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

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Figure 42-3. Array of computer simulated ablation profiles comparing two ablation depths and two spot diameters against five different eye tracker latencies and the referenced ideal profile. It demonstrates that tracker latencies less that 4 msec are required for adequately correcting higherorder aberrations when using a small laser spot (~0.25 mm). Using a larger laser spot (1.0 mm) is less dependent on tracker latency but is less effective in correcting detailed higher order aberrations. (Courtesy of Michael Bueeler, PhD.)

Vision Correction Group (Atlanta, Ga) using the spatially resolved refractometer (InterWave Unit) to subjectively determine the patient’s aberrations in relation to the acceptability of a multifocal corneal shape (see Chapter 20).

Laser Delivery Refinements

Over the past decade, excimer laser delivery refinements have continued to move the field toward smaller spot delivery and faster eye tracking. A decade ago the excimer laser was basically a broad beam delivery system with no tracking device to provide for purely spherical and cylindrical correction of refractive errors in a rapid fashion. Although small spot scanning was introduced with eye tracking in an effort to deliver a more homogeneous beam, the advent of customized corneal ablation for the treatment of higher-order aberrations made the refinement of a smaller beam essential to achieve these corrections. With the reduction of spot size, the need for eye tracking became obvious. Over the course of this past decade, nearly every company providing laser vision correction platforms has now adapted eye tracking into its excimer laser systems. In two recently published studies, mathematically correlating the scanning spot size with the expected resolution of correction, a beam diameter of less <1.00 mm was found essential to correct up to fourthand fifth-order aberrations.10,11 Furthermore, correction of higher-order aberrations beyond the fifth order would require even smaller spot delivery on the order of 0.60 to 0.80 mm full width at half maximum (FWHM) using a Gaussian shape.10 As the spot size gets smaller, the need for faster tracking gets greater. Currently, the LADAR 4000 Excimer Laser System (Alcon Surgical, Fort Worth, Tex) utilizes closed-loop laser radar tracking at a 4000 hertz (Hz) detection frequency, which essentially reduces the tracker response latency to less than to 1 millisecond (ms) (see Chapter 23). Other videocamera-based trackers sampling at a rate between 60 to 400 Hz have an intrinsic delay due to their sampling rate and method of open loop tracking, which leads to a latency of 8 to 40 milliseconds.12

A recent analysis by Bueeler and Mrochen at the Swiss Federal Institute of Technology in Zurich demonstrates the simulated efficacy of scanning spot correction of higher-order aberrations (vertical coma with an error magnitude of 0.60 microns [µm] at 5.70 millimeter [mm] pupil) using spot diameters of 0.25 and 1.0 mm;

ablation depths of 0.25 and 1.0 µm/pulse; and tracker latencies varying from 0 ms, 4 ms, 32 ms, 96 ms, and no eye tracking. Figure 42-3 shows the array of profiles of differing spot diameter and ablation depth vs tracker latency in correcting this thirdorder coma. One can see in the figure that the clear contour lines of an ideal coma profile become increasingly distorted with increasing tracker latency. Even in the case of a perfect ablation without tracker latency, contour lines are distorted as a result of the overlap of a finite number of laser pulses. As one might expect, the case of a small spot diameter with a small ablation depth per pulse provides the best approximation to the ideal correction profile if all eye movements are completely compensated for by the tracker (0 ms latency). A more stable ablation, however, is achieved with a larger spot (approximately 1 mm) together with a small ablation depth per pulse, such that even long tracker latencies on the order of 32 ms and beyond might still be acceptable. However, the maximum detail in fully correcting a profile of coma is only achieved with the smaller spot diameter (250 µm) for which a very fast tracker is essential (0 to 4 milliseconds). One can assume from this data that scanning spot delivery and eye tracking will continue to be refined in the future in an effort to further enhance the predictability and outcome of customized corneal ablations.

The case of a small spot diameter with a small ablation depth per pulse provides the best approximation to the ideal correction profile if all eye movements are completely compensated for by the tracker (0 ms latency).

Most recently, Katena Products Inc (Denville, NJ) has developed a solid state excimer laser with a very small spot and rapid laser delivery rate as an alternative to the relatively larger spot and slower rate found with current excimer laser delivery systems. Although not yet coupled with customized ablation, the future may hold a greater involvement of solid state laser technology in refining our ablation profiles.

One final refinement in laser delivery has to do with proper registration and alignment of the wavefront pattern to the eye. Currently, registration is achieved translationally and rotationally with the Alcon LADARVision system. Similar registration

attempts are now being adapted internationally with the Bausch & Lomb system (Rochester, NY) (Bausch & Lomb has a 250 Hz rotational eye tracker outside the United States) as well as the new iris tracking provided by VISX (Santa Clara, Calif) and NIDEK (Gamagori, Japan). (Note: Neither are commercially available at the time of publication.) Further refinements in registration will certainly need to be pursued by the various custom ablation platforms, including those who currently provide registration. This registration when coupled with very fast tracking will provide a space-stabilized image for effective treatment of complex higher-order aberrations.

Environmental/Interface On-Line Control

Beside adequate registration, scanning spot delivery, and tracking issues, an additional technology requirement for precision customized corneal ablation is that of the environmental interface of the laser to the cornea during ablation. Environmental issues such as temperature, humidity, and physical variables of the cornea (such as pachymetry and topography) all have an impact on how accurately customized corneal ablation can be performed. Attempts at controlling temperature and humidity are already being made by many surgeons who have sophisticated environmental controls in their operating suite. But beyond this control of room temperature and humidity, the microenvironment around the cornea is the most essential for predictability of outcomes. Reproducible techniques, such as maintaining the same time of flap exposure, drying or lack of drying of the stromal bed before treatment, and plume evacuation, all have an impact on the depth of corneal ablation with each pulse. One device that helps control the microenvironment is the Mastel Clean Room (Mastel Precision Inc, Rapid City, SD), a suction ring evacuation system for the removal of particulate debris and maintenance of uniform hydration, giving a predictable ablation response. However, wide spread acceptance of this device has not been established, perhaps because of the cumbersomeness of its use with certain laser systems.

Control of anatomical factors may be of even greater importance, and new devices allowing for on-line measurement of corneal pachymetry during ablation can allow one to stop before going beyond the safety threshold of residual stroma. On-line

Figure 42-4. Real-time intraoperative wavefront monitoring (10 captures/second; spherical equivalent term) during a low energy laser thermal keratoplasty (LTK) procedure to correct +1.50 D of hyperopia (treatment time = 15 sec; R2 = 0.96). (Reprinted with permission from Krueger RR, Gomez P, Kerekar S. Intraoperative wavefront monitoring during laser thermal keratoplasty. J Refract Surg. 2003;19(5):S602S607.)

topography control has also been proposed,13 and this can allow one to see the changes that are being made as they occur. Nevertheless, none of these on-line devices has yet been able to adaptively modify the treatment during its analysis so as to improve the refractive outcome.

Adaptive Corneal Correction

The concept of intraoperative measurement of wavefront error goes a step beyond that of intraoperative pachymetry and topography to give real-time information about the change in the refractive and aberration profile of the eye. Although this would be virtually impossible to do with LASIK or surface laser treatment due to the scatter of the surface during ablation, it can be performed with other modalities such as laser thermal keratoplasty (LTK) (Figure 42-4).14 Adaptive LTK has recently been proposed after success was demonstrated in the real-time measurement of wavefront error during a refractive procedure (Holmium LTK).14 In an ideal situation, scanning spot delivery of Holmium laser energy coupled with a tracking system and registration might allow for real-time wavefront measurement and feedback to adapt the laser pulses to the dynamically changing wavefront during correction. Although adaptive LTK may be a more futuristic procedure, real-time wavefront monitoring during LTK procedures is currently being investigated and can also be useful in influencing the refractive outcome by developing thresholds beyond which treatment would stop (ie, certain value of induced astigmatism or passing a certain refractive spherical outcome). With further adaptation, perhaps even corneal ablation could be linked with real-time wavefront measurement and adaptive corneal correction. This would further refine wavefront corneal ablation by allowing the surgeon to measure the effectiveness of treatment and compensate for treatment variability in real time.

The concept of intraoperative measurement of wavefront error goes a step beyond that of intraoperative pachymetry and topography to give real-time information about the change in the refractive and aberration profile of the eye.

THE QUEST FOR SUPER VISION

IN REFRACTIVE IMPLANTS

When considering the future of customization, one has to consider whether refractive surgery will mitigate toward refractive implants with a particular attention toward intraocular lenses (IOLs). Recent attention has been drawn toward a variety of specialized refractive IOLs that are currently under investigation and being applied with variable levels of success.15-19 When considering wavefront customization of IOLs, the attention narrows somewhat but still holds broad ranging possibilities.

Optimized IOLs (Technis)

One of the most recently adapted specialized IOLs is the Technis aspherical lens, which is specifically designed to reduce spherical aberrations in patients following cataract surgery.15,16 By analyzing a large population of cataract surgery patients regarding the asphericity they experience both preand postoperatively, an optimized aspheric IOL has been designed that will minimize the positive spherical aberrations often noted by these patients following surgery. The Technis lens (Pfizer, New York, NY) has enjoyed some success in its adoption into the field. Yet because it has a population-based optimized lens profile rather than a customized profile (individual-based), it still has errors due to the variability of spherical aberrations seen among individuals. Since approximately 10% of the population has negative spherical aberrations when examined by corneal wavefront sensing, further negative spherical aberration would be conveyed to these patients with implantation of this lens. Additionally, an aspheric lens with greater correction of spherical aberration has greater sensitivity to proper centration and alignment. A decentration of the Technis lens by as much as 0.40 mm or 7 degrees of tilt might induce new aberrations (eg, coma) that could negate the visual benefit of its

asphericity (Figure 42-5).16 Although this amount of decentration or tilt may be unlikely in a well-executed phacoemulsification with implantation procedure, when the surgery is less than optimal, implantation of this lens may not be recommended.

Customized IOLs Preinsertion

Just as the Technis aspheric IOL compensates for spherical aberration based on the mean values of a population, one could also consider correction of patient specific spherical aberration based on corneal topography. This is in contrast to utilizing wavefront-based measurements of preoperative spherical aberration as the unpredictable size and aspheric shape of the aging crystalline lens would make a postoperative spherical aberration less predictable. One could also conceptualize customizing the implant for the total ocular aberrations after cataract extraction but this would require a way of assuring proper alignment of the IOL during its implantation. Phacoemulsification of the crystalline lens with insertion of an IOL scaffold, giving a platform to anchor and align an IOL at a later date, might be one way to customize the IOL preinsertion. This, however, introduces the need to consider a two-step procedure of 1) initial cataract removal and scaffold implantation with subsequent whole wavefront measurements when the eye is stable, followed by 2) a secondary implantation of the customized IOL. Customized phakic IOLs might also be considered in this way. The implantation of a scaf- fold-anchoring element beforehand might be the most ideal way to assure proper alignment and registration of an IOL that is customized prior to its insertion.

Phacoemulsification of the crystalline lens with insertion of an IOL scaffold, giving a platform to anchor and align an IOL at a later date, might be one way to customize the IOL preinsertion.

THE QUEST FOR SUPER VISION

IN LENTICULAR MODIFICATION

Although a somewhat new concept that has yet only been experimentally proposed, lenticular modification opens a new paradigm within refractive surgery, beyond the field of keratorefractive surgery (corneal reshaping), to that of lenticular refractive surgery (natural crystalline lens reshaping and modification).

Can the Lens be Modified

Without Forming a Cataract?

In the mid-1980s, an important paradigm shift in ophthalmology occurred when the excimer laser was introduced for corneal ablation.24 Prior to the introduction of the excimer laser, corneal injury and surgery penetrating beyond Bowman’s layer was known to be associated with the formation of an anterior stromal scar. Therefore, it was a long-held belief that one should never operate on the optical center of the cornea. The early investigational work with excimer laser phototherapeutic keratectomy (PTK) and PRK violated this principle by asking the question, “Can one operate on the optical center of the cornea without creating a scar?” This paradigm shift introduced a new technology and way of thinking about corneal surgery, which led to the birth of a new subspecialty.

In a similar fashion, we are now only beginning to consider the question, “Can the natural crystalline lens be structurally modified without forming a cataract?” Recent investigation by Krueger and associates using femtosecond lasers has demonstrated that rabbit eyes can be irradiated with several hundred thousand pulses within the epinuclear lens tissue without the formation of a visible cataract or increase in lens light scatter (Figure 42-6A).25 Rabbit lenses lasered using a titanium sapphire amplified erbium fiber laser in one eye, with the other eye serving as a control, demonstrated similar or less light scatter in six paired eyes after three postoperative months, when using heli- um-neon (HeNe) laser scanning studies (Figure 42-6B). The ultrastructural morphology of these lenses demonstrated only a

0.50 to 1.00 µm electron dense change adjacent to the laser disrupted lens fibers. Despite the electron-dense border, the surrounding hexagonal lens fibers appear essentially normal.

Photophaco Reduction

Once you can successfully deliver laser energy into the crystalline lens without causing a cataract, you can potentially modify the lens to change its shape or flexural properties. Photophaco reduction (PPR) is a process by which a femtosecond laser or alternative energy source actually reduces the volume of the crystalline lens to change it into a new shape for the correction of refractive error.26 Although this is a concept that has not yet been experimentally demonstrated, PPR proposes to remove lens tissue more peripherally to induce greater refractive lens power or centrally to lessen the refractive power of the crystalline lens. In a similar way, this energy deposition can be customized because of the fine precision of femtosecond laser tissue ablation. However, because such a small volume of lens tissue is ablated with each laser pulse, the efficacy of PPR in changing the lens shape may make it impractical. Experimental and investigational studies need to be performed to realize its full potential.

Photophaco Modulation

The concept of depositing femtosecond laser energy, or energy from an alternative source, into the crystalline lens can also be used to separate lens tissue to increase its flexibility. Photophaco modulation (PPM) is a procedure whereby femtosecond laser pulses can separate lens fibers that are covalently bound during the process of aging, so as to modify the lens’ modulus of elasticity for the correction of presbyopia. In the 1970s, Ronald Fisher demonstrated that one could measure the relative elasticity of human crystalline lenses by placing them on a pedestal that could be rotated at 1000 revolutions per minute (rpm), creating a centrifugal force simulating that of the zonules.27 As a result, the anterior to posterior dimensions of the lens were reduced by the spinning (polar strain), and the amount of this reduction was found to be dependent on the lens’ age, with younger lenses demonstrating a greater polar strain. To verify this observation,

Krueger and associates reproduced this experiment using 40 cadaver lenses, demonstrating a similar age-dependent polar strain as Fisher, using both direct measurement (viewed with a microscopic reticle) and photographic projection (high-speed camera film projected to a magnified view and referenced to a known calibration target).28 Going one step further, 100 pulses of neodymium:yttrium-aluminum-garnet (Nd:YAG) laser energy were then placed within the lens nucleus/epinucleus in an annular fashion in 11 cadaver lenses of various ages, with the fellow eye’s lens remaining as a control. Both the treated and control lenses were subjected to the rotational deformation experiments, demonstrating a statistically significantly greater polar strain in the treated lenses (Figure 42-7). The concept of modulating the elastic properties of the crystalline lens using an external energy source, such as a laser, could provide a simple means of presbyopia correction by restoring the age-dependent loss of accommodation. Further safety and efficacy experiments using femtosecond laser pulses will be required and are under investigation. There will likely be other concepts for customization of vision in presbyopia correction proposed in the future.

Photophaco modulation (PPM) is a procedure whereby femtosecond laser pulses can separate lens fibers that are covalently bound during the process of aging, so as to modify the lens’ modulus of elasticity for the correction of presbyopia.

THE QUEST FOR SUPER VISION IN

NEURAL PROCESSING AND PERCEPTION

Thus far, the quest for super vision in customized visual correction has dealt with an optical modification of ocular aberrations to improve the quality of the image placed on the retina. Since the limitations of super vision are not only optical but also neural, can we go beyond optics and also improve the retinal and neural processing or cortical perception?

Optical Solution to a Retinal Problem

Although customization in patients with retinal disease will likely be quite difficult, one recently proposed optical solution

may provide a unique benefit to patients with focal lesions of the fovea. In a yet unpublished report by Seiler and associates, a patient with central areolar foveal dystrophy and a best-corrected visual acuity (BCVA) of 20/100 underwent a wavefront-guided LASIK procedure to correct not only a small refractive error, but to induce 3.50 degrees of horizontal tilt (personal communication, Theo Seiler, September 2003). Tilt, which is considered a firstorder aberration, is best understood by its prismatic effect, which at the level of the cornea might cause the light rays to focus onto the parafoveal retina rather than the central fovea where the focal lesion is present. In this patient, the BCVA and uncorrected visual acuity (UCVA) improved to 20/40, which suggests that prismatic displacement of a foveal image may induce an eccentric fixation to help some patient’s with focal retinal disease.

Photoreceptor Transplantation and/or Redistribution

Many may remember the television series in the late 1970s and 1980s about the “Six Million Dollar Man”, who was rebuilt to be stronger, faster, and have better senses, including super vision. Although we are a long way off from rebuilding the complex visual system with which we are born, efforts in photoreceptor transplantation research of macular degeneration does lead us to wonder whether photoreceptor transplantation might be performed some day to increase the resolution of retinal image size to levels beyond the current limitations of nature. Perhaps customization of vision will someday find a solution for patients with macular degeneration to customize their visual correction at the retinal level. Retinal translocation surgery has also shown some benefit in transferring the functional photoreceptors in the macula to an area away from an underlying subretinal pathology. Although this has not resulted in super vision, it does represent a form of customization for visual correction.

Neural Adaptation and Amblyopia Correction

The question about neural limitations posed earlier in this chapter (“Would an eye that has never seen better than 20/20 be correctable to 20/8 or 20/10, if the ocular aberrations were sufficiently corrected?”) is again important to ask when considering the quest for super vision in neural adaptation and amblyopia correction. Are we limited by our previous best spectacle-cor- rected vision, following an ideal optical correction, or is there a period of adaptation in which improvement in best-corrected

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vision can be achieved? Many of these questions, although currently unanswered, will likely be more definitively answered as we continue investigating customized corneal ablation of unique cases in which some level of refractiveor aberration-induced amblyopia exists. The previously mentioned work of Seiler and associates, regarding the correction of anisometropia and high astigmatism, respectively gaining two lines of best-corrected visual acuity 1 or 3 years later, suggests that neural adaptation and the correction of refractive amblyopia is possible. Reports of using magnets for amblyopia correction in several Fyodorov clinics in Russia also introduces the possibility of an external influence to encourage neural adaptation, which has yet been unsubstantiated in the Western world (personal communication, Nick Pastaev, MD, 1997).

THE QUEST FOR SUPER VISION IS HERE TO STAY

Although ophthalmology has seen a number of novel concepts reach the realm of clinical practice, many of these have only been an evolutionary step or have been unsuccessful due to a lack of wide spread acceptance. This is especially true in refractive surgery in which dozens of procedures have been proposed and introduced for the correction of refractive error, yet only a handful are commonly used today as part of routine clinical practice. Most of the currently acceptable procedures today will be obsolete in a decade. So we ask this question about customized corneal ablation, wavefront customized visual correction, and the quest for super vision: Will these novel concepts that we are discussing today be here to stay, or will they slip from the scene as so many refractive procedures have done before? Let us take a moment to consider each of these three mentioned concepts in the following subsections.

Customized Corneal Ablation

Just as LASIK has dominated the refractive market in the United States for the past 5 or so years, so will customized LASIK and PRK come to dominate the refractive market for the next 5 years. This is because customized laser vision correction procedures are providing everything that conventional LASIK and PRK provide and more. Satisfactory visual outcomes with speedy recovery and good quality of vision in a safe and relatively simple procedure have been the mainstay of current, conventional laser vision correction. The disadvantage of conventional refractive surgery is that it increases higher-order aberrations and reduces visual quality in some patients.29 Since the introduction of customized corneal ablation, we have seen superior visual outcomes, sharper contrast, and reduction of higher order aberrations, which has created more patient satisfaction and a reduced likelihood of dissatisfied patients. Consequently, customized corneal ablation is taking a widely-accepted procedure and making it better. Hence, customized corneal ablation is here to stay.

What about after 5 years have passed? Will customized corneal ablation continue to be the mainstay of refractive surgery in 10, 15, or 20 years? Although it is likely to remain an option for the next two decades, other forms of customized visual correction may take on a more dominant role.

Wavefront Customized Visual Correction

In looking to the future, other forms of wavefront customization, such as IOLs, onlays, and other synthetic implants, will take on an ever increasing role in the methods of visual correction and customization. Ocular wavefront sensing, as a method of determining refractive error, will increasingly be employed in vision assessment and refractive testing and will likely become routine in the refractive ophthalmic exam, similar to corneal topography. Since wavefront sensing will play such a large role in clinical practice, wavefront customization in visual correction will also continue to play an increasing role. Whatever procedure we choose to use for refractive surgical correction, we will want to employ wavefront customization to optimize this procedure. Hence, we believe some form of wavefront customized visual correction will be here to stay for as long as we are offering refractive surgical procedures, or at least until there is a better technology to optimize the human visual system.

We believe some form of wavefront customized visual correction will be here to stay for as long as we are offering refractive surgical procedures, or at least until there is a better technology to optimize the human visual system.

Quest for Super Vision

Finally, if there is ever a suspicion that customized corneal ablation will be displaced by an alternative method of vision correction or if wavefront customization will somehow be replaced by a superior method of customization, the quest for super vision will never be replaced because patients will always desire to have the best, sharpest vision obtainable. In conclusion, when all other things pass because of superior technology, the quest for super vision will ever be present. Hence, if we continue with subsequent volumes of this book, the titles may change; however, the subtitle—“The Quest for Super Vision”—will remain the same.

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