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

depicts all aberrations in the exiting wavefront, including simple sphere and cylinder. The other map isolates the higher-order aberrations, which constitute a much smaller proportion of the error6. From this information, the WaveScan VSS software calculates the best treatment table for an individualized ablation.

Not only does the WaveScan system function as a high-level autorefractor for lower-order aberrations, it also can be used in combination with topography to help evaluate and understand aberrations in the optical system in order to decide how to best apply refractive surgery technology. Wavefront diagnostics can also be used to follow progressive symptoms inside the eye in the lens—specifically symptoms that cannot be accurately followed with corneal topography or with Snellen acuity alone.

Zernike polynomials are highly complex shapes that represent non-spherocylindrical refractive errors. These complex shapes provide a mathematical description of optical aberration. Higherorder aberrations—such as spherical aberrations, coma, and trefoil—typically account for less than 20% of refractive error. The WavePrint system also provides a way to document fluctuations in a patient’s visual acuity from minute to minute or exam to exam. Ocular surface problems are among the most common postoperative problems following laser in-situ keratomileusis (LASIK). The WavePrint provides a way to observe higher-order aberrations that may be quite variable from one minute to the next. This can help in determining if the patient has an ocular surface problem and not necessarily an ablation-related problem. For example, tear-film status, blepharitis, and lid abnormalities may be the cause of the visual complaints in these cases.4

If a patient has aberrations that are not reflected on topography over time, these aberrations are typically in the lens. These patients may have cataract or other lens abnormalities, such as anterior lenticonus.

Section IV: Aberrations and Aberrometer Systems

Acquisition of Image

The VISX WaveScan device has two modes for focusing the Hartmann-Shack image that appears in the Retina Camera window: AutoFocus and Manual7. Selecting AutoFocus causes the instrument to give the best resolved spot pattern by automatically selecting the correct precompensation. If the AutoFocus mode is not able to sharpen the focus on a patient, the Manual mode can be used.

Using the Reset Precompensation Button automatically sets the spherical precompensation to +8.0 D and the cylindrical precompensation to 0.0 D x 0°. The precompensation is automatically reset for the first eye in a 2-eye acquisition. For the second eye, the spherical precompensation is set to +3.0 D greater than its position for the first eye.

The captured image should appear circular (not oblong) in shape and sharply focused. If the eye has astigmatism, the dots will be oblong rather than circular.

The WaveScan system displays refractive errors and wavefront aberrations as the optical path difference (OPD) between the measured outgoing wavefront and the ideal plane wavefront. Regions of the pupil with positive OPD are in front of the ideal plane and are shown in red. Areas with negative OPD are behind the ideal plane and are shown in blue.

An Acuity map and the All Aberrations map contain both the refractive and higher-order aberrations. The color-coded scale appears on the left side of the map (Figure 5). The wavefront-based refraction, the pupil size to use when calculating the refraction, the RMS (root mean square) and error and Effective Blur measurements are displayed above the maps.

A green center with red edges corresponds to a myopic wavefront-based refraction and a green center with blue edges corresponds to a hyperopic wavefront-based refraction (Figures 6 & 7).

The Higher-Order Aberration map contains only the higher-order aberrations, with spherical and cylindrical refraction errors removed (Figure 8). The range of the color-coded scale that appears on the left side of the map is usually smaller than with the All Aberrations map, reflecting an overall lower magnitude of aberrations.

Aberrometers do have limitations. Looking at spherical aberration, for example, if the aberration "knee" extends outside the radius of the pupil, the aberrometer will not measure it. However, optical aberrations of the eye outside the pupillary zone usually do not affect a patient’s vision. This is in contrast to corneal irregularities outside the pupil, which do have an effect on the vision, and this is because the corneal irregularities outside the pupil can cause optical aberrations central to the corneal irregularity. For example, testing from a 2-mm pupil measurement will not provide any information about what happens optically when the patient’s pupil size gets bigger. Testing should be conducted using maximum pupil size so the most accurate information is collected for both the lower and the higher-order aberrations which can be displayed independently for review.

Section IV: Aberrations and Aberrometer Systems

Refractive Information

The WavePrint system also is going to change the way surgeons look at refractions. It has promise to make refractive surgery outcomes more reproducible by improving the endpoint on the patient’s refraction, providing objective information that the patient can subjectively verify. The WavePrint system also improves on the standard 0.25-D adjustment ceiling inherent with manifest refraction by providing for much higher precision treatments in increments of 0.10 D or smaller. These characteristics provide a more accurate refraction and enable the excimer laser to be used to its fullest capability.

One of the problems with a standard refractive evaluation is that the patient’s ability to discern or discriminate between quarteror half-diopter steps is relatively limited. To get the highest quality vision, even if patients can only intermittently appreciate this higher quality, refractive surgeons have to think in terms of hundredths of a diopter rather than quarters and halves.

When using the VISX WaveScan system, a patient’s optical system can be displayed using a Wavefront Error map that shows wavefront errors measured in microns or a Refractive Correction map that shows the wavefront errors measured in diopters7 (Figure 9).

The effect that changes in pupil size have on the wavefront-based refraction can be demonstrated by changing the pupil size determinant on the map screen. The sphere, cylinder, and axis portion of the wavefront-based refraction based on that pupil size will then be displayed. Refractive or aberrometry information is not available for pupil sizes larger than the maximum available pupil size that was acquired by the instrument. The displayed maps are based on the maximum available pupil size acquired and are unaffected by this setting.

To understand the information provided by the wavefront images, it is important to understand some key terminology. Some of the key terminology used in wavefront mapping is briefly explained below.

The RMS Error is the difference between the exam’s measured wavefront and that of an ideal plane wavefront. It is expressed in microns.

The Effective Blur value is the amount of spherical error (in diopters) it would take to create an RMS error equal to the RMS error created by all the aberrations. This is provided as an aid to assessing the qualitative impact of the higher-order aberrations.

The Higher-order Aberration Index represents the percentage of higher-order aberrations as a fraction of the RMS total of aberrations measured.

The Wavefront Error/Refractive Correction Difference map pairs illustrate the point-by-point differences between 2 exams, taken at different times of the same eye, and are referred to as Ex 1 and Ex 2. Ex 1 is always the first exam selected (Figure 10).

The map pairs are either Wavefront Error maps or Refractive Correction maps, depending on what is selected in the Map Type control. The upper maps always include all aberrations, including sphere and cylinder. The lower maps include higherorder aberrations only, excluding sphere and cylinder.

To display aberrations in a visual format, the WaveScan system includes the Point Spread Function (PSF) view. The PSF attempts to describe the effect of the visual aberrations on a point source of light. The magnification scale is very important in interpreting this map, and is displayed below the diagram. A standard scale is used to help compare maps from one exam to the next. The PSF image can be enhanced by utilizing a logarithmic scale to enhance

low-level details that may be present in PSF images. By selecting this option, optical defects are emphasized that otherwise may not be visible in the PSF image. The Edit control allows the user to control the amount of image enhancement applied to the lowintensity part of the PSF image: 1% corresponds to almost no enhancement, while 100% corresponds to full logarithmic enhancement. The top images show all aberrations, while the images on the bottom of the screen show only the higher-order aberrations. (Figure 11). In the example shown the preoperative total PSF is displayed with a scale spanning 65 degrees, and is compared with the higher-order PSF on the same scale. The point spread function pre and postoperatively can be shown on the same scale spanning 65 degrees, showing a dramatic improve-

ment in the PSF for all aberrations (Figure 12). A similar comparison of pre and postoperative information can be shown with the higher order aberrations expanded to a scale spanning only 6 degrees both pre and postoperatively (Figure 13). There may be some correlation between the appearance of the PSF and Snellen acuity, yet no study has been conducted to assess the correlations in visual quality data between PSF and Snellen acuity results.

Zernike coefficients up to the sixth order are displayed in the Zernike Coefficients table. The Zernike coefficients can be displayed using standard or polar coordinates. Standard Zernike coordinates express the exam data using classical mathematical terms, while polar Zernike coordinates use vector combination to combine similar Zernike terms to represent the optical properties obtained from the

exam data. An Options preference provides for the display of only the higher-order terms. Removing the sphere and cylinder terms from the display allows for a more concise visual comparison of the higher-order terms. Graphical representations of each of the Zernike polynomial terms in polar and standard coordinates can be found in the WaveScan system’s online Help system.

The WaveScan system can measure spherical refractive errors between +6.0 and –8.0 D, and cylindrical refractive errors up to 5.0 D, as well as higherorder aberrations of the eye, which adversely affect vision in some cases8. The role of all higher-order aberrations in vision is unknown at this time.3,9 Typically the more central the higher order aberration is, the more effect that it has on the vision.

Wavefront-Adjusted Manifest

Refraction

While custom ablation is still in the early stage of approval in the United States, many practices are already using WaveScan systems for performing wavefront-adjusted manifest refraction (WAMR), which is essentially a post-wavefront manifest refraction. This concept is similar to the way a post-cycloplegic manifest refraction is used to try to determine how much latent hyperopia can be accepted in the manifest state in a hyperope. The WAMR starts by taking the refractive data from the wavefront, typically the 6-mm refractive information. This is used as the starting point for a refraction, and then the refraction is subjectively refined. With the PreVue lens, the patient can assess whether he or she prefers the wavefront correction to the manifest refraction obtained in the traditional manner.

In most patients, the WAMR is very similar to the MR. Yet, in 5% to 10% of the cases, we find that the patient will prefer the WAMR to the MR and the best-corrected visual acuity (BCVA) is better with the WAMR than with the standard MR. This is because the sphere, axis, or cylinder is one that you and the patient would not have identified through the process of multiple choice testing that is part of the standard manifest refraction process.

Other studies have evaluated the value and accuracy of the VISX WaveScan device is proving to be a valuable and accurate autorefraction tool for measuring the refractive error in normal eyes before refractive surgery. A recent study conducted by the Clearview Eye & Laser Medical Center in San Diego looked at the effect of pupil size on the wavefrontderived refractive error in normal eyes before refractive surgery. WaveScan results for 22 patients were compared with measurements obtained using standard methods of manifest and cycloplegic refraction10.

The results found that the manifest refractions were more myopic than the cycloplegic refractions by a mean standard deviation of 0.13 D ± 0.33 D. Only 3 eyes differed by more than 0.5 D. The manifest refraction was also more myopic than the WaveScan-derived refractions at all pupil diameters ranging from 3 to 6 mm, while the cycloplegic refractions matched the WaveScan-derived refractions more closely at the 6-mm pupil diameter than at any other pupil size.

The cycloplegic refraction measured 0.05 D

± 0.45 D more myopic than the WaveScan-derived refractions for a 6-mm pupil. The largest differences in WaveScan-derived refractions were observed between the 6- and 3-mm pupil sizes. WaveScanderived refractions were approximately 0.25 D more myopic for the 3-mm pupil size. Astigmatism differed between the manifest and cycloplegic refractions by 0.03 D ± 0.15 D. The WaveScan-derived astigmatism appeared to be within 0.25 D of the manifest and cycloplegic refractions for all pupil sizes. The WaveScan astigmatism measurements at all pupil sizes were very similar to the manifest refraction.

One of the most time-consuming processes in refractive practices is that of measuring refractive error. An experienced technician is required to really fine tune the refraction, and there is inherent variability in the skills of technicians. Additionally, patients are frequently afraid that their answers will be incorrect and may respond imprecisely under this stress. Adopting these new WaveScan devices increases our ability to accurately and objectively measure refractions.

In another study, researchers at Baylor College of Medicine’s Cullen Eye Institute in Houston compared the refraction results obtained from the VISX WaveScan Hartmann-Shack aberrometer with those from manifest refraction11.

The Cullen Eye Institute team evaluated the accuracy and reproducibility of the VISX WaveScan

Hartmann-Shack aberrometer, using manifest refraction as the standard. They found that, overall, there was excellent agreement between the measurements of the 2 devices, as demonstrated by their mean differences, values for 95% limits of agreement, and aggregate analysis of astigmatic values.

Initial data for the VISX WaveScan were based on the results from 69 eyes of 42 patients (mean age within 43 years) with mean manifest refractive spherical equivalent of within –1.32 and a range of –8.38 to 3.63 D. The study included normal subjects as well as an array of patients who had LASIK, photorefractive keratectomy, Intacs, and limbal corneal relaxing incisions.

The mean difference between the spherical equivalent of the manifest and the WaveScan refractions indicated that the WaveScan read slightly more hyperopic. The manifest refraction was 0.25 D more

Section IV: Aberrations and Aberrometer Systems

minus on the sphere and cylinder, with a maximal difference for spherical equivalent values of 1.2 D. Reproducibility between 3 consecutive WaveScan measurements was also very close, with a mean difference of 0.13 D between spherical equivalent readings.

A study was done at Minnesota Eye Consultants evaluating 50 eyes preoperatively and following LASIK to determine differences between Manifest, Cycloplegic, WaveScan, and Tracey refractions. The LASIK was performed using the VISX STAR S3 laser using a 6.5 mm optical zone with an 8.2 mm blend zone. There was no difference in the preoperative spherical equivalent between any of the measurements (Figure 14). Postoperatively, at 1 month the WaveScan Refraction showed slightly more myopia than the Manifest Refraction. Preoperatively the WaveScan and Tracey both