Effects of ablation diameter, depth, and edge contour on the outcome of photorefractive keratectomy.
OBJECTIVE
To investigate the effects of the ablation diameter, depth, and edge contour on the outcome of excimer laser photorefractive keratectomy (PRK).


METHODS
A prospective study was conducted in which 60 patients (60 eyes) were randomly allocated to 5.00-mm, 6.00-mm, or 5.00 to 6.00-mm multizone treatment groups. All eyes underwent a -6.00 diopter (D) correction using a Summit Omnimed excimer laser.


RESULTS
In eyes treated with 6.00-mm diameter zones, the initial hyperopic shift was reduced, with significant differences at 1 and 4 weeks (p < 0.01). At 6 and 12 months, the refractive changes were closer to the intended correction with 6.00-mm diameters. The predictability of PRK was improved with 6.00-mm zones, with a significant reduction in variance of the refractive changes, at all stages postoperatively (p < 0.05 to p < 0.001). Objective measurements of haze were significantly less at 1, 3, and 6 months with 6.00-mm ablations (p < 0.05). There were no differences between the 5.00-mm and the 5.00- to 6.00-mm multizone groups. Computerized measurements of "night" halo were significantly smaller in the 6.00-mm treatment group at 1 week and 1 month (p < 0.05). At 12 months, two patients treated with 5.00-mm zones and three with the 5.00- to 6.00-mm multizone complained of severe night vision disturbances. No 6.00-mm eyes were similarly affected.


CONCLUSIONS
Treatment with a 6.00-mm spherical ablation diameter produced less initial overcorrection, improved predictability, and was associated with a reduction in postoperative halos and night vision disturbances. Creating a superficial blend zone with a 5.00- to 6.00-mm multizone treatment had no beneficial effect on the outcome.
O'Brart
D P
DP
Department of Ophthalmology, United Medical School, St. Thomas' Hospital, London.
Corbett
M C
MC
Verma
S
S
Heacock
G
G
Oliver
K M
KM
Lohmann
C P
CP
Kerr Muir
M G
MG
Marshall
J
J
D000328:Adult; D003315:Cornea / Q000503:physiopathology; D003319:Corneal Stroma / Q000528:radiation effects; D004311:Double-Blind Method; D006801:Humans; D054018:Lasers, Excimer; D008027:Light; D008875:Middle Aged; D009216:Myopia / Q000503:physiopathology / Q000601:surgery*; D009755:Night Blindness / Q000209:etiology; D018903:Photorefractive Keratectomy / Q000379:methods*; D011183:Postoperative Complications; D011184:Postoperative Period; D011446:Prospective Studies; D012029:Refraction, Ocular; D012542:Scattering, Radiation; D016896:Treatment Outcome