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Introduction
Wavefront Technology
Computer-Assisted Videokeratography
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Wavefront Corneal Analysis

Charles A. Barsam, MD · Peter J. McDonnell, MD

Introduction

Before the advent of wavefront technologies, refractive error was measured by only three parameters: sphere, cylinder, and axis. Wavefront analysis has facilitated the evaluation of the eye's entire refractive status to discern the higher-order optical abnormalities of the eye beyond just sphere and cylinder. Correction of such aberrations should allow for finer retinal image resolution and thereby allow individuals to perceive the world in higher contrast and resolution.

Wavefront Technology

A wavefront relates to light's property of moving in a unidirectional manner through space. Light waves emanate from a single point source in all directions as a sphere, and the line that connects the points upon the surface of this propagating wave is called the wavefront. As this wave encounters the eye's anterior tear film and passes through to the retina, it is bent and distorted. The sum of the deformation represents the unique refractive error of the eye. The emmetropic eye possesses a wavefront that is a plane, which is perpendicular to the line of sight.^1,2 The wavefront aberration is the difference between the measured eye's wavefront and that of the ideal optical system.¹

The common methods of wave aberration detection and reconstruction use light ray tracing technology. The Hartmann-Shack-inspired devices are the most commonly used of the wavefront aberrometers. These devices project light into the eye; the light converges onto the retina and is then reflected out of the eye.^1,3 As this light emerges out of the optical system, it is plotted and then compared to a calculated reference point for an ideal optical system. It is the deviation of the evaluated eye from the ideal that allows for calculation of the wavefront. Another commonly used wavefront analyzer is the Tscherning aberrometer. This instrument analyzes incoming light. As the light rays emitted from this device travel through the optical planes of the eye, they form a unique pattern on the retina that is captured and analyzed.¹ The distortion of this pattern from the ideal forms the wavefront aberration.

The shapes of the wavefronts that are generated from these aberrometers represent the total of the monochromatic aberrations of the eye.¹ Monochromatic aberrations are defined in clinical terms - spherical aberration, astigmatism, and coma - and can be further broken down into Zernike polynomials, which are mathematical terms that describe the geometrical pattern that elucidates the total wavefront deformation.²

The diminution of vision under scotopic conditions is a frequent complaint of patients undergoing refractive surgery. It is increasingly apparent that these common complaints of glare and halos at night likely represent the surgically induced increase in higher-order optical aberrations; they do not seem to be the byproducts of the surgical incision.⁴ By performing wavefront analysis of refractive surgery on patients both pre- and postoperatively, an increase in higher-order aberrations following surgery can be identified.³ The aberrations are detected by wavefront scanners and characterized by the Zernike polynomials. Current refractive surgical procedures are effective in eliminating the so-called lower-order spherical and cylindrical components of the eye's refractive error, but they do not address the higher-order aberrations.

Conclusion

The implementation of the wavefront technologies to guide an individualized or "custom" corneal ablation represents the next frontier in refractive surgery. By utilizing wavefront aberration to guide surgery, custom laser corneal ablations may become free from the vision-degrading effects of higher-order aberrations.

Preliminary studies have examined the effects of wavefront-guided corneal ablations in refractive surgery. In the first major study, Drs. Seiler and Dastjerdi reported the 3-month postoperative data of wavefront-guided ablations performed on 35 eyes of 28 patients.² The authors of this study demonstrated that only 22.5% of patients experienced a reduction in the higher-order root-mean-square value, which represents a means of quantifying the divergence of light as it passes through the optical system. Furthermore, at 3 months postoperative, the patients treated with wavefront-guided ablations experienced an increase in total optical aberrations by a factor of 1.44. The authors noted, however, that this increase in the sum aberration is one order of magnitude less than what would have been expected with conventional laser ablations. The patients treated with wavefront-guided ablations, unlike the patients who were treated with conventional ablations, did not experience any significant diminution of best visual acuity in either low-contrast or glare lighting conditions. The authors of this study suggest that the elimination of a component of the higher-order corneal aberrations serves to maintain visual acuity during pupillary dilation. Also, light entering the eye with an oblique angle of incidence in conventional ablations seems to degrade the quality and quantity of vision.

Dr. McDonald has also published the results of a small clinical trial aimed at evaluating custom-designed and conventional ablations.⁵ In this study, bilateral LASIK was performed on 20 eyes with each eye randomly receiving either the custom or conventional ablation. The visual acuity results were nearly identical for each group, yet the majority of eyes treated on the custom-ablation platform demonstrated a diminution of higher-order aberrations.

The data in support of these procedures are thus far minimal. It appears, however, that there is a decrease in the measurable higher-order aberrations for eyes treated on the wavefront platform in these preliminary studies. Further studies are needed to evaluate the long-term efficacy of these surgical procedures, yet this modality of evaluating the sum of the eye's optical aberration shows great promise in assisting the treatment of ametropia.

References

1. Maeda N. Wavefront technology in ophthalmology. Curr Opin Ophthalmol. 2001;12(4):294-299.
2. Seiler T, Dastjerdi MH. Customized corneal ablation. Curr Opin Ophthalmol. 2002;13(4):256-260.
3. Yo C. LASIK, Future Advances. E-Medicine [serial online]. Available at: eMedicine.com . Accessed July 31, 2003.
4. MacRae SM, Williams DR. Wavefront guided ablation. Am J Ophthalmol. 2001;132(6):915-919.
5. McDonald MB. Summit-Autonomous CustomCornea laser in situ keratomileusis outcomes. J Refract Surg. 2000;16(5):S617-618.