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M. E. Arnoldussen, L. Zickler, B. Logan; Characterizing the Dynamic Ablation Response. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5352.
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© ARVO (1962-2015); The Authors (2016-present)
Accurate knowledge of the amount of corneal tissue removed per laser pulse is critical for success in refractive surgery. It is difficult to gauge the effects of individual pulses during treatment, so characterizing ablation usually begins with ex vivo testing and is refined based on clinical data. The existence of nomogram adjustments used to compensate for physician-specific practices suggests that delivery style influences laser-tissue interaction, which means that the effective ablation shape is dynamic. To validate that changes occur, an experiment was conducted to compare the effective ablation shapes in plastic in two extreme modes: stationary and scanning. To characterize the moving pulses, we developed a method based on wavefront analysis to infer the pulse shapes created during ablation.
Stationary-mode ablations were performed on optically transparent plastics using multiple beam sizes and measured using stylus profilometry. Scanning-mode lenses of various intended optical powers were generated based on the modeled stationary ablation response. Using a Hartmann-Shack aberrometer, each lens wavefront was input to a treatment simulator to determine which parameters of the stationary model affected the outcome. Subtle adjustments corrected for the difference from intended lens powers, and new lenses were made and measured.
The intended optical powers of the final set of test lenses improved, verifying that a transformation in shape occurs between stationary- and scanning-modes of ablation. The small but measurable differences between the directly measured craters and the modeled shapes extracted from wavefront analysis are not due to randomness or system variation but are related to the manner in which the ablation was created. For plastic ablations, the treatment simulator determined that size and depth were the primary factors and that symmetry-of-shape was a minimal factor. Acoustic side-wall effects may cause stationary-mode ablations to overestimate width and depth compared to the scanning-mode ablation response.
By refining the ablation model with wavefront data, we can converge upon optimal desired surgical performance. That a difference in derived shapes arises depending on the mode of ablation reveals that the dynamic response is a complex mechanism. We validated this effect by comparing surface profilometry with optical wavefront measurements and determined the adjustments needed to better match ablation results with the model. Ultimately, this method only requires differential wavefront data, so we plan to transfer its use to clinical data to better understand laser-tissue interaction.
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