Effects on visual function of approximations of the corneal-ablation profile during refractive surgery

Abstract

We offer an analysis that shows that the approximations made for the ablation depth during practical refractive surgery, in which the square-root terms are replaced by the first two terms of the series expansion, can limit the visual function of the observer by reducing the modulation transfer function (MTF). To simulate the refractive-surgical operation, we considered two groups of myopic patients with different ametropia who were emmetropized with different ablation profiles. We made the MTF calculations by taking the spherical aberration into account. In addition, a fuller analysis showed that these approximations limit the possibility of considering surfaces that are aspherical for reshaping the anterior cornea to optimize the observer's visual function.

Changes in corneal asphericity after laser in situ keratomileusis

“…In our case, at low frequencies (up to 6 cpd), the differences were not significant (PϾ.05). This is consistent with theoretical works 17 that show that variations in corneal asphericity affect the sensitivity of the subject primarily in the middle-and highfrequency ranges.…”

Hyperopic Q-optimized algorithms: a theoretical study on factors influencing optical quality

“…The axial length of the model was modified to provide the initial degree of hyperopia, from 1D to 5D, a range usually used in hyperopic surgery [11][12][13][14][15][16]. Eye models slightly modified to simulate ametropia have been used in the myopia simulations [2,28]. Zemax OpticStudio 16 (ZEMAX Development Corp. Bellevue, Washington, USA) was used for the computations.…”

“…In the case that we consider the eye to be a conic curve (described by radius and corneal asphericity), after simple computations [2,28] and imposing the condition s(0) = 0, we would have the following equation for the ablation depth, s(y):…”

Optimum corneal asphericity of myopic eyes for refractive surgery

“…Several practical [1] and theoretical works [2,[8][9][10][11] have proposed modifications to the Munnerlyn algorithm, incorporating information on corneal asphericity in order to optimize the visual function. Recently, Manns et al [2], based on the criterion of minimization of primary spherical aberration, theoretically deduced that the best post-surgical corneal asphericity ranges from Q ¼ À0.45 to Q ¼ À0.…”

“…In the present work, for the corneal ablation, an expression proposed by several different authors [2,9,11] is used, which takes into account the corneal asphericity by selecting this parameter as a criterion to optimize visual function. For this, we generate a broad range of myopes derived from a schematic eye and from average data of real myopic eyes.…”