Abstract
Purpose: :
Femtosecond lasers have become current in refractive surgery and are presently being extended to corneal grafting. An accurate control of the laser parameters (pulse energy or numerical aperture) and the surgical protocol (surface positioning) is in order to minimise side-effects.
Methods: :
Human corneas were exposed to near-infrared femtosecond laser systems. A clinical system (Intralase FS60) and two laboratory lasers ( = 0.8 and 1.06 µm, delivering fs pulses in the µJ range at 10 kHz repetition rate) have been used. The samples were mounted on an artificial chamber positioned by a PC controlled step motor system. An optical microscope was inserted in the experimental set-up allowing the focalisation of the beam at numerical apertures from 0.15 to 1.2. The samples were subsequently analysed by optical microscopy, Transmission Electronic Microscope (TEM) and with an Electron Energy Loss Spectrometer (EELS).
Results: :
Periodical structures were observed in the cornea before and after the focalisation spot of the laser beam during lamellar cuts. These structures are believed to originate from a filamentation process (autofocalisation and defocalisation). This effect can be minimised using appropriate pulse energies and numerical apertures. It appears that energy control is also important for transfixing cut while effective cut energy is varying with the depth in the cornea. Ultrastructural analysis of the samples also showed the presence of solid fragments of submicron dimensions along the walls of the incised tissue and close to the surface. The EELS technique enables to identify these particles to be silica dioxide (glass). The applanation lens used during the LASIK surgery was examined by optical microscopy: the lens suffered micromachining from the laser and the particles are ejected from the surface of the lens to the cut in the cornea. This can be avoided by ending the protocol before the focalisation spot enter the lens.
Conclusions: :
Future systems will be improved by considering the filamentation process and by considering a better control of the positioning during the laser cut protocol.
Keywords: refractive surgery • refractive surgery: LASIK