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

The VISX WavePrint system includes ActiveTrak, which provides real-time tracking of the entire pupil—without dilation. The eye position is directly measured, increasing system accuracy. In fact, the automated ActiveTrak system greatly reduces the possibility of human error.

Innovations being refined for the ActiveTrak system include a unique feature to anticipate cyclotorsion problems. In the event of cyclotorsion, the tracker continues to function, but the ablation stops until fixation is re-acquired. This technology includes an automatic iris image-registration system for cyclotorsional registration and tracking that links the image to the WaveScan that appears to have significant advantages over ink-mark-based tracking technology. Iris image registration, which has been tested and validated, is being developed as part of the next generation of VISX tracking technology and should be more precise and automatic, reducing the possibility of human error9.

Eye movements fall into several categories, including linear motion in the X-axis, Y-axis, Z-axis, and cyclotorsional motion, which is the rotational movement of the eye around the Z-axis. Cyclotorsional registration of the diagnostic and laser trackers is important because preoperative diagnostic and laser treatment information must be aligned to achieve optimal outcomes. In order to achieve the best results during refractive eye surgery, the ablation should be centered on the part of the cornea that corresponds to the measurement taken by a wavefront device. Rotational alignment of the wavefront is also necessary.

For the most-accurate procedure, torsional motion should be precisely tracked intraoperatively with cyclotorsional tracking. With the VISX Torsional Iris Registration Method, registration is tracked from the diagnostic unit to the laser and during, or intraoperatively, under the laser. Using this automatic method, surgeons can expect tighter alignment of laser ablation to the eye rotation and potentially better clinical results.

Using Wavefront Data

WavePrint data is valuable because it objectively measures the patient’s refractive information and does not depend on a patient’s subjective interpretation10. Much of the information that refractive surgeons have depended on has been subjective, based on patient responses to a manifest refraction or cycloplegic refraction.

Using the results from manifest refractions, higher-order aberrations have been "hidden" in the spherical and cylindrical information obtained from patient exams. But now, wavefront aberrometers enable higher-order aberrations to be addressed directly.

The Optical Aberration Index measurement provided by the VISX WavePrint shows the percentage of the total WavePrint errors that are higher-order aberrations, not just spherical and cylindrical refractive errors. This is valuable for determining which patients are good candidates for traditional LASIK and which would benefit from custom ablation.

WavePrints are also useful for counseling those patients who have low spherocylindrical errors, but significant higher-order aberrations. The Optical Aberration Index provides that missing link of information not available from manifest refraction—the percentage of the visual error related to higher-order aberrations. This is extremely useful for determining a course of treatment that is customized to each patient’s needs.

The higher-order aberrations in most patients represent a fairly small portion of their entire refractive error. For example, a patient who has a refractive error of -8 +3 axis 90 might have 99% of the aberrations or problems related to sphere and cylinder and just 1% related to higher-order aberrations. But if a patient who has a refractive error of -1 +0.50 axis 90 and still desires laser vision correction has significant higher-order aberrations that are responsible for 25% or more of his or her visual problems, then that patient probably would benefit from having those higher-order aberrations corrected with wavefront-

guided custom ablation. A conventional treatment would not address the higher-order aberrations and may, if the patient is symptomatic from these aberrations, result in a less-than-happy patient.

Postoperative evaluation with the WavePrint can be helpful in planning enhancements for patients who aren’t happy with the results of their first treatment. These patients are screened in a manner similar to those undergoing primary treatments. Patients who had an increase in higher-order aberrations following initial treatment may specifically benefit from a wavefront-directed treatment.

The VISX laser has already been used to correct irregular astigmatism through the custom-con- toured ablation pattern (Custom-CAP) method. This is being used in the United States under a humanitarian use device exemption from the U.S. Food and Drug Administration to perform topographically driven Custom-CAP treatment of eyes with decentered ablations as the result of previous excimer laser surgery. With Custom-CAP treatment, the laser is directed to create irregular patterns that match the irregular cornea based on size, depth, and location, making it possible to reshape the cornea for optimum correction. This is the precursor technology to wave- front-directed treatments, where the treatment is based on wavefront rather than topographically derived information, allowing us to understand better the effect of reducing the irregularities on the sphere and cylinder component.

Capturing Wavefront Images

The information captured by the WaveScan is used to produce a unique WavePrint that can be used for a patient’s custom ablation. The HartmannShack aberrometer used in the VISX WaveScan device does not rely on subjective responses from patients. The sensor uses an array of tiny lenslets— small lenses—arranged in a hexagonal array to measure the wavefront emerging across the pupil. The wavefront measurement can then be displayed as a spot pattern.10

Mathematical algorithms are used to analyze how close together or far apart the spots are in any given wavefront pattern. The algorithms make hundreds of thousands of calculations using Zernike polynomials and can be used to create highly complex shapes that represent non-spherocylindrical refractive errors. These complex shapes are the high- er-order aberrations, such as spherical aberration, coma, and trefoil.

Calculation and Treatment Primer

Custom WavePrint treatments using the VISX WaveScan device can be based on from 1 to 3 patient exams. Before the custom WavePrint treatment is calculated, the WaveScan system software will compare the patient’s manifest refraction data entered on the preoperative screen with the WaveScan refraction obtained from the selected exam. If the 2 refractions are significantly different, a message will appear on the computer screen stating that the 2 refractions do not match.2

VISX recommends using the automated exam selection mode when selecting patient exams for custom WavePrint treatments. Many eyes can be characterized with 3 WaveScan measurements; however, additional measurements may be needed, usually no more than 6. The WaveScan device will select the exam with the median spherical equivalent as the basis for the WavePrint treatment.

The automated exam selection mode will warn the surgeon when 1 or more of the selected exams exceeds the following criteria: One or more of the selected exams with a pupil size less than or equal to 5 mm (exams with 6-mm pupils are preferred for treatments); or 2 or more of the selected exams have spherical equivalent values differing by more than 1 D; or the auto-selected exam has a higher-order root mean square (RMS) error that differs from the average high-order RMS error by greater than 0.07 µm. When one of these warnings appear, the surgeon can acknowledge them and continue with the treatment calculation or select another set of exams.

Once the patient exam has been chosen, the physician has to click on a TREAT OD or TREAT OS button. Sub-tabs will appear at the bottom of the screen for entering surgical treatment parameters for the custom WavePrint treatment.

Treatment parameters from the selected WaveScan measurement are shown on the OD or OS DESIGN screen. The OD or OS DESIGN screen shows several interactive displays including a graphic of the planned ablation map, an image of the patient’s pupil with scotopic pupil size, ablation optical zone, and the ablation size displayed in different colors. (Figure 3) Surgical treatment parameters, such as Treatment Type (LASIK or surface PRK) and physician adjustments to the WavePrint treatment can be made before treatment. Treatment dimensions can be changed for Minimum Optical Zone and Ablation Zone. The physician also can adjust surgi-

cal parameters for LASIK Flap Diameter (millimeters) and LASIK Flap Thickness (microns). If any of the treatment parameters are invalid, a red arrow will appear next to the parameter that needs to be changed.

The WaveScan system automatically calculates treatment totals, maximum optical zone, and transition zone. It also calculates results of the treatment, including maximum ablation depth of the planned treatment and estimated residual stromal bed depth. Preoperative data are also displayed for reference, including preoperative corneal thickness (microns) and scotopic pupil size (millimeters). A warning message will be displayed if the difference between the manifest refraction and the selected WaveScan refraction is outside the margin set by the WaveScan software.

After the WavePrint treatment has been successfully calculated, the following treatment statistics will be displayed on the OD or OS CALC subtab: (Figure 4).

direction and adding it to the absolute value of the maximum deviation in the negative direction. All

successfully calculated treatments have a P-V difference of ≤2.5 µm.

• Distribution of VSS pulse diameters of the calculated treatment.

The WaveScan system software generates 2 sets of laser instructions, one for creating a plastic PreVue lens and the other for the patient procedure. Both sets of instructions are loaded onto the STAR S4 ActiveTrak System and are used to define the custom WavePrint treatment.

Custom ablation treatments are accomplished in the same manner as in a non-wavefront driven procedure from the standpoint of surgical technique for flap creation and pre and postoperative care. In these treatments, flap management and epithelial healing are still critical to a good outcome.

Figure 4: The CALC sub-tab shows a variety of information about the custom ablation including the distribution of the Variable Spot Scanning pulses, the solution quality, the treatment time, and the maximal ablation depth.

•Number of pulses for the Custom WavePrint treatment and the PreVue lens ablation.

•Treatment time.

•Time to ablate the PreVue lens.

•Maximum ablation depth for the Custom WavePrint treatment and the PreVue lens ablation.

•The RMS difference, which is a standard mathematical way of measuring the "goodness of fit" of the calculated treatment. It can be interpreted as a kind of average deviation across the optical surface. All suc-

cessfully calculated treatments have an RMS difference of ≤0.4 µm.

• P-V (peak to valley) difference, which is calculated by finding the maximum deviation in the positive

Results

Results from the first 6 months of a multicenter Food and Drug Administration clinical trial for wavefront-guided LASIK using the VISX WaveScan Wavefront System demonstrate that the procedure is highly accurate, predictable, and safe for the treatment of patients with low to moderate myopia with or without astigmatism.3

All patients in the 6-month cohort received bilateral treatments using 1 of 3 microkeratomes— the Moria, Amadeus, or Hansatome. Otherwise, the surgical technique was standardized—with no nomogram adjustment for age, amount of correction, or individual surgeon. All eyes were targeted for emmetropia. The custom ablation zone was out to 8 mm, and there was very tight control in terms of environmental conditions, such as humidity and temperature.

One of the unique aspects of the VISX wavefront clinical trial is the use of the PreVue lens system for preoperative screening. It can identify those patients who may benefit most from a wavefront correction by measuring their potential improvement in best-corrected visual acuity. The PreVue lens provides a guide for verifying the treatment parameters for the patient. When there is a difference between manifest and wavefront refractions due to corneal or lenticular abnormalities, the PreVue lens serves as an excellent means of verifying that the wavefront measurement is optimal for the patient. It also gives the patient and physician a method to validate the type of result they may achieve from the wavefrontguided ablation.

The study examines the effectiveness of the VISX WaveScan device in terms of uncorrected visual acuity, refractive stability, predictability, or intended versus achieved correction, some higher-order analysis, and responses to a subjective questionnaire on night vision, glare, and contrast sensitivity.

The 6-month data are based on 320 eyes of 173 patients with a mean age of 38. The mean preoperative sphere was 3.5 ± 1.3 D, with a range of 0.8 to 6.5 D. The mean cylinder was +0.6 ± 0.7 D, with a range of 0 to +2.8 D, and the manifest refractive spherical equivalent was –3.2 ± 1.3 D, with a range of –0.6 to –6.0 D. All patients in the study were targeted for emmetropia.

Looking at the uncorrected visual acuity at 6 months, 96% of patients in the trial achieved UCVA of 20/20 or better. Moreover, 74% were 20/16 or better and 19% were 20/12.5 or better. Seventy percent of the treated eyes had the same or better postoperative UCVA compared with their preoperative BSCVA. This is a particularly significant result because, in order for them to qualify for this study, patients had to see at least 3 letters on the 20/16 line with the PreVue lens.

Results have been very stable, with 98% of the eyes having less than a 0.50 D change. In fact, at 3 to 6 months, 99% of eyes had less than or equal to

1 D of change. And 100% of the patients were within a 1 D of intended versus achieved correction at 6 months and 95% of the patients were within 0.5 D of emmetropia.

Comparing patients in the trial who received wavefront-guided treatments with a similar cohort of patients who had conventional treatment, the wavefront group had less of an increase in the higher-order changes. At one month, the wavefront group had 0.35 of higher order aberrations compared with 0.47 in the conventional group. The wavefront group also showed 0.16 of spherical aberration compared to 0.21 with the conventional group. (Figure 5)

Figure 5: A comparison of higher order aberrations with the VISX system show slightly higher total higher order and spherical aberration with conventional treatment than standard treatments.

Moreover, 70% of eyes had a decrease or an increase of less than 0.1µm in RMS postoperatively compared to preoperatively. In fact, the majority of custom ablation patients showed an improvement in higher-order terms at 6 months—78% showed some improvement in coma, 84% showed some improvement in trefoil, and 70% showed some improvement in spherical aberration compared to preoperatively.

In response to a subjective questionnaire, patients in the study were more satisfied with their vision postoperatively in terms of night vision, glare, and frequency of halos. For example, when evaluating their vision at night with glare, 60% were very satisfied or satisfied pre-operatively compared to 76% post-operatively.

In terms of contrast sensitivity, there was no significant difference between the preoperative result and the postoperative results at 6 months.

As far as BSCVA, 8% of the custom ablation eyes gained 1 to 2 lines, 58% gained up to a line, 28% had no change, and 6% of the eyes lost up to a line, with no eyes that lost more than 3 letters of BSCVA.

Night vision satisfaction has been higher from wavefront-driven treatments using the VISX WavePrint system. Patients show significant improvement in night vision from their preoperative baseline, improving from 70% satisfied preoperatively to 85% satisfied at 6 months.3

In summary, 96% of the eyes treated with the VISX WaveScan device achieved UCVA of 20/20 or better, 74% were 20/16 or better, 70% of the eyes had the same or better postoperative UCVA compared with their preoperative BSCVA, 95% of the eyes were within 0.5 D of target, 70% of the eyes had a decrease or less than a 0.10 µm change in RMS, and no eyes lost a line of preoperative BSCVA.

Results of several studies being conducted at Moorfields Eye Hospital in London underscore the potential of wavefront-guided correction of higherorder aberrations. In 1 small study, wavefront-guided treatments were done on a group of 5 patients who had lost lines of best-corrected visual acuity as the result of previous refractive surgeries. All 5 patients in the study experienced a reduction of their higherorder aberrations and achieved at least 20/30 UCVA11.

In another study at Moorfields, wavefrontguided therapeutic LASIK were performed on 34 patients who had poor night vision outcomes following primary LASIK surgery12. A third of the patients in the study had higher-order aberrations following their initial LASIK surgery. Overall, the mean high-

er-order aberration before wavefront-guided was 0.68 microns, indicating a significant level of higher order aberrations. Following the wavefront guided treatment, the mean higher order aberration RMS was reduced to 0.58 microns at 1 month, and at 3 months, it was 0.57 microns. While these eyes weren’t brought to a perfect wavefront, they showed a significant reduction. (Figure 6)

Figure 6: Results of higher order aberration changes in 34 eyes with abnormalities following laser vision correction showing improvement in higher order aberrations at 1 and 3 months.

A study of 12 eyes at the University of Ottawa Eye Institute in Ottawa shows promising results for using wavefront-guided LASIK to treat hyperopic patients. The prospective, nonrandomized study included 12 eyes of 6 patients with up to +3.0 D of hyperopia with astigmatism. Following WaveScan analysis, the planned correction was confirmed using the PreVue lens13.

Two-thirds of the eyes were 20/20 uncorrected at 1 week postoperatively. At 1 week postop, 50% of the eyes in study were 20/16 or better. All the eyes had achieved UCVA of 20/20 by 1 month postop. At 1 month, 33% were 20/16 or better, and 92% were within 0.5 D of the intended treatment.

Custom multifocal ablations also are being used to treat presbyopia. Results from another laser vision correction study at the University of Ottawa Eye Institute in Ottawa, Canada, demonstrate that many patients can adapt to multifocal correction14. The VISX VSS ablation technology has proven effective in making the subtle ablation changes necessary for successful multifocal ablation. Using VSS and the WavePrint system, the study found that the central area of the cornea could be steepened for near vision while the periphery is targeted for emmetropia.

The study population included 8 myopic eyes with mean refractive spherical equivalent (MRSE) of -3.30 ± 0.13 D and a mean add requirement of +2.31 ± 0.22 D. The hyperopic cohort included 12 eyes with MRSE of +2.26 ± 0.86 D and a mean add requirement of +2.29 ± 0.18 D. Three months following LASIK treatment, the mean MRSE for the myopic group improved to -0.16

±0.44 D and the hyperopic group improved to -0.67

±0.47 D

The study also found that multifocal correction for presbyopia did not compromise the accuracy of distance correction. At 3 months, 100% of the eyes had 20/40 or better UCVA and 80% had 20/40 or better for near vision. The mean add requirement dropped by 1.0 D at 1 month postoperatively, with some regression to 1.25 D after 3 months.

Best-corrected near visual acuity for 100% of the eyes in the study was 20/25 or better after 1 month, and 20/20 or better after 3 months. Also, 95% of eyes in the study lost less than 1 line of distance BSCVA with one eye losing more than 1 line of BSCVA. It appears that the VSS VISX Star S4 has the capability to improve near vision while also improving near vision in presbyopic patients.

Conclusions

Understanding the relationship between various Zernike polynomials and the symptoms they

cause is in its early stage. As clinicians get a better handle on which aberrations are helpful and which are not, the ability to map these optical anomalies will better translate into improved treatment strategies.

But, already, customized corneal corrections are a proven method for the measurement and treatment of lowerand higher-order aberrations. As the ability to identify the relationship between various higher-order aberrations improves, surgeons will be able to take full advantage of both the diagnostic and treatment capabilities of the WavePrint System.

The WavePrint system will enable fine-tun- ing of diagnostic capabilities that take advantage of the laser’s ability to make corrections of 1/10 or even 1/100 of a diopter. Wavefront technology provides a much more refined way to determine what the best level of correction is for each individual patient, and for those patients who are very discerning about their vision, that’s going to be extremely important. The ability to identify and treat higher-order aberrations is going to help improve results and transform the laser vision correction industry.

Classification of normal corneal topography based on computer-assisted videokeratography. Arch Ophthalmol. 1990;108(7):945-949.

ablation studies for the correction of myopia using the VISX WaveScan system. J Refract Surg 2003; in press.

. Liang J, Williams D: Effect of higher order aberrations on image quality in the human eye. Vision science and its applications. OSA Technical Digest Series Washington DC, Optical Society of America 1995;1:70-

ment tolerances during excimer laser surgery for myopia. Ophthalmic Technologies Proc. SPIE. 1991;1423:140-53

. Lee KE, Klein BE, Klein R: Changes in refractive error over a 5-year interval in the Beaver Dam Eye Study. Invest Ophth Vis Sci. 1999;40:1645-1649.

. Handa T, Mukuno K, Niida T, et. al.: Diurnal variation of human corneal curvature in young adults. J Refract Surg. 2002;18:58-62.

. Tamayo Fernandez GE, Serrano MG: Early clinical experience using custom excimer laser ablations to treat irregular astigmatism. J Cataract Refract Surg. 2000;26:1442-1450.

opia. EyeWorld. Supplement. November 2002

David R. Hardten, MD

Director of Refractive Surgery

Minnesota Eye Consultants

Minneapolis, Minnesota

Associate Clinical Professor of Ophthalmology

University of Minnesota

Minneapolis, Minnesota

Director of Refractive Surgery and Residency

Programs

Regions Medical Center

St. Paul, Minnesota

Address correspondence to: 710 East 24th Street

Suite 106

Minneapolis, MN 55404 Phone: 612-813-3632 Fax: 612-813-3658

E-mail: drhardten@mneye.com

Richard L. Lindstrom, MD Medical Director,

Phillips Eye Center for Teaching and Research; Clinical Professor,

University of Minnesota,

Minnesota, Minneapolis

Introduction

Within the past years, there have been a number of different wavefront sensing devices placed on the market. With that, a number of different models of wavefront customized corneal ablation have been proposed. Each model, however unique, does have essential technology requirements that are necessary to properly correct refractive aberrations. Without the correction of these aberrations, true customized corneal ablation is not possible.

Aberrations have recently been reported to increase up to 17 times after laser vision correction1. Since all laser vision correction surgery to this point has been found to create aberrations, reduction of high order aberrations with customized corneal ablation sets a new precedent. With the recent approval of Alcon’s LADARVision platform of CustomCornea® for the correction of myopia by the US FDA, customized corneal ablation has made its debut in the mainstream of refractive surgery, and together with other similar platform approvals, will likely revolutionize our field in the months and years to come.

In order for the successfulness of customized laser vision correction to reach its full potential, a number of technology requirements must be

addressed and implemented, by the specific laser vision correction platform offering this customized procedure. The aberrations that are induced by conventional laser vision correction have the potential for being minimized and eliminated with customized corneal ablation.(1,2) In addition, pre-existing aberrations may be minimized or eliminated by this same technology.(3)

The technology required to achieve aberra- tion-free laser vision correction may not be successfully implemented by every laser system’s attempt in this area. Therefore, it is important to carefully consider the following requirements.

Scanning Spot Delivery

Spot Size and Shape

Although many of today’s commercially available excimer laser systems have beam diameters which can decrease to as small as 1mm, the shape of this small, 1mm beam varies according to its mode of formation being either a gaussian or top-hat pattern. A top-hat beam created by a concentric iris aperture produces sharp ablation edges which overlap in the laser vision correction profile. A gaussian beam

allows for very uniform overlap in the creation of ablation profile. (Figure 1). A truly customized profile can best be created by a gaussian beam with ideal spot overlap4.

Figure 1: Gaussian beam delivery with ideal spot overlap to produce a very uniform, smooth ablation. (Courtesy of Ronald R. Krueger, MD).

The size of the beam also plays a factor when considering a top-hat or gaussian profile. In a study of small spot scanning, a 2mm top-hat beam profile results in performance degradation of both low and high spatial frequency during custom ablation7. This is in contrast to a 1mm gaussian beam which shows good performance when treating both high and low spatial frequency aberrations4.

Finally, when implementing a gaussian pattern, the size of the spot must correspond to the resolution of aberrations being treated. An optical ablation zone diameter of 6mm would require a spot size of ≤1mm to correct fourth order aberrations. Therefore, scanning spot lasers larger than 1mm would not adequately treat the most common of higher order aberrations, namely spherical aberration and coma.

This leads us to the question of which lasers provide a small, ≤1mm spot. Table 1 lists seven excimer laser systems which attempt to provide a small scanning spot. The first four are used to correct refractive errors with a ≤ 1mm spot during the entire treatment. Of the latter three, the Meditech