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Abstract
Corneal refractive surgery is based on the change in corneal curvature to compensate for refractive errors of the eye. After many mechanical approaches, such as radial keratotomy, myopic and hyperopic keratomileusis, and astigmatic keratotomies, ablative procedures using the excimer laser have become the preferred techniques, and in the past few decades, corneal excimer laser refractive pro¬cedures have been the most popular surgical alternative for the correction of spherocylindrical errors to obtain emmetropia and increase uncorrected visual acu¬ity (UCVA).1 The safety, predictability, and efficacy of these techniques have been demonstrated particularly for the cor¬rection of myopia, hyperopia and myopic astigmatism.2,3 It is mainly the submicron precision and the high repeatability of the ablation of the cornea accompanied by minimal side effects that guarantees such success.4
Concepts of Conventional Laser Refractive Surgery
In photorefractive keratectomy (PRK), the corneal epithelium is removed, and in laser in situ keratomileusis (LASIK) the epithelium is left intact, and a hinged stromal flap is created and folded back. The exposed stroma, in both procedures, is then photoablated using an excimer laser.
In myopia, the corneal stromal tissue is removed so that the curvature of the central cornea is flattened to compensate for the excessive refractive power or longer axial length of the myopic eye, which results in decreased refractive power, with the resultant focusing of the image further back within the eye and onto the retina.5
In hyperopia, the corneal stromal tissue is removed in an annular manner, so that a steepening of the central anterior corneal surface is obtained with creation of a peripheral annular blend zone, so that the central corneal power increases enough to pull the image back and focus it onto the retina.6
In astigmatism, the ablation pattern is directed towards flattening the steep meridian, rendering the corneal surface rotationally symmetric, and bringing the image into one focused location on the retina.
Although the specific algorithms used by laser system manufacturers to design conventional ablation profiles are proprietary,7 with excellent outcomes regarding the uncorrected visual acuity (UCVA), they are generally derived from the pioneering theoretical models of Munnerlyn et al.1 which assume both thin lens theory and paraxial optics. These models predict the change in corneal power by considering the initial unablated and the final ablated corneal surface to be two spherical surfaces, with a single but different radius of curvature, and with the assumption that preoperative corneal surface has a single radius of curvature.8
The Aspheric Human Cornea
The normal human cornea is not spherical, but rather like a conoid,9 and, despite its shortcomings, modeling the corneal shape in cross section as a conic section is a better approximation and has been widely used10-13 since its introduction by Mandell and St. Helen in 1971.14 Historically, corneal asphericity and contour measurement dates back to the 19th century, when the cornea was first described as a perfect sphere.15 In 1860, the German optician Knapp postulated that the cornea was not a sphere, but rather a perfect spherocylinder.16 This picture of the corneal optical contour was again revised in 1929 when Berg noted the cornea to be a prolate asphere with a mean ellipsoid that flattens towards the periphery, in an attempt, teleologically, to reduce the spherical aberrations of the eye.17
The asphericity of the cornea is defined by the shape factor of the conic section that approximates it most closely. A negative Q value indicates a prolate profile with flattening toward the periphery of the surface. A positive Q value, on the other hand, indicates an oblate surface, the slope of which is steeper with increasing distance from the vertex.14 Most normal corneas conform to a prolate ellipse and flattens from the center to the periphery (negative asphericity), with the central part of the cornea having a stronger curvature than the periphery or, in other words, the refractive power of the outer corneal surface decreases from center to periphery. In a minority of corneas, however, the overall shape is oblate and steepens from the center to the periphery (positive asphericity), with the refractive power of the outer corneal surface increasing from center to periphery. Although an asphericity value of approximately -0.26 is generally accepted as the mean cor¬neal shape, the range of measured asphericity values in native corneas is from +0.50 to -0.88.14,15,18,19
Impact of refractive Surgery on Corneal Asphericity
In clinical discussions today, it is commonly assumed that the cornea changes from its natural prolate aspheric optical architecture after excimer laser photoablation. Changes in corneal asphericity can explain some of the induced spherical aberration because when the radius of curvature of a cornea is constant, corneal asphericity is a predominant factor affecting the amount of the aber¬ration.20 The clinical results of a previous study 21 suggest that within the treatment zone, the cornea loses its natural negative asphericity and becomes more oblate after myopic LASIK and PRK, and an increased negative asphericity after photorefractive keratectomy (PRK)22 and laser in situ keratomileusis (LASIK)23 for hyperopia.
A reduction in the natural negative asphericity of the cornea may contribute to a less than optimal visual outcome with possible subjective visual sequelae of glare or halo.24 Investigators have mathematically demonstrated that the prolate asphericity should not change.8 Therefore, maintaining the natural prolate contour may help reduce postoperative optical side effects and has led some investigators to attempt nominally aspheric treatments to optimize visual outcomes.25
The theoretical and actual change in corneal aspher¬icity after refractive surgery has an important clinical significance. The cornea is the main refractive element of the eye, an optimal aspheric corneal shape is important for min¬imizing the eye’s overall aberration profile and maximizing retinal image quality. Moreover, the native corneal asphericity partly corrects the natural spherical aberration of the bundle of light rays entering the eye.24