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C. Dorronsoro, J. Merayo–Lloves, S. Marcos; An Experimental Correction Factor of Radial Laser Efficiency Losses in Corneal Refractive Surgery . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3611.
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© ARVO (1962-2015); The Authors (2016-present)
Standard LASIK surgery for myopia increases corneal asphericity, which results in increased spherical aberration. We have developed and experimental model cornea on polymethyl–methacrilate (PMMA) and experimental protocols to account for the physical factors that produce an increase in corneal asphericity after ablation, and to obtain an experimental correction factor (ρ) for any corneal ablation algorithm to compensate for radial losses in laser energy.
A surgical excimer laser programmed with a standard ablation profile (Bausch & Lomb, Tecnolas 217–C) was used to ablate flat and spherical PMMA surfaces. The spherical surfaces simulated a cornea, with a radius of curvature of 8 mm. Spherical corrections ranging from –3 to –12 D were applied, with optical zones of 5 to 7 mm. Pre– and post–ablation shapes were assessed by contact profilometry (flats) and corneal topography (spheres). The ablation profiles were obtained by subtraction of pre– minus post–ablation surfaces after precise alignment. The ablation profile programmed in the laser system was assessed from measurements on flat surfaces, after multiplication for a factor accounting for the difference in ablation rate on PMMA and corneal tissue. The ablation profiles on spherical surfaces were affected by laser efficiency changes across the curved surface. The correction factor ρ for PMMA was obtained by dividing ablation profiles measured on flat surfaces by those measured on spherical surfaces. Beer–Lambert laws and corneal ablation thresholds on PMMA and corneal tissue were used to obtain ρ for the cornea. Corneal asphericity affected by the experimental ablation profile and laser efficiency losses was compared to clinical corneal asphericities as a function of correction.
1) The ablation rate on PMMA was 0.33 that on corneal tissue. 2) The programmed ablation profile (as assessed on flat surfaces) was consistent with Munnerlyn equation. 3) Experimental ρ showed a decrease in laser efficiency from the center to the 2.5 mm by 0.82 on PMMA and by 0.93 on cornea, slightly larger than previous theoretical computations. 4) These measurements predict at least 70% of the increased corneal asphericity found clinically with this laser system.
An experimental model on PMMA allows estimation of a correction factor of laser efficiency losses for any algorithm, without the approximations involved in theoretical estimations. The increased asphericity after refractive surgery is largely due to this effect, and not to biomechanics.
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