Ординатура / Офтальмология / Английские материалы / Mastering theTechniques of Lens Based Refractive Surgery (Phakic IOLs)_Garg, Alio, Dementiev_2005

waves of various frequencies, amplitudes and orientations. In fact, visual processing in the human nervous system works like Fourier analysis in reverse, with functionally independent neural channels filtering images to create what we see.2 Thus, sine wave gratings are the building blocks of vision, just as pure tones are the building blocks of audition.

Ophthalmologists realize that patients may complain about haziness, glare and poor night vision despite 20/20 Snellen acuity. Contrast sensitivity testing has the ability to detect differences in functional vision when Snellen visual acuity measurements cannot.3 For example, a patient with loss of low frequency contrast sensitivity may be able to read 20/20 but be unable to see a truck in the fog. While blur due to refractive error alone affects only the higher spatial frequencies, scatter of light due to corneal or lenticular opacities causes loss at all frequencies. Glaucoma and other optic neuropathies generally produce loss in the middle and low frequencies. Contrast sensitivity testing thus offers critical information to help explain patients’ complaints.

Numerous studies have demonstrated the relationship of contrast sensitivity and visual performance. From driving difficulty4 and crash involvement,5 to falls6 and postural stability in the elderly,7 to activities of daily living and visual impairment,8 to the performance of pilots in aircraft simulators,9 contrast sensitivity has consistently been found to provide a high correlation with visual performance.

Unfortunately, contrast sensitivity declines with age even in the absence of ocular pathology such as cataract, glaucoma or macular degeneration (Fig. 21.2). The pathogenesis of this decline in vision likely involves changes in the spherical aberration of the crystalline lens.

SPHERICAL ABERRATION

Spherical aberration is a property of spherical lenses. A spherical lens does not refract all parallel rays of incoming light to a single secondary focal point. The lens bends peripheral rays more strongly so that these rays cross the

Figure 21.2

optical axis in front of the paraxial rays. As the aperture of the lens increases the average focal point moves towards the lens, so that a larger pupil produces greater spherical aberration.

Spherical aberration of the cornea changes little with age. However, total wave-front aberration of the eye increases more than threefold between 20 and 70 years of age.10 Wave-front aberration measurements combined with data from corneal topography demonstrates that the optical characteristics of the youthful crystalline lens compensate for aberrations in the cornea, reducing total aberration in younger people (Figs 21.3 to 21.5). Unfortunately, the aging lens no longer compensates so well, as both the magnitude and the sign of its spherical aberration change significantly

Figure 21.3

Figure 21.4

Figure 21.5

(Fig. 21.6).11 Thus a loss of balance between corneal and lenticular spherical aberration causes the degradation of optical quality in the aging eye (Figs 21.7 to 21.9).

It has been documented that the sine wave grating contrast sensitivity of a pseudophakic patient is no better than that of a phakic patient of a similar age who has no cataract. When a 65-year-old patient with cataracts has the cataracts removed and is implanted with IOLs the resulting visual outcome is no better than the visual quality of a 65-year-old without cataracts (Fig. 21.2). The fact that the visual quality of the IOL patients is no better than that of their same-age counterparts may seem surprising because an IOL is optically superior to the natural crystalline lens. However, this paradox is explained when one realizes that the intraocular implant has positive spherical aberration like

Figure 21.6: The spherical aberration of the human crystalline lens increases with age (Glasser)

Figure 21.7

Figure 21.8

Figure 21.9

the aged lens. It is not the optical quality of the intraocular lens in isolation that creates the image, but the optical quality of the intraocular lens in conjunction with the cornea.

The spherical aberration of a manufactured spherical intraocular lens is in no better balance with the cornea than the spherical aberration of the aging crystalline lens (Fig. 21.10). 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 reduction in quality is more severe under low luminance conditions because ocular aberrations increase when the pupil size gets larger.

Figure 21.10

Figure 21.11

TECNIS IOL

The Tecnis Z9000 intraocular lens (Pharmacia), currently undergoing FDA-monitored clinical trials in the United States, has been designed with a modified prolate anterior surface to compensate for the spherical aberration of the cornea (Fig. 21.11). The Tecnis Z9000 shares basic design features with the CeeOn Edge 911 (Pharmacia), including a 6 mm biconvex square-edge silicone optic and angulated cap C polyvinylidene fluoride (PVDF) haptics. The essential new feature of the Tecnis IOL, the modified prolate anterior surface, acts like the youthful crystalline lens and compensates for corneal spherical aberration. The exciting new concept of the Z9000 is the potential for restoration of youthful optical quality and improvement of functional vision. Theoretical calculations and optical bench measure-ments support the hypothesis of improved contrast sensitivity with the Tecnis IOL (Fig. 21.12).

A study performed by Ulrich Mester, MD, of Salzabach, Germany, and reported at the American Society of Cataract and Refractive Surgery Symposium in Philadelphia (June 1-5, 2002), has compared the quality of vision obtained with the Tecnis IOL and a spherical acrylic IOL (Acrysof, Alcon Surgical). A total of 45 patients were enrolled and randomized to receive the Tecnis IOL in one eye and the SI 40 in the fellow eye. The average photopic contrast sensitivity values demonstrated a statistically significant advantage for the

Figure 21.12

Tecnis IOL at all spatial frequencies. The contrast sensitivity curves showed an even greater difference under mesopic conditions.

SUMMARY

As advances in technology allow cataract and refractive surgeons to address higher order optical aberrations, the measurement of functional vision becomes increasingly critical as a gauge of our progress. Sine wave contrast sensitivity testing assumes a prominent place in our evaluation of surgical modalities because it reflects functional vision, correlates with visual performance and provides a key to understanding optical and visual processing of images. The Tecnis Z9000 study represents a first step towards the integration of wave-front technology and lens-based surgery.

REFERENCES

1.Ginsburg AP. The Evaluation of Contact Lenses and Refractive Surgery Using Contrast Sensitivity, in Contact Lenses: Update 2. Grune and Stratton, Inc, 1987;56.5.

2.Spillman L, Wooten DR (Eds). Visual Form Perception Based on Biological Filtering, in Sensory Experience, Adaptation and Perception. Hillsdale, NJ: Lawrence Erlbaum Associates, 1984.

3.Evans DW, Ginsburg AP. Contrast sensitivity predicts agerelated differences in highway sign discriminability. Human Factors 1985;27(12):637.

4.McGwin G Jr, Chapman V, Owsley C. Visual risk factors for driving difficulty among older drivers. Accid Anal Prev 2000;32(6):735-44.

5.Owsley C, Stalvey BT, Wells J et al. Visual risk factors for crash involvement in older drivers with cataract. Arch Ophthalmol 2001;119(6):881-87.

6.Lord SR, Dayhew J. Visual risk factors for falls in older people. J Am Geriatr Soc 2001;49(5):508-15.

7.Lord SR, Menz HB: Visual contributions to postural stability in older adults. Gerontology 2000;46(6):306-10.

8.Rubin GS, Bandeen-Roche K, Huang GH et al. The association of multiple visual impairments with selfreported visual disability: SEE project. Invest Ophthalmol Vis Sci 2001;42(1):64-72.

9.Ginsburg AP, Evans DW, Sekule R et al. Contrast sensitivity predicts pilots’ performance in aircraft simulators. Am J Optom Physiol Opt 1982;59(1):105-09.

10.Artal P, Berrio E, Guirao A et al. Contribution of the cornea and internal surfaces to the change of ocular aberrations with age. J Opt Soc Am A Opt Image Sci Vis 2002;19(1):137-43.

11.Glasser A, Campbell MC: Presbyopia and the optical changes in the human crystalline lens with age. Vision Res 1998;38(2):209-29.

Richard S Hoffman

I Howard Fine

Mark Packer (USA)

Despite the introduction of more accurate intraocular lens (IOL) formulas and biometry instrumentation, cataract and refractive lens surgery have yet to achieve the ophthal-mologist’s ideal of perfect emmetropia in all cases.1-5 This limitation stems from occasional inaccuracies in keratometry and axial length measurements, an inability to accurately assess the final position of the pseudophakic implant in a fibrosing capsular bag, and the difficulty of completely eliminating pre-existing astigmatism despite the use of limbal relaxing incisions and toric IOLs.6,7 A new lens technology offers the hope of taking ophthalmologists one step closer to achieving emmetropia in all cases and also perhaps further improving the final result by addressing higher order aberrations.

THE IDEAL PSEUDOPHAKIC LENS

A pseudophakic lens that could be noninvasively adjusted or fine-tuned following implantation would allow for extreme accuracy in the final refractive outcome. Ideally, this lens would have the ability to be precisely adjusted using a non-toxic external light source and allow for several diopters of myopic, hyperopic, or astigmatic correction should a postoperative refractive surprise occur. Micron precision adjustment would allow for the possibility of modifying not only the lower order aberrations of sphere and cylinder but also higher order optical aberrations such as coma and spherical aberration. The lens should be stable following adjustment and composed of a safe biocompatible material. In addition, a foldable lens that could be inserted

through a 2.5-3.0 mm clear corneal incision would insure control of surgically induced astigmatism.8 Finally, if possible, an injectable flexible polymer design that could be injected through a 1 mm incision would further reduce any surgically induced astigmatism or higher order corneal aberrations and conceivably, depending on its final elasticity, could return accommodative ability to the lens/ciliary body apparatus.

LIGHT ADJUSTABLE LENS (LAL)

This ideal lens technology is no longer science fiction and is currently being developed by Calhoun Vision (Pasadena, Ca). It is termed the light adjustable lens (LAL) (Fig. 22.1). The current design of the LAL is a foldable three-piece IOL with a cross-linked photosensitive silicone polymer matrix, a homogeneously embedded photosensitive macromer, and a photoinitiator. The application of near-ultraviolet light to a portion of the lens optic results in disassociation of the photoinitiator to form reactive radicals that initiate polymerization of the photosensitive macromers within the irradiated region of the silicone matrix. Polymerization itself does not result in changes in lens power, however, it does create a concentration gradient within the lens resulting in the migration of non-irradiated macromers into the region that is now devoid of macromer as a result of polymerization. Equilibration from migration of the macromers into the irradiated area causes swelling within that region of the lens with an associated change in the

Figure 22.1: Calhoun Vision, Light Adjustable Lens (LAL)

(Courtesy of Calhoun Vision, Inc.)

radius of curvature and power. Once the desired power change is achieved, irradiation of the entire lens to polymerize all remaining macromer “locks-in” the adjustment so that no further power changes can occur.9

MODULATING REFRACTIVE POWER

The treatment of residual postoperative sphere and cylinder is fairly straightforward. In a patient whose postoperative refraction reveals residual hyperopia, power will need to be added to the LAL in order to achieve emmetropia (Fig. 22.2). Once postoperative refractive stability has been reached (2-4 weeks), irradiation of the central portion of the lens with the Light Delivery Device (Fig. 22.3) polymerizes macromer in this region. Over the next 12-15 hours, macromer in the peripheral portion of the lens will diffuse centrally down the concentration gradient in order to achieve concentration equilibrium with the central lens which has been depleted of macromers due to their polymerization. This migration results in swelling of the central portion of the lens with an increase in the radius of curvature and an associated increase in the power of the LAL. With variation in the duration and power of light exposure, differing amounts of hyperopia can be corrected. One day or more after this adjustment, the entire lens is treated to lock-in the fine adjustment. Since outdoor ultraviolet light can affect the LAL, patients wear sunglasses to eliminate UV exposure until the final lock-in is performed. Once final polymerization and lockin is executed, no further UV protection is necessary.

In a patient with a myopic postoperative result following primary surgery, power will need to be reduced from the LAL in order to achieve emmetropia (Fig. 22.4). In this scenario, irradiation of the peripheral portion of the lens in a doughnut configuration will result in polymerization of macromers in this region with a resultant diffusion of central lens macromers into the peripheral irradiated portion of the lens. This creates swelling of the peripheral annulus of the lens with a concomitant increase in the radius of curvature and a decrease in lens power (Fig. 22.5).

Similarly, astigmatism can be treated by irradiating the LAL along the appropriate meridian in order to create a toric change in the radius of curvature of the lens and thus increase power ninety degrees from the treated meridian.

ANIMAL STUDIES

Nick Mamalis, MD, from the Moran Eye Center, University of Utah, has been instrumental in documenting some of the early data regarding the efficacy and accuracy of LAL adjustment in animal studies. In his pilot study, five rabbits underwent cataract surgery and LAL implantation followed by irradiation to correct 0.75 D of hyperopia. Each lens was then explanted and its power change analyzed. The mean power change was extremely close to the target correction at 0.71 ± 0.05

D (Fig. 22.6A). Four additional rabbits underwent LAL implantation and treatment to treat -1.00 D of myopia. These eyes also demonstrated precise adjustments averaging –1.02 ± 0.09 D of power reduction (Fig. 22.6B).

In addition to these animal tests documenting the accuracy and reproducibility of LAL adjustments, Calhoun Vision has also performed extensive animal testing demonstrating biocompatibility and safety. Toxicology testing has revealed that no leaching of the macromers embedded in the cross-linked silicone matrix occurs despite experimental transection of the IOL.

RESOLUTION

Although, the ultimate determination of an IOL’s effect on the quality of vision can best be determined by

Figure 22.3: The Light Delivery Device is mounted on a conventional slit lamp. The refractive error and desired refractive outcome are entered on the color console and irradiation is activated using either a foot pedal or the joy stick. (Courtesy of Calhoun Vision, Inc.)

contrast sensitivity testing after human implantation, the resolution efficiency of a lens can be determined utilizing optical bench studies. To monitor the resolution efficiency of the LAL after irradiation, the lens was evaluated on a collimation bench utilizing a standard 1951 US Air Force resolution target. (Fig. 22.7A) demonstrates the quality of the resolution target through the LAL in air prior to irradiation. (Fig. 22.7B) reveals the imaged target 24 hours following treatment of the LAL for -1.58 D of myopia. (Fig. 22.7C) reveals the image through a +20 D AMO SI40 for comparison. Inspection of the images reveals that the resolution efficiency of the LAL is not compromised following irradiation.9

REFRACTIVE LENS EXCHANGE

Perhaps one of the greatest possible uses of a LAL is as a platform for refractive surgery. The concept of exchanging the human crystalline lens with a pseudophakic IOL as a form of refractive surgery is gaining popularity in the ophthalmic community. This stems from several problems inherent in excimer laser corneal refractive surgery including the limitations of large myopic and hyperopic corrections, the need to address presbyopia, and progressive lenticular changes that eventually will interfere with any optical corrections made in the cornea.

Currently acceptable methods of performing refractive lens exchange incorporate multifocal lenses as a means of maximizing the final refractive result.10 Multifocal IOLs allow the presbyopic patient considering refractive surgery to address their distance refractive error in addition to their near visual needs without resorting to monovision with monofocal lens implants. In patients whose nighttime visual demands preclude the use of multifocal technology, monofocal IOLs can still be used with the understanding that monovision or reading glasses will be necessary to deliver functional vision at all ranges.

The LAL is an ideal implant for refractive lens exchanges since emmetropia can be fine-tuned following insertion. In addition, Calhoun Vision has demonstrated in vitro, an ability to irradiate multifocal optics of any near add onto any portion of the LAL (Fig. 22.8). Theoretically, a patient undergoing a refractive lens exchange could have their lens adjusted for emmetropia and then have multifocality introduced to determine if they were tolerant to multifocal optics. If intolerant, the multifocality could be reversed and a trial of monovision could be induced. Once the desired refractive status was achieved, the LAL could then be locked-in permanently. This would allow patients the option of experimenting with different refractive optics and deciding in situ which was best for them.

Until now, the potential drawbacks of refractive lens exchange have included the risk of endophthalmitis,