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R. Berret, D. Kemmner, T. Oltrup, T. Bende, B. Jean; Influence of the Reflection on the Ablation Rate during Excimer Laser Ablation and its Compensation . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2593.
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Purpose: A reduced ablation rate for Excimer laser irradiation was postulated in the corneal periphery: the bigger the distance from the center of the ablation and the steeper the curvature, the higher the amount of reflected Excimer laser irradiation and the lower the amount of ablated tissue per pulse. In nowadays Excimer laser systems an empiric found enhancement factor is used in order to compensate this effect. The goal of this study was to assess the postulated effect in order to optimize Excimer laser ablation. Methods: With a flying spot Excimer laser (Schwind ESIRIS, SCHWIND, Germany) hyperopia treatments up to 4.0 D and myopia treatments up to -8.0 D were performed in PMMA plates and PMMA balls, as well as in porcine cornea. Plates were used to verify the compensation algorithms for reflection, PMMA balls and porcine cornea were used to compare the ablation depth with and without compensation. First results of patients with standard treatment and treatment with reflectivity compensation are presented. Results: The calculated difference for a 6.5 mm treatment diameter was 10% less in power without compensation. The findings showed up to 12% less in power for PMMA balls and up to 8% less for porcine cornea (being equivalent to 0.5 D) in the no-compensation mode. The functional optical zone (FOZ) of the patients with standard treatment is slightly smaller compared to the FOZ of patients with the same amount of correction, but treatment with reflectivity compensation. Conclusions: The difference between the calculated and the experimentally found values for ablated tissue is due to the fact, that the formula is based on a theoretically estimated reflection behavior. The results obtained indicate that the reflection behaviors of the samples used doesn’t match the expected values.More important than the difference in curvature of only 0.5 D is the induced change in surface quality. With reflectivity compensation, a larger functional optical zone (FOZ) can be expected. This may be due to the fact that the preexisting corneal asphericity is not influenced as much as with the no-compensation treatment, enabling to create more prolate corneas. Clinical trials to investigate this effect are performed in our department. This will also help to include biological factors i.e. postoperative wound healing.
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