The youthful, unaberrated human eye has be- come the standard by which we evaluate the results of cataract and refractive surgery to- day. Contrast sensitivity testing has con- firmed the decline in visual performance with age, and wavefront science has helped explain that this decline occurs because of in- creasing spherical aberration of the human lens. Since we have learned that the optical wavefront of the cornea remains stable throughout life, the lens has started to come into its own as the primary locus for refrac- tive surgery. At the same time, laboratory studies of accommodation have now con- firmed the essentials of Helmholtz’s theory and have clarified the pathophysiology of presbyopia.What remains is for optical scien- tists and materials engineers to design an in- traocular lens (IOL) that provides unaberrat- ed optical imagery at all focal distances. This lens must, therefore, compensate for any aberrations inherent in the cornea and either change shape and location or employ multi- focal optics.
Accommodative IOLs have now made their debut around the world (CrystaLens, Eyeonics and 1CU, HumanOptics). Clinical results indicate that restoration of accommo- dation can be achieved with axial movement of the lens optic [1]. However, concerns re- main about the impact of long-term capsular fibrosis on the function of these designs.
Flexible polymers designed for injection into a nearly intact capsular bag continue to show
promise in animal studies [2]. These lens pro- totypes require extraction of the crystalline lens through a tiny capsulorrhexis and raise concerns about leakage of polymer in the case of YAG capsulotomy following the devel- opment of posterior or anterior capsular opacification. A unique approach now in lab- oratory development involves the utilization of a thermoplastic acrylic gel, which may be shaped into a thin rod and inserted into the capsular bag (SmartLens, Medennium). In the aqueous environment at body tempera- ture it unfolds into a full-size flexible lens that adheres to the capsule and may restore ac- commodation. Another unique design in- volves the light-adjustable lens, a macromer matrix that polymerizes under ultraviolet ra- diation (LAL, Calhoun Vision). An injectable form of this material might enable surgeons to refill the capsular bag with a flexible sub- stance and subsequently adjust the optical configuration to eliminate aberrations.
While these accommodating designs show promise for both restoration of accommoda- tion and elimination of aberrations, multifo- cal technology also offers an array of poten- tial solutions. Multifocal intraocular lenses allow multiple focal distances independent of ciliary body function and capsular mechan- ics. Once securely placed in the capsular bag, the function of these lenses will not change or deteriorate. Additionally, multifocal lenses can be designed to take advantage of many innovations in IOL technology, which have Mark Packer, I. Howard Fine, Richard S. Hoffman
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already improved outcomes, including better centration, prevention of posterior capsular opacification and correction of higher-order aberrations.
The fundamental challenge of multifocali- ty remains preservation of optical quality, as measured by modulation transfer function on the bench or contrast sensitivity function in the eye, with simultaneous presentation of objects at two or more focal lengths. Another significant challenge for multifocal technolo- gy continues to be the reduction or elimina- tion of unwanted photic phenomena, such as haloes. One question that the designers of multifocal optics must consider is whether two foci, distance and near, adequately ad- dress visual needs, or if an intermediate focal length is required. Adding an intermediate distance also adds greater complexity to the manufacture process and may degrade the optical quality of the lens.
We have been able to achieve success with the AMO Array multifocal IOL for both cataract and refractive lens surgery, largely be- cause of careful patient selection [3]. We in- form all patients preoperatively about the like- lihood of their seeing haloes around lights at night, at least temporarily. If patients demon- strate sincere motivation for spectacle inde- pendence and minimal concern about optical side-effects, we consider them good candi- dates for the Array. These patients can achieve their goals with the Array, and represent some of the happiest people in our practice.
In the near future, the Array will likely be- come available on an acrylic platform, similar to the AMO AR40e IOL. This new multifocal IOL will incorporate the sharp posterior edge design (“Opti Edge”) likely to inhibit migra- tion of lens epithelial cells. Prevention of pos- terior capsular opacification represents a spe- cial benefit to Array patients, as they suffer early deterioration in near vision with mini- mal peripheral changes in the capsule. AMO also plans to manufacture the silicone Array with a sharp posterior edge (similar to their Clariflex design).
The Array employs a zonal progressive re- fractive design. Alteration of the surface cur- vature of the lens increases the effective lens power and recapitulates the entire refractive sequence from distance through intermedi- ate to near in each zone.A different concept of multifocality employs a diffractive design.
Diffraction creates multifocality through constructive and destructive interference of incoming rays of light. An earlier multifocal IOL produced by 3M employed a diffractive design. It encountered difficulty in accept- ance, not because of its optical design but rather due to poor production quality and the relatively large incision size required for its implantation.
Alcon is currently completing clinical tri- als of a new diffractive multifocal IOL based on the 6.0-mm foldable three-piece AcrySof acrylic IOL. The diffractive region of this lens is confined to the center, so that the periphery of the lens is identical to a monofocal acrylic IOL. The inspiration behind this approach comes from the realization that during near work the synkinetic reflex of accommoda- tion, convergence and miosis implies a rela- tively smaller pupil size. Putting multifocal optics beyond the 3-mm zone creates no ad- vantage for the patient and 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 (Figs. 15.1 and 15.2).
Recent advances in aspheric monofocal lens design may lend themselves to improve- ments in multifocal IOLs as well.We now real- ize that the spherical aberration of a manufac- tured spherical intraocular lens tends to worsen total optical aberrations. Aberrations cause incoming light that would otherwise be focused to a point to be blurred, which in turn causes a reduction in visual quality. This re- duction in quality is more severe under low luminance conditions because spherical aber- ration increases when the pupil size increases.
The Tecnis Z9000 intraocular lens (AMO, Santa Ana, CA) has been designed with a mod- ified prolate anterior surface to reduce or elim- inate the spherical aberration of the eye. The Tecnis Z9000 shares basic design features with the CeeOn Edge 911 (AMO), including a 6-mm
biconvex square-edge silicone optic and angu- lated cap C polyvinylidene fluoride (PVDF) haptics. The essential new feature of the Tecnis IOL,the modified prolate anterior surface,com- pensates for average corneal spherical aberra- tion and so reduces total aberrations in the eye.
Fig. 15.1. The Alcon AcrySof multifocal IOL
Fig. 15.2. Diffractive vs. zonal refractive optics (AcrySof vs. Array)
Clinical studies show significant improve- ment in contrast sensitivity and functional vision with the new prolate IOL [4]. AMO plans to unite this foldable prolate design with their diffractive multifocal IOL current- ly available in Europe (811E) (Fig. 15.3). Im- proved visual performance and increased in- dependence for patients constitute the fundamental concept behind this marriage of technologies. This new prolate, diffractive, foldable, multifocal IOL has received the CE mark in Europe. Introduction of the IOL in the USA will be substantially later. Food and Drug Administration-monitored clinical tri- als were expected to begin in the fourth quar- ter of 2004. Optical bench studies reveal supe- rior modulation transfer function at both distance and near when compared to stan- dard monofocal IOLs with a 5-mm pupil, and equivalence to standard monofocal IOLs with
a 4-mm pupil (Fig. 15.4). When compared to the Array multifocal IOL, the Tecnis IOL has better function for a small, 2-mm pupil at near and for a larger, 5-mm pupil at both dis- tance and near (Fig. 15.5). From these studies, it appears that combining diffractive, multi- focal optics with an aspheric, prolate design will enhance functional vision for pseudo- phakic patients.
Multifocal technology has already im- proved the quality of life for many pseudo- phakic patients by reducing or eliminating their need for spectacles. We (i.e., those of us over 40) all know that presbyopia can be a particularly maddening process. Giving surgeons the ability to offer correction of presbyopia by means of multifocal pseu- do-accommodation will continue to enhan- ce their practices and serve their patients well.
Fig. 15.3. The Tecnis ZM001, CeeOn 911A, Tecnis Z9000, and CeeOn 811E IOLs
Fig. 15.4. Multifocal vs. monofocal IOLs
Fig. 15.5. Diffractive vs. zonal refractive optics (Array vs. Tecnis)
References
1. Doane J (2002) C&C CrystaLens AT-45 accom- modating intraocular lens. Presented at the XX Congress of the ESCRS, Nice, Sept 2002 2. Nishi O, Nishi K (1998) Accommodation am-
plitude after lens refilling with injectable sili- cone by sealing the capsule with a plug in pri- mates. Arch Ophthalmol 116:1358-1361
3. Packer M, Fine IH, Hoffman RS (2002) Refrac- tive lens exchange with the Array multifocal intraocular lens. J Cataract Refract Surg 28:
421–424
4. Packer M, Fine IH, Hoffman RS, Piers PA (2002) Initial clinical experience with an ante- rior surface modified prolate intraocular lens.
J Refract Surg 18:692–696
