[Quantification of the wavelength dependence of laser-induced choroid coagulation]

The penetration depth of laser light into the fundus is dependent on the wavelength. Thereby, the profile of damage depends on the absorption and on the temperature profile. It has been demonstrated earlier that wavelength-dependent differences in penetration depth are hard to detect ophthalmoscopically or histologically in severe lesions in Chinchilla gray rabbits. Therefore in this study, the wavelength-dependent exposure powers necessary for (1) similar retinal effects and (2) similar choroidal effects were compared. The resulting wavelength-dependent differences were smaller than expected-due to at least three reasons: (1) predominance of the temperature profile over the absorption profile for clinically useful exposure times; (2) light scatter in the retina in relationship to the exposure time (and wavelength?): (3) changes in the choroidal geometry in relationship to the exposure time and wavelength. Nevertheless, it appears to be possible to spare the retina somewhat by using longer wavelengths. In order to evaluate the clinical situation experimentally despite the fact that the human choroid is about three times thicker than the rabbit choroid, a geometrical model has been developed. The results indicate that the retina is less damaged using Nd:YAG lasers than using argon lasers when similar choroidal effects are produced.