Corneal endothelial injury thresholds for exposures to 1.54 micro m radiation.

The endothelial injury threshold was determined in rabbit for an 11-s exposure to 1.54 micro m radiation from an Erbium fiber laser. The beam was Gaussian with a 1/e diameter of 7 mm. Cell damage was detected with a wet staining technique. The threshold dose for these conditions is 4.4 x 10(5) J m(-2) (44 J cm) and is only 9% greater than the threshold for epithelial damage for the same exposure conditions. Exposures just above the threshold caused substantial endothelial damage, including loss of cells. The calculated endothelial temperature increase at the threshold is similar to that calculated for the epithelium at its injury threshold for the same exposure conditions. The results suggest that endothelial damage can be correlated with a critical temperature damage model with a critical temperature increase near 40 degrees C. The results also suggest that if a person were to receive an exposure only slightly above the epithelial injury threshold from a beam having a diameter of 7 mm (which is the diameter of the exit pupil of 7 x 50 binoculars) he or she would risk sustaining substantial endothelial damage.

Light-scattering and ultrastructure of healed penetrating corneal wounds.

PURPOSE: To investigate quantitatively for the first time the relationship between light-scattering and ultrastructure of semitransparent scars resulting from penetrating wounds in rabbit cornea. METHODS: Penetrating wounds, 2 mm in diameter, were made in the central cornea and allowed to heal for 3.6 to 4.5 years at which time the rabbits were killed. The scar and cornea thickness outside the scar were measured using ultrasonic pachymetry. Corneas were excised immediately and their transmissivity was measured from 400 to 700 nm. The tissue was then prepared for transmission electron microscopy. Transmission electron micrographs (TEMs) were analyzed to determine fibril positions and radii. Scattering was calculated using the direct summation of fields (DSF) METHOD: RESULTS: Scar thickness averaged 0.26 +/- 0.04 mm, and the scars were flat. Thickness outside the scars averaged 0.40 +/- 0.04 mm. Three scars were moderately transparent, five were less transparent, and one was much less transparent. The wavelength dependence of the measured total scattering cross- section was indicative of the presence of voids (lakes) in the collagen fibril distribution, and lakes were evident in the TEMs. The images showed enlarged fibrils and some showed bimodal distributions of fibril diameters. Calculated scattering was characteristic of that expected from regions containing lakes-a finding consistent with the scattering measurements. CONCLUSIONS: Despite the long healing time, these scars remained highly scattering. A combination of lakes, disordered fibril distributions, and a significant population of enlarged fibrils can explain the scattering. A possible cellular contribution cannot be ruled out.

corneal epithelial injury thresholds for exposures to 1.54 microm radiation-dependence on beam diameter.

Corneal epithelial injury thresholds have been determined for exposures to 1.54 mum infrared radiation from an Erbium fiber laser. Thresholds were determined for beam diameters from 0.05 to 0.7 cm for exposures having durations from approximately 1 to 100 s and for a fixed beam diameter of 0.1 cm for exposures with durations between 0.036 and 0.26 s. Near-threshold damage appeared within 30 min post-exposure. There was no evidence of latent damage from lesser exposures appearing up to 24-48 h post-exposure. The dependence of the threshold radiant exposures on laser beam diameter for exposures >1 s provides strong evidence supporting a critical temperature damage model. However, the shorter exposures are not in accord with a critical temperature damage model. Thresholds for exposures longer than 1 s are greater than 10 times the maximum permissible exposure (MPE) in ANSI Z-136.5-2000; however, the safety factor decreases to less than 10 for exposures less than 0.1 s with a 0.1-cm-diameter beam.