INFRARED-LASER SCLEROSTOMIES

Four solid-state lasers with three fiberoptic delivery systems were used to perform laser sclerostomies in an acute-injury rabbit model and in fresh human globes. The lasers used were continuous-wave neodymium:yttrium aluminum garnet (YAG, 1.06-mu-m) and pulsed holmium:yttrium scandium galliam garnet (YSGG) (2.10-mu-m), erbium:YSGG (2.79-mu-m), and erbium:YAG (2.94-mu-m). Thermal damage to tissue and total laser energy required to produce sclerostomies decreased with increasing wavelength. In human tissue using a 600-mu-m fused silica fiberoptic, maximum thermal damage (greater-than-or-equal-to 100-mu-m) was noted at 1.06-mu-m with a total energy of 21 J at a power density of 2.5 kW/cm2. In addition, focal damage to the iris and ciliary body was noted at this wavelength. The least amount of thermal damage (15-20-mu-m) and lowest total energies needed were found at 2.94-mu-m. A 250-mu-s pulse length and pulse radiant exposures of 3.6 J/cm2 and 14.3 J/cm2 were used for the low hydroxyl-fused silica (500-mu-m) and zirconium fluoride (250-mu-m) fiberoptics, respectively. Although zirconium fluoride fibers have high through-put efficiencies that facilitate study of laser tissue interactions at 2.94-mu-m, problems encountered with fragility and solubility of the bare tip in aqueous media limit its usefulness. A high attenuation rate with the low hydroxyl-fused silica fiber limited its usable length to 35 cm at 2.94-mu-m. Tissue damage during sclerostomy formation was minimized at 2.94-mu-m, reaching a maximum at 1.06-mu-m. Minimizing tissue damage theoretically could decrease subconjunctival scarring and filtration failure. By comparing the neodymium:YAG with the erbium:YAG laser, this hypothesis can be tested in an in vivo model using a minimal number of laboratory animals.