J Korean Ophthalmol Soc.  2012 Jan;53(1):1-10. 10.3341/jkos.2012.53.1.1.

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

• Park DY
• Lee SB

• KMID: 2215239
• DOI: http://doi.org/10.3341/jkos.2012.53.1.1

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 > or =-0.50 D and <-1.00 D, 45 eyes; Group 3 > or =-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.

Keyword

Ablation center; Cylindrical correction; Decentration; Geometric corneal center; Photorefractive keratectomy (PRK)

MeSH Terms

Corneal Topography
Displacement (Psychology)
Eye
Humans
Learning
Pupil
Risk Factors

Figure

• 

  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.

• 

  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).

• 

  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).

• 

  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).

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