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D.W. Hahn, B. Fisher; Dynamic Absorption Coefficient Model For 193–Nm Excimer Laser Ablation Of Corneal Tissue . Invest. Ophthalmol. Vis. Sci. 2006;47(13):525.
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While clinical excimer laser refractive surgery systems are continually evolving, a complete understanding of the physics and mechanisms of corneal ablation and the role of tissue constituents such as collagen remains a topic of research. A better understanding of the ablation process may lead to better refractive procedures.
The previously measured absorption cross–section of collagen peptide bonds is 1.18x10–17 cm2, which translates to a static corneal tissue absorption coefficient of about 16,000 cm–1. A dynamic model, using the peptide bond as the primary chromophore, was formulated that perturbed the static absorption coefficient through interactions of excimer laser photons with collagen bonds, intermediate species, and water molecules. Ablation depths were determined numerically by tracking the progression of an incident laser pulse as influenced by the transient absorption coefficient.
A comparison of reported corneal ablation depths with Beer–Lambert model predictions reveals a lack of agreement over all relevant laser fluences for a single (i.e. static) absorption coefficient. The current transient (i.e. dynamic) absorption coefficient model produces an effective absorption coefficient that changes during the ablation process from the static value of 16,000 cm–1 to nearly 30,000 cm–1. The model generates an ablation depth vs. laser fluence curve that agrees well with experimental data over a range of clinically relevant fluences.
Collagen is the primary chromophore for 193–nm radiation at non–ablative laser fluences; however, the role of collagen alone does not adequately account for observed laser ablation rates near and above clinical laser fluences. The current model provides a framework to further understand the dynamic nature of laser–cornea interactions, thereby helping to provide a clearer understanding of the role of 193–nm photochemistry in ablation.
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