Evaluation of the effective corneal ablation in refractive surgery by two 3-D topographic surface matching methods

摘要

The effectiveness of the corneal ablation process in refractive surgery is mostly evaluated by indirect measures of vision or optical quality such as post-operative refraction or wavefront aberrometry. Yet, the effective amount of corneal tissue removed in the treatment can only be determined by correctly overlapping a pre- and a post-operative topography measurement. However such an overlap is not trivial due to the discrepancy in the centration axes used in the measurement and the treatment, as well as due to the shift of ocular axes through the treatment or tilt between the two surfaces. We therefore present two methods for overlapping pre- and post-operative topographies for the purpose of extracting an effective corneal ablation profile. Method one uses a 3-dimensional profile matching algorithm and cross-correlation analysis on surface rings outside the optical zone of the topographies. Method two employs a surface normal matching routine to align the two surfaces along their common ablation axis. The profile matching method implies the problem that it requires measurement data outside of the optical zone which was found to be uncertain with placido-disk-based topographers. Method number two is more simple and implies the advantage of using measurement data within the optical zone. For regular profiles the extracted ablation profiles showed a very good match with the planned ones. Surprisingly, even for highly irregular profiles of topography-guided laser treatments the method delivered reasonable overlaps when being compared to the planned profiles. Analysis of the effective tissue removal yields valuable information on the quality of the ablation process.