In situ monitoring of second-harmonic generation in human corneas to compensate for femtosecond laser pulse attenuation in keratoplasty

Abstract

The application of femtosecond lasers in corneal transplant surgery requires high pulse energies to compensate for the strong optical scattering in pathological corneas. However, excessive energies deteriorate the quality of the incisions. The aim of this study is to demonstrate the dependence of side effects on local radiant exposure, numerical aperture, and tissue properties, to quantify the penetration depth of the laser for individual corneas, and to provide a method for optimizing the energy in the volume of the cornea. We examine histological and ultrastructural sections of clear and edematous corneas with perforating and lamellar incisions performed at different pulse energies. We demonstrate that the augmented energies in edematous corneas may result in unwanted side effects even when using high numerical apertures. The dependence of the laser beam penetration depth on pulse energy is evaluated by histology and an exponential decrease is observed. We show that the penetration length can be determined by evaluating the backscattered second-harmonic emission associated with the nonlinear optical properties of the tissue. This approach represents a noninvasive method for the in situ quantification of the laser beam attenuation, enabling us to adapt the pulse energy accordingly. Experiments using adapted energies show that the side effects are minimized.

Effects of cavitation bubble interaction with temporally separated fs-laser pulses

“…By tightly focusing an ultrashort laser pulse into the transparent tissue, it is possible to manipulate the medium beyond its surface in an arbitrary three-dimensional pattern. Nowadays, there are diverse clinically well-established applications, such as the fs-LASIK (laser in situ keratomileusis) [1][2][3][4][5] and keratoplasty, 6 and other intracorneal applications 7 as well as the assistance for cataract surgery. 8 Additionally, fs-laser systems for the reversal of presbyopia 9 are under investigation.…”

Deposit of glass fragments during femtosecond laser penetrating keratoplasty

“…Moreover, the modified optical properties of edematous corneas cause spherical aberration and scattering which broaden the beam. As a consequence, keratoplasty requires the use of higher energies than in the case of surgery on transparent corneas [19]. Hence, the surface in the glass in which optical breakdown occurs is increased, and particles are more likely created [14,20].…”

Self-focusing and spherical aberrations in corneal tissue during photodisruption by femtosecond laser

“…Although the beam experiences attenuation, which reduces the radiant exposure in the volume of the tissue, we estimate that the power is in a range where self-focusing is generated, for NA= 0.15 and NA= 0.3. In fact, based on considerations on the laser propagation in opaque corneas reported in a previous investigation, 18 typically for a cornea with mild edema, the 1 / e penetration depth is in the order of 300 m. According to Eq. ͑4͒ ͑see the following͒, the energy at 150 m is attenuated by a factor of 0.6, which would support our estimation.…”

“…It also limits the working depth for a given pulse energy. 18 In the present study, we probe the influence of the numerical aperture on the fs laser cutting of corneal tissue. Focusing the beam with low to moderate NAs for performing incisions in a lamellar plane parallel to the surface induced nonlinear effects outside the focal volume.…”