Algorithms for decision on intrastromal ring segment implantation

Success in improving vision with intracorneal ring segment (ICRS) implantation for keratoconus depends on understanding the phenotype/pattern present in a particular cornea, the relative weight of the different aberrations (astigmatism, coma, spherical), the mechanism of action of the implants, and the possible modalities of implantation. The application of a particular nomogram makes sense only there has been a decision on the location of the implants relative to the selected landmarks (axis of refractive or topographic astigmatism, coma axis, etc.). It is possible to define all possible modalities of implantation of 1 or 2 ICRS based on only two features: whether there is implantation symmetry and the position of the center of the implant relative to one significant landmark (usually the flat topographic meridian) what we call “axiality”. The resulting matrix/classification includes 6 possible modalities. This presentation will develop on the author's methodology for ICRS selection based on this concept.