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

Purpose:Thermal effects on the laser aperture mask map play a major role in the thermal loading of the cornea during nonmechanical trephination in pen etrating keratoplasty. The purpose of this study was to assess the temperat ure increase on the laser mask using the 2.94-mu m Er:YAG laser in order to find suitable parameters for avoidance of thermal damage to the cornea. Me thods: Thermal load measurements were performed on donor (7.5 mm trephinati on diameter, 0.7 mm thickness, central hole 3.0 mm) and recipient (7.5 mm t rephination diameter, 0.7 mm thickness, outer diameter 13.0 mm) aperture ma sks. 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 o f the mask (setup 2) or in the ablation area under dynamic conditions, rota ting the whole globe to simulate a constant trephination speed with the mas k 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 trephi nation margin (half of the beam on the mask and half of it on the cornea) w ith a pulse energy of 200 or 400 mJ and 18CrNi10 stainless steel versus thr ee different types of ceramic masks (silicium carbide, silicium nitrite, al uminum oxide). Temperature was assessed using an infrared pyrometer with au tomatic data acquisition software for a personal computer. Results: Overall , the temperature rise ranged between 43.6 K (metal donor mask at the treph ination area with 400 mJ pulse energy) and 3.3 K (silicium carbide recipien t 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 significa ntly higher (P<0.02) than the heating of the three types of ceramic masks. The silicium carbide masks revealed the lowest temperature rise. Comparing the three setups, the temperature rise was maximal under static conditions in the ablation area and minimal at the opposite side, with the dynamic set up ranging in between. Temperature rise was significantly greater (P<0.04) in donor masks than in recipient masks for each mask material and both ener gy levels. Conclusion: The physical characteristics of silicium carbide mas ks seem superior to those of metal masks with regard to minimizing the ther mal load of the epithelium or superficial stroma during Er:YAG laser trephi nation of the cornea for penetrating keratoplasty.