Stromal thermal effects induced by nonmechanical (2.94-mum) erbium:YAG laser corneal trephination

To determine stromal thermal changes after erbium (Er):YAG laser corneal trephination with the use of 2 open masks.

Corneal trephination was performed in 89 enucleated pig eyes with an Er:YAG laser (400-microsecond pulse duration), 4 open masks (2 metallic and 2 ceramic) for both donors and recipients, and an automated globe rotation device. Different combinations of laser settings were used: pulse energy, 100, 200, and 400 mJ; repetition rate, 2 and 5 Hz; and spot size, 1.3 and 3.2 mm. Thermal effects in corneal stroma and regularity of the cut edges were quantitatively assessed by light microscopy, transmission and scanning electron microscopy.

Best regularity and minimal thermal effects of the cut were observed with the use of ceramic masks at 200 mJ, 2 Hz, and 3.2-mm spot size, with middepth thermal changes of 18 +/- 2 microm. Effects increased with cut depth and were lower in donor corneas and with the use of ceramic masks (P<.001). Regularity of the cut was higher in the donors (P = .05) with lower repetition rates (P<.001).

Even with the "free-running" Er:YAG laser mode, features of the trephination cut resembling those created by the 193-nm excimer laser along metal mask were achieved. Ceramic masks may be more suitable than metal masks. The Er:YAG laser seems to have the potential to be a compact and low-cost alternative in nonmechanical trephination for penetrating keratoplasty.

Thermal effects after corneal trephination with the free-running Er:YAG laser (2.94 mm) are limited and predictable.