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

Figure 28-1. Bausch & Lomb Dual Work Station which combines the Zywave Shack-Hartmann Wavefront Sensor with the Orbscan Corneal Topographer. The Dual Workstation Zylink Software generates a customized excimer treatment profile that is driven by the Zywave Wavefront Sensor.

+6 to -12 D and cylinder from 0 to 5 D with pupil diameters ranging from 2.5 to 8.5 mm. The system uses a 785 nanometers (nm) infrared laser pulse that reaches a minimum beam diameter of 15 µm in focus on the retina with u 100 millisecond (ms) exposure period. A minimum of five Zywave measurements were taken. The wavefront sensing that had the closest correspondence to the manifest and cycloplegic refraction was selected for the laser treatment. The Orbscan and the Zywave readings were entered in the Zylink Software (Bausch & Lomb, Rochester, New York) and the ablation pattern was calculated (see Figure 28-1). The Zylink software uses the corneal pachymetry obtained from the Orbscan, the wavefront error data determined by the Zywave, and the surgeon’s preferred ablation optical zone to construct the ablation pattern. No adjustments were made to the Zywave wavefront values for treatment. (Eyes were not treated with a monovision undercorrection and no correction factors were added.) A postoperative 2.5% neosynephrine-dilated Zywave measurement was performed at 1 week, 1 month, 3 months, and 6 months.

The Bausch & Lomb Dual Workstation combines the Orbscan corneal topographer with the Zywave wavefront sensor into one system, called Zylink, which creates the computer shot pattern for Zyoptix.

In the data analysis of higher-order aberration, we examined the percentage of eyes that had less than 0.10 µm of increase in root mean square (RMS) wavefront error from their preoperative value because we have noted that an increase of 0.10 µm or less of higher-order RMS wavefront error is not clinically significant.

Surgical Procedure

All eyes were marked with a methylene blue pen at the 3:00 and 6:00 limbal positions at the slit lamp. That position was con-

firmed when the patient first lay down on the operating table, and again prior to ablation, using an ocular reticule on the operating microscope. A superior hinged LASIK flap was then created using the Hansatome microkeratome (Bausch & Lomb, Rochester, New York).

The ablation was performed with the Bausch & Lomb Technolas 217Z excimer laser, which uses a 1 mm and a 2 mm spot size. A 120 hertz active eye tracker was used with a passive automatic shut-off system if the eye moved more than 0.5 mm. The system allows the surgeon to monitor the eye tracker while performing the procedure to ensure centration. The majority of the treatment is done with a 2 mm spot and the transition zone and higherorder aberration are treated with a 1 mm spot.

Patients were seen at 1 day, 1 week, and 1 month, 3 months, and 6 months postoperatively. They were asked to fill out a questionnaire quantifying their subjective symptoms and their response to the surgery at the 6-month visit.

Statistical Analysis

Comparisons of preoperative values to postoperative values were done using the McNemar test. Chi-squared tests were performed to compare the proportion of patients who reported improvement to the proportion who reported worsening for each symptom parameter. (Patients reporting “no change” were excluded from the analysis.)

RESULTS

Demographics/Accountability

The mean age of the participants was 34.4 ± 8.29 years and there were 46.1% males and 53.9% females. The accountability was 100% with all 340 eyes being evaluated at the 6-month postoperative interval.

Preoperative Measurements

The mean preoperative mesopic pupil diameter was 5.60 mm

± 1.07 mm. The population was reflective of a representative myopic population where 54.3% of eyes had a preoperative bestcorrected visual acuity (BCVA) of 20/16 or better and 98.2% of eyes were 20/20 or better, which is similar to our previous study on the Bausch & Lomb Technolas 217 using conventional treatment. This indicates that our preselection techniques did not bias our study population toward eyes that saw 20/16 or better.

Efficacy

The mean preoperative myopic sphere was -3.32 ± 1.54 (Table 28-1.) The mean postoperative sphere was +0.29 ± 0.55 at 6 months postoperatively. Refractive stability was obtained at 1 to 3 months with the mean postoperative spherical values ranging between +0.33 ± 0.63 at 1 day postoperatively and +0.29

± 0.55 D at 6 months postoperatively (see Table 28-1).

Of the 340 eyes, 91.5% of eyes had 20/20 or better uncorrected visual acuity (UCVA), while 70.3% had 20/16 and 30.9% had 20/12.5 or better UCVA 6 months after surgery. Most (99.4%) eyes had 20/40 or better vision without correction 6 months after surgery. When 117 eyes treated for sphere only were analyzed, 94% of eyes were 20/20 or better.

Figure 28-2. Comparison of BCVA preoperatively, 1 day, 1 week, 1 month, 3 months, and 6 months postoperatively. At 6 months, 60% of eyes gained one or more lines of vision while 0.6% of eyes lost two lines of vision. No eyes lost more than two lines of vision.

Of the 340 eyes, 91.5% of eyes had 20/20 or better UCVA, while 70.3 had 20/16 and 30.9% had 20/12.5 or better uncorrected vision 6 months after surgery.

Predictability

Of the 340 eyes, 258 (75.9%) were within ±0.50 D of plano while 319 (93.8%) of eyes were within ±1 D of plano.

Astigmatism

Preoperatively, the mean astigmatism was -0.68 ± 0.59 D, while the postoperative mean astigmatism was -0.29 ± 0.32 D at 6 months postoperatively (see Table 28-1). The mean intended refractive cylinder (IRC) was 0.68 D, while the surgically induced astigmatism (SIRC) was 0.75 D. The SIRC/IRC ratio is 1.10 ± 0.74 D, indicating a slight overcorrection of the astigmatic component.

Safety

At 6 months after surgery, 78.3% of eyes had UCVA that was either as good as or better than the BCVA measured preoperatively. Sixty percent of eyes gained one or more lines of BCVA and 19.7% of eyes gained two lines or more lines of BCVA (Figure 28-2). Ninety-nine percent of eyes had 20/20 or better BCVA at 6 months postoperatively. There were no eyes that lost more than two lines of BCVA at 6 months postoperatively. Two eyes lost two lines of BCVA vision 6 months after surgery. One eye had a preoperative BCVA of 20/12.5, which decreased two lines to 20/20 at 6 months postoperatively. The second eye had a preoperative BCVA of 20/16 that was reduced to 20/25 at the 6-month interval.

At 6 months after surgery, 73% percent of eyes had UCVA that was either as good as or better than the BCVA measured. Sixty percent (60%) of eyes gained one or more lines of vision of BCVA and 19.7% gained two lines of BCVA.

Wavefront Analysis

Preoperative

The preoperative wavefront testing demonstrated a variation in the amount of preoperative higher-order aberration with a normal distribution about the mean of 0.414 µm ± 0.162 µm of RMS wavefront error (Figure 28-3). There was no correlation between the amount of preoperative sphere or cylinder and the amount of higher-order aberrations, nor was there any correlation between the amount of sphere or cylinder and the amount of the individual Zernike modes tested (Figure 28-4). The preoperative higher-order aberration was predominantly third-order aberration (coma and trefoil) (Figure 28-5). There was very little fifth-order aberration noted preoperatively or postoperatively on average.

Postoperative

Figure 28-3. The preoperative distribution of the higher-order wavefront aberration. Note that higher-order aberration is being presented as root mean square (RMS). This is the square root of the sum of the individual Zernike terms squared. This makes the values always positive and allows comparison of the magnitude of wavefront aberration from eye to eye. A higher-order RMS value of zero indicates a perfect optical system. The typical amount of preoperative higher-order aberration measured using the Bausch & Lomb Zywave is about 0.40 µm for a 6 mm pupil.

Figure 28-5. Profile of preoperative higher-order aberrations: third, fourth, and fifth expressed in RMS of wavefront error. Note that third-order aberrations dominate. Coma and trefoil are third-order aberrations. Fourth-order (including spherical aberration) and fifthorder aberrations are not as predominant preoperatively.

and fifth-order aberrations when compared to higher-order aberrations preoperatively with a 6.00 mm aperture. When total high- er-order aberrations were analyzed, 49% (Figure 28-7) (6 mm aperture or pupil diameter) and 70% (5 mm aperture) of eyes either showed a decrease or insignificant increase (<0.10 µm root mean square [RMS]) increase in higher-order aberration when compared to preoperatively. Twenty-eight percent of eyes showed a reduction in total higher-order aberrations at 6 months postoperatively, when compared to preoperatively, with a 6 mm aperture or pupil diameter.

Figure 28-4. The relationship between degree of myopia and high- er-order RMS. Note that there is no correlation between myopia and higher-order aberration. Some have speculated that higher degrees of myopia will have more higher-order aberration, but this was not noted in the current study.

Figure 28-6. Third-order aberrations in RMS values (including trefoil and coma) 6 months postoperatively compared to preoperatively. Note that if the eye has 0.3 or more µm of higherorder RMS error, on average, preoperatively, it is likely to have a reduction in third-order aberration. The gray portion of the graph represents the limits of detection of change in the high- er-order aberrations for the typical patient (ie, a typical patient would not notice a decrease or increase of higher-order aberrations by approximately 0.1 µm of higher-order RMS). In the 340 eye cohort, 69% of eyes either had a decrease or insignificant increase (<0.1% increase RMS) in third-order aberration when compared to preoperative mean values.

Postoperatively, there was a decrease or insignificant increase in total higher-order aberration in 49% of eyes with a 6 mm pupil and 70% of eyes with a 5 mm pupil. Third-order aberrations improved the most, with 69% of eyes either having an improvement or insignificant increase in third-order aberrations.

Figure 28-7. Total higher-order aberrations at 6 months postoperatively in RMS values compared to preoperatively. Eyes with 0.5 µm or more RMS error preoperatively were statistically more likely to have either a reduction or insignificant increase in higher-order aberration compared to eyes with less than 0.5 µm of RMS preoperatively. Nearly half (49%) of eyes studied had a decrease or insignificant increase (<0.1 µm RMS) in total higher-order aberration when compared to mean preoperative values.

Postoperatively, there was a mean increase in higher-order aberration in eyes with 0.50 µm or less of RMS wavefront error preoperatively (see Figure 28-7). For eyes that had more than 0.50 µm RMS wavefront error preoperatively, there was a mean decrease in RMS of higher-order aberration.

Contrast Sensitivity

The mean contrast sensitivity measures improved from preoperative to 6 month postoperative values by one patch or 0.15 log units for all spatial frequencies tested (1.5, 3, 6, 12, and 18 cycles/degree), which includes low, mid, and high spatial frequencies (Table 28-2). The one patch postsurgery improvement is a 29% average increase in contrast for the entire population noted at all spatial frequencies under low (mesopic) and high (photopic) illuminations. This was noted to be more striking when larger ablation optical zones were used (6.5 and 7.0 mm compared to 6.0 mm).

Under photopic (daylight) conditions, 25% of eyes were >0.30 log units (two patches) improved compared to preoperatively, while 71% were unchanged and 4.1% were >0.30 log units worse. Under mesopic (nighttime) conditions, 22.1% of eyes were improved >0.30 log units, while 75.9% of eyes were unchanged from preoperative and 2.1% were worse than preoperative.

Table 28-2

Percentage of Patients With Clinically

Significant Change in Contrast Sensitivity

at 6M Postoperative*

*Contrast sensitivity data demonstrating two patch units (>0.3 log units) change comparing the preoperative evaluations with the evaluation at 6 months postoperatively. A change of 0.15 log units (one patch unit) represented a 29% contrast improvement in the postoperative eye cohort overall for both photopic and mesopic conditions.

an epithelial defect (case excluded from the study), one eye had a small (<2 mm) flap hinge tear that healed uneventfully, and one treatment had a laser energy problem that was treated as planned after a laser gas refill. The most common complication postoperatively was debris in the interface. This was noted in 7.9% (27/340) of eyes one day after surgery but only 2.0% (7/340) of eyes had interface debris noted at the 6 month visit. We found that 5.8% (19/340) of eyes had stage 2 or less interface keratitis. Two eyes were taken back to the operating room and the interface washed out for lamellar keratitis. These eyes recovered without sequelae. One eye was noted to have a corneal recurrent erosion at 6 months.

Optical Zone Efficacy

We found a strong relationship between improved outcomes and the use of larger ablation optical zones. The was a statistically significant better outcome in UCVA, photopic and mesopic contrast sensitivity, reduction in higher-order aberrations, and improvement in subjective symptoms when larger ablation optical zones (specifically 6.5 to 7 mm) were used when compared to smaller optical zones (6 mm). The use of a larger ablation optical zone tended to minimize an increase in higher-order aberration. We recommend the use of a larger optical zone if feasible while balancing this with other considerations like corneal thickness and pupil size.

Mean contrast sensitivity improved 0.15 log units when compared to preoperative values for all spatial frequencies tested (1.5, 3, 6, 12, and 18 cycles/degree).

Complications

Subjective Surveys

Nine of 18 subjective symptoms surveyed before and after the surgery were significantly improved overall after surgery,

Patients noted an significant improvement in eight of the 18 subjective categories surveyed including an improvement in dim light vision and in difficulties with night diving

The following complications occurred at the time of surgery: the microkeratome stopped halfway through the cut (case excluded from the study), one eye had a thin flap associated with

including bright light vision, dim light vision, and difficulties with night driving (Table 28-3 [shown in yellow]). Seven of the 18

240 Chapter 28

Table 28-3. Subjective patient symptoms demonstrating symptoms that had no statistically significant change in grey. Patient categories that had a significant improvement are noted in yellow. Patient categories that were significantly worse postoperatively are shown in white. Only two categories were worse, dryness and fluctuation in vision. Nine of the 18 categories surveyed were improved compared to preoperatively. The unchanged group is not shown for simplicity sake but can be derived by subtracting from 100% the sum of the worse and better categories. For instance, light sensitivity was better than preoperative in 36.8% of eyes and worse in 7.6% for a total of 44.4% that were better or worse, while 55.6% were unchanged.

subjective symptoms at the 6 month interval were not significantly different when compared to preoperatively (see Table 28- 3 [shown in grey]). There were two categories that were worse: fluctuation in vision and dryness. The dryness change was primarily caused by shift from 62.1% of eyes reported as an absence of dryness preoperatively to 48.5% at 6 months. The percentage of mild dryness increased 15.6% from 30.0% preoperatively to 45.6% at 6 months. Interestingly, there was no increase in moderate or marked dryness when comparing the preoperative to 6 months postoperative evaluation. Thus, most of the increase in dryness symptoms occurred when eyes moved from the absent category preoperatively to the mild category at 6 months.

Most (98.6%) patients noted moderate to extreme improvement in vision and of these, 84.7% noted extreme improvement. Most (98.8%, 336/340) patients were either very satisfied (90.9%, 309/340) or moderately satisfied (7.9%, 27/340). A few (1.2%, 4/340) patients were neutral in satisfaction and no patients (0/340) were dissatisfied with their results in the study. Most (98.2%, 334/340) would choose the surgery again when asked 6 months after the surgery, 1.2% (4/340) were unsure, and 0.6% (2/340) would choose not to have the surgery.

DISCUSSION

Our study noted a similar pattern of higher-order aberration compared to other studies that evaluated normal populations.6-8 We did not find a tendency for increasing higher-order aberrations with increasing myopia (see Figure 28-4). The current large population FDA study demonstrates the efficacy and safety of treatment guided by a Shack-Hartmann wavefront sensor, that treats lowerand higher-order aberrations.9 This result is better than the 87.3% 20/20 or better UCVA noted in eyes treated with conventional sphere and cylinder (using the Bausch & Lomb Technolas) in a previous FDA study.9 The current results indicate that 91.5% of eyes achieved 20/20 or better, while 70.3% of eyes achieved 20/16 or better uncorrected vision. Preoperatively, only 54.3% of eyes had a best corrected vision of 20/20 or better, indicating that this population was similar to that encountered in the similar populations previously tested.9

We also noted excellent safety results with 60% of eyes gaining one or more lines of vision and 78.3% of eyes obtaining UCVA that was either as good as their BCVA preoperatively. The worst visual loss was one eye, which went from 20/16 to 20/25 BCVA at the 6 month postoperative visit.

The postoperative wavefront analysis indicated that there was a decrease or insignificant increase in higher-order aberrations in 47% of eyes treated when analyzing with a 6 mm aperture (or pupil size). Third-order aberrations (trefoil and coma) were more prominent preoperatively and these aberrations seemed to be those most profoundly reduced with Zyoptix treatment. Fourthorder aberrations are not as large preoperatively, and a smaller percentage of eyes (36%

) had either a decrease or insignificant increase. This is because there is minimal fourth-order aberrations preoperatively, particularly spherical aberration. We have found in a previous study of the microkeratome flap biomechanics that the flap causes a minimal increase in spherical aberration, when 17 eyes were followed after a flap cut.10 When the flap was lifted, after 2 months of observation, and a conventional Bausch & Lomb Planoscan ablation was performed, there was a 40% increase in spherical aberration, indicating that spherical aberration is primarily caused by the laser ablation (Figure 28-8). The current software does not attempt to compensate for the laser-induced spherical aberration. Future studies will incorporate software modification that alters the ablation to further compensate for the laser induced spherical aberration component.

Our current study also noted an improvement in contrast sensitivity. We were also encouraged by the one patch improvement in contrast sensitivity results for low, mid and high spatial frequencies including 1.5, 3, 6, 12, and 18 cycles/degree. An improvement in contrast of more than one patch (0.15 log units) is clinically significant and is associated with images being clearer or sharper. We noted a two patch improvement in up to 25% of eyes, photopic conditions being slightly better than mesopic.

We also noted that patients reported a 98% moderate or very satisfied rate when surveyed after the surgery. When surveyed regarding subjective symptom categories, 9 of 18 patient symptom categories were improved significantly after the treatment including improvement in dim light vision and difficulties with

Figure 28-8. Spherical aberration changes after creating a microkeratome flap (180 µm Hansatome) and following the eye for 2 months and then lifting the flap and ablating the cornea with a conventional ablation. The contralateral eye served as an untouched control for two months and then underwent a conventional LASIK. Note that spherical aberrations changes minimally after the flap only but increases 40% to 67% after the ablation in both eyes.10 (Reprinted with permission from Porter J, MacRae S, Yoon G, Roberts C, Cox IG, Williams DR. Separate effects of the microkeratome incision and laser ablation on the eye's wave aberration. Am J Ophthalmol. 2003;136(2):334, with permission from Elsevier.)

night vision at 6 months postoperatively. Only two symptom categories were worse: dryness and fluctuation in vision at 6 months. Both of these symptom categories are frequently reported as increased after post-LASIK studies and are not unique to this particular study.9,11 We suspect that this symptom is primarily caused by the microkeratome incision and not the type of ablation. The dryness symptom category change was driven by a 13.6% shift from an absence of dryness symptoms preoperatively to mild dryness at the 6-month interval. Interestingly, there was no increase and even a slight nonsignificant decrease in the moderate and marked dryness categories. The marked dryness (worst category) decreased from 1.5% (5/340) preoperatively, to 0.0% at the 6 month postoperative interval. This suggests that the dryness that developed after LASIK was mild in most patients in this study and not severely debilitating.

SUMMARY

The Bausch & Lomb Technolas 217Z Zyoptix results are encouraging. Patient with greater levels of higher-order aberration preoperatively are more likely to benefit from higher-order aberration correction. We noted a high level of patient satisfaction and these results suggest that wavefront driven ablation is a viable option for the correction of myopic refractive error.

REFERENCES

1.Mrochen M, Kaemmerer M, Seiler T. Clinical results of wavefrontguided laser in situ keratomileusis 3 months after surgery. J Cataract Refract Surg. 2001;27(2):201-207.

2.Seiler T, Mrochen M, Kaemmerer M. Operative correction of ocular aberrations to improve visual acuity. J Refract Surg. 2000;16(5):S619S622.

3.Nuijts R, Nabar V, Hament W, Eggink F. Wavefront-guided versus standard laser in situ keratomileusis to correct low to moderate myopia. J Cataract Refract Surg. 2002;28(11):1907-1913.

4.Nagy Z, Palagyi-Deak I, E EK, Kovacs A. Wavefront-guided pho-

torefractive keratectomy for myopia and myopic astigmatism.

J Refract Surg. 2002;18(5):S615-S619.

5.Vongthongsri A, Phusitphoykai N, Naripthapan P. Comparison of wavefront-guided customized ablation vs. conventional ablation in laser in situ keratomileusis. J Refract Surg. 2002;18(3):S332-S335.

6.Porter J, Guirao A, Cox I, Williams D. Monochromatic aberrations of the human eye in a large population. J Opt Soc Am A. 2001;18(8): 1793-1803.

7.Castejon-Mochon J, Lopez-Gil N, Benito A, Artal P. Ocular wavefront aberration statistics in a normal young population. Vision Res. 2002;42(2002):1611-1617.

8.Bradley A, Hong X, Thibos L, et al. The statistics of monochromatic aberrations from 200 healthy young eyes [ARVO Abstract 862].

Invest Ophthalmol Vis Sci. 2001;42(4):S161.

9.FDA/CDRH. FDA-Approved lasers for LASIK. CDRC, 2002. http://www.fda.gov/cdrh/PDF/P970043S010.html.

10.Porter J, MacRae S, Yoon G, Roberts C, Cox IG, Williams DR. Separating the effects of the microkeratome incision and laser ablation on the eye’s wave aberration. Am J Ophthalmol. 2003;136(2):327337.

11.FDA/CDRH. PMA Summary information on Alcon Laboratories, Inc. LADARVision 4000 excimer laser system, P970043/S010. FDA, CDRH, 2002. Accessed October 27, 2003 from: http://www.fda.gov/ cdrh/PDF/P970043S010.html.

INTRODUCTION

routinely in a doctor’s office, but to enable physicians to base corneal refractive surgery on this information.

Historical Perspective

The eye can be thought of as a sophisticated optical instrument that automatically adapts to its environment. It has automatic systems for adjusting the light level (pupil/iris) and focus (accommodative lens). The eye can also have a range of common optical errors that limit its performance. The dominant errors in most eyes are myopia (near-sightedness), hyperopia (far-sighted- ness), and astigmatism (asymmetrical focal power). These errors have been, for centuries, corrected by adding a lens in front of the cornea (glasses or contact lenses). Modern laser refractive surgery has been driven by efforts to modify the structure of the cornea itself. Traditionally, the amount of these errors in the eye has been measured by a trial-and-error process. A trial corrective lens is placed in front of the eye and patient feedback gives information about whether the vision is better or worse. The phoropter, trial lens kit, and, to some extent, the autorefractor are the primary instruments used to gather this refractive error information. The subjective refraction (or manifest refraction) method is the primary means for determining the proper patient corrective lens prescription, and has, for the last 15 years or so, formed the basis for calculating ablation profiles to be applied to the cornea. The errors in the eye’s optical system, however, are not always limited to focus and astigmatic errors. For example, it is possible for the focal power to change at different locations across the pupil. These effects cannot be described purely in terms of focus or astigmatism since they relate to changes in the focus and astigmatism as a function of position. Thus they are called “higher-order” aberrations, while focus and astigmatic error are defined as “lower-order” aberrations. Examples of higher-order aberrations are changes in the focal power as a function of pupil diameter (predominantly spherical aberration), increased power in the upper half of the pupil, decreased power in the lower half (vertical coma), or increased/decreased power left to right (horizontal coma). These effects can be small and have relatively little effect, or they can dominate visual quality, causing starburst, glare, image ghosting, or even monocular diplopia. Until recently, it was only possible to measure the high- er-order aberrations of the eye in controlled laboratory conditions, in vivo. Thanks to the advent of modern aberrometers, it is now not only possible to measure these higher-order aberrations

Definition of Wavefront

The wavefront of the light that is transmitted through an optical system is an imaginary surface that remains normal to the direction of propagation at all cross sectional points within the optical pathway. For a perfect eye focused at infinity, the wavefront of the light collected by the optics of the eye would be part of an aspheric surface, which would converge on the back of the eye to create a (diffraction-limited) spot on the retina. Because of the reciprocal behavior of light (ie, it traverses the same path in either direction), it is possible to measure this wavefront from light that is scattered or reflected from the retina (after it has been projected onto it). This is the primary measurement principle of the Carl Zeiss Meditec Wavefront Supported Customized Ablation (WASCA) device, produced jointly with WaveFront Sciences (Albuquerque, NM), who market the same device as the Complete Ophthalmic Analysis System (COAS) in the United States. In operation, the WASCA emits a small beam of light, projecting this onto the retina. The light scatters from the retinal surface (fovea), it is collected by the lens and cornea, and is then “projected” out of the eye. A perfect, emmetropic eye would completely collimate this light. The resulting wavefront would be a perfect flat plane wavefront, perpendicular to the direction of propagation. Any deviations from a perfect plane wave are the result of optical errors in the optical system (ie, the eye), and as a result are called the wavefront error of the system.

Custom Ablation: Definition

We define custom ablation as an ablation profile designed to specifically meet optical correction requirements for a specific individual eye. The recent introduction of diagnostic aberrometry into refractive surgical practice has put higher-order optics into the standard vocabulary of refractive surgeons. This has also raised new issues concerning the design of ablation profiles. For example, aberrometry has now made it possible for us to study what aberrations are being induced by current treatment profiles.1 As such, in our view, wavefront-guided ablation comprises an important component but not the total sum of custom ablation. In addition, we must also consider a number of independent variables contributing to a scheme designed to give us control over the modification of corneal shape. These will include

Figure 29-1. WASCA aberrometer unit showing narrow head design that enables fellow eye to “see past” the device to minimize instrument myopia and enable fellow eye targets to study dynamic accommodation.

knowing the factors affecting the accuracy of laser energy delivery to the cornea and ablation biophysics, as well as epithelial and biomechanical responses within the cornea. This chapter will describe the current and near-future components of the Carl Zeiss Meditec system, designed to achieve true custom ablation within the cornea.

ABERROMETRY: WASCA

The WASCA aberrometer is based on the Shack-Hartmann wavefront sensing principle (Figure 29-1). The basis of this technology has been described elsewhere in this textbook (Chapter 14) and is based on a large body of research. Briefly, a lenslet array collects incident light emerging from the eye. Each lenslet then creates a focal projection onto a charged-coupled device (CCD) camera array. The position of the spots on the CCD array relative to a reference location (as would be produced by a flat wavefront) is used to determine the actual wavefront slope of the incident light onto a particular lenslet. The combination of this array of slopes in a topographic manner leads to the calculation and digital reconstruction of the incident wavefront. This wavefront can then be either displayed directly from the raw data (as a result of the very high resolution of the WASCA) by zonal reconstruction or broken down into a collection a shapes of varying amplitude (eg, the Zernike expansion series). We will now delineate some of the specific design features of the WASCA that provide increased accuracy and reproducibility of wavefront measurement in practice.

Lenslet Array Resolution and Spot Quality

Simply stated, lenslet resolution directly impacts the accuracy of wavefront detection, the dynamic range within a wavefront to be detected, and the reproducibility of detection. Thus, resolution will certainly affect the concordance between the measured and actual wavefront, but it will also significantly influence whether a wavefront measurement can even be obtained from a particular aberrated eye.

Figure 29-2. Small portion of the image from a lenslet array. (A) shows the irradiance distribution incident on the CCD detector, and (B) is the resulting CCD image pixel-by-pixel. The location of these spots is the key information that is used to determine the wavefront.

Figure 29-3. Scanning electron micrograph of a portion of the WASCA lenslet array. Each lenslet is in fact square and comprises a depression of approximately 1.6 micron (µm) with a diameter of 144 µm. Note the 100% fill (ie, no gap) between lenslets.

The lenslet array incorporated within the WASCA is based on the core patented technology of WaveFront Sciences, Inc and is comprised of the most compact lenslet array (highest resolution) available worldwide. It is constructed using methods similar to those for creating modern integrated circuits. That is, they are designed using a computer-generated mask, which is transferred to the surface of a fused-silica substrate using photolithography and reactive-ion etching. This results in an extremely accurate lenslet array with known and extremely accurate characteristics. Figure 29-2 shows some examples of the focusing data for a small portion of the incident light. Figure 29-3 shows a small portion of the actual lenslet array. The full array consists of lenslets arranged in a rectangular array 44 x 33 (1452 lenslets). The shape of each lenslet surface is spherical, but each lenslet is a 144 microns (µm) square section of this surface. In this way, there is 100% fill of the focusing array, so that no light can “leak” through between lenslets. This reduces stray light that could cause interference or background effects that reduce spot localization accuracy. The full array measures 6.5 x 4.8 millimeters (mm). The WASCA lenslet array enables approximately 800 lenslets to collect light from a 7 mm entrance pupil, with an effective lateral resolution of 210 µm. Comparison of the number of lenslets collecting from a 7 mm entrance pupil and the effective lateral resolution created by several commercial ocular aberrometers is shown in Table 29-1.

The WASCA lenslet array enables approximately 800 lenslets to collect light from a 7 mm entrance pupil, with an effective lateral resolution of 210 µm.

The other most common method of creating lenslet arrays is by production of molded monolithic lenslet modules. These are made by an embossing process in plastic. Some of the weaknesses of this method are that the lenslet shape is less predictable, and it cannot produce a 100% fill, thus potentially enabling stray light to “leak” between lenslets.

To understand why lenslet size affects resolution and how this in turn affects the accuracy of wavefront detection, we present the illustration shown in Figure 29-4. Essentially, the smaller the

Figure 29-4. (A) and (B) The smaller the diameter of the lenslet, the more that can be contained in an array and hence the greater the number of lenslets collecting light from the entrance pupil. (C) Shows a portion of wavefront containing a nonlinear slope entering the catchment of the left-most lenslet, but effectively a linear slope entering the middle and right-most lenslets. The sample of the wavefront in the case of the left-most lenslet has a nonlinear structure; therefore, the lenslet will scatter light over a large region, causing a diffuse and irregular focal spot. In the case of the middle and right-most lenslets, light will be properly focused (F), producing a sharp peak.

(D) Increasing the number of lenslets or decreasing the diameter of the lenslets allows more frequent sampling. Now each lenslet is picking up effectively linear slopes from the same wavefront as in (C); Because of the smaller sampling area the non-planar portion of the incoming wavefront is now divided into quasi-planar segments to produce wellfocused projections (F), onto the CCD array.

lenslets, the more of them can be fit into the catchment area of the pupillary zone, and hence the higher the resolution. Smaller lenslets have smaller diameters and therefore collect a smaller sample of the incoming wavefront. The smaller the sampled area from the incoming wavefront, the less “averaging” of the wavefront slope within one lenslet occurs, and thus the more focused the spot projected onto the CCD detector. The more focused the spot on the CCD detector, the more likely that (A) it can be detected with good positional accuracy and (B) it is a true representation of the slope of the incoming wavefront into that lenslet.

Figure 29-5 demonstrates the CCD capture output obtained from a mild keratoconic eye, using either a 400 or 200 µm resolution lenslet array. This example clearly demonstrates how lenslet size/resolution affects the quality of data acquired and hence the reliability of the wavefront calculated.