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D.W. Hahn, B.T. Fisher; Quantitative Measurement of Corneal Tissue Excimer Laser Ablation Rates . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2633.
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Purpose: Clinical excimer laser refractive surgery systems are characterized by high accuracy and precision. Researchers, however, are still working to better understand the processes of corneal ablation, including the influences of corneal hydration. Such efforts are aided by a means to precisely measure corneal tissue ablation rates. Methods: Paraffin wax was used to make impressions of ablation craters created with an ArF excimer laser. A white-light interferometer was then used to investigate the wax impressions and determine the ablation depth, which combined with the number of ablating laser shots enabled determination of the ablation rate. The technique was validated by comparing the depth measured directly from poly(methyl methacrylate) ablation craters with the depth measured from the corresponding wax impressions of those craters. The technique was then used to measure crater depths, and the corresponding ablation rate, of bovine corneal tissue, including the ablation rate as a function of corneal hydration. Results: The ablation rate was measured as 0.47 µm/pulse from direct measurement of the PMMA ablation craters and 0.48 µm/pulse from measurement of the corresponding wax impressions, with the difference statistically insignificant. The ablation rate of bovine corneal tissue was found to be approximately 0.90 µm/pulse, which was in excellent agreement with the expected clinical ablation rate of human cornea. The ablation rate of bovine corneal tissue was found to depend on corneal hydration, as measured using Raman spectroscopy. Conclusions: Wax impressions provide an accurate and precise representation of ablation craters. Using this technique, the ablation depth of corneal tissue and PMMA was found to increase linearly with increasing number of laser pulses, and the slope of the regression lines provided an accurate measure of the laser ablation rate. CR: F
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