Should Retinal Surgeons Influence the Selection of Intraocular Lens Implants?
ROBERT M. KERSHNER, MD, MS, FACS
In the early days of ophthalmic surgery, patients who had previously undergone cataract procedures would sooner or later enter the domain of the retinal surgeon because of complications caused by surgical techniques and technology. Today, with improved cataract surgery techniques, smaller incisions, and foldable IOLs, the occurrence of cystoid macular edema, vitreous loss, retinal tears, and retinal detachment are much less common consequences of cataract removal.1
When IOLs replaced surgical aphakia in the late 70s, retinal complications concurrently decreased. Unfortunately, widespread acceptance of IOL implantation came with a price. Small lens optics and intact lens capsules all too often resulted in poor visualization of the peripheral retina. IOL implantation was especially discouraged in diabetics. The use of silicone material in the lens optic further raised concerns regarding the safety of using silicone oil for patients indicated for retinal surgery in the material's potential to impede air exchange in vitrectomy.2
Today's IOLs, the product of an innovation developed in 1949 by the late Sir Harold Ridley, have undergone tremendous advances. There are 3-piece IOLs of foldable material, capsular "C" haptics to fixate the lenses within the capsular bag, and square-edged optics to reduce the incidence of posterior capsule opacification (PCO).3
Figure 1. The Tecnis IOL contains a modified prolate anterior surface which neutralizes spherical aberration.
|Figure 2. Image of the fundus preoperatively on the left and postoperatively on the right demonstrating improved image contrast (corresponding histograms below each image)|
|Figure 3. The AcrySof aspheric IOL with blue-light filtering material offers a posterior aspheric surface to reduce spherical aberration.|
Figure 4. The silicone SofPort AO IOL has aspheric anterior and posterior surfaces, as well as uniform power from the center to the edge of the optic.
MATERIAL VS. DESIGN
With increasing advances in the technology of IOL design and manufacture, should retinal surgeons influence the type of IOL used for their patients? Certainly, the problems with visualization of the retina through a small optic should preclude using an IOL with a diameter smaller than 6 mm. Due to influence from their retina colleagues, a consensus exists among cataract surgeons that in patients with diabetes, silicone IOLs should be avoided in favor of IOLs with acrylic lens optics. However, is the preference for acrylic IOLs in patients with diabetes based on sound medical evidence or hearsay?
While acrylic is currently the most popular IOL material in use, no studies support the long-term safety of acrylic in the eye compared with more than 50-years' experience with PMMA, and more than 30-years' experience with silicone. It is possible that cataract surgeons select IOL optic material based more on marketing and ease of implantation than clinical data. Many surgeons believe that IOL optic material affects the incidence of PCO and, thus, the retinal surgeon's view of the fundus. However, recent data have demonstrated that IOL design, optic edge configuration, and haptic insertion are more responsible than the IOL material for whether lens capsular epithelial cells migrate and opacify the capsule.3-5
The use of UV-absorbing materials in the IOL manufacturing process was readily accepted by the ophthalmic community in the 1980s, based on the assumption that the human crystalline lens naturally absorbs wavelengths of light below 400 nm. However, the risk of allowing penetrating wavelengths of light to pass through an artificial lens and damage the retina spurred manufacturers to offer UV-absorbing chromophores in all of their IOL optics. There is little downside to filtering UV light, as it is easy to accomplish and poses no potential adverse effects. Studies show varying degrees of protection offered by today's lenses.6 Regardless, utilizing UV-absorbing IOLs is a widespread and accepted fact.
IOLs that block the visible spectrum of light are also available. Known as the "blue-blocker," the AcrySof Natural IOL (Alcon, Fort Worth, TX), a yellow acrylic IOL, absorbs light at 430 nm, which is a wavelength believed to be potentially damaging to the retinal pigment epithelium in culture.7 However, no population-based epidemiological studies exist to demonstrate that shielding the aging retina from the visible blue wavelength of light offers any protective advantage. Teleologically, it seems counterintuitive to filter out the predominant visible wavelength of light that floods the earth's surface by day. It has been suggested that maximally blocking light at the precise wavelength that provides scotopic luminous efficiency for night tasks, such as driving, walking down a flight of stairs, or viewing obstacles in a dark room, may make elderly patients more prone to falls and potential injury. In fact, in evaluating the blue-blocker IOL, the Food and Drug Administration Centers for Medicare and Medicaid panel denied the manufacturer new technology IOL status on their review of the literature and lack of objective data to support the claims. Further study must be performed to demonstrate a need for an IOL with these potentially significant downsides.
When a retinal surgeon examines the fundus with an indirect ophthalmoscope, he or she uses an aspheric lens (usually a 20-D handheld condensing lens) to bring the light rays emerging from the scope's illumination of the retina to converge in focus in front of the eye. Why would surgeons not use a simple spherical lens instead to accomplish the same purpose? The answer rests in the ability of an aspheric lens to neutralize spherical aberration induced by the cornea to allow viewing of the peripheral retina without distortion.
Aspheric IOLs can accomplish the same task in the human eye. Recent advances in IOL design involve applying wavefront technology to improve the optics and visual quality delivered through an IOL. The Tecnis IOL (AMO, Santa Ana, CA) has a modified prolate anterior surface (Figure 1). While the positive spherical aberration of the cornea combined with a conventional spherical IOL results in reduced contrast sensitivity, the modified prolate optic of the Tecnis successfully neutralizes corneal spherical aberration, resulting in better contrast sensitivity, functional visual acuity (VA), and patient safety.8
A prospective randomized study compared the effects of the Tecnis with those of a conventional silicone lens (AA4207VF, Staar Surgical, Monrovia CA) and a conventional acrylic lens (AcrySof SA60AT, Alcon) on visual performance. The goal of the study was to determine whether the addition of the modified optic improves retinal imaging and functional visual performance. Two-hundred twenty-one eyes of 156 patients were randomized to receive 1 of the 3 IOLs. Test methods included Snellen VA, digital retinal imaging, functional VA, and contrast testing.
Results indicate that all three IOLs improved VA after cataract surgery (with an average improvement of 20/35 or better). However, in the first month after surgery, patients implanted with the Tecnis IOL had better uncorrected VA compared to conventional spherical lenses.
In addition, this study measured retinal image contrast by analyzing the threshold luminance of fundus images taken through the cataract preoperatively and through the IOL at 1-month postoperatively (Figure 2). The Tecnis IOL group demonstrated a 28% improvement in retinal imaging over conventional spherical IOLs.
The Tecnis IOL is designed to neutralize 100% of corneal spherical aberration in over 96% of the population with a nonmodified average corneal curvature. In addition to the Tecnis, 2 other aspheric IOLs that are now available include the AcrySof aspheric optic SN50WF HOA IOL (Alcon) and the aberration-free aspheric SofPort AO IOL (Bausch & Lomb, San Dimas, CA).
The acrylate methacrylate copolymer AcrySof aspheric IOL (Figure 3) with blue-light filtering material offers a posterior aspheric surface to reduce spherical aberration as compared to standard spherical IOLs.
The silicone SofPort AO IOL (Figure 4) offers aspheric anterior and posterior surfaces, as well as uniform power from the center to the edge of the optic. This IOL is unique in that it is aberration-free, which the manufacturer states may provide improved vision for patients who have limited positive spherical aberration in their eyes due to natural causes or previous hyperopic refractive correction procedures. Additionally, if the aberration-free SofPort AO is decentered in the eye, it will not induce coma or astigmatism, which could occur with other aspheric IOLs if they are decentered after implantation.
Studies of the modulation transfer function (MTF) over a series of spatial frequencies for each of the IOLs demonstrate that the AcrySof HOA IOL is capable of reducing over 50% of the corneal spherical aberration in an average eye. The SofPort AO IOL demonstrates an MTF curve comparable to the 911A IOL (AMO), a similarly designed IOL to the Tecnis, but which does not have a modified prolate optic and has not demonstrated improved contrast compare to conventional spherical IOLs. Further clinical studies are currently underway for these new IOLs.
DO NO HARM
As cataract surgeons seriously consider the new options for the correction of presbyopia with multifocal and accommodative IOLs, will the patients be best served if they are implanted with these lenses and are at increased risk for retinal laser or surgical intervention? Is a multifocal optic the best choice when visualization of the fundus may be impaired in a patient with high myopia? Do the smaller lens optics, now decreased to 4.5 mm, necessitated by the new FDA-approved "accommodative" IOL, pose a new threat to retinal surgeons who may be unable to view the peripheral fundus through a scarred and clouded peripheral capsule?
Cataract surgeons must be cognizant of the potential for improvement in VA and functional vision of their patients as a consequence of IOL implantation. In following the primary doctrine, "physician, first do no harm," they must not select an IOL that has the potential to decrease visual potential or compromise the patient in any way from achieving the highest degree of visual functioning available with today's technology. Cataract surgeons should be meticulous to avoid potential problems with lens implantation in cases such as pseudoexfoliation, in which sulcus placement or scleral fixation may avoid the need for retinal intervention from IOL dislocation. A choice of an anterior chamber IOL may be preferable in cases in which adequate sulcus fixation cannot be achieved. New standards are being created with these new IOLs. Retinal surgeons should be aware of these changes and be prepared to argue strongly for that which is best for their patient's vision and for the surgeon's ability to care for the total ocular needs should retinal intervention be required.
1. Kershner, RM. Clear corneal cataract surgery and the correction of myopia, hyperopia and astigmatism. Ophthalmology. 1997;104(3):381-389.
2. Seok H, Chung JW, et al. Phacoemulsification and foldable intraocular lens implantation combined with vitrectomy and silicone oil tamponade for severe proliferative diabetic retinopathy. J Cataract Refract Surg. 2004;30(8):1721-1726,
3. Kiran A, Missier A, et al. Posterior capsule opacification: silicone plate-haptic versus AcrySof intraocular lenses. J Cataract Refract Surg. 2003;29(8):1569-1574.
4. Hayashi K, Hayashi H, et al. Changes in posterior capsule opacification after poly(methyl methacrylate), silicone, and acrylic intraocular lens implantation. J Cataract Refract Surg. 2001;27(6):817-824.
5. Sacu S, Meanpace R, et al. Effect of intraocular lens optic edge design and material on fibrotic capsule opacification and capsulorhexis contraction. J Cataract Refract Surg. 2004;30(9):1875-1882.
6. Laube T, Horst A, et al. Ultraviolet absorption of different intraocular lenses. Ophthalmology. 2004;111(5):880-885.
7. Sparrow J, Miller A, Zhou J. Blue light-absorbing intraocular lens and retinal pigment epithelium protection in vitro. J Cataract Refract Surg. 2004;30(4):873-878.
8. Kershner RM. Retinal image contrast and functional visual performance with aspheric, silicone, and acrylic intraocular lenses. J Cataract Refract Surg. 2003;29(9):1684-1694.
Address correspondence to: Robert M. Kershner, MD, MS, FACS, Eye Laser Consulting, Suite 19E, Two Avery Street, Boston, MA 02111-1003, telephone: (978) 270-3835, Fax: (617) 357-5077, e-mail: Kershner@EyeLaserConsulting.com
From Eye Laser Consulting, Boston, MA. Dr. Kershner has no financial interest in this information.