, a special section on “The Cornea After LASIK” (pages 757–794) demonstrates that more than 10 years after the first LASIK procedure, intensive research is still being undertaken to understand refractive corneal surgery with this most successful technique. Compensating for corneal aberrations with an aspheric IOL is a fascinating concept. Initial results are encouraging, but before these IOLs will replace the standard IOL designs, further research should be performed, including studies to answer the above questions. I look forward to further investigations of aberration-correcting IOLs.

Topic: Ophthalmology and Visual Impairment Studies
Subfield: Epidemiology
Field: Medicine
Domain: Health Sciences