الفهرس 
INTRODUCTION
Aim of the workOnly 14 pages are availabe for public view
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Abstract
Astigmatism is an important and often visually significant optical aberration. In an optical system with astigmatism, rays that propagate in 2 perpendicular planes result in 2 different focal lines.
For years, corneal power calculation has been based on anterior corneal measurements, assuming a fixed posterior–anterior curvature ratio to estimate both the spherical and astigmatic contributions of posterior corneal power. The new options of measuring the power of the posterior corneal curvature may allow us to more precisely measure corneal power and achieve even better results in cataract and refractive surgery.
The posterior surface has greater toricity than the anterior surface. Using various methodologies, studies have reported mean values for posterior corneal astigmatism that range from -0.26 to -0.78 D.
Posterior corneal astigmatism should be considered when trying to optimize visual outcomes in patients with astigmatism, so as to increase predictability and enhance refractive results, as ignoring posterior corneal astigmatism could cause unanticipated outcomes in eyes having toric IOL implantation
The aim of this study was to evaluate the importance of the posterior corneal surface in comparing total corneal astigmatism to refractive astigmatism in different groups based on age, sex, refractive error, corneal thickness & corneal curvature
503 eyes of 273 patients were included in this study. The patients were those seeking LASIK correction for their refractive errors. They were classified into different groups considering age, sex, refractive error, corneal thickness & corneal curvature. The difference between refractive astigmatism and toatal corneal astigmatism were calculated considering two parameters. The first parameter was the cylinder power difference which considered outside accepted range if more than 1 diopter. The second parameter was the cylinder axis which considered unaccepted if > 15°.The percentage of difference were calculated and compared between groups.
Conclusion:
This study concluded that, difference between corneal astigmatism and refractive astigmatism, regarding cylinder power & axis, may be affected by several factors. The percentage of cylinder power difference >1D was higher in high myopia, hyperopia, males & age older than or equal to 40 years groups respectively. While the lowest percentage was at mild myopia, K2 less than 44D, females & corneal thickness groups respectively.Regarding the percentage of axis difference >15°, it was higher at hyperopia, high myopia, K2 more than 46D & corneal thickness from 500 um to less than 550 um groups respectively. While the lowest percentage was at females, mild myopia, K2 44-46D, females & age more than or equal to 40 years groups respectively
Recommendations:
Further studies, with larger sample size and more methods for correlation of axis to cylinder power, are needed to confirm the findings of this study in attempt to predict refractive surprises after doing lens surgeries for patient with previous refractive corneal surgeries.