Corneal ablation profilometry and steep central islands.
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.
Shimmick
J K
JK
VISX Incorporated, Santa Clara, California, USA.
Telfair
W B
WB
Munnerlyn
C R
CR
Bartlett
J D
JD
Trokel
S L
SL
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