MATHEMATICS OF LASER IN-SITU KERATOMILEUSIS FOR HIGH MYOPIA

Purpose: To determine the maximal ablation that can be safely performe d with laser in situ keratomileusis (LASIK) to maintain long-term corn eal integrity. Setting: TLC The Windsor Laser Center, Windsor, Canada. Methods: The pretreatment protocols for the VISX Star, Summit Omnimed , and Chiron Technolas 116 excimer lasers generally apply 1 to 2 mu m per diopter (D) at an optical zone of 3.0 mm or less to avoid the post operative central islands that can occur with broad-beam excimer laser s. The ablation depth per diopter for the VISX Star, Summit Omnimed, C hiron Technolas 116, and Chiron Technolas 217 excimer lasers ranges fr om 10 to 24 mu m per diopter depending on the size and number of ablat ion zones and the excimer laser used. Results: Previous experience wit h lamellar surgery suggests that at least 250 mu m of central posterio r stromal tissue should be preserved to maintain long-term corneal int egrity and avoid postoperative corneal ectasia. If a 160 mu m flap is created for LASIK, the average 550 mu m cornea will have 140 mu m of c orneal stroma available for ablation. Depending on the excimer laser a nd ablation nomogram used, the maximal LASIK correction for the averag e cornea ranges from 9.8 to 15.0 D. Conclusion: The preoperative corne al thickness and the depth of the excimer laser ablation must be evalu ated before LASIK to ensure that adequate posterior corneal stroma is preserved.