Carbon dioxide lasers are used in numerous applications that involve human exposure to
the radiation that can produce ocular injury. The objective of this study is to show that
the thermal gradient produced in the eye by the radiation from an 80 ns CO2 laser pulse
can generate a thermoacoustical tensile pressure wave with large enough magnitude to
rupture the epithelial layer of the cornea. A Gaussian-shaped temperature distribution
will be employed. It is assumed that the corneal tissue is inhomogeneous, with the
density and wave velocity varying slowly in space. Under these conditions, the
acoustical wave equation is decoupled into two first-order partial differential equations,
one that propagates energy into the eye from the point of thermoacoustical wave
generation, and the other toward the front of the eye. These equations are solved
numerically using the Lax-Wendroff numerical method. A compressional wave
generated in the epithelial tissue of the cornea due to the thermal gradient of the laser
arrives at the air-tear layer interface with a pressure amplitude of ~6600 Pa. When this
wave is reflected back into the eye, the resulting tensile pressure wave has a tensile
strength of approximately 4.6 x 108 Pa/m just inside of the epithelial layer of the cornea.
This is an order of magnitude larger than what is necessary to produce cellular damage to
the cornea.
Corneal epithelial damage thresholds for exposures to sequences of pulses of 1.54 μm infrared radiation produced by an Er fiber laser were investigated. Thresholds were determined for sequences of 8 to 128 pulses at a repetition frequency of 10 Hz and 8 to 256 pulses at 20 Hz. The duration of the individual pulses was 0.025 sec and the 1/e diameter of the laser beam was 0.1 cm. The results show that threshold damage is correlated by an empirical power law of the form Hth = CN-β, where Hth is the threshold radiant exposure per pulse, and N is the number of pulses. The constant C is different for the 10 Hz and 20 Hz exposures and, for both cases, is greater than the estimated threshold for a single 0.025 sec pulse. Thus the empirical power law breaks down for small numbers of pulses (viz., N< 8), where it overestimates the damage thresholds. Temperature calculations for the threshold exposure conditions show that a critical temperature model also correlates the multiple-pulse injury thresholds.
Corneal epithelial injury thresholds have been determined for exposures to 1.54 μm infrared radiation having durations from 1 to 100 sec and beam diameters from 0.5 to 7 mm. For 1 sec exposures, measured thresholds range from 12 W/cm2 (5 mm diameter beam ) to 67 W/cm2 (0.5 mm diameter). For 2 sec exposures, they range from 9 W/cm2 (7mm diameter) to 57 W/cm2 (0.5 mm diameter). For 10 sec exposures, they range from 3.7 W/cm2 (7mm diameter) to 33 W/cm2 (0.5 mm diameter). For 100 sec exposures, they are 1.4 W/cm2 (7mm diameter) and 3.7 W/cm2 (2mm diameter). The dependence of the measured thresholds on laser beam diameter provides strong evidence supporting a critical temperature damage model. These measured thresholds are greater than 10 times the maximum permissible exposure (MPE) in ANSI Z-136.5-2000.
Corneal epithelial damage thresholds were determined for exposures to sequences of pulses from a Tm:YAG laser (wavelength 2.02 ?m). Pulse repetition frequencies were 1, 10, 20, and 100 Hz and individual pulse durations were 0.300 sec at 1 Hz, 0.025 sec at 10 and 20 Hz, and 0.005 sec at 100 Hz. Threshold damage is correlated by an empirical power lawof the form '1th= CN?, in which '1th is the threshold irradiance and N is the number of pulses. The constant C differs depending on the pulse repetition frequency and individual pulse duration. The exponent a varies between 0.22 and 0.29. For some exposure conditions the empirical power law underestimates the damage threshold for small numbers of pulses.
We report on the use of a recently developed scatterometer to make objective measurements of haze that develops following excimer laser ablation of cornea. Phototherapeutic keratectomies (PTK), 6.0 mm dia, 0.25 mm transition zone, 150 micrometers stromal depth, were done on 4 pigmented rabbits using the VISX Model 20/20 laser system. Scatterometer measurements were made on the normal cornea prior to ablation and at various times up to 125 days postablation and are compared to slit-lamp photographs. Scattering from 3 of the eyes exhibited nearly identical behavior, peaking 20-25 days after treatment at 10 to 12 times the preablation level and then diminishing slowly. Scattering from the fourth eye peaked 14 days after treatment at 28 times the preablation level and remained well above the level of the other three eyes throughout the experimental period. These results suggest that identical PTK treatments can lead to different degrees of subepithelial haze.
Normal cornea transmits greater than 90% of visible light, but its transmission would be less than 30% if the stroma's collagen fibrils scattered independently of one another. Thus modern transparency theories are based on there being sufficient order in fibril positions for destructive interference to cause cancellation among the scattered fields. Two types of structure have been proposed: long-range crystalline order as used in the earliest theory, and short-range liquid-like order such as that depicted by electron microscopy. Of course structures depicted in electron micrographs may be distorted and other tests are required to determine the nature of the order. Light scattering measurements can afford such a test. Specifically, the two types of order produce different dependencies on wavelength for the scattering cross-section (angular or total) in the long-wavelength limit. Measurements must be analyzed appropriately to obtain the long-wavelength limit. The results reported in this paper demonstrate that measurements of both angular and total scattering cross-sections support short-range order of fibril positions.
We have developed a simple instrument for making objective measurements of haze that develops following excimer laser ablation of the cornea. It consists of an appropriately modified slit-lamp microscope, with a fiber optic pickup, a filter system for wavelength selection, and a photomultiplier detector. The scattered intensity at 120 degree(s) from the forward direction is determined. Preliminary tests were made by measuring the haze following a deep photorefractive ablation on a rabbit cornea under conditions which ensured that rather severe haze would develop. The VISX Model 20/20 laser system was set to produce a 6.0 mm diameter, -15 D correction, with a central depth of 236 micrometers . Measurements were made on the normal cornea prior to ablation and at various times up to 114 days post-ablation and are compared to slit-lamp photographs. Scattering peaked two weeks post-ablation at a value approximately 40X that of the normal (unablated) cornea and gradually decreased to approximately 11X the normal value at 114 days.
Infrared radiation from a CO2 laser at a wavelength of 10.6 im is strongly absorbed by the cornea. Indeed, 99% is absorbed within the first 50 itm of tear film and epithelium. This energy is rapidly converted to heat that initially is concentrated in the volume of absorption and subsequently is conducted to deeper layers of the cornea and beyond. Consequently, various layers of the cornea can sustain thermal damage, depending on the exposure conditions. In this review we summarize very briefly our past work on: epithelial damage thresholds for single- and multiple-pulse exposures having individual pulse durations between 1 ms and 10 s12; endothelial damage thresholds and endothelial temperature histories34; and damage thresholds for stromal cells.5 For very short duration pulses that have extremely high peak irradiance, the possibility for acoustic as well as thermal damage exists. Here we give a more extensive report of new epithelial damage thresholds for single- and multiple-pulse exposures with an individual pulse duration of 80 ns. We note that material is ejected from the corneal surface at near-threshold exposures. This observation is used in conjunction with lesion histology and temperature computations to discuss possible damage mechanisms.
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