Effects of pulse width on erbium: YAG laser photothermal trabecular ablation (LTA)

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Abstract
An erbium (Er):YAG laser can remove trabecular meshwork (TM) by photothermal ablation with minimal contiguous thermal damage. A variable pulse width Er:YAG laser was used to investigate the effect of varying pulse width on ablation of human TM. Trabecular photothermal ablation was performed on tissue obtained from eye bank eyes at pulse widths of 50, 150, and 250μs, with energy held constant at 4 mJ. At this energy, a single laser pulse was sufficient for full-thickness ablation of TM. Laser energy was delivered through a 200-μm diameter optical fiber held in apposition to the tissue sample, which was immersed in physiologic saline. High-speed photography of the resultant steam bubbles also was performed. Light microscopy and scanning electron microscopy of TM ablated at 50 μs revealed the greatest variability in size (0–140 μm) of the full-thickness ablated areas and demonstrated blast effects, tissue shredding and ⩽10 μm thermal damage. At 150 μs, the full-thickness ablated areas were more consistent in size (115–120 μm), showed no blast effects and 10 to 20 μm thermal damage. At 250 μs, the largest ablations were found (180–220 μm) and showed no blast damage; however, a significant amount of thermal damage (⩽50 μm) was evident. The steam bubbles produced by the laser energy were largest at 50 μs and did not begin to collapse until well over twice the original pulse interval. At 150 and 250 μs, the steam bubbles were successively smaller and dissipated at the end of the laser pulse. In single pulse Er:YAG photothermal laser trabecular ablation, a pulse width (total energy of 4 mJ) around 150 μs appears to be optimal. The resultant acoustic shock wave from steam bubble formation is smaller, its duration does not exceed the laser pulse width and tissue thermal damage is minimal.

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