Alignment in Customized Laser in situ Keratomileusis

PURPOSE: To identify fundamentals of beam alignment in customized laser in situ keratomileusis (LASIK) with a special focus on the Nidek NAVEX system. METHODS: Analysis of Nidek specifications and recommendations for beam alignment with regard to a critical case example are presented. The potential impact of misalignment (tilt and defocus) is calculated. Cyclotorsional error evaluation in a normal LASIK population was performed by video image comparison. Potential problems of infrared-based eyetracking systems are discussed. RESULTS: The laser beam should be aligned with reference to the line of sight (LOS) when customized segmental laser ablation is applied. Only in cases with significant offset between the LOS and the visual axis it is recommended not to use segmental ablation and to manually align the beam toward the visual axis. Eye drift (tilt) as well as defocus should be avoided since undercorrection and irregular astigmatism can result. Almost 30% of eyes in a normal LASIK population showed a torsional error of 5 degrees to 10 degrees (9% more than 10 degrees) on video-based image comparison. Eye-trackers not only are defined by take-up speed and latency, but also by their sensibility (picking the real center of the pupil) and robustness (keeping it tracked during surgery). Errors due to parallax and reflex effects can systematically influence the performance of an eyetracker. CONCLUSION: Correct alignment is difficult to achieve but is of fundamental importance in customized LASIK. Strict standardization and further improvement in the alignment strategy is necessary to achieve more consistent results in customized LASIK.