Far-ultraviolet laser ablation of the cornea: photoacoustic studies.

Wide bandwidth piezoelectric transducers made of thin (9 microns) polyvinylidene fluoride film have been used to make time-resolved measurements of the stress-wave generated by far-ultraviolet (193 nm) laser ablation in corneal tissue in vitro. At high fluence (approximately 250 mJ/cm2), ablation commences within 10 ns (+/- 5 ns) of the laser pulse and generates short acoustic impulses (approximately 30 ns). The time profile of the ablation, when coupled to the energy requirements for ablation from earlier work, allows the estimation of a temperature and a half-life for the thermal decomposition of the collagen in cornea. These values do not support a photothermal mechanism for the ablation under the experimental conditions.

Excimer laser surgery of the cornea.

The excimer laser, which produces light in the far-ultraviolet portion of the spectrum, allows precise removal of corneal tissue through a photochemical laser-tissue interaction. This interaction is not thermal and does not involve optical breakdown; rather, it directly breaks organic molecular bonds without tissue heating. We used this process of ablative photodecomposition to remove corneal tissue in a series freshly enucleated cow eyes. Applying the far-ultraviolet light in short intense pulses permitted us to control the depth of the incision with great precision. We found that 1 joule/cm2 ablates corneal tissue to a depth of 1 micron. Adjacent tissue suffered no thermal damage and the stromal lamellae adjacent to the incision showed no evidence of disorganization.