Thermal load of laser aperture masks in nonmechanical trephination for penetrating keratoplasty with the Er:YAG laser : comparison between stainless steel and ceramic masks

Thermal effects on the laser aperture mask may play a major role in the thermal loading of the cornea during nonmechanical trephination in penetrating keratoplasty. The purpose of this study was to assess the temperature increase on the laser mask using the 2.94-microm Er:YAG laser in order to find suitable parameters for avoidance of thermal damage to the cornea. Thermal load measurements were performed on donor (7.5 mm trephination diameter, 0.7 mm thickness, central hole 3.0 mm) and recipient (7.5 mm trephination diameter, 0.7 mm thickness, outer diameter 13.0 mm) aperture masks. The masks were either mounted on a thermal isolator or fixed directly on porcine corneal samples. Temperature increase was measured either under static conditions in the ablation area (setup 1) and at the opposite side of the mask (setup 2) or in the ablation area under dynamic conditions, rotating the whole globe to simulate a constant trephination speed with the mask positioned directly on a porcine cornea (setup 3). We used the NWL Er:YAG solid-state laser in a 1.3-mm free-running spot mode focused on the trephination margin (half of the beam on the mask and half of it on the cornea) with a pulse energy of 200 or 400 mJ and 18CrNi10 stainless steel versus three different types of ceramic masks (silicium carbide, silicium nitrite, aluminum oxide). Temperature was assessed using an infrared pyrometer with automatic data acquisition software for a personal computer. Overall, the temperature rise ranged between 43.6 K (metal donor mask at the trephination area with 400 mJ pulse energy) and 3.3 K (silicium carbide recipient mask at the opposite side of the mask with 200 mJ pulse energy). With all setups and both energy levels, the heating of the metal mask was significantly higher (P