Cataract/IOL
October 2007
by Richard Tipperman, M.D.,
and Jim Schwiegerling, Ph.D.
Each multifocal IOL design uses a different strategy to produce simultaneous near and distance images. Experimental work suggests that the optic design makes a difference for night vision
For surgeons implanting multifocal IOLs, nighttime dysphotopsia in their patients is a concern. If a patient is severely debilitated by glare and halos at night, IOL explantation or exchange may be necessary. No surgeon or patient wants to take another trip to the operating room if it can be avoided. Thus, the best way to ensure against this is to implant a multifocal IOL that has the least chance of creating unwanted nighttime optical effects in our patients. It is up to the clinician to select the presbyopia-correcting lens that will provide patients with the best possible image quality. How are clinicians to evaluate which of these lenses has the least chance of creating nighttime dysphotopsia in our patients? Each of us has independently performed evaluation of optical effects in nighttime situations with three multifocal IOLs—AcrySof ReSTOR (Alcon, Fort Worth, Texas), the ReZoom (Advanced Medical Optics [AMO], Santa Ana, Calif.), and the Tecnis Multifocal (AMO).
Background on multifocal IOL optics
Each of the available multifocal IOL technologies uses a different strategy to simultaneously produce near and distance images on the retina. All multifocal IOLs produce some stray light because they divide incoming light into two or more images on the retina. Each of the three lenses we evaluated uses a different strategy to achieve this. The Tecnis Multifocal uses a full-aperture diffractive design to divide light into near and far images. The ReZoom employs concentric refractive zones to produce near and distance images. The ReSTOR has an apodized diffractive zone that produces both near and distance foci, surrounded by a purely monofocal refractive zone that produces a distance focus. The apodized diffractive design of the ReSTOR lens provides an advantage in nighttime lighting situations. When the pupil expands in dim light, more light is permitted to come through the peripheral refractive zone of the IOL, thus increasing the percentage of incoming light devoted to the distance image. This effect tends to diminish stray light effects from the near visual image in low light situations. The Tecnis Multifocal and ReZoom, because their light-splitting technologies are distributed across the entire lens optic, produce a greater amount of light scatter in these situations.
The human visual system consists of not only the eye, but also the neural processing of the visual cortex in the brain. The models we used in our studies simulated only the “front” portion of the visual system, from the cornea to the retina. We believe that with multifocal IOLs it is important to present as clear an image or images as possible on the retina, so that the visual cortex can best interpret the simultaneous near and distant images. Our experiments were designed to evaluate the quality of the images projected on the retina by these three IOLs.
Visual quality in nighttime scenes
The first study was a pinhole image comparison of the multifocal IOL models. This was performed to evaluate their light-spreading properties.1 A modified ISO model eye with a 5 mm aperture and a white light source were used to evaluate three IOLs: a 22 D AcrySof ReSTOR, a 20 D ReZoom, and a 26.5 D Tecnis. Powers used were based upon the availability of the lenses. The setup was similar to that used to measure an IOL’s modulation transfer function. The results are shown in Figure 1. Defocused light can be seen around the central focused image with all three lens models. Because, with a 5-mm aperture, 85% of the light energy with the ReSTOR lens goes to the distance focus, the defocused near image forms only a small halo around the focused distance image.3 The ReZoom’s energy distribution is about 60% to distance and 30% to near with a 5 mm aperture, and the Tecnis’s is about 41% to each focal point.3,4 The ReSTOR lens provided the sharpest focus of the three. The size of the light spread with the other two lenses was similar, and both were greater in size than the ReSTOR lens. Halos and rings can be seen around both the ReZoom and the Tecnis Multifocal images, with more ring patterns around the ReZoom image.
The second study evaluated the nighttime visual quality of the three multifocal IOL models by photographing a night scene through each one.2 This evaluation used a portable digital imaging system that included an eye model, a macro lens and a commercial SLR digital camera. The eye model included an artificial cornea designed to simulate the spherical and chromatic aberrations of the normal human eye. The eye also accommodated a wet cell in which the IOLs were placed, immersed in saline. Apertures of 4.5 mm and 6 mm were inserted into the eye model to mimic different pupil sizes.
This assembly was mounted on a digital camera and taken to various locations for nighttime photo sessions. An exposure time was determined, and then photographs were taken using three mid-power lenses: a 21.5 D AcrySof ReSTOR, a 21.5 D Tecnis Multifocal and a 22 D ReZoom. The results with a 6-mm aperture in a nighttime scene with car headlights can be seen in Figure 2. One can see that the diffractive lenses—the ReSTOR and the Tecnis Multifocal—cause arcs around light sources. However, the arc effect is relatively smaller in the ReSTOR lens, primarily due to its apodized diffractive technology. The zonal refractive optic of the ReZoom can be seen to cause a continuous blur around light sources.
The take-home message of this study for clinicians is that, in nighttime situations, the apodized diffractive design of the ReSTOR lens devotes more light to the distance image and minimizes light scatter and resulting unwanted optical effects.
Best for patient satisfaction
Separate investigations found that the ReSTOR produces fewer stray light effects than the other two IOL models. These studies suggest that the ReSTOR will provide the clearest image on the retina and therefore fewer visual disturbances for our patients in night-lighting situations.
This is important for our potential multifocal IOL patients. Nighttime glare and halos can be visually debilitating and can lead to lens explantation or exchange and a dissatisfied patient. Even though we may counsel patients that these effects can lessen over time with neural adaptation, some patients are unwilling to wait, and some may never adapt. And a pair of glasses will not make dysphotopsia go away. Unwanted optical images remain the “Achilles heel” for this technology. Although significant clinical symptoms are uncommon this is of little consolation to the patient who is bothered by nighttime glare and halo. Unfortunately at present there is no way to determine which patients are at higher risk for these symptoms. Choosing a multifocal IOL that minimizes nighttime visual problems is critical to clinical success with this technology since the nighttime dysphotopsias rarely respond to conservative management and may ultimately require lens explantation. In contrast, with regard to near and midrange vision, multifocal IOL patients will be able to tolerate functioning with spectacles from time to time for specific visual tasks and it is rare for dissatisfaction with near vision or midrange vision to lead to explantation. Because patient satisfaction is vital in a refractive cataract practice, it is important for ophthalmologists to consider the differences in lens design when making recommendations to patients. The lens that produces the fewest unwanted optical images may be the best choice for these patients who value quality of vision, and it may help the surgeon and patient avoid another trip to the operating room.
Editors’ note
Dr. Schwiegerling has no financial interests related to this article. Dr. Tipperman has financial interests with Alcon (Fort Worth, Texas).
References
- Tipperman R. Pinhole image comparison with white-light source for 3 multifocal IOL designs. Poster presented at: ASCRS Symposium on Cataract, IOL and Refractive Surgery; April 27-May 2, 2007; San Diego, Calif.
- Schwiegerling J, Ye X, Choi J, Ernest P. Night-time visual quality with different multifocal IOLs. Poster presented at: Annual Meeting of the European Society of Cataract and Refractive Surgeons; September 9-13, 2006; London.
- Davison JA, Simpson JA. History and development of the apodized diffractive intraocular lens. J Cataract Refract Surg. 2006. 32: 849-858.
- Advanced Medical Optics, Inc. ReZoom™ Multifocal Posterior Chamber Intraocular Lenses. Santa Ana, CA. September 28, 2004.
