title: Comparison of keratometric changes after myopic ablation: ray tracing versus simulated keratometry.
abstract: OBJECTIVE
  To compare the changes in corneal power (ΔK) induced by myopic ablations estimated by Placido-disk and total corneal power (TCP) ray tracing methods to the refractive change (ΔSE).
  
  
  METHODS
  Manifest refraction, simulated keratometry from a Placido-disk based system, and TCP from a dual-Scheimpflug analyzer were obtained from 35 patients (58 eyes) before and 3 months after a myopic ablation. The change in the manifest refraction that occurred after surgery was then vertex distance corrected and compared to the changes observed in corneal power with the various systems analyzed.
  
  
  RESULTS
  The Placido-based simulated keratometry overestimated the corneal power after the procedure by 0.50 ± 0.53 diopters (D) when compared to refractive change in the corneal plane induced by the laser surgery (ΔSE-ΔK). The ray tracing method showed the opposite trend, with the TCP simulated keratometry showing an underestimation of the corneal power of -0.25 ± 0.48 D. The Placido system showed a direct correlation between the overestimation of the corneal power and the level of myopia, whereas the ray tracing method showed an underestimation, which was more pronounced in higher levels of myopia.
  
  
  CONCLUSIONS
  Ray tracing methods underestimate corneal power as opposed to the overestimation in corneal power after refractive surgery, directly related to the level of myopia, observed in Placido-based systems. They have the potential to overcome the errors in calculations based on anterior curvature alone. However, ray tracing methods need to be validated and optimized before it can be used routinely in IOL calculation.
authors.affiliation: The Cleveland Clinic, Cleveland, OH, USA.
authors.forename: Glauco Reggiani
authors.initials: GR
authors.lastname: Mello
authors.forename: Cynthia J
authors.initials: CJ
authors.lastname: Roberts
authors.forename: David
authors.initials: D
authors.lastname: Smadja
authors.forename: Crislaine C
authors.initials: CC
authors.lastname: Serpe
authors.forename: Ronald R
authors.initials: RR
authors.lastname: Krueger
authors.forename: Marcony R
authors.initials: MR
authors.lastname: Santhiago
mesh_terms: D003315:Cornea / Q000473:pathology* / Q000503:physiopathology / Q000601:surgery; D019781:Corneal Topography / Q000379:methods*; D005500:Follow-Up Studies; D006801:Humans; D020731:Keratomileusis, Laser In Situ / Q000379:methods*; D054018:Lasers, Excimer / Q000627:therapeutic use*; D009216:Myopia / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D011446:Prospective Studies; D012029:Refraction, Ocular