Ординатура / Офтальмология / Английские материалы / Phakic Intraocular Lenses_Hardten, Lindstrom, Davis_2004

Figure 21-11. High resolution ultrasound image of an implanted ThinOptx phakic IOL. Note large corneal/lens distance due to thin lens thickness (courtesy of ThinOptx, Inc).

pupils than other anterior chamber lenses with smaller optics. The company plans to offer a bifocal version to address the presbyopic market. Several lenses have been implanted outside the United States with good success. European and US trials may begin during the third and fourth quarter 2003.20

LIGHT ADJUSTABLE

INTRAOCULAR LENSES

Ametropia following phakic IOL implantation will inevitably occur as these lenses become integrated into the surgical armamentarium of refractive surgeons. The Van der Heijde formula, used to calculate dioptric power of phakic IOL, does not use the axial length, removing one possible source of measurement error present in standard IOL calculations.21 However, it does make use of an estimation of the distance between the corneal plane and the IOL plane, the anterior chamber depth, as well as the patient’s subjective refraction. While lens exchange or bioptics are certainly options for the patient with significant ametropia following phakic IOL implantation, a much less invasive approach may be available in the future.

A Light Adjustable Lens (LAL) (Calhoun Vision, Inc, Pasadena, Calif) is currently under development.22,23 This technology is designed to provide noninvasive adjustment and correction of residual postimplantation refractive errors following cataract surgery by applying near-ultravi- olet light to an IOL composed of a cross-linked silicone polymer matrix, a guest macromer, and a photoinitiator. The application of the appropriate wavelength of light onto the central optical portion of the 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 diffuse into the irradiated region. As a consequence of the diffusion process and the material properties of the

Figure 21-12. LAL: mechanism of action. Schematic illustrating the proposed mechanism of swelling. A. Selective irradiation of the central zone of the IOL polymerizes macromer, creating a chemical potential between the irradiated and nonirradiated regions. B. To re-establish equilibrium, excess macromer diffuses into the irradiated region, causing swelling. C. Irradiation of the entire IOL “locks” the macromer and the shape change (courtesy of Calhoun Vision, Inc).

host silicone matrix, the LAL will swell, producing a concomitant decrease in the radius of curvature of the lens and a corresponding increase in lens power (Figure 21-12). This process may be repeated if the surgeon wants further refractive change in the lens. The surgeon may then irradiate the entire lens, consuming the remaining, unreacted macromer and photoinitiator. This action effectively locks in the refractive power of the lens. The surgeon may induce a myopic change by irradiating the edges of the LAL to effectively drive macromer and photoinitiator out of the lens’ central region, thereby increasing the radius of curvature and decreasing the power. Animal studies in rabbits have found no significant inflammation or signs of toxicity, including no corneal changes, following maximum light exposure.24 Initial human trials are set to begin this year. The company plans to develop the technology for phakic IOLs in the future.

ACCOMMODATIVE MULTIFOCAL

INTRAOCULAR LENSES

The primary stimulus for the development of phakic IOLs was to avoid the loss of natural accommodation resulting from clear lens extraction in young, prepresbyopic eyes. Another concern, particularly in highly myopic eyes, is the risk of retinal detachment, either as a result of surgical manipulation during lenticular surgery and/or from the anatomically altered pseudophakic lens/capsule/zonule/ciliary body apparatus. Multifocal IOLs have been used in cataract surgery for a number of years, providing adequate pseudoaccommodative results for some elderly patients who have long since lost natural accommodation and who are tolerant of compromised near and distance visual function.

Figure 21-13. Array multifocal silicone lens (courtesy of Advanced Medical Optics).

Array Multifocal Intraocular Lens

The Array lens (Advanced Medical Optics, Irvine, Calif) received US Food and Drug Administration (FDA) approval in September 1997 and is the most commonly implanted multifocal lens in the United States (Figure 2113). This silicone lens has a 6.0-mm optic that is a “zonal progressive” design in that it incorporates five blended aspheric zones of power on the anterior surface. The central 2.1 mm is dedicated to distance vision while the ring from 2.1 to 3.4 mm is for near vision. There are three more peripheral zones for distance and near. The Array lens is “distance dominant” because 50% of the light transmission is assigned to distance, 13% to intermediate, and 37% to near vision. A large, multi-center, prospective study by Steinert et al examining the Array lens in 400 subjects with 1-year follow-up found 77% of eyes had both 20/40 or better uncorrected distance vision and J3 or better near vision, compared with only 46% of eyes implanted with a conventional, monofocal lens.25 The study also found that a significantly higher percentage of subjects implanted bilaterally with the Array lens reported they could function comfortably without glasses at near compared with those subjects implanted with one Array and one multifocal lens (81% vs 56%, p <0.001). Subjects receiving the multifocal lens lost about one Snellen line of low-contrast visual acuity compared to those receiving the monofocal implant and also reported more glare and halos. Smaller studies have reported similar findings,26-28 including some decrease in simulated driving performance.29 Effective patient selection is paramount when deciding who will benefit from multifocal lens implantation. Patient characteristics that may suggest good candidacy for multifocal lens implantation include high motivation for spectacle independence, willingness and ability to have bilateral implantation, minimal or easily treatable corneal astigmatism, hyperopia, Alzheimer’s patients (often lose specta-

The Future of Phakic Intraocular Lenses 211

cles), certain occupations (mechanics, musicians, librarians), and certain sports enthusiasts (tennis, golf, shooting sports).30 Many of these patients will be quite happy with the results of multifocal lens implantation.

The ThinOptx ultrathin lens was recently found to produce good near vision, even through distance correction.31 Since this lens is not a true multifocal design, any “accommodation” must result from anterior movement of the lens. Ultrasound studies of the ThinOptx lens in vivo have shown some anterior movement up to the third postoperative month. The mechanism whereby the lens (natural or implant) moves forward to provide near vision has been extensively investigated and utilized in the development of true accommodative IOLs.

True Accommodative

Intraocular Lenses

The functional results of pseudoaccommodation from multifocal lenses do not compare to that afforded by the natural lens. As a result, multifocal lenses do not offer a viable option for young prepresbyopic patients. A truly accommodative IOL would be a unifocal design that, through mechanical deformation or movement, alters that unifocality to a near focal point in response to an accommodative stimulus. The development of an effective, stable, safe, and truly accommodative IOL would not only revolutionize cataract surgery but would also likely supplant the position of phakic IOLs in the armamentarium of refractive surgeons.

History

In 1955, Busacca observed that the ciliary muscle mass encroached on the vitreous cavity during chemically induced ciliary muscle constriction in an aniridic patient.32 He also observed that the zonular insertions on the lens moved anteriorly during ciliary muscle constriction. In 1986, Coleman directly measured a simultaneous increase in vitreous cavity pressure and decrease in anterior chamber pressure following stimulation of the ciliary muscle in 10 primate eyes.33 Subsequently, Cumming noticed that some pseudophakic patients with plate haptic lenses were able to read J2 and J3 through best distance correction, having removed factors that might improve near vision, such as uncorrected myopia and with-the-rule astigmatism. Using A-scan ultrasonography, Cumming observed a forward shift in posterior chamber IOL position by an average of 0.7 mm in 10 patients between installation of pilocarpine and a cycloplegic.34 With previous historical information and his own experience, Cumming set out to design a hinged-plate haptic IOL that would move forward with accommodative effort due to increased vitreous pressure on the optic (Figure 21-14). The first design consisted of a hinged-plate haptic with a 4.5-mm optic, the

Figure 21-14. Increased vitreous pressure from ciliary muscle contraction may push lens forward, increasing lens power during accommodation (courtesy of C & C Vision).

optic diameter reduced to provide a longer lever arm of the hinged haptic mechanism, affording a larger anterior movement of the optic. The first lens was implanted in an 85-year-old patient in 1991. Four months postoperatively, the lens movement was analyzed by A-scan ultrasonography. After instillation of pilocarpine, the anterior chamber depth (cornea to anterior surface of lens) was noted to decrease by 2.5 mm while the vitreous cavity length was noted to increase by 2.5 mm, confirming good efficacy of the hinged-plate haptic design. Unfortunately, the lens had a tendency to dislocate into the ciliary sulcus.

CrystaLens: Silicone

Accommodative Intraocular Lens

Over the next 7 years, Cumming and Kammann revised the design of the lens multiple times to reduce the incidence of anterior dislocation into the ciliary sulcus. The final design, known as the AT-45, to be marketed as CrystaLens (C&C Vision, Aliso Viejo, Calif), is shown in Figure 21-15. The lens is a modified three-piece plate haptic design made from a third generation, high refractive index silicone material containing an ultraviolet filter. The lens features hinges located adjacent to the biconvex 4.5-mm optic, running across the plates, to decrease resistance to forward motion. Fixation within the bag is ensured by small, T-shaped polyamide haptics at the end of the plates. The overall length of the lens is 10.5 mm. The lens may be folded and inserted through a 3.0-mm incision.35-37 Atropine is given at the end of the case and the following day to stabilize the lens during the initial period of fibrosis.

Results of clinical trials of the CrystaLens have demonstrated excellent safety and improved efficacy over multi-

Figure 21-15. CrystaLens silicone accommodative IOL featuring a 4.5-mm optic and hinged haptics, each with two polyamide feet (courtesy of C & C Vision).

focal and conventional IOLs with regard to uncorrected distance, intermediate, and near vision.38 One-year results from the FDA Phase I trial on 369 eyes and 124 bilaterally implanted patients were recently presented.39 Figure 2116 highlights the efficacy data. Bilateral results were somewhat better than unilateral results, underscoring an important factor for patient counseling when considering an accommodative implant. One hundred percent of bilaterally implanted patients saw 20/40 or better at near and intermediate through distance correction. These results represent a new level of efficacy in pseudophakic near vision when compared to previous conventional monofocal implants (Table 21-1).25,40 The study also reports dis- tance-corrected near acuity results on 56 eyes after neodymium:yttrium-aluminum-garnet (Nd:YAG) capsulotomy. Significant improvements were seen at 20/25, 0/32, and 20/40. Pre-Nd:YAG percentages were 17.6%, 29.4%, and 76.5%, respectively. Post-Nd:YAG percentages were 29.4%, 61.8%, and 94.1%, respectively. Zaldivar reports a larger improvement in uncorrected near acuity following large versus small capsulotomy.37 Four-year data on six bilaterally implanted patients has been reported.41. One hundred percent have near visual acuity of 20/40 or better through distance correction. The small optic size of 4.5 mm may not cause scotopic vision problems due to the posterior position the lens assumes in the unaccommodated state. Because of its hinges, the CrystaLens sits very posterior in the capsule. As a result, the lens is closer to the nodal point of the eye where all the light rays converge to a single point, limiting the diameter through which light enters the lens. Contrast sensitivity also appears to be excellent. A study by Colvard found no difference in contrast sensitivity between the CrystaLens and a conventional intraocular lens.42

Figure 21-16A. One-year postoperative results of CrystaLens US FDA Trial. Uncorrected distance visual acuity (reprinted with permission of C & C Vision).

Figure 21-16C. One-year postoperative results of CrystaLens US FDA Trial. Distance-corrected near visual acuity (reprinted with permission of C & C Vision).

Figure 21-16B. One-year postoperative results of CrystaLens US FDA Trial. Uncorrected near visual acuity (reprinted with permission of C & C Vision).

Figure 21-16D. One-year postoperative results of CrystaLens US FDA Trial. Distance corrected intermediate visual acuity (reprinted with permission of C & C Vision).

1CU: Acrylic

Accommodative Intraocular Lens

Based on finite element computer models that simulate changes of the ciliary body-zonule-lens apparatus, KD Hanna and colleagues developed the 1CU (HumanOptics

AG, Erlangen, Germany) posterior chamber accommodative IOL (Figure 21-17). This hydrophobic acrylic lens features a 5.5-mm optic attached to four hinged haptics that allow anterior movement of the optic during accommodation (Figure 21-18). A standard 3.2-mm clear-corneal incision may be used to inject the lens into the capsular

Figure 21-19. Transillumination image of 1CU lens in the capsular bag 1 year after surgery. Note small size of capsulorrhexis necessary to overlap 5.5-mm optic (courtesy of HumanOptics AG).

Figure 21-20B. SmartLens in-vivo dimensions. Thickness varies from 2.0 to 4.0 mm depending on dioptric power; average thickness = 3.5 mm (courtesy of Medennium, Inc).

development of liquid or flexible polymer materials that could be injected into the empty capsular bag.46-49 Multiple difficulties have been encountered: containing the liquid within the capsule without leakage; using balloons50; implanting a full size, flexible polymer lens through a small incision if it is not liquid, preventing capsular opacification from occurring;51 and controlling the shape and, ultimately the accommodative amplitude of the liquid or flexible polymer-filled bag.52 Ultimately, the advances in cataract surgical instrumentation and small incision, capsulorrhexis techniques have reduced the problem to one of material science and biocompatibility. Early attempts examined the safety of in-vivo polymerization of a liquid monomer controlled by light exposure.53,54 Silicone polymeric gels were also used with some success in primates, only to be foiled by lens epithelial cell proliferation and subsequent posterior capsular opacification.55,56 While none of these prototypes were ever developed into a commercial product, both Nishi and Haefliger independently showed that accommodation could be restored in primate eyes by removing the native, presbyopic lens and refilling the capsular bag with a soft gel lens.47,56

Figure 21-20A. SmartLens in-vivo dimensions. Diameter = 9.5 mm (courtesy of Medennium, Inc).

A recently developed lens technology may have the answer to many of these previously unconquered challenges. The SmartLens (Medennium, Inc, Irvine, Calif) is made of a thermodynamic, hydrophobic acrylic material. At body temperature, the biconvex lens is 9.5 mm in diameter and from 2 to 4 mm thick, depending on the specific dioptric power requirements to which it was manufactured (Figure 21-20). The lens is highly flexible and completely elastic, returning to its original shape when deforming forces are released (Figure 21-21). The lens is packaged as a 30-mm long, 2-mm wide cylinder at room temperature (Figure 21-22). Once injected into the capsular bag through a sub-3-mm incision, the thermodynamic, hydrophobic material is transformed by exposure to body temperature and assumes the precise shape for the dioptric power “imprinted” earlier during the manufacturing process (Figure 21-23). The transformation process from the rod into the biconvex, flexible lens that completely fills the capsular bag takes about 30 seconds (Figure 2124). After this transformation process, the lens is dimensionally and optically stable.

Several other useful properties have been engineered into this novel material. The hydrophobic acrylic has a high tackiness that will adhere closely to the capsule, minimizing lens epithelial cell migration and virtually eliminating mechanical stability and decentration concerns. The material also has a high index of refraction that should provide significant accommodative effect from only small changes in lens shape or position. The lens is in its early stages of development, currently undergoing toxicity and optical bench studies with initial clinical implantation projected for 2005.

Many issues remain to be resolved regarding true accommodative IOLs, particularly with regard to use in prepresbyopic eyes. First and foremost, a reproducible amplitude of accommodation of at least 3.0 D will be required before such a lens would be considered an appropriate replacement for the natural lens in a prepresbyopic

Figure 21-22. SmartLens cylinder configuration. The lens is packaged as a 30-mm long, 2-mm wide cylinder at room temperature (courtesy of Medennium, Inc).

patient. Secondly, it stands to reason that the accommodation afforded by these lenses will depend on the power of the lens implanted: for the same shift in lens position, a lens of high power would afford a larger amplitude of accommodation when compared with a lens of lower power. This issue, relevant mainly to lens designs requiring an axial shift of a nondeformable optic to provide a power change (eg, CrystaLens and 1CU designs), may be a significant issue in high myopes who typically require a very low power lens implant to achieve emmetropia. Finally, a surgeon must feel comfortable that the risk of retinal detachment is sufficiently low to justify invading the natural lens/capsule/zonule/ciliary body apparatus. Until such time that these issues are sufficiently resolved, phakic IOLs will play a significant role in the treatment of highly ametropic eyes.

Many of the complex issues involved with phakic IOL design have been resolved with the latest generation lenses. As FDA trials near completion for several lens designs, upcoming years will provide an exponential increase in clinical experience with phakic IOLs, undoubtedly uncovering unforeseen benefits and difficulties. The develop-

Figure 21-23. SmartLens implantation. Once injected into the capsular bag through a sub-3-mm incision, the thermodynamic, hydrophobic material is transformed by exposure to body temperature and assumes the precise shape for the dioptric power “imprinted” earlier during the manufacturing process (courtesy of Medennium, Inc).

ment of safe, effective keratorefractive techniques over the last decade and the promising advancements afforded by wavefront technology in recent years are truly remarkable for ophthalmology. It may be the addition of phakic IOLs, however, that will augment the armamentarium of the refractive surgeon to a point where no refractive challenge will be left unconquered.

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The Future of Phakic Intraocular Lenses 219

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