Deposit of Glass Fragments at the Lens-Cornea Interface During Femtosecond Laser Treatment

Abstract

Purpose:: Using a femtosecond laser to perform incisions in corneal tissue produced solid fragments close to the surface of the treated region. By the electron energy loss spectroscopic technique (EELS), it could be demonstrated that the particles were composed of silicon dioxide, which came from the applanation lens in contact with the cornea.

Methods:: Human corneas were exposed to a femtosecond laser, composed of an oscillator and a CPA system with a regenerative amplifier delivering pulses at a wavelength of 1.06 µm, pulse durations of 400 fs, a maximum energy of 6 µJ, and a repetition rate of 10 kHz. The experimental set-up provided a diffraction limited beam at a numerical aperture of 0.15, leading to peak power densities in the order of 10¹³W/cm². Samples were mounted on an anterior chamber system, installed on a computer controlled step motor positioning system. This set-up allows to treat cornea in situ, reproducing the in vivo environment of the eye. Samples were analysed, a posteriori, by electron energy loss spectroscopy in an electron microscope.

Results:: Posterior-to-anterior incisions were performed in human corneas with femtosecond pulses at energies equal or superior to the threshold energy for tissue disruption. The ultrastructural analysis of the samples showed the presence of solid fragments of nanometric dimensions, along the walls of the incised tissue and close the surface. The EELS technique enabled to discover the nature of these nanoparticles, which proved to be composed of silicon dioxide. In parallel, an applanation lens used during a LASIK surgery was examined by optical microscopy. The side of the lens in contact with the cornea was not completely smooth and the micromachining path of the laser corresponding to the creation of the corneal flap could be found and observed.

Conclusions:: When performing a LASIK procedure with a femtosecond laser, nanoparticles from the applanation lens may settle on the corneal surface.

Keywords: refractive surgery • laser