Comparion of change in manifet refraction and corneal power after photorefractive keratectomy

PURPOE: To determine which corneal curvature value mot cloely correlate to change in manifet refraction after excimer laer photorefractive keratectomy. METHOD: In a propective tudy at the Cullen Eye Intitute, excimer laer photorefractive keratectomy wa performed on 27 eye of 27 patient (mean age, 38.07 ± 6.65 year). Preoperative refractive error ranged from −2.25 diopter to −8.75 diopter (mean, −5.74 ± 2.09 diopter). Preoperatively and 1 month potoperatively, we determined the pherical equivalent of the ubjective manifet refraction (corrected for a 12-mm vertex ditance) and meaured corneal power uing tandard keratometry (Bauch & Lomb Keratometer; Rocheter, New York) and computerized videokeratography (Eyey Corneal Analyi ytem; Premier Laer ytem Inc, Houton, Texa). We collected 15 corneal value: tandard keratometry and 14 computerized videokeratography value calculated uing the axial, intantaneou, and refractive formula. All calculation were performed with 1.3375 and 1.376 for the refractive index of the cornea. For each of the corneal value, we ubtracted the change in corneal power from the change in manifet refraction and calculated for thi difference the mean, D, correlation, and regreion. REULT: Mean difference between change in refraction and change in corneal power were lower when for a refractive index of 1.376 than for 1.3375, were lowet for the mot central meaurement point, and diplayed a high D. A value of 1.408 for the refractive index would be required to optimize the correlation between change in manifet refraction and effective refractive power of the central 3 mm of the cornea. CONCLUION: For individual patient who have undergone photorefractive keratectomy, change in corneal value determined by computerized videokeratography or by tandard keratometry do not reliably predict change in manifet refraction.