title: Corneal ablation profilometry and steep central islands.
abstract: BACKGROUND
  Photorefractive keratectomy with large diameter ablations using a uniform laser beam has produced central undercorrections, or "steep central islands" in patients, as seen with videokeratography.
  
  
  METHODS
  Using a custom optical profilometer to measure corneal ablation profiles and a VISX excimer laser system, we measured the effect of ablation algorithms, diameter, depth, and dioptric correction on enucleated porcine eyes and living rabbit eyes. Our profilometer was verified using a 43.00 diopter (D) spherical surface and a 35.00 and 43.00 D bicurve test surface as a model for the ablated cornea.
  
  
  RESULTS
  The profilometer measured the test surfaces to within 3 microns of predicted values. Photorefractive keratectomies showed over-ablation peripherally and under-ablation centrally which increased with ablation diameter and dioptric correction. Fixed diameter ablations 2 to 6 mm in diameter and 10 to 80 microns deep showed stromal ablation rates vary spatially but not with ablation depth. These spatially variant ablation profiles were used to re-engineer the ablation algorithm and to produce photorefractive keratectomies with improved sphericity.
  
  
  CONCLUSIONS
  Steep central islands are caused by the spatial variance of tissue ablated with a uniform laser beam irradiance. This aberration can be corrected by modifying the laser ablation algorithm to correct for the spatial variance of stromal ablation.
authors.affiliation: VISX Incorporated, Santa Clara, California, USA.
authors.forename: J K
authors.initials: JK
authors.lastname: Shimmick
authors.forename: W B
authors.initials: WB
authors.lastname: Telfair
authors.forename: C R
authors.initials: CR
authors.lastname: Munnerlyn
authors.forename: J D
authors.initials: JD
authors.lastname: Bartlett
authors.forename: S L
authors.initials: SL
authors.lastname: Trokel
mesh_terms: D000465:Algorithms; D000818:Animals; D003315:Cornea / Q000473:pathology* / Q000601:surgery*; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D054018:Lasers, Excimer; D018903:Photorefractive Keratectomy / Q000379:methods*; D011237:Predictive Value of Tests; D011817:Rabbits; D012029:Refraction, Ocular; D013552:Swine