When determining which IOL to implant, it makes sense to assess the patient's needs. Doing so, in turn, requires an understanding of what each lens offers.
With any of the IOLs discussed herein, the surgeon's target refraction should be emmetropia to ensure good distance vision. The days of targeting ±0.25D for the postoperative refraction are essentially gone. Instead, the surgeon should determine what kind of vision the patient desires and needs as well as his tolerance for compromise by means of a questionnaire such as that designed by Robert Maloney, MD,1 or, more recently, Steven Dell, MD (see his article in this issue on page 79). The optics of these IOLs dictates the right choice.
Mechanism of Action
Studies have shown that the Crystalens (Eyeonics, Inc., Aliso Viejo, CA) provides better near vision than monofocal IOLs but that the mechanism is increased depth of field from a small aperture rather than movement of the IOL.2 Simple Gaussian optics as well as the use of a camera show that the depth of field is inversely related to the size of the pupil. If an IOL's aperture is 25% smaller, then the depth of field will be 25% longer. Unlike a standard IOL's 6.0-mm optic, the Crystalens' optic has a diameter of 4.5mm. As a result, it provides a 25% greater depth of field.
Studies have shown that the majority of patients in the cataract age group, when asked to choose, will select excellent distance and intermediate vision with pretty good near vision over excellent distance and near vision without intermediate vision (data from the Array lens' [Advanced Medical Optics, Inc., Santa Ana, CA] premarket approval on file with the FDA). Such patients will likely be pleased with the Crystalens, which provides crisp distance and intermediate vision. Moreover, because the lens has one focal point, patients need not adapt to nighttime halos or mildly reduced contrast sensitivity, unlike with multifocal lenses.
Patients who desire sharp near visual acuity without spectacles, including in dim lighting, are unlikely to be happy with the Crystalens, however. Additionally, individuals who have lattice degeneration, a history of peripheral retinal holes, or a long axial length are not ideal candidates for the lens. The reason is that the IOL causes object displacement when retina specialists attempt to examine the peripheral retina through a microscope or to perform retinal surgery. As a result, they may have to remove the lens during surgery.
THE REZOOM AND ACRYSOF RESTOR IOLs
These two multifocal IOLs employ different optical strategies. The Rezoom IOL (Advanced Medical Optics, Inc.) is a significant improvement on the original Array lens. The former is designed to provide clear vision at distance, intermediate, and near. The add is approximately 2.70D (working distance of 37.0cm or 14.6 inches) (Figure 1).
In comparison, the Acrysof Restor lens (Alcon Laboratories, Inc., Fort Worth, TX) has a bifocal that provides approximately 3.00D of add power (working distance of 33cm or 13 inches) at the spectacle plane, so it provides a very close near focus. The IOL is also apodized (no diffractive rings in the periphery), a design that is intended to reduce the halo effect in lower light but also decreases the intensity of the near image.
With both the Rezoom and the Acrysof Restor lenses, one image will be blurred when another is in focus. In time, the brain adapts to simultaneous vision, and patients become less bothered by unwanted nighttime visual phenomena. Studies have shown that most patients adapt to multifocal lenses by 1 year postoperatively, but some will adapt in 3 to 6 months (data from the Array lens' premarket approval on file with the FDA).3 Although most laboratory studies show that multifocal IOLs decrease contrast sensitivity by 30% to 40%,4 this amount tends to improve with time due to neural adaptation. Ulrich Mester, MD, has also demonstrated an improvement in patients' contrast sensitivity and quality of vision.5
When choosing between the two multifocal lenses, the Rezoom is likely the better choice for patients who wish to see at all distances. For someone who is less concerned with intermediate distance vision and highly values near visual acuity, the Acrysof Restor IOL may be a better choice. Its apodization does mean that at least medium illumination is needed for reading.
THE SELECTION PROCESS
Almost everything in life has trade-offs, and people have various attitudes and needs. An engineer who requires excellent distance vision may be happiest with the aspheric, prolate Tecnis IOL (Advanced Medical Optics, Inc.) and a pair of glasses for reading. An elderly patient who wants to play bingo and read without glasses might prefer a multifocal or accommodating lens. It would be naïve to think that multifocality requires too great a compromise in terms of increased aberrations; surgeons would not implant them if all patients who received the IOLs were unhappy with their outcomes. Ophthalmologists need to assess the needs of each patient and determine which IOL (all with their advantages and disadvantages) is the best fit. The aforementioned questionnaire designed by Dr. Dell is the best this author has seen.
Jack T. Holladay, MD, MSEE, FACS, is Clinical Professor of Ophthalmology at Baylor College of Medicine in Houston and President of the Holladay LASIK Institute in Bellaire, Texas. He is a consultant to Nidek, Inc., and Advanced Medical Optics, Inc. Dr. Holladay may be reached at (713) 668-7337; firstname.lastname@example.org.
1. Maloney WF. Presbyopia success depends on comprehensive preop evaluation. Ocular Surgery News. Available at: http://www.osnsupersite.com. Accessed: January 25, 2005.
2. Findl O. Intraocular lenses for restoring accommodation: hope and reality. J Refract Surg. 2005;21:321-323.
3. Holladay JT. A prospective, randomized, double-masked comparison of a zonal-progressive multifocal IOL. Ophthalmology. 1992;99:853-861.
4. Holladay JT, Van Dijk H, Lang A, et al. Optical performance of multifocal intraocular lenses. J Cataract Refract Surg. 1990;16:413-422.
5. Holladay JT, Packer M, Mester U, et al. Defining and optimizing functional vision. Course presented at: The XXIII Congress of the ESCRS; September 12, 2005; Lisbon, Portugal.