title: Keratometric index, videokeratography, and refractive surgery.
abstract: OBJECTIVE
  To clarify the confusion resulting from the use of slightly different refractive indices in calculations related to optical modeling of the cornea for refractive surgery, corneal diagnostics, and cataract surgery.
  
  
  METHODS
  Scientific Institute H.S. Raffaele, Milan, Italy.
  
  
  METHODS
  The cornea is represented as a centered optical system composed by 1, 2, or 3 spherical interfaces, in progression of modeling accuracy. Optical analysis is performed with the usual formulas of paraxial geometrical optics as well as with ray tracing. Simple models are also provided for corneas having both incisional and photoablative refractive surgery. Values of geometrical parameters are taken from the Gullstrand eye model.
  
  
  RESULTS
  Using the keratometric index of refraction of 1.3375 is validated for estimating optical power differences on untreated corneas or after incisional keratotomy. It is not as accurate in assigning absolute values of dioptric power, where the value 1.3315 is more appropriate. For photorefractive keratectomy (PRK), however, the proper stromal index of refraction, 1.376, must be used for ablation calculations and dioptric change estimates.
  
  
  CONCLUSIONS
  Videokeratographic instruments should include three distinct values of refraction index (1.3375, 1.376, and 1.3315) for an accurate and complete characterization of dioptric power maps. In cataract surgery, corrections must be introduced in the calculation of intraocular lens power for patients who have previously had PRK.
authors.affiliation: Department of Ophthalmology and Visual Sciences, Scientific Institute H.S. Raffaele, University of Milan, Italy.
authors.forename: P G
authors.initials: PG
authors.lastname: Gobbi
authors.forename: F
authors.initials: F
authors.lastname: Carones
authors.forename: R
authors.initials: R
authors.lastname: Brancato
mesh_terms: D002387:Cataract Extraction; D003315:Cornea / Q000473:pathology* / Q000601:surgery; D019781:Corneal Topography; D006801:Humans; D007646:Keratotomy, Radial; D054018:Lasers, Excimer; D008433:Mathematics; D018903:Photorefractive Keratectomy; D054140:Refractive Surgical Procedures; D012031:Refractometry