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EF Jarade, FC Abinader, KF Tabbara; IntraOcular Lens Power Calculation Following LASIK . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4141.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Accurate intraocular lens power (IOLP) calculation following LASIK depends on accurate K reading (Kr). Most equipment measure with accuracy the anterior corneal curvature (Ra) only, and use a modified index of refraction (1.3375) to compensate for the posterior corneal surface power depending on a constant ratio between the anterior and posterior surface curvature of the normal cornea. Following LASIK, this ratio is disrupted and the Kr is overestimated with subsequent low IOLP calculation. The purpose of this study is to find the new corneal relative index of refraction (rN) following LASIK in order to be used for accurate K-reading. Methods: Patients that underwent myopic LASIK were divided into two groups (group-A and group-B), in each group patients were divided into four subgroups according to the amount of LASIK ablation.in each subgroup of group-A, Kr was measured by the clinical history method (CHM)and rN was determined by using the formula:[Kr = (rN-1)/Ra]. Then,in group-B, Kr was measured in each subgroup by using rN in the same formula after measuring the Ra, CHM and automated kr (auto-k). Results: A total of 332 eyes that underwent myopic LASIK were included in this study.There were 137 eyes in group-A (47 eyes in subgroup I(<-4D),30 eyes in subgroup II (-4 to -8D),30 eyes in subgroup III (-8 to -12D) and 30 eyes in subgroup IV (≷-12D)) to determin rN in each subgroup and 195 eyes in group-B (40,59,62 and 28 eyes in subgroups I,II,III and IV respectively) to measure Kr by using rN ,CHM and auto-K . In group-A rN was 1.3355,1.3286,1.3237 and 1.3172 in the four subgroups respectively. In group-B the mean of Kr measurement by using rN,CHM and auto-K were 40.33(SD ±1.68),40.33(SD ±1.67)and 40.54(SD ±1.69)respectively in subgroup I; 37.96(SD ±1.26),38.03(SD ±1.38) and 38.98(SD ±1.28) respectvely in subgroup II; 35.78(SD ±1.75),35.84(SD ±1.85) and 37.29(SD± 1.83) respectively in subgroup III and 34.03(SD ±1.67),34.15(SD± 2.1) and 36.21(SD ±1.77) respectively in subgroup IV. In each subgroup of group-B, Kr measured by using rN and CHM were statistically similar (p≷ 0.45), but different from automated Kr (p<0.000). Conclusion: The use of the new corneal relative indexes of refraction (rN) following LASIK in each subgroup gives an accurate Kr. This may be accomplished by changing the index value in the equipment algorithm for Keratometry and Videokeratography according to the amount of LASIK ablation.This will lead to accurate, simple and objective measurement of Kr following LASIK with subsequent accurate IOLP calculation.
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