Nomogram, corneal topography, and final prescription relations for corneal refractive therapy

To analyze the relationships between the first diagnostic lens and the lens finally prescribed for corneal refractive therapy with Paragon CRT and to correlate them with corneal topography, and refractive parameters.

The files of 86 patients who were successfully fitted with corneal refractive therapy using Paragon CRT were reviewed. Differences between diagnostic vs. finally prescribed lens parameters of base curve radius (BCR), return zone depth (RZD) and landing zone angle (LZA), as well as the number of lenses required to achieve the final fit were correlated with corneal topographic parameters and attempted refraction.

There were statistically significant differences between CRT parameters for the first lens suggested by the nomograms and the lens finally prescribed (Wilcoxon signed ranks, p<0.05). On average, those changes did not achieve 1 full step difference (i.e., 0.10 mm for BCR and 25 microm for RZD). Only LZA achieved a full step change (1 degrees). According to our data, 92% of the fittings were achieved by changing only two parameters or less. The majority of changes required were within+/-1 step of the initially selected (96% for BCR, 99% for RZD, and 100% for LZA). Changes in lens parameters are related with some ocular parameters, but mainly with first lens parameters. RZD was the parameter requiring more changes. The amount of change in RZD was correlated with steep keratometry, BCR, and RZD of the first lens.

Nomograms used for lens selection with Paragon CRT have a high first-fitting success with a relatively simple fitting procedure. Correlations between final lens parameters and ocular and initial lens parameters obtained in this study can assist clinicians to anticipate the level of difficulty in achieving optimal fitting with Paragon CRT sigmoid contact lens for corneal refractive therapy and the most frequent changes needed to achieve it.