Spherical Aberration and Its Symptoms

To view the tables and figures related to this article, please refer to the print version of our May issue, page 55.

Current excimer photoablative procedures have become a popular surgical option for correcting refractive errors, and ophthalmologists worldwide perform a rapidly increasing number of laser refractive surgeries each day. Recently, new technology capable of measuring the pattern of optical wave aberration (including higher-order aberrations) showed that, although laser refractive surgery reduces or eliminates conventional refractive errors, it generally induces higher-order aberrations.1-4

NEW ATTENTION TO ABERRATIONS
The goals of today's refractive surgery procedures have changed. Rather than merely shift the location of the image formed by the eye onto the retina, surgeons also now attempt to ensure the image formed on the retina is not aberrated, especially in eyes with large pupils. The amount of spherical aberrations increases following standard laser treatments to correct myopic and hyperopic refractions.5,6 Conventional laser ablation modifies corneal asphericity, a fact that may explain the observed increase in spherical aberration.

WHAT IS SPHERICAL ABERRATION?
Spherical aberration is a fourth-order aberration that varies with the radial distance from the center of the pupil. Therefore, an optical system may have no refractive error in the center of the pupil and an increasing error in the annular zones that surround the pupil's center. The resultant image may appear sharp for small pupil diameters but degrade as the pupil expands. Figure 1 shows the effects of spherical aberration on paraxial and marginal rays. The former travel close to the optical axis and are minimally affected by the aberration. These rays, therefore, focus at the rear focal point F of the eye. Marginal rays pass through the pupil's edge. In the presence of spherical aberration, they will not focus at F but at the marginal focus M. The level of longitudinal spherical aberration, measured in diopters, equals the difference in vergence between the marginal and paraxial rays.

Spherical aberrations can be positive or negative. Normal preoperative eyes usually contain low amounts of positive spherical aberration, which has a central accelerated area (focus of hyperopia) surrounded by a retarded ring of light (annulus of myopia). With normalization in the far periphery, the three-dimensional wavefront profile resembles a flat sombrero (Figure 2). A small percentage of the normal patient population can have negative spherical aberration with an inversion in the shape pattern. Negative spherical aberration has a central retarded area (focus of myopia) surrounded by an accelerated ring of light (annulus of hyperopia). We describe this pattern as a flipped-over sombrero hat or a doughnut-shaped profile (Figure 3).

SPHERICAL ABERRATION MAGNITUDE AFTER LASIK
Patients who undergo myopic treatments develop an oblate corneal pattern.7 The wavefront pattern shows a flattening or concavity to the otherwise bowl-shaped wavefront of myopia. Analyzing the wavefront of these myopic treatments reveals that the spherical aberration increases in numerical value and size on the 3-D wavefront profile.

For a hyperopic ablation, the surgeon removes tissue from the peripheral area, thereby flattening this region and producing an increased central corneal curvature as a final result. Patients who undergo hyperopic treatments have an accentuated prolate corneal pattern that results in a dampening or inversion of the otherwise hill-shaped hyperopic wavefront.8,9 When we analyze the spherical aberration pattern of these eyes, the magnitude of spherical aberration decreases, and the shape pattern becomes inverted (the flipped sombrero or doughnut shape).

CLINICAL STUDY
We analyzed 60 healthy virgin eyes with a Shack-Hartmann aberrometer (LADARWave Wavefront System; Alcon Laboratories, Inc., Fort Worth, TX) and found the mean value of spherical aberration for a normal population is 0.36 mm (SD = 0.31) root mean square (RMS). Our team conducted another study analyzing 105 post-LASIK eyes and found a spherical aberration mean value of 1.27 mm (SD = 0.60) RMS. This value agrees with other papers in the literature. One study published in 2001 analyzed 14 eyes following standard LASIK procedures and verified that patients' total aberrations and corneal aberrations increased by a factor of 1.92 (total) and 3.72 (corneal) on average. Spherical aberration was the main higher-order aberration created by standard LASIK. Interestingly, anterior corneal spherical aberration increased more than the amount of total spherical aberration, a finding that suggests a change in the spherical aberration of the posterior corneal surface.5

Another study published in 2002 reported a significant increase in spherical aberration after LASIK, even when analyzing pupils as small as 4 mm.10 Other investigators analyzed spherical aberrations induced after myopic LASIK and found an increased factor of 3.9.2 Still another study published in 2002 examined 100 eyes that underwent myopic LASIK correction, and found that spherical aberrations increased significantly when compared with the preoperative value for a 3-mm pupil and a 6-mm pupil. There was a significant correlation between the amount of achieved myopic correction and the surgically induced changes in spherical aberration.11

SYMPTOMS CORRELATED WITH SPHERICAL ABERRATIONS
Standard laser refractive surgery performed on patients with large scotopic pupil sizes is associated with nighttime vision problems such as halos.12 The increased amount of higher-order aberrations after standard LASIK is consistent with the relatively common patient comment, “I can read 20/20, but my vision is not as good as it was before.”13

We analyzed 105 eyes that underwent LASIK correction and correlated their symptoms with higher-order aberrations. Our analysis of optical symptoms and measured aberrations for a scotopic pupil size showed a statistically significant correlation between higher-order aberrations and glare (P=.041) as well as starburst (P=.004). When we broke down these aberrations into individual Zernike components, spherical aberration was the predominant cause, with a statistically significant correlation to glare (P=.010) and starburst (P=.014). Halos seemed to be associated with spherical aberration for the scotopic pupil size (P=.053). Table 1 shows the relationship of spherical aberration and coma with patients' symptoms.

SPHERICAL ABERRATION PREVENTION AND CORRECTION
Surgeons must exercise care when treating eyes with larger scotopic pupils, especially if the procedure is expected to induce higher levels of spherical aberration (patients with large pupils will experience more symptoms with higher levels of spherical aberration). Customized laser ablations attempt to minimize these symptoms by more effectively avoiding laser-induced spherical aberrations. The ideal ablation profile for correcting refractive error without generating spherical aberration is to reshape the cornea with a lesser radius of curvature in the midperiphery rather than in the center. This difference in asphericity corrects the spherical aberration of the eye, because the flatter surface will cause less refraction of the peripheral rays.14

Surgeons have attempted to correct spherical aberrations by increasing both the blend zone and the ablation in the midperiphery and periphery of the cornea. One study published in 2002 analyzed the postoperative reduction in spherical aberration by using software featuring an aspheric algorithm that increased the ablation in the midperiphery. The researchers found that spherical aberrations were reduced or maintained with this new technology.15

In the same year, other investigators tried to reduce spherical aberrations with a compound ablation. This technique consisted of treating myopia by increasing the preoperative myopic sphere by 25% and applying a hyperopic ablation of 25% of preoperative myopic sphere to theoretically increase the optical zone and eliminate spherical aberration. The results were frustrating; they actually increased the amount of optical aberrations.16

Customized wavefront ablations have large optical and blend zones in order to compensate for and treat spherical aberrations. The initial results with customized ablations showed that, even if spherical aberration is not eliminated, its induction is significantly less than with standard laser ablation. Patients noted better visual quality with the eye that underwent wavefront-guided correction.9

IN SUMMARY
Spherical aberration is an optical complication of laser vision correction that results in the visual symptoms of glare, starburst, and halos. They can be minimized by proper patient selection (avoiding patients who have large pupils and require high myopic corrections). However, customized laser ablation can safely treat even these patients by minimizing induced spherical aberration. It can also treat symptomatic, induced spherical aberration in previously treated eyes.

Maria Regina Chalita, MD, is a refractive surgery fellow at the Cole Eye Institute, Cleveland Clinic Foundation, in Cleveland. She holds no financial interest in any product or technology mentioned herein. Dr. Chalita may be reached at (216) 444-8158; chalitm@ccf.org.
Ronald R. Krueger, MD, MSE, is Medical Director of the Department of Refractive Surgery, Cole Eye Institute, Cleveland Clinic Foundation, in Cleveland. He receives travel support and research funding from Alcon Laboratories, Inc. Dr. Krueger may be reached at (216) 444-8158; krueger@ccf.org.
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