Intraocular Lens Power Calculation Following LASIK: Determination of the New Effective Index of Refraction

Abstract

PURPOSE: To determine the new corneal effective index of refraction (rN) following LASIK to be used for accurate keratometry reading (K- reading). METHODS: A total of 332 eyes that underwent myopic LASIK were divided into two groups (group A [n=137] and group B [n=195]). In each group, patients were divided into four subgroups according to the amount of spherical equivalent refraction of myopic LASIK ablation (subgroup 1 [<-4.0 D], subgroup 2 [-4.0 to <-8.0 D], subgroup 3 [-8.0 to -12.0 D], and subgroup 4 [>-12.0 D]). In each subgroup of group A, K- reading was measured by the clinical history method and the new corneal effective index (rN) was determined using paraxial formula: (Kreading=(rN-l)/Ra), where Ra is the radius of curvature of the anterior corneal surface. In group B, the anterior radius of curvature of the cornea was determined by automated K- reading, and K- reading was measured in each subgroup using the new effective index in paraxial formula, clinical history method, and automated K- reading. RESULTS: In group A, the new effective index of refraction was 1.3355, 1.3286, 1.3237, and 1.3172 in the four subgroups, respectively. In group B, the mean K- reading measurements using rN in paraxial formula, clinical history method, and automated K-reading were: 40.33±1.68 D, 40.33±1.67 D, and 40.54±1.69 D, respectively, in subgroup 1; 37.96±1.26 D, 38.03±1.38 D, and 38.98±1.28 D, respectively, in subgroup 2; 35.77±1.75 D, 35.84±1.85 D, and 37.29±1.83 D, respectively, in subgroup 3; and 34.03±1.49 D, 34.15±1.84 D, and 36.21±1.59 D, respectively, in subgroup 4. In all subgroups of group B, the results of K-reading obtained using the new effective index of refraction were statistically similar to the results obtained by clinical history method (P>.05). Automated K-reading statistically overestimated the K-reading values in subgroups 2, 3, and 4 of group B (P<.001). CONCLUSIONS: The use of the new corneal effective index of refraction allows for an accurate derivation of K-reading from the anterior radius of curvature. [J Refract Surg. 2006;22:75-80.]