It was assumed originally that a uniformly distributed pulsed excimer laser profile would ablate a uniform thickness of the cornea as it does with isotropic plastic materials, and the result of ablation would be the same as the original surface profile translated to some depth that was a function of total fluence applied.
34 However, it was noted early that the central cornea appeared to be under-ablated significantly, creating a severe asphericity termed “central islands.”
35 These central irregularities were correlated with visual disturbances, such as decreased best corrected visual acuity, glare, and halos.
36,37 Excimer-induced asphericity has been the accepted reason that the original Munnerlyn formula works well on a plastic sphere, but not as well on a human cornea. Increased asphericity has been shown following commercial excimer myopic correction. Before the use of central overablation algorithms, the incidence of clinically evident central asphericity measured by Placido ring topography in human eyes was between 50% and 88%.
38 Empiric adjustments to the ablation algorithm were made that mostly solved the problem at the time, without a clear understanding of the cause.
20,39–41 Even with these interventions, research has noted that the largest contributor to post-ablation ocular aberrations, by a factor of 4, is spherical aberration.
42 Despite the empiric changes to the excimer ablation algorithms, irregularities persist, albeit less frequently and with less severity. The major causes in symptomatic patients with poor visual outcomes are linked to persistent central corneal irregularities with associated aberrations, mostly spherical aberration.
42–44 Also, excimer laser ablations still lead to a decrease in the quality of vision, particularly at night with decreased contrast sensitivity.
43,45,46 Even recent studies highlight the postablative change in asphericity and the need to understand better its etiology.
23,47–50