PHOTOFRAGMENTATION OF LENS NUCLEI USING THE ER-YAG LASER - PRELIMINARY-REPORT OF AN IN-VITRO STUDY

The energy of the erbium:YAG laser (2,940-nm wave-length) can be used for minimally traumatic photoablation due to its high absorption at th e tissue water and its consequently low penetration depth. Laser scler ostomy ab externo, an application of this principle, has undergone adv anced clinical investigation. Another potential application is photofr agmentation of the lens for cataract extraction. A laboratory model Er : YAG laser (flashlamp-pumped, 200-mu s pulse length, 5-Hz repetition frequency) was coupled to a short low-OH quartz fiber (400 mu m in dia meter). The laser energy was applied by direct contact of the fiber ti p to human lenses with very dense cataract. The lenses rested in a sma ll cuvette filled with an aqueous-humor-analogous fluid. The fragmenta blation rate was evaluated in relation to the number of pulses and the pulse energy. A laser-triggered flash-photography unit was engaged to visualize the ablation dynamics. We found tissue-ablation rates to ra nge from 4 to 19 mu g/pulse, depending on the nucleus density and ulse energy. The maximal size of the removed fragments was always below 50 0 mu m During ablation, rapidly increasing and collapsing cavitation b ubbles were photographed at the distal tip of the application fiber. T he impact radius of these cavitation effects markedly exceeded the pur e penetration depth of laser radiation at a 2.9-mu m wavelength. A cli nical application of the method should be possible as judged by the re sults obtained for tissue-ablation rate and fragment size. Cavitation- bubble dynamics seems to be responsible for the high fragmentation eff iciency. Special application probes have to be developed to optimize a blation and to prevent inadvertent destruction of the posterior lens c apsule by cavitation effects.