Intraocular lens power calculation following LASIK: Determination of the new effective index of refraction

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: (K-reading= (rN-1)/Ra), where Ra is the radius of curvature of the anterior corneal surface. In group 13, 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.