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

deduce that latency due to processing delay and mirror realignment contributes a notable limitation to adequate tracking during laser vision correction. Even passive eye tracking, which typically shuts off the laser for >1.50 mm excursions in some laser systems, does not protect for fixation-related saccades.

Latency is the time required to determine the eye’s location, calculate the required response, and move the laser tracker mirrors to compensate for the new location. The latency period is therefore due to both the processing delay and the mirror readjustment delay.

Tracker Type

Table 23-2 illustrates the comparative features of eye tracking among small spot scanning excimer lasers. The table lists only two columns, referring to LADARVision tracking using laser radar and video camera-based tracking using an infrared video image. The difference in sampling rates are included in this table and even though latency periods are not listed, the previously illustrated figures illustrate the important contributions of latency for video camera vs laser radar trackers. Typical video camera eye tracking uses infrared light illumination of the iris against a dark pupil in most refractive surgical systems. This video-based tracking captures a new image without maintaining a reference of the eye’s position previously so that no feedback of the eye’s current position exists. It simply sees a deviation from the intended position and moves to respond to that deviation.

Closed vs Open Loop Tracking

In open loop (video) tracking, once a new image is taken, the change from the previous image location is calculated and an error signal is sent to then move the mirrors. This is in contrast to

closed loop tracking, represented by the laser-radar–based system (LADAR) where the rapid sampling rate together with this system’s closed loop servo response feeds back information on the eye’s new position continually, thus maintaining a space stabilized image and accurate tracking without latency.

When initiating tracking, the LADARVision system scans the eye to find four contrast boundaries (A, B, C, D) that reflect the 905 nm laser signal back to the detector device in the eye tracking system. When the detected spots vary in size, the eye position is immediately readjusted so that all reflections are of equal magnitude. An equal reflected spot magnitude is essentially maintained, providing a space-stabilized image throughout the engagement of the tracker. Figure 23-9A demonstrates the spot reflection magnitude when the eye has moved down and to the left. This is adjusted instantaneously to allow an equal spot reflection magnitude to be maintained, as in Figure 23-9B. This maintenance of a space-stabilized image is the essence of closed loop eye tracking, such that the controlled variable (eye movement) can be fed back and compared to the reference input (eye position) in order to minimize the position error. In contrast, open loop eye tracking has no comparison of the controlled variable with the desired input. For closed loop eye tracking to be maintained, the sampling rate needs to be at least 10 times the tracker bandwidth, which for eye tracking is approximately 100 Hz.18 Hence, a tracker sampling rate of at least 1000 Hz is sufficient for maintaining a space-stabilized image with closed loop eye tracking.

For closed loop eye tracking to be maintained, the sampling rate needs to be at least 10 times the tracker bandwidth, which for eye tracking is approximately 100 Hz.18 Hence, a tracker sampling rate of at least 1000 Hz is sufficient for maintaining a space-stabilized image with closed loop eye tracking.

Clinical Significance of Eye Tracking

When discussing the important components of eye tracking, such as sampling rate, latency time, tracker type, and open loop vs closed loop, the clinical significance of these components are demonstrated by comparative studies of laser vision correction when the eye tracker is on, compared to the same when the eye tracker is off. In a presentation at the American Academy of Ophthalmology Meeting in 2001, Hardten et al used the VISX ActiveTrac, which is a 60 Hz video camera tracker with open loop tracking, to treat a cohort of 202 eyes in comparison to 110 eyes in which the tracker was not engaged. Statistically significant improvement in vision was noted with the active tracker engaged (mean SCVA = 20/19.3) in comparison to no tracker (mean SCVA = 20/20.2) (p = 0.020). This study is in comparison to that reported by Mrochen et al,19 in which a faster, 250 Hz, video camera tracker employed by the Wavelight Allegretto (Erlangen, Germany) laser demonstrated a statistically significant improvement in vision (mean SCVA = 20/17.7) when tracking 20 eyes in comparison to 20 eyes with no tracker (mean SCVA = 20/20.8) (p = 0.013). Both studies showed clinical improvement with eye tracking, yet the latter study required a smaller number of eyes to achieve statistical significance. Performing a similar comparison with the LADARVision eye tracker system at 4000 Hz cannot be done because the laser will not fire with the tracker turned off. Nevertheless, one can infer that significant clinical improvement would also be achieved when using the LADARVision tracker. In conclusion, we recommend the use of a high sampling rate tracker with minimal or no latency and closed loop servo response to achieve the maximum benefit.

ACCURATE WAVEFRONT DEVICE

Wavefront vs Corneal Topography

Although customized corneal ablation and especially customized visual correction, as specified by this book, refer predominantly to information gathered from a wavefront measurement device, computerized corneal topography can also be used in customizing corneal ablation by breaking down the corneal topographic map into the same aberrations and irregularity terms presented in ocular wavefront analysis. The concept of topographic customized corneal ablation was first proposed by Gibralter and Trokel in the mid-1990s.20 Although a number of companies have attempted to make a successful clinical link between corneal topography and laser ablation, wide spread adaptation has been limited due to difficulties in the registry of the topographic information with the laser. Nevertheless, highly irregular corneas after previous laser vision correction, or other corneal surgery, can be in part corrected with several different customized topographic platforms presented later in the book. With many of these platforms, however, although the cornea can be more successfully regularized, a precise refractive outcome is yet difficult to achieve as clinical topography gives no information about the patient’s refraction and this must be implemented as a separate step.

The more popular focus of customized corneal ablation, as well as customized visual correction, utilizes ocular wavefront sensing as the refractive information being used in wavefrontguided customized ablation. The wavefront measurement device gives a two-dimensional (2-D) profile of refractive error much in

the same way as computerized corneal topography gives a 2-D mapping profile of keratometry. Therefore, wavefront-guided customized corneal ablation should be more precise than topog- raphy-guided ablation, as the wavefront not only attempts to smooth the cornea, but provides a sharp focus of all corneal points on the retinal fovea.

The wavefront measurement device gives a two dimensional profile of refractive error much in the same way as computerized corneal topography gives a two-dimensional mapping profile of keratometry.

Principles of Wavefront Measurement Devices

Much in the same way that computerized corneal topography devices became available during the past decade, there are now a number of different types of wavefront measurement devices being made available on the market. Although it is often difficult to adequately categorize new products in an understandable fashion, there appears to be four different principles by which wavefront aberration information is collected and measured. A summary of the principles of measurement and their devices are included in Table 23-3.

Outgoing Refractive Aberrometry (Shack-Hartmann)

At the turn of the past century, Hartmann first described the principles by which optical aberrations in lenses could be characterized.21 This was later modified by Shack and found practical application in adaptive optics telescopes to eliminate the aberrations of the earth’s atmospheres for the past 20 years. It was finally introduced into ophthalmology by Liang and Bille in 1994.22 The Shack-Hartmann wavefront sensor was used to objectively measure the wave aberrations of the human eye. Further application of adaptive optics in ophthalmology found use in viewing retinal structures with greater detail than ever before. In 1996, images of the cone photoreceptors were viewed in the living human eye by adaptive optics defined by a ShackHartmann wavefront sensor.23 This first attempt at customizing the optics of the eye to increase the resolution of structures within it defined the need for the measurement specificity provided by wavefront technology in achieving better resolution when viewing structures outside the eye.

The typical Shack-Hartmann wavefront sensor utilizes approximately 100 spots or more, created by approximately 100 lenslets that focus the aberrated light exiting the eye onto a CCD detection array. Figure 23-10 demonstrates the principles of Shack-Hartmann wavefront sensing, in which a low energy laser light reflects off the retinal fovea, passing through the optical structures of the eye and creating an outgoing wavefront. After the light passes through the array of lenslets, each small segment of the wavefront is then focused to a small spot on the detection array, and the distance of displacement of the focused spot from the ideal very accurately defines the degree of ocular aberration. Figure 23-11 shows a segment of the wavefront going through a single lenslet and focusing at a distance (dx) from an ideal perpendicular point of focus. To the left of this figure, the entire aberrated wavefront spot profile in red is slightly more peripheral than the ideal wavefront spot profile in blue. This particular wavefront spot profile is representative of a patient with myopia.

Although the Shack-Hartmann method of wavefront aberrometry is a very accurate method, the level of accuracy is

188 Chapter 23

Table 23-3

Categorical Listing of Commercial

Wavefront Devices

OUTGOING REFLECTION ABERROMETRY

(SHACK-HARTMANN)

•Alcon LADARWave

•VISX Wavescan

•Schwind Aberrometer

•Bausch & Lomb Zywave

•Meditec WASCA

RETINAL IMAGING ABERROMETRY (TSCHERNING)

•WaveLight Analyzer

•Tracey Ray Tracing

INCOMING ADJUSTABLE REFRACTOMETRY (SCHEINER)

• InterWave SRR

DOUBLE PASS ABERROMETRY (SLIT SKIASCOPY)

• Nidek OPD-Scan

dependent on the number of spots that are collected within a 7 mm pupil area. Commercially, Shack-Hartmann wavefront devices vary from as little as 70 spots (Bausch & Lomb Zywave) to as many as 800 spots (WaveFront Sciences COAS). In general, it appears that approximately 200 spots within a 7 mm pupil area appears to be sufficient for accurately measuring up to eighthorder aberrations (see Chapter 21). The potential limitations of Shack-Hartmann wavefront sensing may include multiple scattering from choroidal structures beneath the fovea, but this is likely to be insignificant in comparison the axial length. Also, highly aberrated eyes may find a crossover of the focused spots. Although potentially a concern with some devices,24 this has not been found to be of concern with other devices, even in capturing the most complex wavefronts (see Chapters 26 and 29).

Retinal Imaging Aberrometry (Tscherning and Ray Tracing)

The next type of wavefront sensing was characterized by Tscherning in 1894, when he described the monochromatic aberrations of the human eye.25 Tscherning’s description, however, was not supported by the leaders of ophthalmic optics, including Gullstrand, and was not favorably accepted. It was not until 1977 that Howland & Howland used Tscherning’s aberroscope design together with a crosscylinder lens to subjectively measure the monochromatic aberrations of the eye.26 This same concept was more recently modified by Seiler using a spherical lens to project a 1 mm grid pattern unto the retina.27 This, together with a paraxial aperture system, could visualize and photographically record the aberrated pattern of up to 168 spots as a wavefront map. This 13 x 13 spot grid of laser light is projected through a 10 mm corneal area and represents an analysis of approximately 100 spots within a 7 mm pupillary area. Figure 23-12 illustrates the optical principals of the modified Tscherning aberrometer, demonstrating the optics of retinal imaging (A) and image capture (B).

Figure 23-10. Principle of Shack-Hartmann wavefront sensing in which a low energy laser light reflects off the retinal fovea, passing through the optical structures of the eye to create the outgoing wavefront. The wavefront passes through a lenslet array to define the deviation of focused spots from their ideal. (Courtesy of Raymond Applegate, PhD.)

Figure 23-11. A small segment of the wavefront passing through a single lenslet demonstrating the deviation (dx) of the focused spot (red) from its ideal location (blue). When presented as an array of spots, the more peripheral location of the focused spots (red) from the ideal spots (blue) represents a myopic wavefront spot pattern.

Although the Shack-Hartmann method of wavefront aberrometry is a very accurate method, the level of accuracy is dependent on the number of spots that are collected within a 7 mm pupil area.

The potential limitation of this type of wavefront sensing is the use of an idealized eye model (Gullstrand Model I) to perform the ray tracing computation. The Gullstrand model, which varies with refractive error, however, is modified within the commercial device according to the patient’s refractive error to maintain an accurate assessment of the axial length.

An alternative form of retinal imaging has been introduced over the past several years by a Ukrainian scientist, Vasyl Molebny.28 Tracey retinal ray tracing is a slightly different form of retinal imaging in that it uses a sequential projection of spots onto the retina, in a “gatling gun” fashion, that are captured and traced to find the wavefront pattern. A total of 64 sequential retinal spots (recently increased to nearly 100) can be traced within 12 ms, and converted to a wavefront pattern on the pupillary plane. The number of spots with this method of analysis is nearly the same as that of the Tscherning aberrometer within a

7 mm pupillary area, but the Tracey technology has the flexibility of analyzing a greater number of spots within a critical area, expanding the specificity of resolution, especially in more aberrated eyes.

Ingoing Adjustable Refractometry

(Spatially-Resolved Refractometer)

The third method of wavefront sensing is based on the 17th century principle of Sheiner and described by Smirnov in 1961 as a form of subjectively adjustable refractometry.29 Peripheral beams of incoming light are subjectively redirected toward a central target to cancel the ocular aberrations from that peripheral point. This was modified by Webb, Penny, and Thompson in 1992 as a subjective form of wavefront refractometry of the human eye.30 Figure 23-13 demonstrates the optical schematic of the spatial resolved refractometer (SRR), which measures the wavefront pattern according to these principles. The InterWave (Emory Vision, Atlanta, Ga) device commercializes the SRR technology utilizing approximately 37 testing spots that are manually directed by the observer to overlap the central target in defining the wavefront aberration pattern. Although this technology is unique and potentially beneficial by subjectively verifying the aberration seen by the patient, the limitation of this technique is a lengthy time required for subjective alignment of the aberrated spots.

Figure 23-13. Principles of spatially resolved refractometry: a peripheral light source projected through a wheel (mask) can be subjectively redirected by a joystick to overlap a reference point in the center of the pupil. The movement varies the angle of incidence of the peripheral ray and can be used to mathematically characterize the wavefront error at that point. Multiple peripheral rays are tested to construct the wavefront profile. (Courtesy of Stephen Burns, PhD.)

Double Pass Aberrometry (Slit Skiascopy)

The final method of wavefront sensing is based on methods of double pass aberrometry or retinoscopic aberrometry which considers both the passage of light into the eye and reflection of light out of the eye in defining the wavefront pattern. Slit retinoscopy (skiascopy) rapidly scans a slit of light along a specific axis and orientation. The fundus reflection is then captured to define the wavefront aberration pattern onto a parallel array of photodetectors and one set of perpendicular photodetectors to define the orientation.31 A total of four spots can be defined at each meridian of scanning with 360 meridia being scanned (at each degree of 360 degrees) for a total of 1440 data points. Figure 23-14 shows the retinoscopic scanning slit of light into the eye and reflected light out of the eye. The temporal delay of photo voltage peaks from the photodetector signify the points of wavefront information. Although this technique, utilized by the Nidek OPD-Scan (Gamagori, Japan), is also sequential at various axes, the objective capture of the reflex makes it possible to acquire the information in a rapid sequence. The potential limitation of this technology includes the small amount of information collected axially within a given meridian (four spots) and the sequential nature of the capture.

Figure 23-14. Principles of slit skiascopy used in the Nidek OPDScan. A scanning slit of light (aqua blue) enters the eye and is reflected out (yellow) with a temporal delay. An array of photo detectors capture the temporal delay, which defines the refractive or aberration error specific to that meridian. Multiple meridia characterize the total ocular wavefront profile.

Practical Aspects of Wavefront Measurement Devices

Table 23-3 outlines the categorical listing of commercially available wavefront devices. Each has a representation by at least one or several companies. Each company has made its own proprietary modifications to provide a practical device for clinical use. Careful comparison of these various wavefront measuring principles and their specific devices are only beginning to be performed clinically, and this is represented in Chapter 21. The practical utility of one device vs another will continue to be the subject of future investigation. A more detailed description of these various principles of wavefront sensing is discussed in Chapters 15 to 20.

LASER/WAVEFRONT INTERFACE

Wavefront Capture and Comparison

The first step to properly linking up the wavefront device and measurement with the actual laser treatment is to ensure the most accurate and reproducible wavefront has been captured and implemented. Many commercial wavefront devices have steps to assure an accurate wavefront capture. For example, with the Alcon LADARWave aberrometer, each patient exam requires five consecutive wavefront measurements, each of which is saved, with the three closest in agreement being compared and used to generate a composite profile (Figure 23-15). This new composite wavefront map can be used diagnostically or used to create a wavefront-guided laser ablation profile and spot pattern for treatment. Whether other wavefront/laser link-ups require the creation of a composite map is unknown, but a very reproducible and accurate map can best be achieved with multiple captures, comparisons, and the generation of a composite map.

Figure 23-15. Capture and comparison of five consecutive wavefront maps in a myopic eye before custom cornea LASIK surgery. The three closest in agreement were used to generate a composite profile map to be used in creating the wavefront-guided laser ablation profile.

Just prior to the wavefront capture, small ink marks may be placed along the limbus at two locations using an eye marking pen. This step is specifically done with the LADARVision platform for custom cornea, where the eye geometry of the markings is captured together with the wavefront information. At this point, the wavefront and geometry information is electronically transferred to the treatment laser. This link-up is achieved by a computer disc that downloads the information from the wavefront device to the treatment laser. This transfer includes not only the wavefront error information, but also the orientation data gathered during the wavefront measurement and defined by the position of the ink marks and the limbus.

Conversion to Ablation Profile

The next step in the wavefront to laser link-up is the conversion of the wavefront measurement to an actual ablation profile. The tissue that needs to be removed from the cornea to correct the refractive error and higher-order aberrations is determined from an ablation profile that is fundamentally the inverse of the wavefront error map. When implementing this step, it is important to have a wavefront measurement that has been captured through a diameter at least 0.50 mm larger than the scotopic pupil size. To achieve a pupil diameter of this size, pharmacological dilation is necessary. Although subtle variations in the wavefront pattern have been demonstrated with the use of pharmacologic agents,32 a customized laser vision correction requires a large ablation profile in excess of the typical scotopic pupil, hence necessitating pharmacologic dilation. The actual conversion process of the wavefront into the ablation profile is a complex mathematical inversion of the three dimensional maps that often factors in other variables, such as corneal curvature33 and biomechanics (Figure 23-16A).34 The actual ablation profile diameter used in customized laser vision correction may vary somewhat for different custom laser platforms. For instance, with the Alcon LADARVision platform, the CustomCornea ablation profile is defined by a 6.5 mm optical zone together with a 1.25 mm blend zone for a total ablation diameter of 9 mm.

Figure 23-17. Registration process, which includes a limbus ring for xy alignment and orientation marks for cyclorotation. The latter can be overlapped by virtual sputniks to register the wavefront data to the actual eye and laser tracker.

A customized laser vision correction requires a large ablation profile in excess of the typical scotopic pupil, hence necessitating pharmacologic dilation.

In every instance of customized ablation, a blend zone is necessary to produce a smooth transition between the correction of higher-order aberrations at the edge of optical zone and the residual unablated cornea. With the Tscherning aberrometer linkup to the Wavelight Allegretto laser, a blend zone of at least 0.5 mm is added to the calculated ablation zone. In cases where the residual stromal thickness after LASIK makes it unsafe to meet a full 7 mm optical zone, a slightly smaller optical zone diameter is implemented.

The final step in the conversion process is determining the excimer laser shot pattern (Figure 23-16B). This step is generally an automatic process in which the typical ablation depth per pulse is used, corresponding to the measured fluence of the laser during the calibration process. Subtle variations of laser ablation

technique and environmental conditions, including temperature and humidity, can affect the ablation depth per pulse. For the most part, the currently approved customized ablation platforms have a spherical offset to compensate for individual surgeon variability, while maintaining the same shot pattern for cylinder and higher-order aberrations.

For the most part, the currently approved customized ablation platforms have a spherical offset to compensate for individual surgeon variability, while maintaining the same shot pattern for cylinder and higher-order aberrations.

Dynamic Registration

Once the ablation profile and laser shot pattern is in place, the patient is ready for customized laser vision correction. For most laser vision correction platforms, this means aligning the geometric center of the laser vision ablation profile with the center of the undilated pupil. For many of these systems, infrared video camera eye tracking can also be engaged to maintain this position in the midst of low grade eye movements.

For CustomCornea and the LADARVision platform, this goes one step further. Dynamic registration can be achieved by engaging the laser radar eye tracker, which registers the wavefrontdetermined laser shot pattern to its corresponding position on the cornea by overlaying the identification reticles. The first reticle is the limbus ring which provides the xy alignment, and dynamically maintains that alignment throughout the tracking of the dilated pupil margin. The second reticle is the cyclorotation alignment which is implemented by rotating the image of the limbus ring in such a way that virtual “sputniks”, taken from the orientation marks recorded during the wavefront capture, are overlapped with the actual ink marks that still remain on the eye. In this way, true registration can be achieved dynamically, not only in XY orientation, but also statically with regard to cyclorotation. The graphic user interface on the LADARVision 4000 laser demonstrates the alignment of the limbus and overlap of the orientation marks for verification prior to treatment (Figure 23-17). In the future, registration and tracking based on iris detail will provide a possible alternative for dynamic capture of cyclorotation, as well.

The process of registration is actually the most important technology requirement for accurate customized ablation of higherorder aberrations. The importance of registration regarding lateral movement and cyclorotation can be seen in Table 23-4. Here the criteria for registration and tracking requires: 1) less than 50 µm of decentration to maintain an ideal wavefront ablation; 2) less than 200 µm of decentration to achieve results consistent with the best 10% of the untreated, normal population; and 3) approximately 400 µm of decentration to only duplicate the mean preoperative image quality. These same lateral decentrations can be noted with tortional misalignment of 1 degree, 4 degrees, and 10 degrees, respectively, for the aforementioned criteria.

The process of registration is actually the most important technology requirement for accurate customized ablation of higher-order aberrations

Algorithm Development

A final consideration in the wavefront/laser interface requires an understanding of the variable of the ablation process. With many of the existing customized laser vision correction platforms, a complex algorithm needs to be implemented to insure the effectiveness of the custom treatment. Fortunately, the software received by the user already has the appropriate algorithm in place. This makes it even easier for the surgeon, who merely has to download the wavefront information rather than develop a nomogram based on refraction data. Subtle adjustment can be implemented depending on the surgeon factor and environmental conditions. However, this offset is not intended to be an actual nomogram adjustment, but rather a preset spherical refinement that considers variables based on an individual surgeon and location. Both the actual laser algorithm and the offset are to be carefully established.

With a proper algorithm and “nomogram” offset, together with the technology requirements of a small, scanning spot, very fast eye tracking system, accurate wavefront device and state of the art wavefront/laser interface, it is truly possible achieve the optimum results with customized corneal ablation. These technology requirements serve as a foundational basis in our current attempts to provide state of the art wavefront-guided customized corneal ablation.

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Invest Ophthalmol Vis Sci. 1999;38:S538.

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12.Arevalo JF, Ramirez E, Suarez E, et al. Incidence of vitreoretinal pathologic conditions within 24 months after laser in-situ keratomileusis. Ophthalmology. 2000;107:258-262.

13.Ozdemar A, Aras C, Sener B, et al. Bilateral retinal detachment associated with giant retinal tear after laser-assisted keratomileusis. Retina. 1998;18:176-177.

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18.Krueger RR. In perspective: eye tracking and Autonomous laser radar. J Refract Surg. 1999;15:145-149.

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INTRODUCTION

With the increasing demands of customized corneal ablation— smaller beam sizes, faster repetition rate, and greater precision of correction—exact positioning of each laser shot onto the eye becomes increasingly more important. This need for greater positioning accuracy has provided the impetus for several refractive laser companies to implement eye tracking as the ”target acquisition system,” to position the ablation beam accurately onto the corneal surface and compensate for patient head and eye movements during the surgery procedure. As the sophistication of the laser delivery systems and the whole treatment process increases, so do the requirements of this eye tracking system.

This chapter will outline the overall accuracy requirements of beam placement for accurate and repeatable customized ablation; how the physiological parameters of eye movements can affect the accuracy of beam placement, and hence the overall outcome; and the requirements and design of the overall eye tracking system to be considered to achieve accurate, repeatable customized surgery.

This requirement of 0.45 mm may seem coarse and simple to achieve, in comparison to the micron resolution of the diagnosis and tissue ablation per shot. However, this offset to the perfectly aligned procedure may have many causes: either systematic errors such as misalignment in the axis of the laser beam or incorrect initial alignment of the patient’s eye; or random or dynamic errors such as the eye movements of the eye. Each of these factors must be carefully controlled to ensure that the desired refractive correction is aligned correctly to the eye accurately enough to ensure a successful and repeatable visual outcome.

It is the purpose of the eye tracking system to measure the eye position at the beginning of the surgery; measure movements throughout the surgery; to send the offset data to the laser system; and to ensure that the position of the laser beam is compensated for changes in eye position. It is a requirement of the eye tracking system that it can have an overall accuracy that ensures an improved visual outcome, that is giving an overall offset of less than 0.45 mm in translation (ideally 0.05 mm), and a rotary offset of less than 15 degrees (ideally 1 degree).

OVERALL ACCURACY REQUIREMENTS

How accurate does beam placement need to be for the quest for ideal refractive corrections? Several studies1,2 have used computers to calculate the effect of displacement of the correction on the cornea or cyclotorsion on a simulated refractive outcome. Results indicated that a maximum constant decentration during surgery of 0.45 millimeters (mm) ensured that the optical quality of the model eye was not decreased by the simulated surgery. This requirement was reduced to a mere 0.05 mm when the aim is to achieve a diffraction-limited eye (ie, optically perfect). The results for cyclotorsion or rotatory displacement of the correction on the cornea were 15 degrees to ensure the optical quality was not further reduced and 1 degree to ensure a diffraction limited outcome.

Editor’s note:

Work by Guirao and coworkers demonstrated that 50% of the visual benefit of higher-order aberration is lost with a 250 microns (µm) decentration or a 10 degree eye rotation.1

S. MacRae

EFFECT OF EYE MOVEMENTS

This section will first describe eye movements in general situations, and then describe eye movements as they occur during the laser surgery and to some extent also during the diagnostic procedure. For correct measurement of eye movements, it is essential to understand the principles of the static and dynamic behavior of the eye’s high performance actuator system—the muscular structure that holds and controls the eye and its movements— and the neurological and vestibular control system that controls the actuator system.

Kinematics of Eye Movements

The eye can be modeled as a ball and socket mounted, rigid spherical body and eye movements can be described as a series of infinitesimal rotations around three orthogonal axes that intersect in the center of the sphere. Three pairs of extraocular muscles, acting according to the push-pull principle, rotate the eye around these three axes, therefore performing eye movements in three dimensions of rotation: horizontal rotation around the vertical axis, vertical rotation around the horizontal axis, and torsional rotation around the line of sight (Figure 24-1). Eye move-