Intracorneal ring segment in keratoconus: a model to predict visual changes induced by the surgery.

PURPOSE We detected keratoconus cases with a significant potential for poor outcomes following intracorneal ring segment implantation (ICRS). We attempted to predict the potential of a case in terms of gain or loss of corrected distance visual acuity (CDVA). METHODS In this retrospective and prospective, consecutive, nonrandomized, multicentric study, 58 keratoconic eyes (aged 1756) were implanted with the keraring using the femtosecond laser technology. The follow-up period was 6 months. keratometric, biomechanical, aberrometric, refractive, and visual variables were measured for two different groups: Group A included eyes that gained 0.2 or more in corrected distance visual acuity (CDVA, decimal scale) and group B included eyes that lost more than 0.15. Correlations between clinical parameters and changes in visual acuity were investigated. In addition, a linear regression model was developed using CDVA, apical keratometry (AK), and a new keratometric parameter defined and named by us as K-factor (K(F)). RESULTS Significant differences between groups preoperatively were found for CDVA (P = 0.002), AK (P = 0.013), and K(F) (P = 0.025). The following predictive model was obtained using these variables: DeltaCDVA = -0.511 + 0.0007K(F)(p)-0.849CDVA(p)+0.008AK(p).$$ Predictability of the model was 0.797. Sensitivity was 88.1% and specificity 83.3%. CONCLUSIONS The mathematical model predicts that this surgery is very effective in patients with preoperative CDVA (decimal scale) in the range of 0.01 to 0.3, predicting a gain of 3, 4, or even 5 lines. Gains are predicted and confirmed for CDVA between 0.3 and 0.5. For preoperative CDVA between 0.5 and 0.75 visual outcomes are doubtful. Higher values of CDVA often are related to a decrease in CDVA. This model may help surgeons to select the best cases for ICRS implantation and exclude the worst in terms of visual outcomes.