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K. R. Huxlin, L. Ding, W. Knox; Using a Femtosecond Laser to Alter the Refractive Index of the Cornea and Lens. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2446. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To assess the optical effects of high-repetition-rate, low-pulse-energy femtosecond laser irradiation on the stroma and cortex of excised cat corneas and lenses.
Eight corneas and five lenses were extracted post-mortem from normal, adult domestic short-hair cats. They were lightly fixed in 1% paraformaldehyde in 0.1M PBS and stored in a 30% sucrose/30% ethylene glycol/0.1M PBS solution to prevent swelling and opacification. An 800nm Ti:Sapphire femtosecond laser with a 27fs pulse duration and a 93MHz repetition rate was used to micromachine gratings into the corneal stroma and the lens cortex. Refractive index (RI) changes in the micromachined regions were determined immediately and after one month of storage in an aqueous solution by measuring the intensity distribution of diffracted light when the gratings were irradiated by a 632.8nm wavelength He-Ne laser.
Periodic gratings consisting of 20-40 lines, each 1µm thick, 100µm long and 5µm apart, were created 100µm below the tissue surface. The laser pulse energy was adjusted so that diffraction gratings were micromachined without the induction of scattering, absorbing features or bubbles. Pulse energies were 0.3nJ in the cornea and 0.5nJ in the lens. The resulting gratings could be visualized by phase, but not bright field microscopy. Within the laser-micromachined regions, average RI changes ranged from 0.005 to 0.01 in the cornea and from 0.015 to 0.021 in the lens. Based on published values for feline corneal power (39D) and RI (1.376), the RI changes induced by micromachining should alter corneal power by 0.14-0.28D if the RI changes were uniform across the entire cornea. Similarly, for the cat lens (power=53D, RI=1.554), the RI changes induced by micromachining should alter lenticular power by 0.5-0.7D. The gratings and associated RI changes remained constant, even after storing the micromachined tissues in an aqueous solution for one month.
The present results describe a novel method of modifying the corneal stroma and the lens cortex using a low-pulse-energy MHz femtosecond laser. This method allows spatially precise changes in the refractive index and optical power to be made without significant tissue disruption or destruction, a potentially significant advance in the field of optical vision correction. Ongoing experiments are measuring the impact of femtosecond micromachining in living corneas and lenses, in addition to assessing the molecular and structural substrates of the induced refractive index changes, with the ultimate goal of expanding the dioptric range of treatment.
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