[Assessment of refractive difference after photorefractive keratectomy. A comparison between keratometry and computer-assisted video keratography].
The purpose of this study was to determine which corneal curvature values most closely correlated to change in subjective manifest refraction following excimer laser photorefractive keratectomy (PRK).


METHODS
Excimer laser PRK was performed on ten eyes of ten patients (mean age: 37.3 years). Preoperative refractive errors ranged from -2.25 to -8.75 diopters. Preoperatively and 1 month postoperatively, we determined the spherical equivalent of the manifest refraction (corrected for a 12 mm vertex distance) and measured corneal power using standard keratometry (Bausch and Lomb keratometer) and computerized videokeratography (EyeSys Corneal Analysis System). We collected five corneal values: standard keratometry, videokeratography-derived simulated keratometric readings calculated using the axial, instantaneous and refractive formulas, and corneal refractive power over the central 3-mm zone (effective refractive power); apart from the traditional refractive index of the cornea (n = 1.3375), we used the refractive value of the anterior corneal stroma (n = 1.376). For each of the five corneal values, we subtracted the change in corneal power from the change in manifest refraction and calculated the means and standard deviations.


RESULTS
The mean differences between the refraction and the corneal values for a refractive index of 1.3375/1.376 were: 0.89 +/- 0.54*/1.26 +/- 0.59* for standard keratometry; 1.64 +/- 0.75*/1.37 +/- 0.7*, 4.03 +/- 1.86*/ 3.86 +/- 1.87*, and 1.16 +/- 0.76*/0.91 +/- 0.74* for the axial, instantaneous, and refractive videokeratography values, respectively; and 0.83 +/- 1.03*/0.39 +/- 1.08 for the effective refractive power (*,p < 0.05).


CONCLUSIONS
In our series, only the values for the effective refractive power, calculated with the refractive Index of the anterior stroma of the cornea, were not statistically different from the change in manifest refraction.
Hugger
P
P
Cullen Eye Institute, Department of Ophthalmology, Baylor College of Medicine, Houston, Texas, USA.
Kohnen
T
T
Koch
D D
DD
D000328:Adult; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation*; D054018:Lasers, Excimer; D008297:Male; D009216:Myopia / Q000145:classification / Q000175:diagnosis / Q000601:surgery*; D018903:Photorefractive Keratectomy / Q000295:instrumentation*; D011183:Postoperative Complications / Q000145:classification / Q000175:diagnosis; D012029:Refraction, Ocular; D012680:Sensitivity and Specificity; D016896:Treatment Outcome; D014741:Video Recording / Q000295:instrumentation*