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

showed more astigmatism than the Manifest or Cycloplegic Refractions, and this difference persisted postoperatively (Figure 15).

Patients with corneal irregularities or who have undergone LASIK or other refractive procedures clearly present a challenge for wavefront. Because wavefront based device averages the data over the whole pupil, zones of irregularity near the edge of the pupil that would not affect the manifest refraction may significantly influence the sphere and cylinder calculated using wavefront.

PreVue Lens

The PreVue lens is a diagnostic tool that uses a plastic lens ablated with a wavefront-driven shape. This lens is being utilized in the VISX wavefront clinical trials and provides a powerful tool for patient screening and subjective verification of wavefront measurements.

In order to create a PreVue lens, a patient’s wavefront data is acquired on the WaveScan. Based on the measurements, the 2 wavefront-driven treatment profile parameters (PreVue and cornea) are created and transferred from the WaveScan to the VISX Star S4 laser.

To make the lenses, the treatment table for plastic medium is entered into the STAR S4 laser. The lens material is positioned on the calibration arm of the laser and ablated. The lenses are then fitted into a frame and precisely aligned on the patient for testing.

Precise alignment of the PreVue lens while it is being ablated and when it is fitted on the patient is extremely important. To ensure proper alignment, the lens disk is inked on both sides. A black mask applied to one side of the disk limits the patient’s field of view to a 6-mm aperture and prevents him or her from looking through the unablated portion of the lens. The other side of the disk is inked with an align-

ment reticle and some areas for identifying information to be recorded. The alignment reticle is keyed so that it cannot be transposed. One arm of the alignment reticle is missing so that it can be positioned inferiorly on the patient. The PreVue lens is placed on the calibration arm of the laser with the marks aligned to the surgeon’s reticule in the operating microscope (Figure 16). The missing mark is inferior on the patient; therefore, the correct view through the surgeon’s microscope places the missing mark at the top of the field. After ablation of the lens, the patient is fitted with the lenses and asked to read the eye chart (Figure 17).

Diagnostic Applications

In addition to having valuable applications for wavefront-adjusted manifest refraction, the VISX WaveScan device is a valuable tool for preoperative screening and diagnostics.

Wavefront technology can identify those patients who would benefit from custom ablations to treat their higher-order aberrations. In most patients, higher-order aberrations comprise fairly small portions of the entire refractive error. But for some patients higher-order aberrations account for a significant amount of refractive error.

Section IV: Aberrations and Aberrometer Systems

Figure 17: The patient is fitted with the PreVue lenses in a trial frame and asked to read the eye chart.

For example, a potential LASIK patient who was evaluated at our office had a fairly low refractive error of approximately –1 D and, based on manifest refraction, appeared to be a good candidate for standard LASIK correction. He did note upon careful questioning that he had some mild night time aberrations that he wished would be better than what he had in his glasses.

Wavefront analysis with the VISX WaveScan revealed that the patient had a sphere of –0.83 D and a cylinder of –1.21 D. His BCVA was 20/20. Most important, the wavefront results indicated that a fairly high portion (27.6 %) of his refractive error was related to higher-order aberrations (Figure 18). Moreover, this patient’s higher-order RMS reading was 0.42 µm, and his overall RMS was 1.52 µm. Usually, if the higher-order RMS is less than 0.50 µm, most patients are satisfied with their vision in spectacles, contact lenses, or with refractive

surgery, and if their higher order RMS is less than 0.25 µm, most patients are extremely satisfied with their vision. In this case, a large portion of the high- er-order RMS resulted from coma. Based on the wavefront data, in combination with his expectations and desires, it appeared that this patient would benefit from waiting for wavefront-guided custom ablation treatment as his primary procedure.

Another example is a patient who has a refractive error of -3.50 + 0.50 x 125 with 20/20 BCVA that is happy in spectacle correction. Even though the patient has a higher-order RMS of 0.49, the higher-order aberrations represent only 12.6% of the patient’s total aberrations (Figure 19). The patient was quite happy with standard ablation

refractive surgery and the result was such that he ended up with slightly less higher order aberrations postoperatively, even with a standard treatment (Figure 20).

Wavefront diagnostics can be useful in screening for corneal abnormalities, such as keratoconus, and for assessing treatment options. There is a gradient of visual abnormalities in patients that may be suspected of having keratoconus. Some eyes will have a relatively central corneal abnormalities that will cause distortion of their optical pathway that is apparent on topography and wavefront analysis (Figures 21 & 22). In other eyes the cone is more peripheral and the wavefront may be relatively normal in the central 6 mm.

The exact management of these eyes is not clear, as no one understands completely the additional risk of progressive ectasia from LASIK or photorefractive keratectomy treatment. But some cases of keratoconus clearly progress to ectasia soon after surgery, suggesting that the surgery may have led to the progression12. Other patients with keratoconus may remain stable for years after LASIK. Clearly, though, those who have abnormal optics have an increased incidence of irregular astigmatism and higher-order aberrations postoperatively. But this is usually because they have irregular astigmatism and higher-order aberrations preoperatively that were not corrected with the surgery.

An example of the usefulness of wavefront in corneal disorders, as well as the stability of LASIK

Section IV: Aberrations and Aberrometer Systems

in some keratoconus patients, is the case of a patient with keratoconus who had previously had LASIK. The corneal topography of this patient—who had undergone LASIK in his right eye 3 years previous- ly—indicated that an area of ectasia from keratoconus remained postoperatively. He had a relatively small post-LASIK spherical refractive error of –0.29 D and –0.27 D of cylindrical refractive error as measured by the WaveScan. This correlated well with his manifest refraction of –0.50 D of sphere and +0.25 D of cylinder. The results then can be displayed as a Zernike map with the lower-order aberrations selected out or included (Figure 23). Viewing just the higher-order aberrations will isolate the irregularities that will not be corrected with spectacle correction or standard laser treatment. The map shows

some coma, indicating that the eye is processing light faster and steeper on the 2 different axes. There also is some trefoil. Typically, the most important factor for determining visual satisfaction with best correction in spectacles, or in determining the patient’s candidacy for standard refractive surgery, is the percentage of higher-order aberrations. The total amount of higher-order aberrations (RMS value) is also important, but the large effect of sphere and cylinder on patients with high lowerand high higher-order aberrations often dampens the importance of the higherorder aberrations. WaveScan measurements indicated that 73% of the patient’s residual problems were the result of higher-order aberrations.

To address this particular patient’s symptoms, which fortunately were minimal, either the laser would have had to ablate the cornea asymmetrically or a strengthening procedure, such as conductive keratoplasty or intracorneal ring segment place-

ment, would be needed to treat the ectatic area at the inferior portion of the cornea. In this case, the patient had UCVA and BCVA of 20/15, was happy with the success of treatment in this eye, and retreatment was not indicated. However, in a cornea with normal stability with this much aberration, a wavefront-guided ablation using WaveScan results would most likely have been the best course of treatment.

Another example illustrates how wavefront diagnostics can be used for preoperative screening. A potential refractive surgery patient showed an anterior polar cataract and wavefront data helped determine how some of the visual aberrations are from the lens and some are from the cornea (Figure 24). The topography was normal, yet the wavefront showed an increased amount of trefoil, coma, and negative spherical aberrations suggesting that the lens is the source of these aberrations.

In addition to isolating the cause of the optical aberrations, wavefront data provided a baseline for examining the patient again in 3 to 6 months to determine refractive stability as well as any changes that might be noted in higher-order aberrations that, in the absence of changes in topography, would be attributable to changes in the lens.

The patient in this case was myopic, with 31% of the refractive error from higher-order aberrations. In general, a good rule of thumb is that if more than 20% of a patient’s visual problems relate to higher-order aberrations, that patient is going to be just 80% satisfied with the treatment. If the patient does not see well in glasses or has an unstable endpoint on refraction, he or she will probably benefit from a wavefront-driven custom ablation, or in this case, lens extraction, because of the large amount of higher-order aberrations that are likely attributable to the lens.

The best solution for this patient was to wait 3 to 6 months to see if the percentage of total RMS for the higher-order aberrations of 0.67 increased. If the RMS value is increasing progressively over time, it may indicate that the patient is better off with lens extraction and not corneal surgery.

Ray Tracing

Another useful diagnostic tool for use in conjunction with wavefront is ray tracing. Hartmann-

Section IV: Aberrations and Aberrometer Systems

Shack technology is occasionally unable to collect information on widely disparate spots when measuring highly aberrated eyes. In these cases, ray tracing, developed by Tracey Technologies, is more versatile. The ability of excimer laser correction to correct these highly aberrated eyes has still not been verified, but diagnostically this is, at times, quite useful.

In general, if the patient has significant high- er-order aberrations, he or she will most likely derive the greatest benefit from a WaveScan-driven custom ablation. But, again, the Hartmann-Shack sensors have difficulty breaking down images in eyes with more than 40% higher-order aberrations—up over 1 to 1.5 RMS. This is where ray-tracing technology may provide a solution.

For example, a patient presented with previously unexplained loss of vision. He had 20/20 BSCVA, but the tip-off that there was an underlying problem was that the patient had a change in refractive error from a –6 D to a –9 D-. This patient was 20/20 refractable at –6 D, –9 D, and even –12 D. Clearly, the patient had an extremely variable or multifocal optical system.

Looking at a refractive map generated by the Tracey ray-tracing system the very center of the cornea, or the very center of the optical system, had a refractive power is –12 D (Figure 25). But moving out into the mid-periphery the refractive error was about –3 D. So, the patient had 9 D of difference in the refractive error between the center and the periphery. More careful slitlamp examination of this

patient revealed an oil droplet cataract. This patient underwent cataract extraction and lens implantation with a very successful result.

Summary

A wavefront assessment is helpful as part of the preoperative evaluation for every refractive surgery patient. The WaveScan can serve as both a diagnostic device and a tool for guiding custom ablations. The WaveScan device can provide a wealth of information by generating an objective mathematical calculation of refraction. In addition to regular sphere, cylinder, and axis, it produces an acuity map that provides information based on the Zernike polynomials for the lowerand higher-order aberrations. It also can produce a Bille aberration map looking only at the higher-order aberrations. The system provides quantitative data on RMS errors. Moreover, the user can also look at point spread function, Zernike coefficients, Hartmann-Shack images, and difference maps.

The WaveScan device also can serve as a confirmation tool for sphere, cylinder and axis, or manifest refractions. Finally, it is proving useful in the postoperative setting, where it can be used to identify and describe specific higher-order aberrations that may be consistent with a patient's subjective visual symptoms.

REFERENCES

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2.Panagopoulou S, Pallikaris IG: Wavefront customized ablations with the WASCA Asclepion workstation J Refract Surg 2000;17(5):S608-612.

3.Kraff C: Results of six-month wavefront-guided ablation studies for the correction of myopia using the VISX WaveScan system. J Refract Surg 2003; in press

4.Hardten D: Wavefront Analysis with the VISX WaveScan. Ophthalmology Times. Supplement. August 15, 2001

5.Harner, C: New VISX technology, Presented at the 2001 Annual Meeting of the American Society of Refractive and Cataract Surgeons.

6.Marcos M: Refractive surgery and optical aberrations. Optics and Photonics News. 2001;12(1):22-25.

7.VISX WaveScan Wavefront System Operator’s Manual, VISX, Inc., 2002

8.Liang J, Williams DR: Aberrations and retinal image quality of the normal human eye. J Opt Soc Am Acad 1997;14:2873-2883.

9.Thibos L: The prospects for perfect vision. J Refract Surg. 2000;16:S540-546.

10.Feldman S: Comparing refraction measurements. Cataract and Refractive Surgery Today. Supplement. November 2002

11.Koch D: The current state and future direction of wavefront science. EuroTimes. Supplement. September 2002

12.Rao SN, Epstein RJ: Early onset ectasia following laser in situ keratomileusis: Case report and literature review. J Refract Surg 2002; 18:177-184.

____________________________

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