Current laser ablation techniques have developed with an increase in frequency and improved tracking system enabling more precise corneal surface remodeling . Furthermore, customized optic design has reduced postoperative mechanical complications and pain, and yielded faster recovery times with better clinical outcomes [6, 8, 23]. The newer excimer lasers with advanced profile have allowed surgeons to perform surface ablation surgeries with a larger optical zone by enhanced projection error compensation .
Previous studies have shown advantages of a larger optical zone for laser refractive surgery. Kim et al.  reported more night vision problems and ablation decentration with a 5.0 mm optical zone than with a 6.0 mm optical zone. The study by O'Brart et al.  showed less initial overcorrection and regression with a 6.0 mm optical zone than with a 5.0 mm optical zone over 6 months of follow-up. Rajan et al.  reported a larger optical zone had better results in terms of early hyperopic shift, regression, and night haloes for patients who had PRK with 4.0 mm optical zone, 5.0 mm optical zone, and 6.0 mm optical. We compared a 6.0 mm optical zone and the larger optical zone (6.5 mm) using more clinical factors and found no significant differences between the groups. This result may due to the fact that we used the newer excimer laser with a more advanced ablation profile than previous studies.
Seo et al.  compared aberration factors after PRK with 6.0 mm optical zone and 6.5 mm optical zone. In their study, RMS of HOAs in the larger zone (6.5 mm) group were less than those in the 6.0 mm optical zone group, and both groups showed a significant increase in SA from preoperatively to three months postoperatively. In the present study, we found that the RMS of HOAs and SA in the 6.5 OZ group were lower than in the 6.0 OZ group at 3, 6, and over 12 months after surgery which was longer follow-up period than the previous study of Seo et al . HOA wavefront of both groups was measured under the same condition by using TAPCON regardless of the pupil size. We also focused on intragroup changes after surgery, and found that there was a decrease in the RMS of HOAs from 6 months to beyond 12 months post-operatively in the 6.0 OZ group, but not in the 6.5 OZ group, and no significant changes in SA after 3 months post-operatively in either group.
Based on early clinical studies of PRK, a deeper ablation depth was thought to increase corneal haze [3, 17]. However, Rajan et al.  reported a larger zone yielded less corneal haze, and recent studies pointed out that a wound edge with a larger ablation zone had a more gradual slope, which enabled smooth epithelial migration and minimized hyperplasia and corneal haze [25, 26]. In terms of biomechanical stability, reducing residual bed thickness is related to the weakening of cohesive tensile strength, tangential tensile strength and shear strength [27-29], which is more likely to result in mechanical postoperative adverse effects such as keratoectasis [14, 18, 30]. Therefore, when surgeons set the optical zone for PRK, there should be a limitation on optical zone size based on biomechanical stability and other individual factors such as refractive error, pupil size and age.
Manifest refraction is considered a clinically significant factor influencing patient satisfaction after PRK. In our study, there was no significant regressive change in either the 6.0 OZ group or 6.5 OZ group, and no significant differences between the two groups at each visit. This was consistent with the outcomes for UDVA in the 6.0 OZ group and 6.5 OZ group.
Sim K in the 6.0 OZ group increased significantly from six months to beyond twelve months postoperatively in this study (by paired t-test). However, the difference was 0.3 D, which is needed to be assessed through a survey for measuring the discomfort in daily life in the further study, and we found that the regression patterns from preoperative visit to any postoperative visit for the 6.0 OZ group and 6.5 OZ group were not significant (p = 0.118, 2 way repeated measured ANOVA). Therefore, there was no significant clinical regression in Sim K in either group.
On the other hand, the RMS of HOAs, SA, and Q value were lower in the 6.5 OZ group than those in the 6.0 OZ group, which meant PRK with 6.5 OZ might yield better visual quality. Corneal haze with epithelial hyperplasia and stromal remodeling can occur after laser refractive surgery , and this thickening can be evident up to 12 months after PRK [31-34], which makes clinical outcomes measured in this period unstable. In this study, the last visits were at least 12 months postoperatively, thus PRK with 6.5 mm optical zone showed lower aberration values than PRK with 6.0 mm zone even in the long term.
The RMS of HOAs showed significant changes between 6 months and beyond 12 months postoperatively likely because it is more reflective of the numerous Zernike’s coefficients (SA, Coma, trefoil, etc.), but SA is more a clinically interesting factor generally caused after myopic and hyperopic correction, and it might be related to variety of visual symptoms. Moreover, there is individual variation in neural transfer function, and common clinical measures of visual function are not sensitive at low levels of aberration [35-37]. Therefore, better aberration values are not necessarily correlated with improved visual performance.
The thinnest corneal thickness in this study increased significantly until the last follow up visit, but the differences ranged from 3 to 6 μm, which is required to evaluate and conduct surveys prospectively, in order to see if these differences are clinically significant or not in their daily activity. The differences may be due to measurement error and/or intra-examination error in thinnest pachymetry, depending on which point of the cornea was selected for measurement. Results would be more reliable if the thinnest corneal thickness was measured at a designated point with ultrasound repeatedly at every visit.
To evaluate the regressive pattern after laser refractive surgery precisely, it requires corneal epithelial and stromal map analysis using optical coherence tomography (OCT) . However, this study was proceeded retrospectively, where we had to estimate the regression based on the routine follow-up exams that were conducted after PRK, therefore we had to rely on the corneal thickness map of corneal topography instead of OCT, thus further study is required to measure corneal epithelial and stromal change to estimate regressive pattern following PRK.
In this study, we performed PRK using the Mel 90 excimer laser with the Triple-A profile which is improved from previous ASA and TSA profiles in that it has better error compensation function and target asphericity control, which minimizes ablation depth, adjusts for spherical aberration and prevents myopic regression . The surgeon can also choose a frequency up to 500 Hz across the surgery in Mel 90 excimer laser, which enables faster ablation time for correction [39, 40]. Thus, clinical outcomes after PRK with the same optical zone size can differ according to which excimer laser used, and it therefore seems inappropriate to compare this study with clinical data from studies that used the previous version of the excimer laser for PRK in the same way.
Previous long-term follow up studies after PRK were up to 12 years . However, that study compared different optical zones (4.0 mm, 5.0 mm, 6.0 mm), and in our study (6.0 mm, 6.5 mm) we used a different excimer laser and ablation profile. A longer study seems necessary in that we noted a significant increase in the thinnest corneal thickness from 6 months postoperatively to beyond 12 months, although this study had the longest follow-up period of all clinical studies after PRK with the Mel 90 excimer laser to date.