Elsevier

Ophthalmology

Volume 107, Issue 12, December 2000, Pages 2140-2143
Ophthalmology

The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis

https://doi.org/10.1016/S0161-6420(00)00407-3Get rights and content

Abstract

Objective

To evaluate the relationship between predicted flap thickness and actual flap thickness and between predicted tissue ablation and actual tissue ablation.

Design

Prospective, nonrandomized comparative (self-controlled) trial.

Participants

A total of 60 patients (102 eyes) who underwent laser in situ keratomileusis (LASIK).

Main outcome measures

Subtraction pachymetry was used to determine actual corneal flap thickness and corneal tissue ablation depth. Other measurements included flap diameter and keratometry readings.

Results

Actual flap thickness was significantly different (P < 0.0001) from predicted flap thickness. Fifteen eyes had a predicted flap thickness of 160 μm and a mean actual flap of 105 μm (standard deviation [SD], ±24.3 μm; range, 48–141 μm). Sixty-four had a predicted flap of 180 μm with an actual flap mean of 125 μm (SD, ±18.5 μm; range, 82–155 μm). Seventeen eyes had a predicted flap of 200 μm, with an actual flap mean of 144 μm (SD, ±19.3 μm; range, 108–187 μm). In addition, we found that significantly more tissue (P < 0.0001) was ablated than predicted. Linear regression of the observed ablation on predicted ablation yielded the following relationship: actual ablation = 14.5 + 1.5 (predicted ablation). Neither flap diameter nor flap thickness were found to increase with respect to steeper corneal curvatures.

Conclusions

Actual corneal flap thickness was consistently less than predicted regardless of the depth plate used; actual tissue ablation was consistently greater than predicted tissue ablation for the laser used in this study.

Section snippets

Materials and methods

The study protocol was reviewed and approved by the institutional review board at the University of Colorado Health Sciences Center. A total of 60 patients (102 eyes) were enrolled in the investigation. All procedures were performed by the same surgeon (LS) using the same laser. Exclusion criteria included previous corneal surgery, history of corneal dystrophy, previous refractive surgery, corneal scarring from corneal infection or trauma, or a history of glaucoma.

Preoperative measurements

Results

Data from 102 eyes of 60 patients who had undergone LASIK for myopia were available for the initial analysis. Five eyes (four patients) were excluded from all analyses because of missing data. Of the remaining 97 eyes, seven eyes had missing data from some of the analyses. Each analysis performed had a minimum sample of 92 eyes of 55 patients.

Discussion

This study examines both the relationship between predicted corneal flap thickness and actual corneal flap thickness and between predicted tissue ablation and actual tissue ablation. Subtraction pachymetry was used to measure corneal flap thickness and the amount of tissue ablated. Accurate predictability of the refractive and visual results of LASIK requires an accurate laser algorithm. As such, predicted corneal flap thickness and predicted tissue ablation are important entities.

The ability

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