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    New approach to corneal power estimation in eyes with previous corneal refractive surgery and keratoconus eyes

    The use of a keratometric index to estimate total corneal power calculation is imprecise, especially in corneas with keratoconus or after myopic laser refractive surgery. The errors can be reduced by using an adjusted keratometric index, consisting of a variable keratometric index that depends on the radius of the anterior corneal surface.

    The calculation of the exact value of corneal power is a key factor in obtaining a precise calculation of the power of IOL to implant in cataract surgery. The anterior and posterior surfaces of the cornea contribute to its total power; therefore, for very accurate calculation of corneal power, the radii of curvature of the anterior and posterior corneal surfaces should be considered as well the central corneal thickness and the true refractive index of the cornea. With all these parameters, the Gaussian corneal power can be calculated assuming paraxial optical conditions.

    In clinical practice, total corneal power is usually calculated from the radius of curvature of the anterior corneal surface only. In this context, the concept of keratometric index (nk) was developed, assuming a corneal model consisting of a single spherical surface with the radius of curvature of the epithelial surface, and a fictitious index of refraction that provides a correction for this simplified model. The simplification assumes that the contribution of the posterior corneal surface to the ocular refractive power is limited, but this may be untrue when anomalous or surgically modified corneal curvatures are present. Our research group demonstrated that the keratometric estimation over- or underestimated the Gaussian corneal power in a range from –1.12 D to +1.80 D in the normal healthy eye.1 Furthermore, the use of keratometric corneal power with the classical keratometric index of 1.3375 was found to induce a maximum underestimation of –3.01 D in IOL power calculation in normal eyes with cataract.2 Therefore, the best approach to obtaining the corneal power is to measure anterior and posterior corneal curvature and to perform a Gaussian calculation.

    Adjusted keratometric index

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