|
Antiviral, antibacterial, antitumoral effects of laser coagulation are now of great importance in clinical ophthalmology for different corneal diseases treatment. One can note, that nowadays solid-state free-mode pulsed near-and-mid-infrared lasers with the 1.3 - 3.0 micrometers wavelengths may become widespreadly accepted in this field of ophthalmosurgery. The corneal tissue absorption coefficients for these wavelengths are lying from some parts to several thousand cm-1. Thus, by proper selection of laser wavelength, possibility is appeared to coagulate precisely the seat of pathological alterations without damaging of more deeper, non changed corneal tissue. In our work the comparative experimental study results of rabbit cornea coagulation after exposure with using of different IR lasers (with 1.32; 1.54; 1.96; 2.09 and 2.84 micrometers wavelengths) are presented. Pulse duration was about 10-3 s. The subthreshold, threshold and superthreshold energy levels were studied. Threshold energies were registered if slightly noticed coagulative changes appeared in exposed volume of corneal tissue. The cornea response resulting from laser radiation exposure was evaluated by clinical examination, slit lamp biomicroscopy, light microscopy, fluorescent staining and biochemistry. For all wavelengths initial damage was found within the cornea layers. Superthreshold injury revealed the same picture except that for 1.32 micrometers the threefold increase of the threshold energy caused additionally iris injury. The damages of the cornea were marked near the surface, stretched to more deeper layers, and damage zones were extended from the epithelium for 2.84 micrometers wavelength to whole cornea thickness for 1.32 and 1.54 micrometers wavelengths and have intermediate position for 1.96 and 2.09 micrometers wavelengths, depending on the corneal absorption coefficient. Quantitative results of threshold energy levels were calculated by probit-analysis. The radiant exposure (HD50), that cause the threshold coagulation changes in cornea with 50% probability was determined for the beam diameter (irradiance diameter) of 2 mm and for 1.32; 1.54; 1.96; 2.09 and 2.84 micrometers wavelengths. Values determined were (2.4 +/- 0.2) X 10; 7.2 +/- 0.6; 2.9 +/- 0.5; 3.7 +/- 0.2; (1.9 +/- 0.4) X 10-1 J/cm2. These values are well-correlated to corneal absorption coefficients. The absorption coefficient the higher, the value HD50 the lower and the `danger' for ocular structures damage also the lower. This fact is needed to be taken into account, because of normative documents did not reflect it in 1.3 - 3.0 micrometers wavelength range. The obtained results may be used for development of new clinical laser applications and refinement of the laser safety standards.
|
