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

this level of accuracy and predictability, with 94% of eyes seeing 20/20 or better and 97% seeing 20/25 or better (100% seeing 20/32 or better). Orbscan II using software version 3.00 E, was used to derive front surface asphericity (Q factor) from the best fit central 4 mm zone before and after surgery. The mean Q-fac- tor changed from a preoperative mean value of +0.030 to a postoperative mean value of +0.015 with a RMS change of +0.075 from before to after surgery. This is in contrast to results of a similar myopic cohort of 53 eyes treated with a conventional myopic ablation profile where the Q value changed from -0.009 to +0.089 with an RMS change of +0.531 from before to after surgery. Prolate optimization function it appears served to decrease the increase in Q, or decrease in prolateness of the cornea. This in turn would be expected to reduce the induction of higher order (mainly spherical) aberrations.

These MEL80 outcomes are significantly better than those achieved by LASIK performed with the MEL70, and it is believed that much of the improvement comes from a better understanding of corneal tissue responses to flap creation and photoablation. Such an optimized nonwavefront-guided system, we believe, forms a substantial platform for the overlay of the high quality wavefront data provided by the WASCA aberrometer. Studies to integrate all components are underway and results will be reported.

These MEL80 outcomes are significantly better than those achieved by LASIK performed with the MEL70, and it is believed that much of the improvement comes from a better understanding of corneal tissue responses to flap creation and photoablation.

CONCLUSION

The Carl Zeiss Meditec platform for custom ablation incorporates a suite of technology for WASCA, TOSCA, sophisticated excimer laser delivery (MEL80), and surgeon-controlled individualization of treatment protocol (CRS-Master). Together, these components promise to deliver increasingly higher accuracy and control over corneal sculpting—a dream come true for the father of keratomileusis, the late Jose Ignacio Barraquer.

REFERENCES

1.Oshika T, Klyce SD, Applegate RA, Howland HC, El Danasoury MA. Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol. 1999;127(1):1-7.

2.Neal DR, Armstrong DJ, Turner WT. Wavefront sensors for control and process monitoring in optics manufacture. SPIE. 1997.

3.Neal DR, Alford WJ, Gruetzner JK. Amplitude and phase beam characterization using a two-dimensional wavefront sensor. SPIE. 1996;72-82.

4.Pallikaris IG, Panagopoulou SI, Vogelsang H. Dynamic aberrometry and accommodation. Paper presented at: IX Congress of the European Society of Cataract and Refractive Surgery; September 2001; Amsterdam, Netherlands.

5.Salmon TO, West RW, Gasser W, Kenmore T. Accuracy, repeatability and instrument myopia with the COAS Shack-Hartman aberrometer. 3rd International Congress of Wavefront Sensing and Aberration-Free Refractive Correction; February 15, 2002; Interlaken, Switzerland.

6.Salmon TO, West RW, Gasser W, Kenmore T. Measurement of refractive errors in young myopes using the COAS Shack-Hartmann aberrometer. Optom Vis Sci. 2003.

7.Barraquer JI. Keratomileusis. Int Surg. 1967;48(2):103-117.

8.Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg. 1988; 14(1):46-52.

9.Barraquer JI. Queratomileusis y queratofakia. Bogota: Instituto Barraquer de America; 1980.

10.Reinstein DZ, Silverman RH, Rondeau MJ, Coleman DJ. Epithelial and corneal thickness measurements by high-frequency ultrasound digital signal processing. Ophthalmology. 1994;101(1):140-146.

11.Wygledowska-Promienska D, Zawojska I, Gierek-Ciaciura S, Sarzynski A. Correction of irregular astigmatism using excimer laser MEL 70 G-Scan with the TOSCA program—introductory report. Klin Oczna. 2000; 102(6):443-447.

12.Carl Zeiss Meditec AG. Data on file.

13.Mrochen M, Seiler T. Influence of corneal curvature on calculation of ablation patterns used in photorefractive laser surgery. J Refract Surg. 2001;17(5):S584-S587.

14.Oliver KM, O'Brart DP, Stephenson CG, et al. Anterior corneal optical aberrations induced by photorefractive keratectomy for hyperopia. J Refract Surg. 2001;17(4):406-413.

15.Oliver KM, Hemenger RP, Corbett MC, et al. Corneal optical aberrations induced by photorefractive keratectomy. J Refract Surg. 1997;13(3):246-254.

16.Reinstein DZ, Silverman RH, Raevsky T, et al. A new arc-scanning very high-frequency ultrasound system for 3D pachymetric mapping of corneal epithelium, lamellar flap and residual stromal layer in laser in situ keratomileusis. J Refract Surg. 2000;16:414-430.

17.Reinstein DZ, Silverman RH, Sutton HF, Coleman DJ. Very high-fre- quency ultrasound corneal analysis identifies anatomic correlates of optical complications of lamellar refractive surgery: anatomic diagnosis in lamellar surgery. Ophthalmology. 1999;106(3):474-482.

18.Reinstein DZ, Silverman RH, Coleman DJ. High-frequency ultrasound measurement of the thickness of the corneal epithelium.

Refract Corneal Surg. 1993;9(5):385-387.

19.Reinstein DZ, Srivannaboon S, Silverman RH, Coleman DJ. The accuracy of routine LASIK: isolation of biomechanical and epithelial factors. Invest Ophthalmol Vis Sci. 2000;2000:S318.

20.Reinstein DZ, Srivannaboon S, Silverman RH, Coleman DJ. Limits of wavefront customized ablation: biomechanical and epithelial factors. Invest Ophthalmol Vis Sci. 2002;43:E-Abstract 3942.

21.Roberts C. The cornea is not a piece of plastic. J Refract Surg. 2000; 16(4):407-413.

22.Roberts C. Biomechanics of the cornea and wavefront-guided laser refractive surgery. J Refract Surg. 2002;18(5):S589-S592.

23.Hersh PS, Shah SI, Holladay JT. Corneal asphericity following excimer laser photorefractive keratectomy. Summit PRK Topography study group. Ophthalmic Surg Lasers. 1996;27(5 Suppl): S421-S428.

24.Holladay JT, Janes JA. Topographic changes in corneal asphericity

and effective optical zone after laser in-situ keratomileusis.

J Cataract Refract Surg. 2002;28(6):942-947.

25.Liang J, Williams DR. Aberrations and retinal image quality of the normal human eye. J Opt Soc Am A. 1997;14(11):2873-2883.

26.Barraquer JI. Cirugia Refractiva de la Cornea. Bogota: Instituto Barraquer de America; 1989.

27.Patel S, Marshall J, Fitzke FWd. Model for predicting the optical performance of the eye in refractive surgery. Refract Corneal Surg. 1993; 9(5):366-375.

28.Seiler T, Genth U, Holschbach A, Derse M. Aspheric photorefractive keratectomy with excimer laser. Refract Corneal Surg. 1993;9(3):166172.

29.Reinstein DZ, Goes F. Prospective, controlled comparison of Hansatome vs. M2 aberrations: can we control for flap induced aberrations in primary wavefront guided LASIK? Paper presented at: XX Congress of the European Society of Cataract and Refractive Surgery; 2002; Nice, France.

30.El-Agha MS, Johnston EW, Bowman RW, Cavanagh HD, McCulley JP. Excimer laser treatment of spherical hyperopia: PRK or LASIK?

Trans Am Ophthalmol Soc. 2000;98:59-66.

31.Pallikaris IG, Kymionis GD, Panagopoulou SI, et al. Induced optical aberrations following formation of a laser in situ keratomileusis flap. J Cataract Refract Surg. 2002;28(10):1737-1741.

32.Reinstein DZ, Srivannaboon S, Sutton HFS, Silverman RH, Shaikh A, Coleman DJ. Risk of ectasia in LASIK: revised safety criteria.

Invest Ophthalmol Vis Sci. 1999;40(Suppl):S403.

33.Reinstein DZ, Dausch D, Schroder E. Custom Ablation with the Asclepion Meditec WASCA and MEL70 G-scan Excimer Laser. Paper presented at: 3rd International Congress of Wavefront Sensing and Aberration-Free Refractive Correction; February 15, 2002; Interlaken, Switzerland.

34.Nagy Z, Palágyi-Deak I, Kovács A, Kelemen E. First results with

wavefront-guided photorefractive keratectomy for hyperopia. J Refract Surg. 2002;18(5):S620-S623.

35.Reinstein DZ, Srivannaboon S. Aberrometry and Custom Ablation Principles: Surgeon controlled custom ablation; The Carl ZeissMeditec CRS-Master. Paper presented at: American Academy of Ophthalmology Annual Meeting: Sub Speciality Day in Refractive Surgery; October 19, 2003; Orlando, Fla.

INTRODUCTION

The quality of vision a patient experiences is the ultimate gauge for the success of a refractive surgical treatment. It is believed that bright, crisp, saturated, high-contrast images actually enhance the satisfaction derived from proper refractive adjustment.1 This area of refractive surgery continues to undergo constant development and refinement. Diagnostic procedures for the evaluation of visual quality and of the eye itself change constantly, as does surgical methodology. It is, therefore, paramount that tools such as the excimer laser be as flexible as possible in order to evolve with those changes. This was an important prerequisite for the development of the Allegretto Wave Analyzer (WaveLight Laser Technologies AG, Erlanger, Germany), with software-controlled flying-spot technology that drives a modular design that adapts easily to changes in the ablation pattern. A rapid repetition rate, high-frequency eyetracker, and a small spot size with a Gaussian beam profile are the preconditions for enabling ablation profiles to correct more than just standard spectacle prescriptions.

Wavefront-optimized and wavefront-guided treatments became possible within a very short time frame. From the day the company was founded, WaveLight Laser Technologies AG relied heavily upon the input of Theo Seiler, MD, PhD, thus incorporating clinical, as well as engineering, considerations throughout the entire product development process. This task was not an easy one. The development phase was characterized not only by adherence to strict external quality controls, but also by a commitment to flexibility and to continual reassessment in the pursuit of technology that would translate well into clinical practice. Despite being conceptually sound and “passing the muster” in the laboratory, many ideas were discarded in the early stages of development if it seemed they might not meet the high standards set for practical application. This approach helped to avert some of the “teething problems” still seen with some of the new lasers in the market today.

TECHNOLOGY:

MEASUREMENT, MAPPING, AND TREATMENT DESIGN

The Allegretto Wave Analyzer technology is based on the Tscherning principle of wavefront measurements. One hundred sixty-eight rays of visible light are emitted by a 660 nanometer

(nm) laser diode, projecting onto the retina a grid pattern with a diameter of only 1 millimeter (mm).2 A highly sensitive chargedcoupled device (CCD) camera captures the retinal image of the grid pattern and that image is then compared to the ideal, nonaberrated image. The extent to which each spot deviates from its ideal position is calculated and that calculation is used to determine the wavefront shape (Figure 30-1).

The Allegretto Wave Analyzer ensures proper alignment of the measurement to the pupil center with an integrated eyetracking system that tracks the pupil in x,y and the eye in a z direction during the measurement. It allows measurements in a 100 micron (µm) x,y range from the center of the pupil and in a 200 µm z range from the eye itself. It is only when the eye has been centered within this range that the data can be used for treatment.

During the measurement process, the patient’s pupil should be dilated to at least 7 mm in order to obtain an optimally accurate map of all aberrations, including those in the corneal periphery. Once the eye is centered, a measurement can be taken manually or with the automatic mode. Farand near-point accommodation can be considered by moving the accommodation target before taking the measurement. For custom laser in-situ keratomileusis (LASIK) treatments, the far-point has to be measured. To study the change of aberrations during accommodation, the patient can be measured within any part of the accommodation process. During the measurement, patients are able to see their own aberrations according to the deformation of the ideal grid pattern, which is displayed in the visible range (660 nm) (Figure 30-2).

Once the measurement has been taken, the retinal image is displayed and analyzed, and the wavefront map is determined. The Allegretto Wave Analyzer calculates Zernike coefficients up to the sixth order. The wavefront map can be displayed in several different ways: wavefront map, higher-orders map, tangentialpower map, and sagittal power map. A display is possible in twoand three-dimensional maps. Also available is a chart comparison of the individual Zernike coefficients, displaying the root mean square (RMS) values for the individual orders, the total wavefront (RMSG), and the higher orders (RMSH).

As many as four different maps for an individual patient can be compared simultaneously in an overview window. This enables comparison of the wavefront, as well as the Zernike coefficients, for each map in order to determine reproducibility of the measurements. After review and selection of up to four meas-

Figure 30-1. Display of colored wavefront map—higher orders only.

urements per patient, the data are exported onto a floppy disk that is inserted into the Allegretto Wave Excimer Laser System for a custom ablation treatment. The surgeon can devise a treatment from a single map or from an average data combination of up to four maps.

THE WAVEFRONT-ADJUSTED ABLATION PROFILE

Since Watkin’s introduction of the first corneal models, it has been known that the native corneal surface flattens toward the periphery.3 When central and peripheral corneal radii are compared, measurements (in mm) toward the limbus increase and diopters of refractive power decrease. It can, therefore, be anticipated that the natural corneal contour would offset the spherical aberrations that are created with an open pupil.4

A two-dimensional surface plane of a meridial cut made through the corneal axis would resemble a prolate ellipsoid in shape. If the cornea were to become steeper toward the periphery, an oblate ellipsoid would be created. The issue becomes more complicated when the cornea is viewed three-dimensional- ly, especially since the extent to which the cornea flattens toward the individual semimeridians (nasal, temporal, superior, and inferior) can vary and, in many cases, some degree of astigmatism is present. The normal corneal contour is, therefore, a nonrotation symmetrical, toric, prolate ellipsoid, which has been described by Wilms simply as natural torus (toroid).5,6

Implications for Laser In-Situ Keratomileusis Surgery

Even the most modern and sophisticated laser systems are only able to emit light vertically toward the corneal center. The cornea, however, changes in both shape and radius toward the periphery. Light rays that strike an optical surface at a nonperpendicular angle are partially reflected and have less impact than rays that strike at a perpendicular angle. The degree of reflection increases with the degree of inclination of the surface, and energy density decreases as the light rays are spread over a larger surface area.7 Naturally, this increasing reflection effect toward the corneal periphery has an impact on the ablation profile of the laser; if this is not taken into account in the design of the ablation

Figure 30-2. Retinal image display of a Tscherning grid pattern.

profile, a larger amount of tissue will be removed in the center than in the periphery. Under these conditions, a spherical ablation profile will flatten the central cornea to a greater extent than the periphery, creating an oblate ellipsoid with increasing refractive power toward the corneal edges.

Light rays that enter the cornea through an open pupil toward the periphery will be refracted more strongly and will create a spherical aberration that impacts the wavefront. In the past, this was a common occurrence, causing the well-known mesopic and scotopic vision problems frequently seen with classic LASIK treatments. These effects are taken into account in the Allegretto Wave system’s standard wavefront-optimized ablation profile; compensation for the energy loss in the periphery of the cornea is accomplished with a specific shot pattern based on a patentprotected algorithm.5 As a result, the surgeon is able to achieve the desired ablation depth without creating problematic spherical aberrations.

Named for Zernike, spherical aberration is also called Zernike 12 (Z12 or Z04) of the fourth order and is likely the best-known of the higher-order aberration errors in geometric optics. All high- er-order optical errors change the wavefront of passing light in an optical system. When these errors are corrected, the wavefront is automatically corrected as well; therefore, the ablation profile used for the correction is known as a wavefront-optimized ablation profile.8

Going one step further, WaveLight has introduced the Q- value assisted treatment mode, in which it is possible to preselect the postoperative corneal asphericity over the ablation area by choosing an appropriate Q-value.

Patient Selection

At present, complete wavefront-guided ablations with the Allegretto Wave are limited to a spherical equivalent of no greater than -7 diopters (D). At higher diopter levels, the passing point of the light rays through the natural lens deviates too greatly from the original entrance point into the optical system of the eye, thus creating additional higher-order aberrations. This is due to the change in refraction on the cornea, the first refractive plane in the eye.

Figure 30-3. Comparison screen of multiple measurements.

As discussed previously, a successful measurement is based primarily on a centered, sharp, and wide-spread image of the dot pattern on the retina. The quality depends significantly on the transparency of the media. The dot pattern must travel undisturbed through the optical pathway from the tear film to the retina. If light is dispersed or absorbed by irregularities or opacities, the quality of the image cannot be assured. As the first optical plane of the eye, the tear film also has a strong impact on the quality of the measurement. Lack of tear film or poor quality can be improved by applying artificial tears; this in turn improves the overall result of the measurement. For standardization purposes, we routinely apply hyaluronic acid repeatedly before a measurement is undertaken.

The transparency of the media can deteriorate with age, but a specific age at which aberration measurement is no longer possible cannot be determined. Therefore, in our clinic, we measure every patient, regardless of age, as long as the patient’s refraction qualifies for a wavefront-guided treatment.

Patients with limited or poor contrast sensitivity benefit most from a wavefront-guided treatment. Contrast sensitivity and glare should be evaluated along with all other preoperative measurements. If these evaluations should be repeated during the postoperative examinations, they can be used for quality control.

Most importantly, aberration measurements must be reproducible; otherwise, it will be uncertain if the measurement actually reflects true aberrations. Under no circumstances should a treatment be based on a single measurement. In fact, it is the first author’s recommendation that at least four similar, validated measurements be obtained per eye. At times, the surgeon may find it advantageous to take even more qualifying measurements to enable selection of the best and most similar results for the actual treatment.

Measurement: The Diagnostic Process

As stated above, the surgeon should strive to obtain four similar, high-quality measurements of each eye, which can then be validated and averaged. While several criteria are important in this regard, some of these are not quantitatively measurable.

The essential steps involved in a measurement evaluation are as follows:

1.Evaluation of quality and point distribution of the retinal image

2.Evaluation of the colored wavefront map

3.Evaluation of the wavefront-calculated refraction and comparison of this objective refraction to the subjective refraction (at a 7 mm pupil and a 4 mm pupil)

4.Evaluation of the RMSH value

5.Evaluation of Zernike polynomials in terms of size, distribution, and structure up to Z27 (the Zernike value structure should be within the same tolerance at all four measurements)

6.Comparison of at least four measurements, per the steps outlined above (Figure 30-3)

When evaluating individual measurements, the measured refraction and the distribution and value of the individual Zernike values are most important. Currently, there are no standardizations of fixed values that can be used for orientation. For small cylinders rather than larger cylinders, higher tolerances in the axis can be accepted. With increasing Zernike values, the importance and the size of the actual measured value decrease, as does the impact of the polynomial on the optical quality. It appears that the most important values to be considered for the diagnostic evaluation are coma (Z7 and Z8) and spherical aberrations (Z12). Our experience has shown that good, validated measurements allow even higher tolerances to the subjective refraction for sphere and cylinder. Although these deviations were originally defined as exclusion criteria and the values were adjusted toward the subjective refraction, we currently accept the measurements without further adjustments. In these cases, wavefront measurements have been shown superior to subjective measurements. The Allegretto Wave Analyzer provides the operator and diagnostic staff with extremely detailed insight into the process of image acquisition and processing. Measurements can easily be validated as optical disturbances in the optical pathway and will not result in false measurement values.

Initiating Treatment: The Therapeutic Process

Once the data have been exported to the computer that controls the laser, the operator has the opportunity to reconfirm all parameters. Again, it is possible to view the individual maps and Zernike polynomial charts for each measurement. If desired, the operator can make final adjustments to the treatment. Then when executed, the laser in-situ keratomileusis (LASIK) treatment itself is entirely wavefront-guided, meaning that the ablation algorithm does not correct higher-order aberrations in a separate pass but rather treats the entire wavefront error at once. The shot pattern is controlled in order to treat a complete optical zone from the beginning. Importantly, from a safety standpoint, an interruption in treatment results only in spherocylindrical undercorrection.

CLINICAL RESULTS

It had been hoped (industry-wide) that wavefront-guided LASIK would allow surgeons to provide super vision (ie, uncorrected visual acuity of 20/12) for most or all of their patients. The approach employed by WaveLight, however, was different from the very beginning. They set out to design a laser that would provide improved quality of vision in all lighting conditions or at all of the pupil sizes encountered during the day and night.

262 Chapter 30

Since improved quality of vision was already being achieved in wavefront-optimized LASIK with the Allegretto Wave, wave- front-guided LASIK would need to further improve mesopic and scotopic visual results, while also maintaining the excellent refractive and visual acuity results established with classic (ie, wavefront-optimized) LASIK.

The Allegretto Wave was engineered to optimize the optical beam path in order to produce a natural, prolate ablation pattern by increasing the number of pulses delivered to the corneal periphery. The resulting corneal contour has yielded excellent contrast sensitivity and visual performance in low-light conditions, even with the classic treatment. The challenge involved in attempting to surpass these outcomes with wavefront-guided treatment was both enormous and daunting.

It had been hoped (industry-wide) that wavefront-guided LASIK would allow surgeons to provide super vision (ie, uncorrected visual acuity of 20/12) for most or all of their patients. The approach employed by WaveLight, however, was different from the very beginning. They set out to design a laser that would provide improved quality of vision in all lighting conditions or at all of the pupil sizes encountered during the day and night.

Since improved quality of vision was already being achieved in wavefront-optimized LASIK with the Allegretto Wave, wavefront-guided LASIK would need to further improve mesopic and scotopic visual results, while also maintaining the excellent refractive and visual acuity results established with classic (ie, wavefront-optimized) LASIK.

Wavefront-Guided vs Wavefront-Optimized LASIK

Author 2 began using wavefront-guided LASIK in September 2001 and, as of November 2002, has performed more than 170 such procedures. Wavefront-optimized LASIK (referred to as classic LASIK with the Allegretto Wave) and wavefront-guided procedures performed during the same period and with the same preoperative criteria (ie, myopia <-6 D, astigmatism <-3.75 D), were compared with respect to outcomes at day 1, 6 weeks, 3 months, and 6 months post-treatment. This discussion will be limited to the visual and refractive results from 150 eyes treated with wavefront-guided LASIK and 240 eyes treated with classic LASIK. The same surgeon performed all of the procedures, and all treatments were carried out using the Hansatome microkeratome (Bausch & Lomb, Rochester, NY) (8.5 and 9.5 mm rings) and a 6.5 mm optical zone. The preoperative data from the two groups were very similar. In the wavefront-guided group, the average preoperative spherical refraction was -3.34 and the average cylindrical refraction was -0.75. In the classic LASIK group, the average preoperative spherical refraction was –3.37 and the average cylindrical refraction was -0.82. Day 1 refractive results showed an average spherical refraction of 0.23 in the wavefrontguided group and zero in the classic group. The average cylindrical refraction in the wavefront-guided group was -0.33 compared to -0.36 in the other group. Anecdotally, the glare factor was already noticeably smaller in the wavefront-guided group by day 1. Patients in the wavefront group consistently reported a reduced area of glare around a Snellen chart projected on a wall.

At the 6-week visit, the average spherical equivalent in the wavefront-guided group was +0.19 and it was -0.14 in the classic group. The cylinder in the wavefront-guided group was -0.26, almost identical to the classic group with -0.28.

Uncorrected visual acuity (UCVA) was 1.07 in the wavefrontguided group versus 0.97 in the classic group.

At 3 months, the results were on average still better in the wavefront-guided group. The spherical equivalent in the wave- front-guided group was +0.105 and -0.085 in the classic group. Cylinder was identical at -0.31 in both groups. The UCVA was 1.05 in the wavefront-guided group versus 0.99 in the classic group.

At the 6-month interval, the average spherical equivalent was still slightly hyperopic in the wavefront-guided group (+0.165) and slightly myopic in the classic group (-0.11). The UCVA was very similar, with 1.02 in the wavefront-guided group and 1.03 in the classic group. This was the first interval at which the results in the classic group were marginally better than in the wave- front-guided group. It is also important to note that there were smaller numbers of patients in these groups at the 6-month interval. The 20 eyes in the wavefront-guided group were the very first 20 eyes ever treated by author 2 with wavefront-guided LASIK and they were being compared with eyes in the classic group, where the overall surgeon experience is infinitely more.

At 6 weeks and 6 months, 67.4% and 82% of eyes, respectively, were within the -0.25 D to 0.25 D range, demonstrating a high degree of accuracy in the wavefront-guided group. The results were even more accurate in the classic group, with 76.3% and 80%, respectively, being within ±0.25 D of plano. The greater degree of accuracy seen in the classic LASIK group is to be expected, since this procedure has been in use much longer than wavefront-guided LASIK. Simple nomogram adjustments will improve wavefront accuracy further (Figure 30-4).

Overall, at 6 months postoperation, only 2.4% of the classic LASIK group had lost more than one line of best-corrected visual acuity (BCVA), 5.9% had lost one line, 57% were unchanged, 35.9% had gained at least one line, and 7.1% had gained two or more lines. In the corresponding wavefront-guided group, none of the patients had lost more than one line of BCVA, 1% had lost one line, 50% were unchanged, 49% had gained at least one line, and 11% had gained two or more lines (Figure 30-5).

Closer analysis of the refractive results shows that eyes with less than -4 D benefit more from wavefront-guided LASIK as they retain the superior results over classic LASIK at every interval up to and including the 6-month mark. In the range of -4.25 D to -6 D, wavefront-guided LASIK achieves better visual results at an early stage, but between the intervals at 3 and 6 months, classic LASIK starts to outperform wavefront-guided LASIK. This may be due to corneal modeling

Closer analysis of the refractive results shows that eyes with less than -4 D benefit more from wavefront-guided LASIK as they retain the superior results over classic LASIK at every interval up to and including the 6-month mark.

CONCLUSIONS AND OUTLOOK

The priorities of the engineers who design technology and the surgeons who rely upon that technology can sometimes be at odds. Quality of clinical results is paramount to the practitioner, while the engineer must consider a host of other factors. WaveLight has succeeded in bringing about a symbiosis of these priorities in the design of its Allegretto Wave system, as evi-

Figure 30-4. Results: Accuracy (±0.25 D) wavefront-guided vs classic LASIK.

denced by Professor Seiler’s use of the laser to conduct the world’s first wavefront-guided LASIK treatment in 1999.7

The purpose of wavefront-guided LASIK is to improve the quality of vision, especially in mesopic and scotopic conditions. The challenge was to achieve this goal without compromising the excellent results that were being achieved with classic LASIK using the Allegretto Wave. In the authors’ hands, wavefrontguided LASIK has clearly outperformed classic LASIK, both in terms of visual acuity and accuracy of refractive results.

What does the future hold? It is our belief that this technology will increasingly be used for patients with specific problems, including complaints of glare, large scotopic and mesopic pupils, and refractive errors of <6 D of myopia, especially <-4 D. We further believe that, as the technology evolves, it may find a primary role in enhancement procedures.

We can anticipate that there will be fine-tuning of existing nomograms, smaller spot sizes, faster lasers, reduction of cut impact (possibly with laser cutting), and an expansion of wave- front-guided LASIK treatments to values higher than -7.00 D spherical equivalent, accomplished by dividing aberrations into those created by the cornea and those created by the lens. Topography could play a greater role than before. Aside from central K-values, meridial eccentricities could gain a stronger impact on the algorithms.

Quality improvement and risk minimization will be critical in determining the future of refractive surgery. Wavefront-guided LASIK is a step in this direction. Measurements of vision and contrast sensitivity, as well as the subjective quality of visual perception (especially night vision), illustrate this impressively. We remain, however, far from fully understanding higher-order aberrations and their impact on the diverse visual qualities. We

Figure 30-5. Classic vs wavefront-guided LASIK safety (6 months) (n = 170 + 100).

have yet to determine if aberration-free optics are an ideal precondition for the physics and physiology of vision, or if there are, in fact, useful aberrations that support visual comfort and physiologic perception. The goal remains to ensure the best possible results in visual quality with significantly improved predictability and minimized effort and risk.

REFERENCES

1.Pinker S. How The Mind Works. New York, NY: WW Norton; 1997:535.

2.Mierdel P, Krinke HE, Wiegard W, Kaemmerer M, Seiler T. Messplatz zur bestimmung der monochromatischen aberration des menschlichen avgeo. Ophthalmologe. 1997;96:441-445.

3.Watkins JR. Corneal topography and contact lens relationship. JAOA. 1966;37(3):224-228.

4.Mrochen M, Seiler, T. Influence of corneal curvature on calculation of ablation patterns used in photorefractive laser surgery. J Refract Surg. 2001;17:584-587.

5.Wilms KH. Ein einfaches verfahren zur erfassung der corneaform mit dem klassischen pphthalmometer. Die Kontaktlinse. 1973;7(1): 24-30.

6.Wilms KH. Praxisnahe verfahren der corneatopometrie. Die Kontaktlinse. 1977;11(4):20-24.

7.Mrochen M, Kaemmerer M, Seiler T. Wavefront-guided laser in situ keratomileusis: early results in three eyes. J Refract Surg. 2000; 16:116-121.

8.Tscherning M. Die monochromatischen aberrationen des menschlichen auges. Z Psychol Physiol Sinne. 1894;6:456-471.

OPTICAL PATH DIFFERENCE

Device Description

The optical path difference (OPD)-Scan (Nidek, Gamagori, Japan) is a combination aberrometer and topographer that uses the principles of spatial dynamic skiascopy to measure the aberrations of the eye and placido disk topography to measure the corneal shape. The measuring principle (Figure 31-1) is a projecting system that consists of an infrared light-emitting diode housed within a chopper wheel with slit apertures.

The receiving system consists of a photodetector array that converts the time differences of stimulation in dioptric power maps. The dioptric power maps or refractive maps are displayed as OPD maps from which traditional Zernikebased maps can be derived.

The receiving system consists of a photodetector array that converts the time differences of stimulation in dioptric power maps. The dioptric power maps or refractive maps are displayed as OPD maps from which traditional Zernike-based maps can be derived. Using the traditional maps as an analogy, the OPD map is a total wavefront aberration map expressed in diopters (D) of refractive power rather than microns (µm) of light or elevation. A detailed description of this device and its advantages over traditional aberrometers is available in Chapter 19.

Nidek Advanced Vision Excimer Laser System—Scope

of Treatments

The Nidek Advanced Vision Excimer Laser System (NAVEX) (Gamagori, Japan) consists of the following units:

1.The OPD-Scan aberrometer and topographer

2.The Final Fit interface software

3.The EC-5000 CX excimer laser employing both scanning slit and spot ablation capabilities to deliver the treatment onto the cornea (Figure 31-2)

The Final Fit software uses both topography and aberrometry to develop the ablation algorithms. A cyclotorsion module allows compensation for cyclotorsion that may occur between the sitting to supine positions. The spot size is 1 millimeter (mm), the optical zone can be varied to 6.5 mm and the transition zone can be varied out to 10 mm. NAVEX allows the treatment of primary refractive surgery candidates in addition to the treatment of patients who have had suboptimal outcomes for previous refractive surgery. The experience of Howard Gimbel, MD with the latter group of patients was presented at the American and European Society of Refractive Surgery Meetings in 2002. These treatments included the treatment of irregular astigmatism and decentered ablations.

The Final Fit software uses both topography and aberrometry to develop the ablation algorithms. A cyclotorsion module allows compensation for cyclotorsion that may occur between the sitting-to-supine positions.

Customized Aspheric Transition Zone

Various excimer laser manufacturers have reported large increases in spherical aberration after excimer ablation. Spherical aberration has been implicated in a variety of night vision disturbances such as halos, glare, and starburst along with a generalized decreased in best-corrected visual acuity (BCVA). The quality of vision decreases with large increases in higher-order aberration.

The customized aspheric transition zone (CATZ) method was developed to specifically address this issue. This is accomplished by an increase in the transition zone, reduction in optical zone, and a seamless transition between the two treatment zones. The advantages of this unique treatment method are the increase in the effective optical zone and the maintenance of the prolate shape of the cornea postoperatively (Figure 31-3). By moving what Vinciguerra refers to as the “red-ring” on corneal topography past the pupillary excursion diameter and reducing the severity of contour change, the spherical aberration is very effectively reduced. The red ring on instantaneous (or tangential)