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Carlos Dorronsoro, Silvia Schumacher, Pablo Pérez-Merino, Jan Siegel, Michael Mrochen, Susana Marcos; Effect Of Aspiration Air-flow Speed On The Effective Refractive Surgery Ablation Patterns. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5790.
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To study the effect of air-flow speed on the ablation of artificial polymer corneas used for testing refractive surgery patterns.
An Allegretto EyeQ laser platform (Wavelight, Germany) equipped with an adjustable air flow generator was used to ablate flat samples of PMMA and Filofocon A at four different air flow conditions. Repeated ablations of -9 and -6 D were performed with the WF-Optimized algorithm. The shape and profile of the ablated surfaces were measured with a precision non-contact optical profilometer (Plu, Sensofar, Spain).
Important irregularities and asymmetries in the measured profiles are found without air flow, and with the clinical air aspiration levels. Increasing air-flow produces deeper ablations and increases the repeatability of the ablation pattern. Shielding of the laser by the plume of smoke during the ablation of plastic corneas reduces the central ablation depth by over 40% with no-air flow, 30% with clinical air aspiration, and 5% with 1.15 m/s air flow. A simple model based on non-inertial dragging of the particles by air flow predicts no shielding with 2.3 m/s air flow, and accurately predicts (within 2 µm) the decrease of central ablation depth by shielding. The shielding effects for PMMA and Filofocon are similar despite the differences in full-shielding transmission coefficient (number of particles ejected and/or their associated optical behavior) and the differences in the ablation properties of the materials.
Air flow is a key factor in the evaluation of refractive surgery lasers with plastic models. Shielding effects are found with the typical air-flow used clinically. The effects of insufficient air-flow speed (and therefore insufficient air-debris removal) include a decrease in ablation depth, irregularities in the ablation pattern and increased ablation variability. Shielding effects can be avoided by tuning air flow to the laser repetition rate. In particular, an air flow of at least 3 m/s is recommended for a 400 Hz repetition rate laser.
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