Ординатура / Офтальмология / Английские материалы / Wavefront Customized Visual Correction The Quest for Super Vision II_Krueger, Applegate, MacRae_2003

tomization of the functional optical zone to a specific pupil size is also possible within the Final Fit software. The advent of this unique ablation algorithm allows refractive surgeons to taper treatments that maintain quality of vision in photopic and scotopic conditions. A more detailed explanation of CATZ is available elsewhere in this book.

were 20/20 or better, and 14% were 20/15 or better. At 1 month postoperative 100% of the eyes had an uncorrected visual acuity (UCVA) of 20/40 or better, 89% were 20/20 or better, and 22% were 20/15 or better (Figure 31-5). As the majority of the treated patients were low and moderate myopes, magnification effects are an unlikely explanation for the gain in UCVA of 20/15 or better.

Editor’s note:

The CATZ system allows the surgeon to use an aspheric ablation that reduces the positive spherical aberration.

S. MacRae, MD

CATZ Clinical Results

One of the authors (AC) treated 35 consecutive eyes with a preliminary version of the CATZ software. At the time of this writing, 1 month results were available. Patients with up to -5.75 D of myopia and -3.25 D of cylinder preoperatively were treated. All eyes were targeted for emmetropia. The following results are for primary treatments only.

Prior to treatment, 100% of the eyes had best spectacle-cor- rected visual acuity (BSCVA) of 20/40 or better, 91% of the eyes

At 1 month postoperative 100% of the eyes had UCVA of 20/40 or better, 89% were 20/20 or better, and 22% were 20/15 or better.

The accuracy of this automated platform was well within accepted criteria. For example, at the day 1 postoperative visit, 100% of the treated eyes were within +0.5 D of emmetropia; at the 1 month visit, this changed to 100% within -1 D (Figure 31-6). Thirty-three percent of the treated eyes gained at least one line of vision postoperatively and 7% lost only one line of vision (Figure 31-7). Mesopic contrast sensitivity was measured using the Vector Vision chart (Precision Vision, Inc, LaSalle, Ill) with sinusoidal gratings. At the 1 month mark, average values showed no loss of mesopic contrast sensitivity. At 6 cycles per degree, 100% of the

268 Chapter 31

Figure 31-7. Change in BSCVA, 1 month postoperative.

patients maintained or gained lines of vision 1 month postoperatively. At 12 cycles per degree, 95% of patients either maintained or gained lines of vision. This preliminary data does demonstrate

safety and efficacy of the CATZ treatment along with the added benefit of either maintaining or enhancing the quality of vision in most patients.

INTRODUCTION

The development and manufacture of intraocular lenses (IOLs) for cataract surgery is evolving rapidly, and lenses that can be inserted/injected through incisions smaller than 2 millimeters (mm) will soon be marketed. However, the ability of the lens to be implanted through very small incisions is not the only feature sought by researchers, manufacturers, and surgeons. Modern cataract surgery is now in the realm of refractive surgery and patients expect almost perfect results. Thus, the most energy and funding is probably being spent on the development of new and complex IOLs that not only restore the refractive power of the eye after cataract surgery, but also provide special features, including multifocality, toric corrections, pseudoaccommodation, etc.1 Some of these aspects may also apply to the development of new phakic IOLs, which are increasing in popularity, since they can potentially be used for the correction of different refractive errors.2,3

Since the development of wavefront aberrometry, it is possible to clinically measure the optical defects of the eye beyond spherical and cylindrical aberrations (lower-order aberrations). This technology can be used to quantify ocular aberrations and then design the ideal refractive correction for each patient, which may include a customized corneal ablative correction but also a customized contact lens, an IOL, or a combination of the above. It is expected that increasing understanding and availability of wavefront technology will greatly influence the manufacture of IOLs.4

The aim of this chapter is to present, in the first part, a brief overview of the biomaterials that can potentially be used for the manufacture of IOLs in association with wavefront technology to provide patients with a quality of vision beyond that with currently available lenses. In the second part, we describe two examples of recently developed “customized” IOLs for cataract surgery.

BIOMATERIALS FOR INTRAOCULAR

LENS OPTICS: BRIEF OVERVIEW

Biomaterials (polymers) currently used for the manufacture of IOL optics can be divided into two major groups, namely acrylic

and silicone.5-9 Acrylic lenses can be further divided as follows:

•Rigid (eg, manufactured from polymethyl methacrylate [PMMA]) (Figure 32-1)

•Foldable

–Manufactured from hydrophobic acrylic materials (eg, AcrySof [Alcon Laboratories, Fort Worth, Tex] and Sensar [Advanced Medical Optics, Santa Ana, Calif]) (Figures 32-2 and 32-3)

–Manufactured from hydrophilic acrylics also known as hydrogels (eg, Hydroview [Baush & Lomb, Rochester, NY], MemoryLens [CIBA Vision, Duluth, Ga], or Centerflex [Rayner Intraocular Lenses Ltd, BrightonHove, East Sussex, England]) (Figure 32-4)

Polymerization is the process by which the repeating units forming a polymer (the monomers) are linked by covalent and therefore stable bonds. Methyl methacrylate, for example, is the monomer used for the manufacture of PMMA. The latter is a rigid, linear acrylic polymer. Three-dimensional, flexible acrylic polymers can be created by a process known as crosslinking. When different monomers are polymerized together, the process is called copolymerization. Each currently available foldable acrylic lens design is manufactured from a different copolymer acrylic, with different refractive index; glass transition temperature (above this temperature the polymer exhibits flexible properties and below it remains rigid); water content; mechanical properties; etc. Hydrophobic acrylic lenses have a very low water content, lower than 1%. Most of the currently available hydrophilic acrylic lenses are manufactured from copolymer acrylics with a water content ranging from 18% to 38%. One exception is represented by a lens manufactured in Brazil (Acqua, Mediphacos, Belo Horizonte, MG, Brazil), which has a water content of 73.5%. This expandable lens is inserted in a dry state and attains its final dimensions within the capsular bag after hydration and expansion. The Collamer material (STAAR Surgical, Monrovia, Calif) can also be included in the category of hydrophilic acrylic materials. This is composed of a proprietary copolymer of a hydrophilic acrylic material and porcine collagen, with a water content of 34%.

272 Chapter 32

Figure 32-1. Chemical structure of polymethyl methacrylate (PMMA), and gross photograph from a human eye obtained postmortem implanted with a one-piece PMMA lens, with blue-col- ored PMMA haptics (posterior or Miyake-Apple view).

Figure 32-3. Chemical structure of the Sensar material (ethyl acrylate/ethyl methacrylate), and clinical photograph of a patient implanted with a Sensar lens. (Courtesy of Advanced Medical Optics).

Polymerization is the process by which the repeating units forming a polymer (the monomers) are linked by covalent and therefore stable bonds. Three-dimensional, flexible acrylic polymers can be created by a process known as crosslinking. When different monomers are polymerized together, the process is called copolymerization.

Silicones are known chemically as polysiloxanes based on their silicon-oxygen molecular backbone, which confers mechanical flexibility to the materials. Pendant to the silicone backbone are organic groups, which determine mechanical and optical properties. The first silicone material used in the manufacture of IOLs was polydimethyl siloxane, which has a refractive index of 1.41. Polydimethyl diphenyl siloxane is a later generation silicone IOL material that has a higher refractive index than polydimethyl siloxane (1.46) (Figure 32-5). As these polymers exhibit elastic behavior, they are also called elastomers. While foldable acrylics display glass transition temperatures at around room temperature, the glass transition temperature of silicones can be significantly below room temperature. Another differentiating property between foldable acrylics and silicones is the refractive index, which is higher in the first group (1.47 or greater) so acrylic lenses are thinner than silicone lenses for the same refractive power.

Figure 32-2. Chemical structure of the AcrySof material (phenylethyl acrylate/phenylethyl methacrylate), and gross photographs from human eyes obtained postmortem implanted with a three-piece (top), and with a one-piece (bottom) AcrySof lens.

Figure 32-4. Chemical structure of a generic hydrophilic acrylic/hydrogel copolymer and gross photograph from a human eye obtained postmortem implanted with a MemoryLens. The material of the optic of this particular lens design is in fact composed of hydroxyethyl methacrylate/methyl methacrylate.

Another differentiating property between foldable acrylics and silicones is the refractive index, which is higher in the first group (1.47 or greater) so acrylic lenses are thinner than silicone lenses for the same refractive power.

Other important elements of the IOL optic component are represented by the ultraviolet-absorbing compounds (chromophores). These are incorporated to the IOL optic in order to protect the retina from ultraviolet radiation in the 300to 400nanometer (nm) range, a protection normally provided by the crystalline lens. Two classes of ultraviolet-absorbing chro-

Figure 32-5. Chemical structures of two silicone elastomers used in the manufacture of IOLs, polydimethyl siloxane and polydimethyl diphenyl siloxane, and gross photographs from human eyes obtained postmortem implanted with a one-piece silicone lens with large fixation holes (STAAR Surgical, Monrovia, Calif, top), and with a 3-piece silicone optic-PMMA haptic lens (SI-40 NB, AMO; bottom).

Figure 32-7. Gross photograph showing the design of the Calhoun LAL.

mophores are used in general for the manufacture of pseudophakic IOLs, namely benzotriazole and benzophenone (Figure 32-6).

“CUSTOMIZED” INTRAOCULAR

LENSES FOR CATARACT SURGERY

Light Adjustable Lens

Calhoun Vision (Pasadena, Calif) is developing a three-piece silicone lens with photosensitive silicone subunits that move within the lens upon fine tuning with a low intensity beam of near-ultraviolet light (the Calhoun Light Adjustable Lens [LAL]).10,11 The refractive power of the lens can be adjusted noninvasively after implantation to give the patient a definitive

Figure 32-6. Chemical structures of two classes of ultravioletabsorbing chromophores used in the manufacture of IOL optics.

refraction. This new technology is a potential way to correct an important postoperative complication associated with IOL implantation. Indeed, according to the American and European Societies of Cataract and Refractive Surgeons (ASCRS/ESCRS) survey on IOL explantation conducted by Nick Mamalis, MD, in the last 4 years, incorrect lens power was the most important reason for explantation of modern foldable IOLs.12

In the last 4 years, incorrect lens power was the most important reason for explantation of modern foldable IOLs.12

The Calhoun LAL is a foldable three-piece lens; the diameter of the biconvex optic is 6 mm and the overall diameter of the lens is 13 mm (Figure 32-7). The optic component is manufactured from a silicone material, polydimethyl siloxane with a refractive index of 1.43. The optic rim of this lens has square truncated edges. The haptics are manufactured from PMMA. The haptic design is a modified C, with an angulation of 10 degrees. The optic lens material has an incorporated ultraviolet absorber to protect the retina from radiation in the 300to 400-nm range (benzotriazole).

According to Robert K. Maloney, MD, who is working with the prototype of the Calhoun LAL developed by Daniel Schwartz, MD, when the eye is healed 2 to 4 weeks after implantation, the refraction is measured and a low intensity beam of light is used to correct any residual error. The mechanism for dioptric change is akin to holography.

The application of the appropriate wavelength of light onto the central optical portion of the Calhoun LAL polymerizes the macromer in the exposed region, thereby producing a difference in the chemical potential between the irradiated and nonirradiated regions. To re-establish thermodynamic equilibrium, unreacted macromer and photoinitiator diffuses into the irradiated region. As a consequence of the diffusion process and the material properties of the host silicone matrix, the Calhoun LAL will swell, producing a concomitant decrease in the radius of curvature of the lens. This process may be repeated if further refractive change in the Calhoun LAL is desired or an irradiation of the entire lens may be applied, consuming the remaining, undif-

fused, unreacted macromer and photoinitiator. This action has the effect of “locking” in the refractive power of the Calhoun LAL. It should be noted that it is possible to induce a myopic change by irradiating the edges of the Calhoun LAL to effectively drive macromer and photoinitiator out of the central region of the lens, thereby increasing the radius of curvature of the lens and decreasing its power. Astigmatism is treated by using a band-shaped pattern of irradiation across the center of the lens, orienting the light beam along the astigmatic axis. After verification of the new refraction, the surgeon “locks in” the power by irradiating the entire lens optic, a procedure that does not affect the final lens power obtained. This step is very important, as some remaining photosensitive unpolymerized macromers may polymerize in ambient light. Indeed, during the interval between lens implantation and light adjustment, patients need to wear sunglasses with ultraviolet absorbers while performing outside activities. This is necessary in order to avoid unwanted, noncontrolled polymerization of the silicone macromers with unpredictable results regarding change in the IOL power. However, at least 1 hour of exposure to ambient light would be necessary for any significant polymerization to occur.

An in vitro example of the treatment process is shown in Figure 32-8. To simulate the human eye environment, a Calhoun LAL was fixtured in a water cell maintained at 35°C. The Calhoun LAL was placed one focal length away from the focus of a 4 inch (in) transmission sphere fitted to a Fizeau interferometer. Figure 32-8A displays the interference fringes of the Calhoun LAL at its best focus position prior to irradiation. The periphery of the Calhoun LAL was irradiated, causing diffusion of macromer and photoinitiator from the central portion of the lens out to the edges. Figure 32-8B displays the interference fringes 24 hours postirradiation at the original best focus position. The most striking feature is the addition of approximately 12 (in double pass) fringes of defocus (or wavefront curvature) added to the lens, which corresponds to -1.5 diopter (D) of myopic correction. After the Calhoun LAL is irradiated, it is imperative that the lens maintains a resolution efficiency that is acceptable to the patient.

The design of the device for light application is similar to a slit lamp coupled to a computer system. The preparation of the patient for the procedure involves pupil dilation and topical anesthesia. A television monitor helps to control the focus of the

system, directed at the optic-haptic junctions of the lens, and a reticule target with a diameter of 6 mm is aligned with the edge of the optic component of the lens. After entering the base power of the Calhoun LAL and the correction needed, the computer selects the appropriate mask in order to deliver the light beam at the center or the periphery of the lens, as well as the intensity of the light beam and duration of the treatment. The light application for postoperative adjustment of IOL power is of short duration. For example, if a correction of -2 D for a Calhoun LAL with a base power of +20 D is necessary, a light beam with an intensity of 10.00 milliwatts per square centimeter (mw/cm2) is applied for 120 seconds. The time necessary for the treatment varies slightly according to the correction necessary. For the lock-in of the IOL power, a light beam with a higher intensity is used to irradiate the whole lens, and the treatment in general lasts approximately 2 minutes.

The light application for postoperative adjustment of IOL power is of short duration. For example, if a correction of -2 D for a Calhoun LAL with a base power of +20 D is necessary, a light beam with an intensity of 10 mw/cm2 is applied for 120 seconds.

The biocompatibility of the lens and the reproducibility of the results after light adjustment have been evaluated in animal models. Maloney et al implanted the Calhoun LAL in rabbits after phacoemulsification.10 The power of the lenses was then adjusted at 2 weeks postoperatively, followed by explantation. The authors reported that corrections of +2.68 ± 0.26 D, +0.98 ± 0.16 D, +0.71 ± 0.05 D, -0.64 ± 0.21 D, and -1.02 ± 0.09 D were achieved in vivo. They did not note any evidence of keratitis or chronic inflammatory reaction in the anterior chamber.

Mamalis et al also evaluated the biocompatibility of the Calhoun LAL as well as the change in power following light irradiation in a rabbit model (Figures 32-9 and 32-10).11 In this study, the lenses underwent power adjustment using low-level light exposure, but also maximum dosage exposure was performed to assess the safety of the light exposure. The Calhoun LALs were then evaluated with optical bench testing of the power adjustment, and the rabbit eyes underwent complete histopathologic