Effect of excimer laser beam delivery and beam shaping on corneal sphericity in photorefractive keratectomy.
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.
Müller
Bert
B
Department of Ophthalmology, Charité-Campus Virchow Hospital, Humboldt University of Berlin, Berlin, Germany. bertmueller@hotmail.com
Boeck
Thomas
T
Hartmann
Christian
C
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