Cavitation bubble dynamics and shock wave generation in eye surgery using the pulsed neodymium:YAG laser

Abstract

In a model experiment using polyethylene membrane in balanced salt solution the processes which take place during posterior capsulotomy with pulsed Nd:YAG lasers were investigated by means of high-speed photography and hydrophone measurements. It was found that cavitation bubbles with a maximum diameter of 1.5 to 2.3 mm develop at the focal point even with a pulse energy of 5 mJ; i.e., their spatial extent is much greater than that of the ruptures observed. Furthermore, during optical breakthrough and when the bubble collapses shock waves are generated with a pressure amplitude of 9 to 16 bat at a distance of 18 mm or 162 to 288 bar at 1 mm. Previous estimates ranged from more than 0.1 bat at 15 mm to several kilobar at 1 mm. Above a certain threshold value the energy of the cavitation bubble and of the shock waves is approximately proportional to the energy of the laser pulses with which they are generated. At the same time pulse energy the diameter of the bubbles generated by Q-switched lasers is around 1.5 times as large as those generated by model-locked lasers--the bubble energies are more than three times as great. The shock wave amplitudes after O-switched pulses are likewise 1.5 times as great as after mode-locked pulses and the energy of the shock waves is approximately double. Nevertheless, the size of the holes caused in a polyethylene membrane with a pulse energy of 2 mJ to 5 mJ does not depend on the laser operating mode. Thus, with the polyethylene membrane, no great correlation was found between the mechanical energy acting on it and the size of the holes thus created.(ABSTRACT TRUNCATED AT 250 WORDS)