title: Algorithm for correcting the keratometric error in the estimation of the corneal power in eyes with previous myopic laser refractive surgery.
abstract: OBJECTIVE
  To calculate theoretically the errors in the estimation of corneal power when using the keratometric index (nk) in eyes that underwent laser refractive surgery for the correction of myopia and to define and validate clinically an algorithm for minimizing such errors.
  
  
  METHODS
  Differences between corneal power estimation by using the classical nk and by using the Gaussian equation in eyes that underwent laser myopic refractive surgery were simulated and evaluated theoretically. Additionally, an adjusted keratometric index (nkadj) model dependent on r1c was developed for minimizing these differences. The model was validated clinically by retrospectively using the data from 32 myopic eyes [range, -1.00 to -6.00 diopters (D)] that had undergone laser in situ keratomileusis using a solid-state laser platform. The agreement between Gaussian (Pc) and adjusted keratometric (Pkadj) corneal powers in such eyes was evaluated.
  
  
  RESULTS
  It was found that overestimations of corneal power up to 3.5 D were possible for nk = 1.3375 according to our simulations. The nk value to avoid the keratometric error ranged between 1.2984 and 1.3297. The following nkadj models were obtained: nkadj = -0.0064286r1c + 1.37688 (Gullstrand eye model) and nkadj = -0.0063804r1c + 1.37806 (Le Grand). The mean difference between Pkadj and Pc was 0.00 D, with limits of agreement of -0.45 and +0.46 D. This difference correlated significantly with the posterior corneal radius (r = -0.94, P < 0.01).
  
  
  CONCLUSIONS
  The use of a single nk for estimating the corneal power in eyes that underwent a laser myopic refractive surgery can lead to significant errors. These errors can be minimized by using a variable nk dependent on r1c.
authors: *Grupo de Óptica y Percepción Visual, Department of Optics, Pharmacology and Anatomy, University of Alicante, Alicante, Spain; †Foundation for Visual Quality (Fundación para la Calidad Visual), Alicante, Spain; and ‡Department of Ophthalmology (Oftalmar), Medimar International Hospital, Alicante, Spain.
authors: Vicente J
authors: VJ
authors: Camps
authors: David P
authors: DP
authors: Piñero
authors: Veronica
authors: V
authors: Mateo
authors: David
authors: D
authors: Ribera
authors: Dolores
authors: D
authors: de Fez
authors: Francisco J
authors: FJ
authors: Blanes-Mompó
authors: Antonio
authors: A
authors: Alzamora-Rodríguez
mesh_terms: D000328:Adult; D000465:Algorithms; D003315:Cornea / Q000503:physiopathology* / Q000601:surgery; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D053844:Lasers, Solid-State / Q000627:therapeutic use; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000175:diagnosis / Q000503:physiopathology / Q000601:surgery*; D012029:Refraction, Ocular / Q000502:physiology*; D012030:Refractive Errors / Q000503:physiopathology; D012189:Retrospective Studies; D055815:Young Adult