EFFECTS OF PULSE-WIDTH ON ERBIUM-YAG LASER PHOTOTHERMAL TRABECULAR ABLATION (LTA)

An erbium (Er):YAG laser can remove trabecular meshwork (TM) by photot hermal ablation with minimal contiguous thermal damage. A variable pul se width Er:YAG laser was used to investigate the effect of varying pu lse width on ablation of human TM. Trabecular photothermal ablation wa s performed on tissue obtained from eye bank eyes at pulse widths of 5 0, 150, and 250 mus, with energy held constant at 4 mJ. At this energy , a single laser pulse was sufficient for full-thickness ablation of T M. Laser energy was delivered through a 200-mum diameter optical fiber held in apposition to the tissue sample, which was immersed in physio logic saline. High-speed photography of the resultant steam bubbles al so was performed. Light microscopy and scanning electron microscopy of TM ablated at 50 mus revealed the greatest variability in size (0-140 mum) of the full-thickness ablated areas and demonstrated blast effec ts, tissue shredding and -10 mum thermal damage. At 150 mus, the full- thickness ablated areas were more consistent in size (115-120 mum), sh owed no blast effects and 10 to 20 mum thermal damage. At 250 mus, the largest ablations were found (180-220 mum) and showed no blast damage ; however, a significant amount of thermal damage (less-than-or-equal- to 50 mum) was evident. The steam bubbles produced by the laser energy were largest at 50 mus and did not begin to collapse until well over twice the original pulse interval. At 150 and 250 mus, the steam bubbl es were successively smaller and dissipated at the end of the laser pu lse. In single pulse Er:YAG photothermal laser trabecular ablation, a pulse width (total energy of 4 mJ) around 150 mus appears to be optima l. The resultant acoustic shock wave from steam bubble formation is sm aller, its duration does not exceed the laser pulse width and tissue t hermal damage is minimal. (C) 1993 Wiley-Liss, Inc.