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D.A. Chernyak, J. Wong; Use of Corneal Geometry and Pupil Position to Adjust Ablation Target in Refractive Surgery . Invest. Ophthalmol. Vis. Sci. 2003;44(13):963.
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Purpose: The aim of this work is to quantify the effect of surface geometry of the cornea and the offset between pupil center and corneal apex on the laser-tissue interactions during refractive surgery. The work also describes the method of data acquisition and the negative effect on the achieved ablation shape when the pupil is assumed concentric with the corneal vertex. Methods: An image processing method for finding the offset between pupil center and corneal vertex is presented and described for the VISX WaveScan® device. Based on the spatial relationship between the pupil and the vertex and the two keratometry values of the corneal curvature, target ablation profiles are adjusted to account for the so-called cosine effect (the reduction in ablation efficacy as the incident angle of the laser beam with the corneal surface becomes more acute). Results: The average offset of the corneal vertex with respect to a mesopic pupil center was 0.289 mm nasally with about 0.165 mm standard deviation for the 51-eye data set. The offset was also about 0.044 mm superiorly with standard deviation of 0.254 mm. If the offset is not accounted for, the laser may induce coma-like aberrations of about 0.25 µm RMS or more. Conclusions: This work addresses the expected errors in the ablation shape if the relative positions of the pupil center and the corneal vertex are not taken into account.
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