Your doctor may have ablated so much tissue in your first surgery that you may be stuck with 'induced spherical aberration' for life.

There are variations among the companies in terms of how good they are and no-one today has a radial compensation function that is 100% effective.” Dr. Holladay noted that while wavefront and topography-guided systems appear to induce the least amount of spherical aberrations, standard LASIK with lasers with highly accurate radial compensation functions do nearly as well in that respect. Moreover, most of the wavefront and topography-guided treatments still induce spherical aberration as the amount of correction increases, if the radial compensation functions are not sufficient. “That means that they need the radial compensation function to be improved.

Why else does the functional optical zone differ from the treated zone? Variation in the original steepness of the cornea is one factor; the nature of the engineered excimer beam is another. The beam's function to ablate corneal tissue will vary according to whether it affects the relatively flat center of the treatment zone or the slope of the prolate cornea, where its effect will be less and contribute to "that blend within."

Mrochen et al. [11] proposed that the increase of asphericity with the refractive surgery procedure is due to changes in the ablation efficiency as the laser spot moves from the center to the periphery of the cornea. As the angle of incidence α increases, both the reflected energy and the illuminated area increase and therefore, the ablation depth per pulse decreases.

Customized ablation algorithms already compensate for induction of spherical aberrations by placing additional pulses in the periphery of the ablation profile based on previous work with aberrations and RMS values.

The problem with all excimer lasers on the market today is twofold. First, the engineers assumed that the cornea is spherical rather than prolate. Second, they assumed that their job was to reshape a relatively steep sphere into a relatively flat sphere, rather than to reshape a steep prolate into a flatter prolate.

As a result, excimer lasers actually reshape prolate corneas into what is known as an oblate. An oblate is shaped like the cross-section of a hamburger bun, flatter in the center but steeper in the periphery (see figure 3). This shape is actually optically worse than a sphere, because now the peripheral rays are bent even more powerfully than in the periphery of a sphere, causing even more pronounced spherical aberration when the pupil dilates.

Unlike the haloes that result from a too-small optical zone, this problem affects every patient who undergoes an excimer laser procedure to some extent. Like the halo problem, the oblate dilemma is worst for patients whose pupils dilate widely at night.

The first step is to make sure you take an accurate measurement of pupil size. As I mentioned, the larger the pupil, the more optical aberration patients will experience from the oblate cornea effect. Large-pupil patients are also much more likely to see haloes at night, because their pupil exceeds the ablation zone.

Though contrast sensitivity loss is undesirable, especially in a procedure designed to improve vision, the effects are not debilitating, and many patients are very willing to make the trade-off. We have a responsibility to inform patients of this potential problem, however. We also have a responsibility to continue to push laser manufacturers to improve their software so that we can reduce these adverse effects.