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

356 Chapter 41

with one exception*, all such types currently available are RGP contact lenses, which suffer the problems of poor initial comfort that is worsened by the need to ensure lens movement. Hence, they generally have poor acceptance rates by patients. Nevertheless, given the potential optical benefits, this modality merits further investigation and development.

Bifocal and aspheric (progressive) contact lenses are generally more difficult to fit than standard contact lenses. For reason of simplicity, among other advantages, the majority of contact lens fittings for presbyopes are currently achieved with the monovision principle, in which one eye of the patient is corrected for far vision and the other eye is corrected for near vision.21,22 Not all patients tolerate monovision corrections, and there are a number of adverse visual effects,23 including a potential deficit in binocular function24 and a significant degradation of visual performance at night or under scotopic conditions. The relative acceptability and clinical success of this modality is not so much an indication of their acceptable performance as it is a reflection of the poor performance of current forms of bifocal and aspheric contact lenses.

Yet, from the point of view of super vision, contact lenses do have potential advantages for the presbyope. They remain relatively concentric with the axis of the eye, providing the possibility of optimized, aberration-corrected designs at all directions of gaze, while offering the benefits of simple replacement or change in design/prescription that is unlike many surgical approaches, as they are irreversible. Some implementation issues have been studied, demonstrating that wavefront aberration-corrected or controlled contact lenses can return useful visual benefits.25 With the availability of sophisticated lens design software and multiaxis, numerically controlled contact lens lathes that can produce nonaxisymmetric designs, we should expect further developments in the area of aberration optimized multifocal contact lenses.

In summary of the above discussion, while the near focus that is required for reading can be achieved optically with bifocal spectacles or monovision contact lenses, these methods do not truly restore the accommodative function of the eye.

Multifocal and Accommodating IOLs

Conventional IOLs implanted following cataract surgery are designed to provide clear distance vision. Patients implanted with these traditional IOLs must wear corrective glasses for near vision. Several multifocal IOLs designed to provide a clear image at both near and far distance are currently available on the market. In general, multifocal IOLs are designed with a central optical zone of lowest power that provides clear distance vision, surrounded by two or more concentric zones of higher optical power that allow near distance vision (see Figure 41-3). Other designs use diffractive surfaces to provide multifocality.

In general, multifocal IOLs are designed with a central optical zone of lowest power that provides clear distance vision, surrounded by two or more concentric zones of higher optical power that allow near distance vision.

Figure 41-3. The principle of presbyopia correction using a simultaneous vision-type multifocal contact lens or multifocal IOL. Top: distance vision; center: near vision; bottom: simulated views of a distant object as seen clearly through the peripheral/annular distance vision zone (A), out of focus through the central near vision zone (B), and the resulting blurred retinal image (C).

Similar to the simultaneous vision bifocal contact lenses, the retinal image produced by multifocal IOLs is the superposition of the individual images produced by each of the optical zones. In principle, these implants can be tolerated as long as the patient can suppress the unwanted image. A number of clinical studies have shown that multifocal IOLs provide a significant improvement of near vision acuity when compared to conventional IOLs. However, patients with multifocal IOLs generally have lower contrast sensitivity and often report ghosting and halos, especially during scotopic conditions.26-29

Figure 41-4. The theory and principle of ACS.

A number of clinical studies have shown that multifocal IOLs provide a significant improvement of near vision acuity when compared to conventional IOLs. However, patients with multifocal IOLs generally have lower contrast sensitivity, and often report ghosting and halos, especially during scotopic conditions.26-29

The current trend in the search for a technique to correct presbyopia is the design of accommodating IOLs. A number of ingenious, and sometimes complex, accommodating IOL designs have recently been patented or are currently under clinical investiga- tion.30-34 Today, the most common type of accommodating IOL is a design that provides pseudoaccommodation by translation of the implant along the optical axis of the eye. A displacement of the implant toward the anterior chamber increases the total power of the cornea-IOL system and simulates the increase of power provided by accommodation in the normal eye. Several implants have been designed according to this principle, using mechanical forces of the ciliary muscle, capsule, vitreous, or even magnets to displace the implant.

These implants generally provide a low amplitude of accommodation, typically 1.00 to 2.00 D, and their long-term safety and efficacy remains to be demonstrated.

Customized Visual Correction of Presbyopia 357

Presbyopic Laser Corneal Surgery (PRK, LASIK, LTK)

A number of modified ablation algorithms have been developed and tested for the combined correction of myopia or hyperopia and presbyopia.35-37 These include bior multifocal ablation patterns that mimic the optical design of spectacles, contact lenses, or IOLs, or even decentered ablations. These treatments are generally more complex to deliver than traditional PRK or LASIK treatment and their safety and efficacy remains to be demonstrated. Most commonly, laser correction of presbyopia is achieved by using traditional treatment algorithms of PRK, LASIK, or laser thermokeratoplasty (LTK), but with a monovision prescription.35 Naturally, visual outcome and patient satisfaction are comparable to what is obtained with traditional monovision correction using contact lenses.

Phacomodulation

Recently Myers, Krueger, and colleagues38,39 proposed an original concept for the correction of presbyopia using lasers. They suggested that photodisruption of the crystalline lens with a Q-switched Nd:YAG laser can either reduce the lens volume (photophako reduction [PPR]) or soften the lens nucleus (photophako modulation [PPM]). Preliminary experimental studies of PPM on cadaver eyes demonstrated that application of laser pulses at energies above the threshold for cavitation bubble formation in an annular pattern increased the elasticity of lenses from old donors. Additional studies are needed to confirm these findings and to demonstrate the feasibility of these procedures.

Scleral Expansion Surgery

In the early 1990s, Thornton proposed anterior ciliary sclerotomy (ACS) as a procedure to reverse presbyopia.40 ACS relies on the hypothesis that accommodation is caused by a forward movement of the lens when the ciliary muscle contracts, instead of a change in lens shape. According to this theory, presbyopia results from continuous lens growth with age, rather than lens hardening. Due to lens growth, the space between the lens and ciliary body progressively decreases with age, which loosens zonular tension. In the presbyopic eye, the decrease in zonular tension is such that ciliary muscle contraction and relaxation can no longer stretch the zonules and produce anterior lens displacement (Figure 41-4).

ACS is a purely incisional technique. A series of equallyspaced radial incisions are performed in the sclera near the limbus to increase the circumference of the globe at the level of the ciliary body. Due to expansion of the globe, the space between the ciliary body and lens increases, and normal zonular tension and ciliary body action is restored (see Figure 41-4). ACS has been used as a treatment for both presbyopia and glaucoma.41

According to some reports, the procedure provides 2 to 3 D of accommodation postoperatively, but the effect is temporary due to wound closure and loss of the expansion effect, with a return to the initial state 4 to 8 months after surgery. To avoid regression, the procedure has been modified to include insertion of silicone implants (ie, scleral expansion plugs, or SEPs) to avoid wound closure.41 The effectiveness of the modified ACS procedure in maintaining globe expansion and the measured accommodative effect remains to be demonstrated.

In recent years, Schachar introduced and developed another concept for scleral expansion surgery (ie, surgical reversal of presbyopia, or SRP) that relies on his controversial (and disputed) theory of accommodation.42 The underlying hypothesis of SRP is the

Figure 41-5. The key steps for restoring accommodative function to the crystalline lens using the surgical technique of phaco-ersatz include the extraction of the nucleus and cortex through a small hole (minicapsulorrhexis) in the capsule while leaving the capsule intact.

same as for ACS: an increase in space between the ciliary body and lens will restore sufficient accommodation range for near vision. In the current version of this procedure, the globe is expanded at the level of the ciliary body by inserting four polymethyl methacrylate (PMMA) implants (ie, scleral expansion bands, or SEBs) in the sclera near the limbus. According to some undocumented or unreviewed reports, the procedure is capable of restoring 3.00 D to 8.00 D of accommodation.43,44 The validity of these outcomes has been contested45 and contradicted by several other clinical studies that found no gain in accommodation.46-48

Lens Refilling: Phaco-Ersatz

Given our understanding of the mechanism of accommodation and the origin of presbyopia, it seems appropriate that a most direct strategy for restoring accommodation in presbyopia is to restore the elasticity of the crystalline lens. Early work by Kessler49 and Agarwal and colleagues50 suggested the feasibility of removing the lens contents and refilling the empty capsule with an optically similar substance. Since then, there have been several attempts made to restore accommodation using this approach.

The most notable series of work is from the group led by Nishi, who fabricated an injectable balloon that could be inserted into the crystalline lens capsule following extraction of the nucleus and cortex.51,52 This balloon is then filled with a viscous material such as silicone oil. Using a rabbit model, Nishi and colleagues demonstrated that an average of 1.1 D of refractive accommodation can be observed after pilocarpine injection.51 Another study using primates resulted in an even greater result: accommodation amplitude ranging from 1 D to 4.50 D.52 However, due to problems associated with biocompatibility and the leakage of silicone oil into the anterior chamber, the studies using this approach have been discontinued. Nevertheless, Nishi’s studies demonstrated the relative merits of restoring accommodation by restoring the mechanical properties of the crystalline lens.

A group led by Jean-Marie Parel introduced the surgical technique of phaco-ersatz in 1979.53,54 Phaco-ersatz is a direct lens refilling procedure that involves removal of the lens material (ie, nucleus and cortex) through a small opening in the capsule (a

Figure 41-6. The current preferred implementation of phacoersatz.

minicapsulorrhexis) and then injection of a suitable polymeric gel into the capsule through the same opening (Figure 41-5). During this procedure the capsule, zonules, and ciliary body remain intact. Ideally, the properties of the polymeric gel are chosen to be equivalent to that of the young natural lens. Studies performed in young53 and senile54 primates with a siloxanebased polymer showed the restoration of accommodation.

In 1997, an international collaborative project led by Brien Holden’s group at the Cooperative Research Centre for Eye Research and Technology (CRCERT) in Sydney and by JeanMarie Parel’s group at Bascom Palmer Eye Institute’s Ophthalmic Biophysics Center in Miami was established to further develop a surgical realization of the phaco-ersatz technique. This project led to the development of a refined surgical approach (Figure 41-6), and currently addresses some of the remaining issues of the phaco-ersatz procedure, including the development of a material with the required optical and mechanical characteristics, prevention of secondary cataract, and intraoperative control of the shape of the phaco-ersatz implant.

WAVEFRONT-GUIDED

CUSTOMIZED PRESBYOPIA CORRECTION

In their current state, none of the clinical techniques for the correction of presbyopia is of sufficient predictability or flexibility to be suitable for wavefront-guided customization. Of the techniques discussed in this chapter, most likely only diffractive lenses, laser corneal reshaping, and lens refilling may eventually allow wavefront-guided correction of presbyopia with simultaneous correction of ametropia and higher-order aberrations.

Wavefront-Guided Presbyopic Laser Corneal Surgery

In theory, wavefront-guided treatment algorithms could be calculated for presbyopic LASIK or PRK. For instance, in monovision treatments, the ablation pattern for each eye could be optimized based on wavefront measurements. Wavefront analysis could perhaps also help reduce some of the common undesired

A B

C

Figure 41-7. Addition of a supplemental endocapsular lens (SECL) during phaco-ersatz surgery for simultaneous correction of presbyopia, ametropia, and higher-order aberrations. (A) Insertion of the SECL. (B) The SECL in place before polymer injection.

(C) Injection of the polymer.

visual effects of multifocal ablations. In the end, however, optical effects inherent to multifocality (reduced contrast) and monovision (binocular effects) will limit potential improvements.

Wavefront-Guided Diffractive Lens Design

Within practical limits, diffractive lenses can be designed to produce any desired type of monofocal or multifocal aberration pattern in the image plane. In theory, diffractive contact lenses or IOLs could therefore be patient customized using ocular wavefront aberrometry. However, the feasibility of tailored diffractive lenses will eventually be determined by the cost of fabrication of custom diffractive optics and by the required fitting and/or positioning accuracy of the contact lens or implant. Many implementation issues need to be resolved. For example, especially in the case of contact lenses but also in IOLs, biofouling over time of the diffractive echellettes that provide the diffractive performance will need to be either eliminated or compensated. In addition, even if customized, diffractive optics will not restore true dynamic accommodation.

Wavefront-Guided Lens Refilling

The ultimate goal of the phaco-ersatz procedure is not only to restore the normal opto-mechanical response of the lens during accommodation in the presbyopic eye, but also to optimize the visual outcome of the procedure, including ametropia correction and control of ocular aberrations.55

In principle, direct lens refilling is an adjustable procedure. Early studies by Parel and colleagues56 have already demonstrated that the shape of the implant can be modulated by controlling the injected volume of polymer. The refractive index of the lens refilling material can also be modulated, within limits, to adjust the optical power of the implant. A number of materials with different refractive indices could thus be made available to the surgeon to adjust postoperative refraction. At least in theory, a phaco-ersatz procedure with preoperative selection of the refractive index and intraoperative wavefront-guided volume

Customized Visual Correction of Presbyopia 359

control could be envisioned. Such a procedure would provide the adjustability required to control postoperative ametropia and aberrations. However, optical modeling studies55 have shown that the precision on volume or refractive index required to control ametropia during phaco-ersatz is not achievable in practice.

The concept of an adjustable supplemental endocapsular lens (SECL) has been introduced to address this issue. The SECL would be a thin implant that can be placed in the capsular bag to adjust the refractive power and aberrations of the lens refilling implant. Preliminary animal experiments (in unpublished reports) have demonstrated the feasibility of phaco-ersatz surgery with addition of a SECL (Figure 41-7). Another feasible, although arguably less desirable, option to adjust the outcome of phaco-ersatz surgery would be wavefront-guided laser corneal reshaping to correct residual ametropia and wavefront aberrations.

CONCLUSIONS

The correction of presbyopia is considered to be “the last frontier of refractive surgery.”57 The ideal technique for presbyopia correction is one that would restore the accommodation function of the normal, young eye. Most of the techniques available today do not truly correct presbyopia, but attempt to compensate for the loss of accommodation by using monovision or multifocal approaches, or pseudoaccommodation. The current trend is the development of accommodating IOLs, as well as lens refilling or lens modifying procedures.

Since the current technologies for correction of presbyopia are still in their infancy, it appears unlikely that techniques to restore accommodation will be ready for a customized wavefront-guid- ed approach before 2010. If successfully developed at that time, given the demographics of presbyopia, wavefront-guided presbyopia correction could well become the main driving force of wavefront technology.

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53.Haefliger E, Parel, J-M, Fantes, F, Norton EWD, Anderson DR, Forster RK, Hernandez E, Feuer WJ. Accommodation of an endocapsular silicone lens (phaco-ersatz) in the nonhuman primate. Ophthalmology. 1987;94:471-477.

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In looking to the future, a great challenge exists: a challenge to forecast a realistic view of how our field may change with wavefront technology. A challenge to be bold and pursue innovative concepts beyond the status quo. Sometimes in dreaming big dreams, one can actually change the future by being actively involved in the process of innovation. Yet, innovation requires a certain aspect of realism, which is predicated on the stepping stones of science. We look to the future of customization through the prism of science and technology with cautious optimism. The quest for super vision is not about visual perfection; it is accepting the challenge of continuously improving the technologies that optimize our patient’s vision. Building blocks we have currently established may allow us to step into new areas as we move into the future.

Sometimes in dreaming big dreams, one can actually change the future by being actively involved in the process of innovation.

THE QUEST FOR SUPER VISION

Beside the stepping stones of science required for us to move from our first book, Customized Corneal Ablation, to the second book, Wavefront Customized Visual Correction, one resounding theme ties both books together: the quest for super vision. As our field continues to advance, so will our concept and knowledge of customization. Even though the various customized titles that we use in subsequent editions of this book will likely change, the subtitle—“The Quest for Super Vision”—will remain the same.

Customization will continue to play a central role in this quest. In order to achieve super vision, which is defined as “exceptionally good quality of vision beyond that represented by the normal population,” we will need to understand the visual needs, perceptions, and expectations of the individual patient within that normal population. Hence, customization takes on the goal to meet specific needs of an individual patient, as outlined in Chapter 1: functional vs anatomical. The functional needs of the patient include outcomes based on age, occupational vs recreational needs, and psychological acceptance of the current correction and resulting residual optical defects. Anatomical needs include corneal thickness limitations, ocular surface issues, pupil size, corneal curvature’s influence on ablation efficiency, microkeratome safety, and anterior chamber depth when consid-

ering implants. Although these are rather specific, customization in the quest of super vision could also take on a more general optical and neural focus, as can be seen in the following sections.

THE QUEST FOR SUPER VISION IN DIAGNOSTICS

Optical Limitations of Super Vision

In meeting all of the functional needs noted above, we will certainly encounter limitations. Only by understanding these limitations and trade-off and incorporating them into our custom designs will we truly optimize our wave aberration corrections. For example, did the human optical system evolve to have aberrations to provide biological advantages? That is, do we have optical systems optimized for modulation transfer in specific bands of spatial frequencies important to survival at the expense of others? Do we have optical aberrations like mild amounts of positive spherical aberration to increase the range of functional vision (depth of field) at the sacrifice of contrast at high frequencies? Will we be creating new classes of amblyopia by improving retinal image quality? Will aliasing be a problem? How big a role do individual differences play in designing optimized optical corrections?

Moving these and other questions aside for the moment, an accurate measurement and diagnostic representation of the wave aberration needs to be first obtained before an optical design can be generated. Further, it is crucial that wave aberration metrics be developed that can predict reliably the likely visual function resulting from treatment.1 Current wavefront devices and aberrometers can measure wave aberration to the sixth and eighth Zernike order and higher (depending on the device). However, errors in capturing, sampling, and wave aberration representation, as well as errors intrinsic with wavefront variability over time and in differing environments, make the challenge of wavefront customization a matter of achieving a proper wavefront measurement to be used in designing an optimal correction.

To this end, the future of wavefront customization will employ better ways of measuring and expressing the whole eye wave aberration as well as the corneal wave aberration and surface shapes. It is likely that these calculations will become increasingly invisible to the clinician by becoming internal to wave aberration correction system. Expressing a complex wave aberration in terms of the Zernike expansion works well for normal eyes where the

Figure 42-1. Complex wavefront shape of a highly aberrated eye expressed in sixthand 10th-order Zernike modes as well as in a new reconstruction algorithm developed by VISX. The location of centroids back calculated from these representations and compared with the actual centroids are seen in the gradient field difference maps. These demonstrate the relative failures of Zernike expansion in a complex eye and improvements achieved with zonal reconstruction. (Courtesy of Doug Koch, MD.)

wave aberration is well represented by the underlying Zernike modes. In fact, there is a distinct advantage to using a smoothing function to reduce noise in the fundamental data from erroneously influencing the representation of the wave aberration.

However, in cases where the wave aberration is particularly complex, and the underlying modes of the expansion do not represent the actual small details in the wave aberration, then a Zernike approach that is limited in the available modes to fit the error can, and will, fail to properly represent the wave aberration. Such a failure can be seen in Figure 42-1, where a complex wavefront shape is expressed in sixthand 10th-order Zernike modes and the location of the centroids back calculated from these two Zernike representations reveal significant differences between the actual data samples and its Zernike representation by the gradient field difference maps. This is in contrast to alternative, non-Zernike algorithms proposed by VISX and others, also seen in this figure. To minimize adverse consequences of using a Zernike representation that does not accurately reflect the underlying measurement, we anticipate the inclusion of “fit error” limits in measurement devices to warn the user when the underlying data set is not being well-represented by the Zernike, or for that matter any selected basis, function expansion.

In clinically abnormal eyes (eg, penetrating keratoplasty, keratoconus, bad refractive surgery outcomes), the fit error can be high. To solve this potential problem, several commercial devices (VISX and Wavefront Sciences, Albuquerque, NM) are beginning to address these fitting errors using zonal reconstruction. Zonal reconstruction, as well as other methods, can provide an accurate representation of the underlying data set. The liability is that one has to believe that each point in the underlying data set accurately represents the actual wave aberration. Typically, as complexity of the measurement increases, so does the noise in the measurement. Said differently, one has to be careful to prevent measurement noise from overly influencing the wave aberration representation. A strategy to minimize noise in the underlying data prior to a zonal reconstruction is to take multiple measurements and determine an average location for each data point (eg, individual centroids in Shack-Hartmann wavefront sensing).

Figure 42-2. High density centroid pattern (600 lenslets in 3.50 mm pupil area) in an experimental Shack-Hartmann device, which also has multiple samplings over time (12 captures per second) and adaptive correction using a deformable mirror. (Courtesy of Michael Mrochen, PhD.)

Optimizing our ability to measure the wavefront error at any one moment in time is only a first step. Variability of the ocular tear film and accommodation cause ocular wave aberration to vary from one measurement to the next, suggesting that some form of temporal averaging will likely be important in determining the ideal wave aberration correction. Consequently, future research efforts will likely seek to develop an even more sophisticated clinical aberrometer with a great density of lenslets; multiple samplings over time; adaptive optic capabilities for high resolution, dynamic sampling with adaptive correction to verify the accuracy of wavefront representation; and potential refractive surgical outcome. Figure 42-2 demonstrates the centroid pattern of an experimental Shack-Hartmann device with 600 lenslets within a 3.50 mm pupil that captures 12 times per second, and has a deformable mirror for adaptive correction to demonstrate optimal visual performance in real time.

Future research efforts will likely seek to develop an even more sophisticated clinical aberrometer with a great density of lenslets, multiple samplings over time, and adaptive optic capabilities.

Another limitation in the representation of optical aberrations is whether we target a perfectly flat wavefront shape for high fidelity or a compensated, adjusted shape to enhance the dynamic range of vision in presbyopic patients. Although this limitation is not fundamental to adequate wavefront measurement and representation, it does impact visual correction and will certainly be involved in customization of the patients’ individual optical and visual needs. As any refractive surgeon knows, there are tradeoffs to treating presbyopic patients fully for distance vision or giving them monovision. Future research may well focus scientifically on the various functional advantages and disadvantages of these and other strategies.

THE QUEST FOR SUPER VISION

IN CORNEAL ABLATION

The current clinical proliferation of laser vision correction and customized corneal ablation demonstrates a high level of acceptance of customization in the area of corneal ablation. Besides needed improvements in the fidelity of wavefront devices, the future will likely hold further improvements in spot delivery, as well as tracking and registration of the delivery devices associated with corneal laser ablation.

LASIK vs Surface Ablation

One of the biggest questions in our field is that of LASIK vs surface ablation. Will the limitations of biomechanical predictability associated with LASIK and flap creation be adequately overcome, or will a form of surface laser ablation be adapted because of improvements and predictability in corneal wound healing? Efforts are being investigated in both areas, with the former seeking to find a level of predictability in flap induced aberrations with specific microkeratomes and flap diameter and thickness parameters.3,4 In a recent, yet unpublished study by Krueger et al, a statistically significant shift toward hyperopia was noticed when creating a “flap only” in 12 eyes using the Moria M2 microkeratome (Moria, Antony, France), while a “flap

Finding a gene that could be pharmacologically or interventionally turned on and off in an effort to better control the postlaser activation of keratocytes and other wound healing mediators would go a step beyond immunomodulation for wound healing to that of gene modulation.

Multifocal Ablation

In the quest for super vision, customization in an effort to correct presbyopia in addressing a patient’s functional needs has also been investigated in multifocal corneal ablation. It may seem counter-intuitive to induce aberrations in order to create multifocality because of the potential loss of contrast sensitivity and quality of visual function. Yet, because of the functional needs of the patient, attempts are being made at steepening the central or inferior cornea in order to induce a level of multifocality. Previous attempts were unsuccessful, but they did not have the help of sophisticated eye trackers, corneal registration, and wavefront mapping to determine the best multifocal profile.8,9 In the future, we will likely see wavefront mapping and adaptive optics simulation of multifocality to demonstrate to the patient how certain aberrations might expand the physiologic range of vision without significant loss of contrast and visual function. One group that is actively working in this area is the Emory