Mid-infrared laser ablation of the cornea: a comparative study

The ablation thresholds and patterns of collateral damage in cornea produced by Er:YAG (2.94 microns) and Er:YSGG (2.79 microns) lasers were measured. Two different pulse durations, 200 microseconds (normal spiking mode) and 100 ns (Q-switched mode), were used at both wavelengths. In the normal spiking mode, damage zones of 16 +/- 2 microns and 39 +/- 7 microns and ablation thresholds of 250 +/- 20 mJ/cm2 and 420 +/- 35 mJ/cm2 were measured at 2.94 microns and 2.79 microns, respectively. In the Q-switched mode, damage zones of 4 +/- 2 microns and ablation thresholds of 150 +/- 10 mJ/cm2 were found irrespective of the laser used. The similarity between the results using the Er:YAG and Er:YSGG lasers in the Q-switched mode suggest that either laser can be used with equal effectiveness for corneal trephination.

Absorption of 308-nm excimer laser radiation by balanced salt solution, sodium hyaluronate, and human cadaver eyes

Absorption of the excimer laser radiations of 193-nm argon fluorine and 308-nm xenon chloride in balanced salt solution, sodium hyaluronate, and human cadaver eyes was measured. The absorption of these materials as considerably different for the two wavelengths; we found that 308-nm light experienced much less absorption than the 193-nm light. The extinction coefficient (k) for 308 nm was k = 0.19/cm for balanced salt solution and k = 0.22/cm for sodium hyaluronate. In contrast to this, the extinction coefficient for 193 nm was k = 140/cm for balanced salt solution and k = 540/cm for sodium hyaluronate. Two 1-day-old human phakic cadaver eyes showed complete absorption with both wavelengths. Using aphakic eyes, incomplete absorption was noted at the posterior pole with 308 nm and complete absorption was noted with 193 nm. The extinction in the anterior part of aphakic eyes (the first 6 mm) was 4.2/cm for 308 nm, meaning that the intensity of the light is reduced by a factor of 10 after traveling the first 5.5 mm. However, we observed that the material in the eye fluoresces, meaning the 308 nm is transformed into other (longer) wavelengths that travel through the total eye with minimal absorption. Conclusions drawn from this experiment are that the use of the 308-nm wavelength may have undesirable side effects, while the use of the 193-nm wavelength should be consistent with ophthalmic use on both the cornea and the lens.

Fibronectin on excimer laser and diamond knife incisions

The experiment was designed to illustrate the difference between the diamond knife and the excimer laser (ArF 193 nm) on keratotomy incisions. The histopathologic effect of fibronectin applied to these wounds was observed. Fibronectin demonstrated a chemotactic effect on fibroblasts and increased the amount of newly synthesized collagen deposited at the incision sites. The excimer laser incisions treated with fibronectin had 43% more fibroblasts per 2,500 microns 2 than the excimer laser incisions not treated with fibronectin. The adjustable diamond knife incisions treated with fibronectin had 42% more fibroblasts than the adjustable diamond knife incisions not treated with fibronectin. The excimer laser ArF 193 nm incisions had 18% more fibroblasts in the incision area than the adjustable diamond knife incisions independent of fibronectin usage. This indicates that the excimer laser incisions and the diamond knife incisions may have different healing mechanisms--perhaps because of the distinctive modes of action of these instruments.