This article is more than 5 years old. Information may no longer be current.
Aspheric IOLs can improve image quality for cataract surgery patients
Click Here to Manage Email Alerts
In the current age of refractive cataract surgery, we measure the quality of vision in addition to the quantity of vision. The patient’s image quality is as important as his or her Snellen acuity, particularly when we operate on patients with high expectations and high demands for their vision.
 Uday Devgan |
Major manufacturers have developed aspheric IOLs to improve image quality and functional vision for pseudophakic patients. These new aspheric IOLs have different designs, and understanding these differences is instrumental to their use.
We already use aspheric lenses in nearly every aspect of our practices, from the surgical and slit lamp microscopes to the handheld lenses used for binocular indirect ophthalmoscopy. Even higher-quality glasses and contact lenses use aspheric optics because the image quality is better. So why not IOLs?
Aspheric IOLs
Manufacturing an IOL with a constant curvature is easier than making one with a variable curvature. The new designs of aspheric IOLs require a variable curvature surface in order to achieve the optical results that they produce. This results in increased complexity and cost of manufacturing.
Most standard IOLs induce positive spherical aberrations. The newer-design aspheric IOLs can be divided into two groups: IOLs that have zero spherical aberration and IOLs that induce negative spherical aberration in an effort to offset the average cornea (Figure 1).
Spherical aberration changes with age
The cornea has a small amount of positive spherical aberration, which tends to be fairly constant throughout life. The human lens is more kinetic, with a negative spherical aberration in youth, which slowly becomes zero spherical aberration in midlife and then progresses to positive spherical aberration as nuclear sclerosis develops. This means that the eye as a complete optical system has zero spherical aberration in our youth, but this slowly increases as we age. When we implant a traditional IOL during cataract surgery, we dramatically increase spherical aberration (Figure 2).
|
Images: Devgan U |
Importance of image quality
Early LASIK patients who underwent large degrees of myopic correction often presented with the complaint of, “Yeah, I know I can see 20/20, but it’s still not clear.” This likely represented the effects of a loss of contrast sensitivity caused when the ablation induced a large amount of positive spherical aberration as the cornea changed from prolate to oblate in curvature.
Using traditional IOLs that induce positive spherical aberration into the eye can also degrade image quality as compared with aspheric IOL designs. Night driving simulators have shown an increase in functional vision in patients implanted with aspheric IOLs. For the best image quality, it is best to achieve an eye with zero spherical aberration, but this comes at a price — the loss of depth of field.
Because we don’t yet have true accommodative IOLs, it can be beneficial to keep a larger depth of field. However, image quality and depth of field are inversely related: The higher the image quality is, the lower the depth of field will be. A traditional IOL that induces positive spherical aberration has the worst image quality at the focal point but the largest depth of field. Zero aberration aspheric IOLs leave the eye with a mild amount of natural corneal spherical aberration, thereby giving good image quality at the focal point and good depth of field. Negative aberration aspheric IOLs aim to counteract natural corneal spherical aberration, thereby giving the best image quality at the focal point but with a narrower depth of field.
Importance of IOL centration
In order for a negative aberration aspheric IOL to effectively cancel out the natural corneal spherical aberration, it must be well centered with the visual axis. Keep in mind that the center of the pupil, the center of the cornea and the center of the visual axis are not the same. And while it is customary in LASIK to have the patient fixate on a light in order to center the ablation with respect to the visual axis, this is not the case in lens-based refractive surgery or cataract surgery.
When the negative aberration aspheric IOL is misaligned from the visual axis by approximately 0.5 mm or more, its benefits are largely lost and it may induce coma aberration into the visual system and degrade the image quality.
The design of zero aberration aspheric IOLs are much like the handheld lenses we use for binocular indirect ophthalmoscopy – they are minimally affected by decentration and will not induce coma or other higher-order aberrations.
Clinical experience
In my practice, I have tried all three of the aspheric IOL designs that are available in the United States (Tecnis IOL with negative spherical aberration, Advanced Medical Optics; AcrySof IQ IOL with negative spherical aberration, Alcon; SofPort AO IOL with zero spherical aberration, Bausch & Lomb), and I have been happy with the results. Some of the patients with an aspheric IOL in one eye and a traditional spherical IOL in the other eye are cognizant of the difference in image quality.
The difference in image quality between aspheric IOLs and traditional spherical IOLs is real and significant, and it can be amplified in certain situations. Patients with larger pupils, patients with high demands of their vision (such as night driving) and patients with refractive surgery mindsets seeking perfect vision are more likely to perceive and appreciate the benefits of aspheric IOLs.
Aspheric IOLs, of one design or another, are likely to become the standard in ophthalmology.
For Your Information:
- Uday Devgan, MD, FACS, is an assistant clinical professor at the Jules Stein Eye Institute at the University of California, Los Angeles, and in private practice in Sun Valley, Calif. He can be reached at 9375 San Fernando Road, Sun Valley, CA 91352; 818-768-3000; fax: 818-504-4463; e-mail: devgan@ucla.edu.