From wavefront device to laser: an alignment method for complete registration of the ablation to the cornea

PURPOSE: When customized laser refractive surgery is based on a wavefront measurement, correct registration of the laser ablation to the prescribed treatment area is vital. If the treatment is not ablated exactly where it was measured, the error could negate some benefits of the procedure. Currently, treatments are aligned to the pupil center, which has been shown to shift between measurement and treatment. This article describes an alignment method that uses an image-based coordinate system to compensate for shifts of the pupil center as well as cyclorotation. METHODS: Images of patients' eyes were taken during wavefront measurements and again when patients were in position for surgery. An image-processing method, which matches multiple ocular landmarks in both image sets, was used to align the laser beam to the ablation target. RESULTS: Analyses of the data collected from both instruments suggest that alignment errors caused by movement of the pupil center and cyclorotation could be significantly reduced by using the proposed alignment method. CONCLUSIONS: Because the pupil center undergoes significant movement with changes in lighting conditions, it is unsuitable as the sole alignment point for custom ablations. Several reference points are necessary to compensate for changing pupil size as well as the cyclotorsional eye position changes that occur between wavefront measurement and laser procedure. By providing multiple points for alignment, the registration method described herein increases safety and effectiveness of wavefront-based laser vision correction.