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V. Nuzzo, M. Merano, M. Savoldelli, K. Plamann, D. Donate, O. Albert, Gé. Mourou, J.-M. Legeais; Adaptation of the Femtosecond Laser Energy to the Pathology of the Cornea During Keratoplasty by Monitoring the Optical Second Harmonic Emission. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3518.
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© ARVO (1962-2015); The Authors (2016-present)
When performing femtosecond laser corneal surgery in strongly scattering tissue, the laser energy density decreases with increasing penetration depth. To obtain regular incisions and to avoid side effects it is necessary to adapt the energy density to the pathology of the cornea when performing keratoplasty. This is possible by monitoring the second harmonic light generated (SHG) by the laser-tissue interaction at energies below the threshold for tissue disruption.
Human corneas were exposed to a near-infrared femtosecond laser system, delivering pulses with a duration in the fs range, energies of several tens of µJ, and a repetition rate of 1-10 kHz. This laser system is technically similar to the laser used in clinical settings. To reproduce the in vivo environment of the eye, samples were mounted on an anterior chamber positioned by a PC controlled step motor system. A focusing lens or an optical microscope were inserted in the experimental set-up allowing the focalisation of the beam at numerical apertures from 0.15 to 1.2 and the in situ observation of the laser effects on the tissue. A photomultiplier connected to a lock-in amplifier tuned to the laser repetition rate recorded the SHG signal.
When fs laser incisions were performed in pathological tissue, an attenuation of the energy density was observed and attributed to the light scattering due to tissue irregularities. When the laser is set at constant energy high enough to produce transfixing cuts in oedematous corneas, the disruption effects are more pronounced towards the surface of corneal tissue, and non-linear side-effects may occur in the posterior region due to the too high energy and depending on the numerical aperture. Consequently, femtosecond laser keratoplasty requires to be performed at energies densities adapted to the pathology of the cornea. By acquiring the backscattered SH photons generated by the non-linear optical fs laser-collagen interaction, an exponential attenuation of the laser energy was observed. The in situ quantification of the attenuation constant was used for the correction of the laser energy, thus producing regular incisions.
SHG enables to quantify in situ the attenuation of the laser which occurs in opaque corneas during keratoplasty. Future systems should also compensate for the physical mechanisms of the beam attenuation.
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