by Guy M. Kezirian, MD, MBA, FACS
Limits on laser refractive surgery for myopic correction extend from several potential sources. Optics, ocular surface considerations, keratome / flap concerns and tissue availability all factor in to the decision. With modern lasers, however, the ablation design has reduced the importance of the optics. In other words, it is possible to leave a cornea with flatter central keratometry readings and still achieve good visual performance.
Geometrically, the term “prolate” refers to a surface that has the steepest slope (smallest radius) centrally. Most (but not all) virgin corneas are prolate in shape, although the amount varies. The average eye has a corneal asphericity of about -0.3 but there is a significant range around this value in normal eyes. (The actual number by Kiely et al.(1) was -0.26 ± 0.18 with a mean central radial curvature of 7.72 ± 0.27 mm, showing that some virgin eyes are actually oblate.)
The optical result of this in the average unoperated eye is that the corneal contributes positive spherical aberration to the wavefront. The lens does the opposite, and the lens effect is generally reciprocal to the cornea so that the optics are improved, as Michael Mrochen described some years ago (2). The amount varies with pupil dilation, with the net result that the optics vary with ambient illumination and our vision is much better in mesopic conditions than it would be if we were limited to spherical optics.
(Of course, in any single eye, these shapes are seldom perfect. Coma is an example of an aberration produced with off-center optics. The optical impact of coma is easily visualized by thinking of what happens with a decentered spherical aberration.)
The application of aspheric shapes to laser treatments was also described by Michael Mrochen, PhD (who is now in Zurich) and the WaveLight engineers in 2004 (3). The article is worth reading for every WaveLight surgeon. The goal of Wavefront-Optimized treatments is to change the central curvature of the cornea to effect the refractive change, while keeping the asphericity of the eye the same. I call this a “pure refractive treatment” which is not technically accurate but conceptually stands up. On topography, this looks like a larger OZ than is obtained using older lasers. Put differently, the “blue spot” after treatment goes all the way out to the nominal optical zone, rather than tapering to create a smaller effective optical zone. During treatment, it means removing more tissue from the mid-peripheral cornea. Optically, the corneal asphericity is changed, but the optical spherical aberration is not.
Working with an aspheric eye model, Manns showed that this can be done in a physiologic cornea for treatments up to about 6 D, working with a 6.5 mm OZ. After that, there isn’t enough tissue left to work with. We showed that both Mrochen and Manns were correct in the FDA Study that I ran for WaveLight to compare Wavefront-Optimized LASIK with Wavefront-Guided LASIK. On average, they did an outstanding job with the calculations. Wavefront-Optimized ablations can correctly be called aspheric, because the shape of the treatments is not round, but has a curvature that varies as it goes outward from the center.
With prior lasers that used mostly spherical ablation patterns, the amount of spherical aberration that was induced by a treatment was proportionate to the treatment amount. The old dogma to limit how much the cornea is flattened was not based on the flatter shape per se, but on the magnitude of the treatment that would result in such flattening, as that amount would result in very symptomatic spherical aberration.
With Wavefront-Optimized treatments, this is not the case. Induced spherical aberration is nearly nil up to about 7 D, and minimal beyond that. The benefit is seen in improved optics at night, with minimal night glare. Table 16 in the FDA study results (5) showed the impact, with 70% of eyes showing no change in glare with night driving after myopic LASIK, 9% somewhat worse, nobody much worse, and 20% improved. We also published a paper in 2003 about the relationship between glare and pupil size from our initial FDA study for myopic Wavefront-Optimized LASIK, and showed that there was no correlation between glare and pupil size if the refractive outcome is good with this laser. This is because minimal if any spherical aberration is induced by the treatment. (6)
As Manns (4) predicted, however, even with Wavefront-Optimized treatments, some spherical aberration is induced with higher treatment amounts, but the amount is small. Surgeons should consider this, of course, but it isn’t an issue in most eyes.
To return to the “how flat is too flat” question, it is important to understand that the shape of the front optic (the cornea) is only one piece of the optical system. Nearly any shape can be combined with other shapes to produce good vision, including a totally flat cornea, or even (to employ reductio ad absurdum) a concave shape. The prior recommendations not to flatten beyond a predicated K reading of 32 or 34 or 36 D (depending on the speaker) were based on concerns of inducing spherical aberration and did not extend from any inherent issues with corneas becoming “too flat”.
(Extrapolating minds beware – this is not true of hyperopia, where “too steep” definitely exists, and can result in problems with apical drying due to inadequate tear film coverage. Most surgeons won’t steepen corneas beyond 50 D and many won’t go beyond 48 D).
I recognize that this is a potentially vast topic and hope this brief overview is helpful. Every refractive surgeon should be aware of these concepts as they are fundamental to our work. Every patient should be aware of the improved capabilities of modern lasers, and should carefully avoid centers that use older technologies.
1. Kiely PM, Smith G, Carney LG. Meridional variations of corneal shape. Am J Optom Physiol Opt. 1984 Oct;61(10):619-26. PubMed PMID: 6496638.
2. Mrochen M, Jankov M, Bueeler M, Seiler T. Correlation between corneal and
total wavefront aberrations in myopic eyes. J Refract Surg. 2003
Mar-Apr;19(2):104-12. PubMed PMID: 12701714.
3. Mrochen M, Donitzky C, Wüllner C, Löffler J. Wavefront-optimized ablation profiles: theoretical background. J Cataract Refract Surg. 2004 Apr;30(4):775-85. PubMed PMID: 15093638.
4. Manns F, Ho A, Parel JM, Culbertson W. Ablation profiles for wavefront-guided
correction of myopia and primary spherical aberration. J Cataract Refract Surg.
2002 May;28(5):766-74. PubMed PMID: 11978453.
6. Kezirian GM, Stonecipher KG. Subjective assessment of mesopic visual function after laser in situ keratomileusis. Ophthalmol Clin North Am. 2004 Jun;17(2):211-24, vii. PubMed PMID: 15207563.