title: Intraocular lens power calculation following LASIK: determination of the new effective index of refraction.
abstract: OBJECTIVE
  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 = 1951). 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 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.
authors.affiliation: Cornea Service, International Medical Center, Dubai, UAE. ejarade@yahoo.com
authors.forename: Elias F
authors.initials: EF
authors.lastname: Jarade
authors.forename: Françoise C
authors.initials: FC
authors.lastname: Abi Nader
authors.forename: Khalid F
authors.initials: KF
authors.lastname: Tabbara
mesh_terms: D003315:Cornea / Q000473:pathology / Q000503:physiopathology*; D019781:Corneal Topography / Q000379:methods*; D005500:Follow-Up Studies; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D007910:Lenses, Intraocular; D009216:Myopia / Q000503:physiopathology / Q000601:surgery; D012029:Refraction, Ocular; D015203:Reproducibility of Results