Photorefractive keratectomy for correction of myopia: Our one-year experience
Authors: Resan Mirko , Vukosavljevic Miroslav , Milivojevic Milorad 
Published: October 2012

Background/Aim. Photorefractive keratectomy (PRK), after laser in situ
keratomileusis (LASIK), is commonly performed refractive surgical method
worldwide. The aim of this study was to examine the effectiveness and safety
of PRK in correction of various strengths of myopia and to assess how much
corneal tissue is being removed with one diopter sphere (Dsph) correction by
using different optical zones (OZ). Methods. A prospective study with a
follow-up period of 6 months included 55 patients of which 100 myopic eyes
were treated by PRK method (one eye was included in 10 patients). Myopic eyes
with a preoperative best corrected visual acuity (BCVA) = 1.0 (20/20) were
analysed. In order to assess the effectiveness of PRK operated myopic eyes
were divided into four groups according to the dioptric power: 1)? -1.75 Dsph
(n = 26); 2) from -2 to -3.75 Dsph (n = 44); 3) from -4 to -6.75 Dsph (n =
23), and 4) ? -7 Dsph (n = 7). Myopic eyes with preoperative BCVA ? 0.9
(amblyopic eyes) were excluded from the study, as well as eyes with
astigmatism > -1.5 Dcyl. To assess the effectiveness of PRK we examined the
percentage of eyes in the mentioned groups, which derived uncorrected visual
acuity (UCVA) 6 months after the intervention to the following: a) UCVA = 1.0
(20/20) and b) UCVA ? 0.5 (20/40). To assess the safety of PRK we examined
the frequency of intraoperative and postoperative complications. To estimate
how much corneal tissue was removed with one Dsph correction by using
different OZ, we used preoperative and postoperative (after 6 months) central
pachymetry values expressed in ?m and volume of cornea (central 7 mm)
expressed in mm?. In that sense, we used only the myopic eyes with clear
preoperative spherical refraction. The total number of these eyes was 27, of
which 16 eyes were treated using a 6.5 mm OZ and 11 eyes using a 7 mm OZ.
Results. Refractive spherical equivalent (RSE) for all eyes was in the range
from -0.75 to -8.75 Dsph, and preoperative mean value of RSE with standard
deviation (mean RSE ? SD) was -3.32 ? 1.83 Dsph. Six months after PRK, 91% of
eyes had UCVA = 20/20, and 99% of eyes had UCVA ? 20/40. In the first group
(? -1.75 Dsph) preoperative mean RSE ? SD was -1.34 ? 0.32 Dsph, six months
after PRK, 96% of eyes had UCVA = 20/20, and 100% of eyes had UCVA ? 20/40.
In the second group (from -2 to -3.75 Dsph) preoperative mean RSE ? SD was -
2.95 ? 0.57 Dsph, six months after PRK, 89% of eyes had UCVA = 20/20, and
100% of eyes had UCVA ? 20/40. In the third group (from -4 to -6.75 Dsph)
preoperative mean RSE ? SD was - 4.93 ? 0.70 Dsph, six months after PRK, 100%
of eyes had UCVA = 20/20. In the fourth group (? - 7 Dsph) preoperative mean
RSE ? SD was -7.71 ? 0.67 Dsph, six months after PRK, 57% of eyes had UCVA =
20/20, and 86% of eyes had UCVA ? 20/40. There were no intraoperative
complications while postoperative complications occurred in 2 patients - in
both cases in one eye (2%). In that cases, epithelial defects were detected.
In the group of eyes that were treated by 6.5 mm OZ mean RSE ? SD was -2.45 ?
0.99 Dsph, the ablation depth per 1 Dsph was 17.54 ? 5.58 ?m and ablated
volume of central 7 mm cornea by 1 Dsph was 0.43 ? 0.18 mm?. In the group of
eyes that were treated by 7 mm OZ mean RSE ? SD was -3.32 ? 2.26 Dsph, the
ablation depth per 1 Dsph was 23.73 ? 6.91 ?m and ablated volume of central 7
mm cornea by 1 Dsph was 0.61 ? 0.31 mm?. Conclusion. PRK is effective and
safe refractive surgical method for correcting myopia up to -8 .75 Dsph. OZ
size is the main factor determining the depth of the excimer laser ablation
of the corneal tissue volume consumed by 1 Dsph. Higher OZ value determines
higher consumption of cornea tissue.