الفهرس 
Corneal anatomy and opticsOnly 14 pages are availabe for public view
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Abstract
Sphere, cylinder and axis are the clinical refraction components that describe what many refer to as lower order aberrations. Other optical aberrations in the visual pathway of the eye, such as coma and spherical aberration, which used to be referred to as corneal irregularity or irregular astigmatism, are now collectively called higher-order aberrations
These HOA could be measured by different devices and described mathematically by Fourier transformation or, more commonly, by using a series of polynomials named Zernike polynomials (Schallhorn et al., 2008)
Different ablation profiles are designed to achieve a perfect optical system:
The conventional ablations is 1st FDA approved using Munnerlyn formula, the laser profile is based on subjective and objective measurements of refraction. The pattern is designed to eliminate spherical and cylindrical refractive error LOAs With the drawback of ignoring aberrations of the eye, resulting in unwanted effects such as glare and halos, thus degrading postoperative visual performance. (Schallhorn et al., 2008)
Wavefront-optimized LASIK is a treatment profile designed to reduce or eliminate the induced spherical aberration of conventional LASIK. The wavefront-optimized treatment is based on a spherocylindrical correction that is adjusted by an internal algorithm to remove additional tissue in the periphery of the ablation zone, thereby creating a more prolate corneal shape. (Schallhorn et al., 2008)
Corneal-topography-guided ablation has been attempted on patients with regular and irregular astigmatism, decentered ablations, and central islands. Once the topography is taken, data are copied and the ablation profile is calculated on site using a special software called TopoLink ,The results have been encouraging with central islands, regular astigmatism, and decentered ablations, but the system requires refinement with irregular astigmatism. (Pangtey et al, 2005).
Wavefront-guided (WFG) LASIK, also called custom LASIK, is a variation of the surgery in which the excimer laser is instructed to ablate a sophisticated pattern based on measurements from an aberrometer
The goal of WFG LASIK is to achieve a more optically perfect ablation based on all of the optical aberrations measured with the wavefront aberrometer, not just sphere and cylinder. Achieving this goal depends on appropriate patient selection, high-quality wavefront data, successful surgery, and accurately predicting and managing the changes that occur during healing.
Custom LASIK is a corneal refractive procedure that requires more attention to detail than conventional LASIK. In addition to standard preoperative exams and measurements, important factors to consider include the capture of high-quality wavefront images over the entire low-light entrance pupil and consistency between the manifest and wavefront refractions. As with conventional LASIK, the surgeon must observe proper precautions with regard to corneal topography, pachymetry, and other measures of ocular health.
Incorporating advances in wavefront analysis, flap creation, eye tracking, wavefront registration, and accurate centration, wavefront LASIK offers significant improvements in objective and subjective visual outcomes. It has revitalized corneal refractive surgery with unprecedented success. Although still elusive, the concept of attaining super-normal vision is an intriguing possibility with wavefront-guided LASIK and one that is under active investigation.
WFG LASIK is both safe and effective. Compared with conventional LASIK, WFG surgery seems to result in improved outcomes. This is particularly true for contrast sensitivity, night vision, and visual symptoms. (Schallhorn et al 2008)
No doubt, all ablation profiles including the conventional are of good, we cannot, hence choose one as the universal perfect, otherwise, based on the great variability between individuals, in the eye as an optical system and in the visual requirements and expectations from refractive surgeries, we can choose the ablation profile suiting each individual according to his eye’s nature & visual needs. (Schallhorn et al., 2008)