title: Estimation of corneal power after myopic laser refractive surgery: comparison of methods against back-calculated corneal power.
abstract: OBJECTIVE
  To compare methods of corneal power estimation after myopic laser refractive surgery with back-calculated corneal power (K).
  
  
  METHODS
  Private practice, Edmonton, Alberta, Canada.
  
  
  METHODS
  Case series.
  
  
  METHODS
  Patients with previous myopic laser surgery followed by cataract extraction were studied. Back-calculated K obtained with the Holladay IOL Consultant was compared with that obtained by the clinical history method (CHM), the modified Maloney method, an adaptation of the Maloney method using individualized Orbscan IIz-derived posterior corneal power values, Orbscan IIz quantitative area topography, and the Gaussian optics formula. A mixed effects linear model was used for analysis.
  
  
  RESULTS
  The mean spherical equivalent (SE) before laser treatment was -6.43 diopters (D) ± 3.52 (SD). The estimated means of all methods except those obtained with the CHM, modified Maloney method, 2.0 mm total axial map, 1.5 mm total mean map, and 1.5 mm total optical map were significantly different from the mean of the back-calculated K. Estimates from the 1.5 mm total mean map were generally 0.06 D higher. The 2.0 mm total axial map, modified Maloney method, 1.5 mm total optical map, and CHM underestimated corneal power by 0.11 D, 0.13 D, 0.22 D, and 0.26 D, respectively. Unit increases in optical zone and pre-laser myopic SE were associated with decreases in corneal power of 1.58 D (P = .047) and 0.55 D (P = .0001), respectively.
  
  
  CONCLUSIONS
  The modified Maloney method, 2.0 mm total axial map, 1.5 mm total mean map and 1.5 mm total optical map of the Orbscan IIz may provide estimates closer to the back-calculated K than the CHM.
  
  
  BACKGROUND
  No author has a financial or proprietary interest in any material or method mentioned.
authors.affiliation: Singapore National Eye Centre, Singapore. dr_eugenetay@yahoo.com
authors.forename: Eugene
authors.initials: E
authors.lastname: Tay
authors.forename: Cindy
authors.initials: C
authors.lastname: Lim
authors.forename: Howard
authors.initials: H
authors.lastname: Gimbel
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Kaye
mesh_terms: D003315:Cornea / Q000502:physiology*; D048988:Corneal Surgery, Laser; D019781:Corneal Topography; D003941:Diagnostic Techniques, Ophthalmological; D005260:Female; D006801:Humans; D019654:Lens Implantation, Intraocular; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000503:physiopathology / Q000601:surgery*; D018918:Phacoemulsification; D019591:Pseudophakia / Q000503:physiopathology; D012029:Refraction, Ocular / Q000502:physiology*; D012189:Retrospective Studies; D014065:Tonometry, Ocular