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I. Pentari, H. S. Ginis, G. Kounis, N. Tsiklis, D. Tsatsaronis, I. G. Pallikaris; Ablation Rate of Corneal Stroma as a Function of Hydration at 193nm and 213nm. Invest. Ophthalmol. Vis. Sci. 2009;50(13):588.
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The ablation rate of corneal tissue depends primarily on the fluence of the laser and corneal hydration. These factors may affect the accuracy of refractive corrections as changes in corneal hydration affect the ablation rate and therefore the amount of tissue removed in photorefractive keratectomy. It was the purpose of the current experimental study to compare the effect of corneal hydration on the ablation rate between a SSL (Pulzar Z1 laser system) of 213 nm wavelength and an EL (Allegretto WaveLight) of 193 nm wavelength in porcine corneas.
Fresh porcine eyeballs were obtained from a local abattoir. The corneas were excised from the eyes within 24h post mortem. Each cornea used in the study had no visible epithelial defects. The epithelium was removed with a blunt spatula. Corneal buttons, 8mm in diameter, were excised with a trephine (BARRON-Radial vacuum trephine) from the centers of the corneas and rinsed with balanced salt solution (BSS). Following, the corneas were situated to laser room conditions (Temperature, Humidity) in order to gradually dehydrate. Ablation was performed at different hydration states with both lasers in order to establish the ablation rate-hydration relationship. A total of thirteen porcine corneas were exposed to the ArF Excimer laser and a total of twelve porcine corneas were exposed to the Nd:YAG 5th Harmonic CustomVis laser. The disks received ablation equivalent to the ablation of 100microns at a circular zone having a diameter of 7mm. Before and after ablation of each experimental button, each disk was weighted and subsequently dried in order to calculate its dry mass and therefore hydration during ablation. The hydration ranged from 66% to 80% w/w. The ablated mass was calculated by a simple subtraction of the button’s mass before and after ablation.
Normalized ablated mass exhibited a positive linear correlation with corneal hydration for both laser wavelengths. The corresponding normalised dry mass, which seems to be more important determinant of refractive outcome than hydrated mass, exhibited a negative linear correlation with hydration with similar slopes for the two lasers tested.The slope was -0.0651% per unit change hydration (R2 =0.85) for the 193 nm exhibited and -0.0568% per unit change hydration (R2 =0.83) for the 213 nm.
Results of this study revealed similar behavior in the dependence of ablation rate from the hydration of cornea at the 193 and 213-nm laser wavelengths. Although the hydration changes due to exposure to air for the duration of preparation for ablation seems to be of minimal importance similar care should be taken for both laser platforms.
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