[Studies of the development of refractive effects in intrastromal refractive corneal surgery with the picosecond laser].

BACKGROUND

Picosecond laser intrastromal photorefractive keratectomy (ISPRK) aims at achieving a flattening of the central cornea by plasma-mediated tissue evaporation without affecting the anterior or posterior corneal layers. We investigated the laser-induced tissue effects to establish a functional relationship between laser parameters and tissue removal and to assess their influence on the healing process and long-term refractive changes.

MATERIALS AND METHODS

A modified ISL 2001 System with a cone angle of 30 degrees was used for in vitro investigations of the laser effects in water and porcine cornea. Photographic methods were used to determine the plasma volume and the thickness of the laser-generated intrastromal bubble layer as a function of the pulse energy and the number and separation in which the pulses were applied (216 eyes). Histological evaluation was done by polarization microscopy (9 eyes).

RESULTS

Polarization microscopy revealed only minor signs of thermal tissue damage. The maximum amount of tissue that can be evaporated without damaging the outer corneal layers corresponds to a layer about 10 microns thick. With a 6-mm optical zone, this tissue removal yields an immediate refractive effect of only 0.85 dpt. Stronger long-term refractive changes observed in animal experiments and clinical studies must thus be due to the healing response of the cornea. The healing response may be induced by mechanical distortion due to intrastromal bubble formation affecting about one third of the corneal thickness.

CONCLUSION

Since the refractive effects are apparently strongly influenced by corneal healing, they are poorly predictable and can probably not be used for clinical purposes.