This is the most commonly used measure of safety because it answers the question, “If the refractive outcome is not totally acceptable, can the patient put glasses on again and see as well as they did before surgery?” A change of 1 Snellen line is within the range of normal biological variability for repeated measures, and therefore is not a meaningful change. A change of 2 or more Snellen lines has been generally adopted as the standard for safety.(1)
This function calculates the standardized Graph C detailed in the Journal of Refractive Surgery(2) from a two column matrix. First column should contain the preoperative visual acuity with correction (CDVA) and the second column the postoperative CDVA. The plot is only applicable to laser refractive surgery procedures.
Two additional values are returned from the function, the Safety Index (SI) known as the ratio between postoperative and preoperative CDVA(3) and the percentage of eyes with CDVA worse than 20/40 for the subgroup of eyes with CDVA of 20/20 or better preoperatively.(4)
The Refractive Analysis is an Open Source Toolbox for Matlab developed by Rodríguez-Vallejo M for the Analysis of Vision Science Results. It is freely available for the students of our online course but you need the Matlab software to use it. Matlab software is not provided with the course.
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