The youthful, unaberrated human eye has become the standard by which patients and ophthalmologists evaluate the results of cataract and refractive surgery today. Contrast sensitivity testing confirms an age-related decline in visual performance that has been shown by wavefront technology to result from increasing spherical aberration of the human lens. The determination that the optical wavefront of the cornea remains stable throughout life1 has prompted many ophthalmologists to view the crystalline lens as the superior locus for refractive surgery.
Additionally, laboratory studies of accommodation confirm the essentials of the theory put forth by Hermann von Helmholtz, MD, of Germany that the ciliary muscle moves forward and axially in the eye during contraction, which releases tension on the anterior zonular fibers and allows the lens to become more spherical and thicken axially.2 The studies also clarify the pathophysiology of presbyopia.3 This research has prompted optical scientists and materials engineers to attempt to design an IOL that provides unaberrated optical imagery at all focal distances. This lens must compensate for any aberrations inherent in the cornea and either change its shape and location or employ multifocal optics.
IOL INNOVATION
The creation of accommodative IOLs such as the CrystaLens (C&C Vision, Aliso Viejo, CA) and the Akkommodative 1CU (HumanOptics AG, Erlangen, Germany) has excited ophthalmologists worldwide. Clinical results indicate that axial movement of the lens optic can restore accommodation.4 The impact of long-term capsular fibrosis on the function of these lens designs is unknown. Capsular opacification can continue to increase 3 to 6 years postoperatively.5 As part of the US FDA-monitored clinical investigation, we have implanted 96 of the CrystaLenses, and we have been pleased to see that distance-corrected near vision is retained and even enhanced following an Nd:YAG capsulotomy. For this lens, at least, we have begun to answer the question of long-term stability.
Meanwhile, flexible polymers designed for injection into a nearly intact capsular bag continue to show promise in animal studies.6 These lens prototypes require the surgeon to extract the crystalline lens through a tiny capsulorhexis. Consequently, they raise concerns about whether the polymer will leak during an Nd:YAG capsulotomy for the development of posterior or anterior capsular opacification.
A unique lens now in laboratory development involves a thermoplastic acrylic gel, which may be shaped into a thin rod and inserted into the capsular bag. In the aqueous environment at body temperature, the SmartLens (Medennium, Inc., Irvine, CA) unfolds into a full-sized, flexible IOL that adheres to the capsule and may restore accommodation. Its designers believe that, like the youthful crystalline lens, this IOL may increase its surface curvature and thicken in response to changes in the shape of the lens capsule.
Another intriguing design involves the Light Adjustable Lens (LAL) (Calhoun Vision, Inc., Pasadena, CA), a macromer matrix that polymerizes under ultraviolet radiation. An injectable form of this lens material might enable surgeons to refill the capsular bag with a flexible substance and subsequently use light to adjust the optical configuration in order to eliminate aberrations.
CONSIDERING MULTIFOCAL LENSES
Advantages
Multifocality also offers a variety of potential solutions to the loss of accommodation and to the increased aberrations associated with aging. For example, multifocal IOLs allow patients to perceive images at several focal distances without relying on ciliary body function and capsular mechanics. Once the lenses are securely placed in the capsular bag, their function will not change or deteriorate. Additionally, multifocal lenses may be designed to take advantage of many improvements in IOL technology, an approach that has already enhanced visual outcomes. These improvements include better centration, the prevention of posterior capsular opacification by means of a square-edged design and posterior haptic angulation, and the ability to correct higher-order aberrations.
Technological Hurdles
The major remaining challenge of multifocality is preserving optical quality—as measured by modulation transfer function on the bench or contrast sensitivity function in the eye—upon the simultaneous presentation of objects at two or more focal lengths. Another challenge for this technology is to reduce or eliminate associated, unwanted, photic phenomena such as halos. The designers of multifocal optics must consider whether two foci (distance and near) adequately address an individual's visual needs, or if an intermediate focal length is required. Adding an intermediate distance increases the complexity of the manufacturing process and may degrade the optical quality of the lens. For example, the Array multifocal lens (Advanced Medical Optics, Inc., Santa Ana, CA) has an intermediate focal length that consumes 13% of incoming light.
The Array Lens
We have had success recently with the Array lens for both cataract surgery and refractive lens exchange, largely due to careful patient selection.7 We inform all patients preoperatively of the likelihood that they will temporarily see halos around lights at night. If patients demonstrate a sincere motivation for spectacle independence and display minimal concern about optical side effects, then we consider them to be good candidates for the Array. These individuals can achieve their goals with the Array lens and they represent some of the happiest patients in our practice.
In the near future, the Array will probably become available on an acrylic platform, similar to that used with the Sensar OptiEdge AR40e IOL (Advanced Medical Optics, Inc.). The new multifocal IOL will incorporate a sharp posterior-edge design, which is likely to inhibit migration of lens epithelial cells.8 This means of preventing posterior capsular opacification represents a special benefit to Array patients, because minimal peripheral changes in the capsule cause early deterioration in their near vision. Advanced Medical Optics also plans to manufacture the silicone Array with a sharp posterior edge similar in design to that of its ClariFlex IOL.
Pharmacia's New Multifocal IOL
Recent advances in aspheric monofocal lens design may lead to related improvements in multifocal IOLs. The spherical aberration of a manufactured spherical IOL tends to worsen a patient's total optical aberrations. Aberrations reduce visual quality, a problem that worsens under low-luminance conditions, because spherical aberration increases along with the pupil size.
The design of the Tecnis lens (Pharmacia Corporation, Peapack, NJ), which is currently undergoing an FDA-monitored New Technology IOL investigation in the US, possesses the basic design characteristics of Pharmacia's CeeOn Edge 911A IOL, including a 6.00-mm, biconvex, square-edged silicone optic and angulated, cap C, polyvinylidene fluoride haptics. By compensating for average corneal spherical aberration, however, the Tecnis IOL's modified prolate anterior surface reduces the total amount of aberration in the eye. Clinical studies show significant improvement in contrast sensitivity and functional vision with this new IOL.9
Pharmacia plans to apply this foldable prolate design to its diffractive multifocal 811E IOL, which is currently available only in Europe. This new prolate, diffractive, foldable, multifocal IOL will be available in Europe early this year, and Pharmacia expects to receive CE Mark approval for it at that time (Figure 1). The FDA-monitored clinical trials of the IOL are expected to begin in the third quarter of this year.
A diffractive design creates multifocality by generating an interference pattern. Based on the Huygens-Fresnel principle, the diffractive portion of the lens uses discrete zone steps to control the wave propagation of light and produce multiple powers for distance and near focus. This concentric, diffractive microstructure acts like a series of slits, which creates multiple, advancing optical wavefronts. As the waves intersect, they produce constructive interference at the desired focal lengths and thereby create an image. By contrast, the Array lens features a zonal progressive refractive design. Altering the surface curvature of the lens increases the effective lens power. The dioptric power changes with the radius in each annular zone.
The 811 LE, an earlier multifocal IOL produced by 3M (St. Paul, MN) that featured a diffractive design, failed to gain wide acceptance due to poor production quality and the relatively large incision size required for its implantation. The FDA-monitored study of the 3M lens reported a UCVA of 20/40 and J3 or better in only 50.4% of best-case patients.10
Not all diffractive lenses are created equal, however. A prospective comparison of Pharmacia's new 811E lens with the Array's predecessor, the PA154N (Allergan, Inc., Irvine, CA)—both one-piece, 6.00-mm, PMMA IOLs—found significantly better uncorrected near visual acuity with the diffractive lens (mean = J1) than with the refractive lens (mean = J4). There was no significant difference in unaided distance acuity between the group, nor were there any differences in glare visual acuity or contrast sensitivity.11
Additionally, a recent model eye study comparing the 811E with the Array lens revealed comparable modulation transfer functions at distance and superiority of the diffractive lens at near.12 Pharmacia's optical bench studies reveal a superior modulation transfer function at both distance and near when comparing its new multifocal lens with a standard monofocal IOL on a 5.00-mm pupil and equivalence on a 4.00-mm pupil (Figure 2). Clinically, a study of reading performance with the Array versus the 811E demonstrated acceptable results for both IOLs with regard to reading acuity and speed, but the diffractive lens had a definite advantage in a standardized-test setting.13 These studies indicate that combining diffractive, multifocal optics with an aspheric, prolate design will enhance functional vision for pseudophakic patients.
Alcon's New Multifocal IOL
Alcon Laboratories, Inc. (Fort Worth, TX), is currently completing FDA phase III clinical trials of a new diffractive multifocal IOL, the design of which is based on its 6.00-mm, foldable, three-piece AcrySof acrylic IOL (Figure 3). The diffractive region of the new lens is confined to the center, so the IOL's periphery is identical to a monofocal acrylic lens. The inspiration for this approach is that, due to the synkinetic reflex, most patients will experience miosis when performing near work. Placing multifocal optics beyond the 3.00-mm optical zone offers no benefit to the patient and, in effect, diminishes optical quality. In fact, bench studies performed by Alcon show an advantage in modulation transfer function for this central diffractive design, especially with a small pupil at near and a large pupil at distance.
CONCLUSION
Multifocal technology has already improved many pseudophakic patients' quality of life by reducing or eliminating their need for spectacles. Presbyopia can be a particularly maddening process, but multifocal pseudoaccommodation can simplify its correction.
Mark Packer, MD, is Clinical Assistant Professor at the Casey Eye Institute, Department of Ophthalmology, Oregon Health and Science University, and is in private practice at Drs. Fine, Hoffman & Packer, LLC. He is a consultant for Pharmacia. Dr. Packer may be reached at (541) 687-2110; mpacker@finemd.com.
I. Howard Fine, MD, is Clinical Professor at the Casey Eye Institute, Department of Ophthalmology, Oregon Health and Science University, and is in private practice at Drs. Fine, Hoffman & Packer, LLC. He is a consultant for Advanced Medical Optics. Dr. Fine may be reached at (541) 687-2110; hfine@finemd.com.
Richard S. Hoffman, MD, is Clinical Instructor at the Casey Eye Institute, Department of Ophthalmology, Oregon Health and Science University, and is in private practice at Drs. Fine, Hoffman & Packer, LLC. He holds no financial interest in any of the products mentioned herein. Dr. Hoffman may be reached at (541) 687-2110; rhoffman@finemd.com.
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