Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

REFERENCES

1.MacRae S, Fujieda. M, Customized Ablation using the NIDEK Laser, Customized Corneal Ablation. Slack 2001; 17:211-217

2.Campbell C E, Benjamin W J, Howland H C: Objective Refraction: Retinoscopy, Autorefraction and

3.Thibos L N, Applegate R A, "Assessment of Optical

Quality" in Customized Corneal Ablation: The Quest for SuperVision. Chapter 6, Slack Incorporated, New Jersey, USA, 2001.

4.Thibos L N, Applegate R A, Schwiegerling J T, Webb R, et al., "Standards for Reporting the Optical Aberrations of Eyes", Vision Science and Its Applications (OSA Trends in Optics and Photonics, Vol. 35), pp232-244, Optical Society of America, Washington, D.C., 2000

5.Wachler B S, Durrie D S; Assil K K; Krueger R R. Role of clearance and treatment zones in contrast sensitivity: significance in refractive surgery. J Cataract Refract Surg 1999; 25 NO.1: 16-23

6.Miller E F, "Counter-rolling of the human eyes produced by head tilt with respect to gravity." Acta Otolaryngol. 54, pp479-501, 1962

7.Wachler B S, Krueger R R. Agreement and repeatability of pupillometry using videokeratography and infrared devices. J Cataract Refract Surg 2000; 26 NO.1: 35-40

Masanao Fujieda, MA,

Mukesh Jain, Ph D

Nidek Ltd., Co., Japan

Peter Keller, PhD,

Department of Optometry &

Vision Sciences

University of Melbourne,

Australia

INTRODUCTION: THE PATH TO CUSTOMIZATION

During the last two decades refractive surgery has undergone an evolution from incisional procedures such as radial keratotomy (RK), to failed lamellar procedures such as automated lamellar keratoplasty (ALK), to the enormously successful laser ablation techniques photorefractive keratectomy (PRK), laser subepithelial keratectomy (LASEK), and laser in situ keratomileusis (LASIK). Still there is room for improvements, especially with regards to patients with higher levels of myopia, hyperopia, or astigmatism where correction often results in the induction of excessive higher order aberrations and compromised vision quality. This chapter will provide an overview of the current state of wavefront analysis and its application to customized corneal ablation.

Since the introduction of the 193 nm argonfluoride excimer laser into ophthalmology1,2 continuing efforts have been made to improve the technology and its application. Many of these improvements occurred with the development of more advanced laser hardware and software. Thus, more sophisticated systems that use scanning or slit beams eclipsed broad beam lasers with diaphragms that created small diameter ablation zones. Further development in laser technology occurred with the introduction of smaller beam delivery systems linked with eye-track-

ers. These systems made it possible to create more complex ablation profiles along with larger blend zones and smoother surfaces. Modern eye trackers enhance the precision of the laser registration with the cornea and facilitate custom ablation to alter corneal contour in a way that corrects aberrations and enhances vision or at a minimum reduces the aberrations that are generated by correction of high myopia, hyperopia, or astigmatism.

Major advances related to image enhancement in astronomy lead to applications in optical analysis in ophthalmology. In wavefront-supported post-processing techniques used in astronomy, images are deconvolved with wavefronts measured using an instrument called a Hartmann-Shack wavefront sensor (Figure 1). This concept has been used to more completely characterize the refractive errors of the human eye using Zernike polynomial terms (Figure 2) developed by Frederik (Frits) Zernike.3 Zernike was born in Amsterdam in 1888, the son of two mathematics teachers.3 He is probably best known for Zernike polynomials, mathematical functions describing the individual contributions of different aberrations to the total optical aberration. The Zernike functions are similar to polynomials in geometry used to mathematically describe the best fitting curve. The more terms that are included in the equation the better the fit. In optical terms, this means the more Zernike terms that are calculated the more completely the aberrations for a particular eye will be characterized. At some point, however, the

Fig. 1. The Hartmann-Shack grid used for wavefront analysis with many current instruments.

Fig. 2. A Zernike pyramid showing different aberrations to the sixth radial order. The names of some of the specific aberrations like coma and trefoil are indicated. Provided by Ed Sarver of CTView.

benefits of including more terms become negligible due to factors such as retinal structure and function. Most of the wavefront sensors currently used in ophthalmology are based on the Hartmann-Shack wavefront sensor. Other approaches, such as those based on ray tracing, are also being developed.

Other methods of qualitatively analyzing optical aberrations besides Zernike polynomials are being explored and may possibly come to prominence. At the present time, however, Zernike polynomials are considered the current standard (Figure 2).

Wavefront measurements have been integrated with adaptive optics and deformable mirrors to correct lower and higher order aberrations in the eye. In one study these efforts resulted in an improvement of two to six lines in vision in normal individuals under low-light conditions.4 This led to the concept of super vision. These advances provided a more complete understanding of refractive error and stimulated efforts to improve refractive surgical treatments for eyes that are normal, eyes that have physiological wavefront anomalies, and eyes that have disorders of vision quality related to prior refractive surgery.

Objective data obtained from wavefront measurements facilitate a better understanding of issues related to the optics of the eye and quality of vision. Thus, optical abnormalities that were former-

ly grouped as "irregular astigmatism" can now be more precisely characterized using higher order aberration terms like coma and trefoil (Figure 2). For example, the sources of visual disturbance in eyes with uncorrected Snellen visual acuity of 20/20 that have ghosted images and other symptoms related to quality of vision can be better understood. Now the aberrations resulting in the visual disturbance can be mathematically defined and represented. Once specific features of the aberrations in a particular eye are precisely defined it will be possible to devise methods for correcting them using spectacles, contact lenses, or refractive surgery.

Corneal ablation profiles have traditionally been designed based on the sphero-cylindrical refraction. The principles proposed by Barraquer for keratomileusis5 were embraced for excimer laser systems.6 There is a link between the excimer laser platform and measurements that go beyond conventional refraction in customized corneal ablations. The data used in custom ablations can be derived either from corneal topography (anterior corneal surface) or wavefront (total eye aberrations) analysis. The concept of using customized treatments to remove clinically significant corneal irregularity based on corneal topography was proposed much earlier independent of recent efforts linked to progress in wavefront analysis.7-9

Studies of optical aberrations and wavefront measurements have quickly led to clinical applications.10 Seiler used the Allegretto laser system (Wavelight Laser Technologie AG, Erlangen, Germany) to perform customized corneal ablation based on the Tscherning wavefront measurements. McDonald treated patients with the Alcon Summit-Autonomous laser (Alcon, Fort Worth, TX). In this study, one eye was treated with the traditional treatment, while the contralateral eye was treated with a customized treatment based on a Hartmann-Shack wavefront sensing. Both preliminary experiences were considered encouraging by the investigators.10

Many manufacturers are now developing wavefront measurement systems along with excimer laser platforms for custom corneal ablation surgery. These efforts are providing refractive surgeons with the tools to address not only spherical and cylindrical refractive errors (low-order aberrations), but also higher-order ocular aberrations such as trefoil and coma. This, in turn, provides hope that vision can be optimized to the limits determined by pupil size (diffraction) and retinal structure and function. This

chapter will focus on exploring the possible benefits and limitations of this new technology, as well as strategies that may be used to improve the outcomes of laser corneal refractive surgery.

CRITICAL STEPS FOR SUCCESS IN CUSTOMIZED ABLATIONS

There are several basic steps to performing custom ablations. These are listed in Table 1.

The first step in custom ablation is to acquire information from the patient’s eye for use in directing customized treatments that enhance vision quality. Customization can be based on either wavefront or corneal topographic data (Figures 3, 4, and 5). Quantitative information regarding lower order and higher order aberrations present throughout the visual system from the tear film to the retina can be obtained from wavefront sensors. Elevation data from the cornea can be obtained from either Placido disc or scanning slit-based computerized videokeratography. Measurements used to direct custom

corneal ablations must have greater accuracy and precision than those used for diagnostic purposes. The reliability of the measurements obtained from either wavefront sensors or corneal topographers are the subject of ongoing debate and experimentation.

Reliable measurements of aberrations or corneal contour can be used to mathematically derive an ablation profile to reduce aberrations or limit induced aberrations during correction of myopia, hyperopia, or astigmatism. Intuitively one might assume that lasers with smaller beam diameters would be best suited to custom ablation and to a certain extent this is true. However, there are also relationships between the length of ablation and changes in corneal hydration and patient fixation that can affect the efficacy of customized ablation. There is insufficient data available at the present time to determine whether the overall result of customized ablation will be better when a uniform small beam diameter is used to perform the ablation or a variable beam diameter is used such that gross changes are made with a larger beam and refinements are made with a smaller beam.

Once an optimal ablation profile has been designed, either lamellar or surface ablation techniques can be used for surgery. Each approach has benefits and limitations and it is not clear which is superior.11 Proper registration of the ablation on the cornea relative to the aberrations requires compensation for eye movements through eye tracking. Openlooped and closed-looped active eye tracking systems are available. These systems should not only compensate for saccades and drifts, but also for cyclotorsion. Cyclotorsion-related misalignment of 4 degrees or 10 degrees would theoretically result in a 14% or 35% undercorrection of astigmatism, respectively.12 Compensation for cyclotorsion requires accurate marking with the patient in the upright position. With most laser systems this is achieved by rotating the patients head to align a lenticule within the optical system with the pre-placed marks. The LADARVision excimer laser (Alcon Summit Autonomous, Orlando, Florida) rotates the axis of treatment according to the marks. Novel systems are under development to use iris detail to guide axis alignment.

All current tracking systems are dependent on patient fixation. It is likely this will be a significant problem leading to suboptimal customized ablation results in some patients. It seems likely that future systems will be able to register a good approximation of the line of sight by monitoring the position of the macula relative to the center of the entrance pupil.

Factors that affect laser-tissue interactions are also important. Stromal hydration is a factor that will remain a challenging problem. Thus, stromal hydration, and therefore ablation efficiency, is dependent on room humidity, the timing of application of topical anesthetics that affect epithelial barrier function, the time from insertion of the lid speculum to beginning ablation, and the duration of the ablation itself. Stromal hydration even varies between surface ablation and ablation beneath a lamellar flap. Clearly consistency of technique will be even more important in custom corneal ablation than it is in traditional PRK, LASEK, or LASIK. The depth of the lamellar cut is relevant for different levels of stromal hydration on the cornea.

Postoperative monitoring of the results of customized corneal ablation over time will be critical for improving the success of wavefront or topogra- phy-guided refractive surgery. Modifications designed to refine results can only be made if outcomes are carefully and systematically monitored.

A major limitation of current efforts to measure and manipulate aberrations is that the systems only monitor from the tear film to the retina. No information about central processing of the information in the brain is provided. Similarly, little is known about compensation for aberrations in the brain or plasticity in the central nervous system once the eye is changed. Thus, for example, some patients with higher order aberrations detected with a wavefront sensor might have developed compensatory image processing in the brain. Such patients might actually feel vision is worse after custom ablation to "correct" higher order aberrations. Ophthalmologists are used to dealing with a similar situation when prescribing spectacle. It is not uncommon to find an optimal prescription that improves best-corrected vision, but which is not accepted by the patient due to feelings

of disorientation or strain. It will be interesting to see whether similar situations occur following customized corneal ablation.

MEASURING ABERRATIONS: PRINCIPLES AND TECHNIQUES

Wavefront sensing or aberrometry is a diagnostic method that enables mapping the aberration profile of the eye beyond lower order aberrations measured with a standard refraction. The difference between classical refraction and wavefront sensing is analogous to the difference between keratometry and corneal topography.13,14,15 Most refractive surgeons are even less skilled at interpreting wavefront maps than they are topographic maps. Just as with corneal topography, the use of standardized displays will facilitate understanding and interpretation. At the present time the understanding of wavefront is so rudimentary that it is difficult to fix on particular presentations of the data. For example, of the many aberrations evaluated by a typical wavefront sensor which are most relevant to vision quality? Studies are underway to answer these types of questions and, therefore, focus on what data is critical to diagnostics and custom corneal ablation.

Several types of wavefront sensing devices are currently under investigation. It is important to note that wavefront sensing is an evolving technology. Thus, all wavefront sensors are undergoing modification and improvement. Table 2 provides a classification system for wavefront systems that are currently available.

There are pros and cons for each type of wavefront sensing system. The means through which different devices measure the wavefront profile are the subject of other chapters in this book. However, a basic understanding of the differences between these systems is needed for comparison of strengths and weaknesses.

The Hartmann-Shack (Figure 1) wavefront sensor is the most commonly used for custom corneal procedures at the present time. Alcon (Fort Worth, TX), Bausch & Lomb (Rochester, NY), VISX (Santa Clara, CA), and Zeiss-Meditec (Dublin,CA) have adopted this device for their initial efforts in

customized ablations (Figures 3, 4, and 5). The Hartmann-Shack sensor uses light from a monochromatic infrared laser that passes into the eye and focuses at a point on the retina. The point light source bounces off the retina and passes back through the vitreous, lens, and cornea. These structures introduce aberrations in the light. The emerging beam of light passes through a lenslet array. The lenslet array is a disc containing 70–800 small lenses or lenslets, which focus the light onto a grid-like pattern that is captured by a video camera. The captured pattern is then compared to an ideal pattern, which is a symmetric equidistant grid that represents a flat wavefront. The calculated difference between the measured and ideal represents the "wavefront error." Since the wavefront obtained with the Hartman-Shack system is captured as it exits the eye, it is classified as an "Outgoing Optics" device.

Another wavefront instrument that is currently in use for designing custom ablations is the Tschering Aberrometer. This aberrometer is used in Wavelight (Erlangen, Germany) and Schwind (Kleinostheim, Germany) custom ablation systems.

This system projects a grid onto the retina and then captures the image of the grid that is formed on the retina (Ingoing Optics) for digitalization and analysis.16

An important limitation of either the Tschering Aberrometer or the Hartmann-Shack sensor is that highly aberrated eyes cannot be measured. Thus, eyes most in need of wavefront diagnostics or custom corneal ablation are frequently impossible to study. For the most part this is attributable to difficulties in obtaining reliable images. Future advances in the acquisition systems, along with improvements in interpretation software, may improve measurement consistency in highly irregular eyes.

Slit skialoscopy is the method used by Nidek (Gamagori, Japan). In this system a scanning slit is bounced off the retina. It is also considered an "Ingoing Optics" system. Nidek is also working on a hybrid system that incorporates Placido disc-based corneal topography with wavefront sensing for custom ablation.17

Another "Ingoing Optics" system that uses retinal imaging is the Tracey Retinal Ray Tracing

system (Houston, TX). This system works by firing a rapid sequence of individual parallel rays of light into the eye. The system can be adjusted to focus in on irregular areas.18 An important advantage of the Tracey system is that it should be able to accurately evaluate highly irregular corneas. Currently, the Tracey system is used exclusively for diagnostics (Figure 6). However, VISX Inc. (Santa Clara, CA) has obtained the rights for using this device in the future applications to custom ablations.

Spatially resolved refractometry uses Smirnov’s method to measure aberrations in the eye.19 With this method a reference beam enters the center of the pupil and forms a reference mark at the fovea. The patient lines up a second beam of light with the first. A potential advantage of this system is that it includes subjective input from the patient. Thus, it is currently the only system that has the capacity to provide information about how the wavefront is altered by processing in the central nervous system. This system is used only for diagnostics at the present time.

All wavefront analyses, even those in normal eyes, are very complex. Wavefront data are typically broken down into combinations of simpler surfaces. This allows the component aberrations to be described graphically. Thus, with currently available wavefront systems, the aberration errors are described using Zernike polynomials. The Zernike terms (Figure 2) comprise a new alphabet for representing ocular aberrations. The aberrations are classified in terms of orders (i.e., second, third, fourth, fifth, etc. order) (Figure 2). The "lower-order aberrations" include sphere (defocus) and cylinder. Higherorder aberrations (third order and above) include coma, trefoil, tetrafoil, and other terms. One can calculate tenth-order aberrations or higher, but measurements above the sixth order are rarely needed to describe even the most complex aberration patterns when one takes into account relevance to human visual performance.