Mathematics of laser in situ keratomileusis for high myopia

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Abstract
Purpose: To determine the maximal ablation that can be safely performed with laser in situ keratomileusis (LASIK) to maintain long-term corneal 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 pm per diopter (D) at an optical zone of 3.0 mm or less to avoid the postoperative central islands that can occur with broad-beam excimer lasers. The ablation depth per diopter for the VISX Star, Summit Omnimed, Chiron Technolas 116, and Chiron Technolas 217 excimer lasers ranges from 10 to 24 pm per diopter depending on the size and number of ablation zones and the excimer laser used. Results: Previous experience with lamellar surgery suggests that at least 250 pm of central posterior stromal tissue should be preserved to maintain long-term corneal integrity and avoid postoperative corneal ectasia. If a 160 μm flap is created for LASIK, the average 550 pm cornea will have 140 pm of corneal stroma available for ablation. Depending on the excimer laser and ablation nomogram used, the maximal LASIK correction for the average cornea ranges from 9.8 to 15.0 D. Conclusion: The preoperative corneal thickness and the depth of the excimer laser ablation must be evaluated before LASIK to ensure that adequate posterior corneal stroma is preserved.

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