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Park and Lee: Influences of Cylindrical Correction Amount of PRK on Accuracy of Geometric Corneal Center-Adjusted Ablation Centration
Original Article
Journal of the Korean Ophthalmological Society

Journal of the Korean Ophthalmological Society 2012; 53(1): 1-10.

Published online: 16 January 2012

DOI: https://doi.org/10.3341/jkos.2012.53.1.1

Influences of Cylindrical Correction Amount of PRK on Accuracy of Geometric Corneal Center-Adjusted Ablation Centration

Dae-Young Park, MD, Sang-Bumm Lee, MD, PhD

Department of Ophthalmology, Yeungnam University College of Medicine, Daegu, Korea.

Address reprint requests to Sang-Bumm Lee, MD, PhD. Department of Ophthalmology, Yeungnam University College of Medicine, #317-1 Daemyeong-dong, Nam-gu, Daegu 705-717, Korea. Tel: 82-53-620-3445, Fax: 82-53-626-5936, sbummlee@med.yu.ac.kr

Received 27 June 2011       Accepted 13 December 2011

Copyright © 2012 The Korean Ophthalmological Society

Abstract

Purpose

To evaluate the effect of the amount of cylindrical correction on the accuracy of geometric corneal center-adjusted ablation centration selected by the surgeon in advanced surface ablation-photorefractive keratectomy (ASA-PRK).

Methods

Ninety-five myopic eyes of 62 patients who underwent ASA-PRK were divided into three groups based on the amount of myopic cylindrical correction: Group 1 <-0.50 D, 22 eyes; Group 2 ≥-0.50 D and <-1.00 D, 45 eyes; Group 3 ≥-1.00 D, 28 eyes. The distances and distribution of the ablation centers from the entrance pupil center were analyzed by corneal topography at one month after ASA-PRK. Risk factors for decentration (>0.3 mm) were determined from patient-related factors such as gender, age, laterality, central corneal thickness, and order of procedure and surgery-related factors such as amount of spherical correction, ablation depth, vertical scale bar percentage, and learning effect of surgeon.

Results

Mean decentration was 0.26 ± 0.15 mm (0.04 to 0.83 mm). There was no statistically significant difference in the amount of decentration among the three groups (p = 0.879). Superior-nasal displacement (53.7%) of the ablation center most frequently occurred after ASA-PRK. The amount of decentration among the three groups was not dependent on either patient-related or surgery-related factors.

Conclusions

ASA-PRK using the surgeon-selected ablation center with adjustment toward the geometric corneal center was found to be highly accurate in ablation centration and did not influence the amount of ablation decentration regardless of the amount of myopic cylindrical correction.

Keywords: Ablation center, Cylindrical correction, Decentration, Geometric corneal center, Photorefractive keratectomy (PRK)

Figures and Tables

Figure 1
The relationship between the ablation zone center and the center of the entrance pupil. The ablation zone center was defined as the intersecting point of the maximum edges of the ablation zone in the X and Y axes. A layer of a red solid ellipse/circle (specific to each patient's ablation zone size) with red crosshair (representing the center of the ellipse/circle) was manually positioned over the patient's topographic ablation zone. Then, the distance and degree from the center (white cross) of the entrance pupil (white circle) to each patient's ablation zone center was measured using the scale tool in photoshop.
jkos-53-1-g001
Figure 2
Cumulative percentage of decentration from the center of the entrance pupil. There are no statistically significant differences in the number of eyes among the three groups for each decentration group by 0.1 mm steps (p = 0.879, linear by linear association test). Group 1: cylindrical correction < -0.50 D (n = 22), Group 2: -0.50 D ≤ cylindrical correction < -1.00 D (n = 45), Group 3: -1.00 D ≤ cylindrical correction (n = 28).
jkos-53-1-g002
Figure 3
Distance of ablation zone center from entrance pupil center and distribution of ablation zone center. Mean decentration was 0.26 ± 0.15 mm. No statistically significant difference in the amount of decentration (Group 1: 0.28 ± 0.13 mm, Group 2: 0.26 ± 0.15 mm, Group 3: 0.26 ± 0.16 mm) among the three groups (p = 0.879, one-way ANOVA). No statistically significant difference in the distribution of ablation center for each quadrant among the three groups (p = 0.128, linear by linear association test). Superior-nasal displacement of the ablation center most frequently occurred (51 eyes, 53.7%) after the ASA-PRK. Group 1: cylindrical correction < -0.50 D (n = 22), Group 2: -0.50 D ≤ cylindrical correction < -1.00 D (n = 45), Group 3: -1.00 D ≤ cylindrical correction (n = 28).
jkos-53-1-g003
Figure 4
Scatterplot and linear regression of amount of decentration and amount of cylindrical correction. No significant correlation was observed between the amount of cylindrical correction and the amount of decentration (r2 = 0.000, p = 0.992, simple linear regression analysis). The regression equations were y = 0.000x + 0.264 (x = amount of cylindrical correction and y = amount of decentration). Group 1: cylindrical correction < -0.50 D (n = 22), Group 2: -0.50 D ≤ cylindrical correction < -1.00 D (n = 45), Group 3: -1.00 D ≤ cylindrical correction (n = 28).
jkos-53-1-g004
Table 1
Patient demographics and baseline characteristics of 95 eyes according to the amount of myopic cylindrical correction before ASA-PRK
jkos-53-1-i001

OD = right eye; OS = left eye; SD = standard deviation; D = diopters.

*χ2-test; †One-way ANOVA test.

Table 2
Risk factors for decentration in the three groups divided by the amount of myopic cylindrical correction (univariate analysis)
jkos-53-1-i002

Group 1: cylindrical correction < -0.50D (n = 22), Group 2: -0.50D ≤ cylindrical correction < -1.00D (n = 45), Group 3: -1.00D ≤ cylindrical correction (n = 28).

OR = odds ratio; CI = confidence interval; D = diopter.

*Percentage of eyes which had significant large amount of decentration (i.e., decentration larger than 0.3 mm); †There was no elucidated related factor (patient-related factors such as gender, age, laterality, CCT, and order of procedure and surgery-related factors such as amount of spherical correction, ablation depth, vertical scale bar percentage, and learning effect of surgeon) influencing the significant amount of decentration exceeding 0.3 mm11 according to the amount of myopic cylindrical correction before ASA-PRK in this study (univariate logistic regression analysis).

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