title: A steep peripheral ring in irregular cornea topography, real or an instrument error?
abstract: OBJECTIVE
  To demonstrate that the steep peripheral ring (red zone) on corneal topography after myopic laser in situ keratomileusis (LASIK) could possibly due to instrument error and not always to a real increase in corneal curvature.
  
  
  METHODS
  A spherical model for the corneal surface and modifying topography software was used to analyze the cause of an error due to instrument design. This study involved modification of the software of a commercially available topographer.
  
  
  RESULTS
  A small modification of the topography image results in a red zone on the corneal topography color map.
  
  
  CONCLUSIONS
  Corneal modeling indicates that the red zone could be an artifact due to an instrument-induced error. The steep curvature changes after LASIK, signified by the red zone, could be also an error due to the plotting algorithms of the corneal topographer, besides a steep curvature change.
authors.affiliation: King Khaled Eye Specialist Hospital, Al Urubah Branch Rd, Umm Al Hamam Al Gharbi, Riyadh 12329, Saudi Arabia; Department of Ophthalmology, Complejo Asistencial Palencia, Donantes de sangre s/n, 34005 Palencia, Spain.
authors.forename: Alicia
authors.initials: A
authors.lastname: Galindo-Ferreiro
authors.affiliation: King Khaled Eye Specialist Hospital, Al Urubah Branch Rd, Umm Al Hamam Al Gharbi, Riyadh 12329, Saudi Arabia.
authors.forename: Alberto
authors.initials: A
authors.lastname: Galvez-Ruiz
authors.affiliation: King Khaled Eye Specialist Hospital, Al Urubah Branch Rd, Umm Al Hamam Al Gharbi, Riyadh 12329, Saudi Arabia; Department of Ophthalmology Faculdade de Medicina de Botucatu - UNESP, São Paulo CEP 18618-970, Brazil.
authors.forename: Silvana A
authors.initials: SA
authors.lastname: Schellini
authors.affiliation: Galindo Clinic, Francisco Zarandona St 10, 47003 Valladolid, Spain; Rio Hortega University Hospital, Dulzaina St 2, 47012 Valladolid, Spain.
authors.forename: Julio
authors.initials: J
authors.lastname: Galindo-Alonso
keywords: Irregular corneal astigmatism; Red zone; Spherical aberration; Topography