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The refractive power of the human eye depends on the power of the cornea and the lens. Cornea one of the Important parts of IOL Calculation to achievement of true IOL Power and patient satisfaction  

Manual Keratometry and Placido based corneal topography are the most frequently used methods in Measurement the corneal power for calculation of intraocular lens (IOL) power. Although both instruments have proved to be fairly accurate in determining the refractive power of regular and unoperated corneas, they have limitations in measuring corneas that have irregular astigmatism and in corneas that have had refractive surgery.

With keratometry or topography the cornea is not regular or symmetrical in some areas

Common Causes: Irregular Cornea

Keratoconus • Contact lens-induced corneal warpage • Corneal transplants (Post-penetrating keratoplasty) • Post refractive surgery • Pellucid marginal degeneration • dystrophy, trauma

Corneal Pterygium, Previous Keratorefractive surgery Like Radial Keratotomy and corneal scarring. That's are challenging in true measurement of corneal power for IOL Power calculation.

One of the common corneal irregularity sources is Keratoconus, Keratoconus (KCN) is bilateral corneal ectasia characterized by progressive thinning and conical protrusion of the cornea. IOL power calculation in KCN patients unpredictable and the most challenging part of preoperative considerations. calculating an accurate IOL power, it is imperative to precisely determine K and AL readings regardless of which calculation formula is used.