title: Effect of excimer laser beam delivery and beam shaping on corneal sphericity in photorefractive keratectomy.
abstract: OBJECTIVE
  To evaluate the impact of beam delivery and beam shaping on corneal profiles after myopic excimer laser photorefractive keratectomy (PRK).
  
  
  METHODS
  Department of Ophthalmology, Charité-Campus Virchow Hospital, Humboldt University of Berlin, Berlin, Germany.
  
  
  METHODS
  Standard myopic 193 nm excimer laser PRK of -3.0 diopters (D) and -6.0 D was performed in porcine eyes using 2 commercially available broad-beam lasers with band-mask and fractal-mask beam shaping, 2 flying-spot lasers, and a scanning-slit laser. A silicone replica was obtained to preserve the corneal profile and was measured with a dynamic focusing topometry system.
  
  
  RESULTS
  The scanning-slit and flying-spot lasers created uniform profiles comparable to those in an untreated control group. Both broad-beam lasers with band-mask and fractal-mask beam shaping created central islands and paracentral profile valleys of 15.10 microm and 17.00 microm maximum height after -3.0 D PRK and 26.45 microm and 24.31 microm after -6.0 D PRK. An anti-central-island program, which applied a series of laser pulses centrally to compensate for the central profile elevations, did not eliminate the islands. Stromal surface roughness increased with ablation depth and was significantly worse after scanning-slit ablation than after broad-beam ablation.
  
  
  CONCLUSIONS
  Laser-induced deviations from the intended uniform corneal profiles were associated with broad-beam ablation and increased ablation depth and therefore lessened the predictability of the refractive outcomes. Scanning-slit and flying-spot systems produced predictably uniform corneal profiles.
authors.affiliation: Department of Ophthalmology, Charité-Campus Virchow Hospital, Humboldt University of Berlin, Berlin, Germany. bertmueller@hotmail.com
authors.forename: Bert
authors.initials: B
authors.lastname: Müller
authors.forename: Thomas
authors.initials: T
authors.lastname: Boeck
authors.forename: Christian
authors.initials: C
authors.lastname: Hartmann
mesh_terms: D000818:Animals; D002370:Casts, Surgical; D003315:Cornea / Q000473:pathology* / Q000601:surgery*; D019781:Corneal Topography; D054018:Lasers, Excimer; D023421:Models, Animal; D009216:Myopia / Q000601:surgery; D018903:Photorefractive Keratectomy; D012029:Refraction, Ocular; D013552:Swine