A comparison of keratoconus progression following collagen cross-linkage using standard or personalised keratometry thresholds

Abstract

Abstract Objective: To define how estimates of keratoconus progression following collagen cross-linking (CXL) vary according to the parameter used to identify changes in corneal shape. Materials and Methods: We estimated progression following CXL in 1677 eyes. We compared standard definitions of keratoconus progression based on published thresholds for Kmax, front K2, or back K2, or progression of any two of these three parameters, with the option of an increased threshold for Kmax values ≥55D. We excluded corneal thickness from the main analysis as this reduces unpredictably after CXL. We repeated the analysis using novel adaptive estimates of progression for Kmax, front K2, or back K2, developed separately from 6463 paired readings from keratoconus eyes, with a variation of the Bland-Altman method to determine the 95% regression-based limits of agreement (LoA). We created Kaplan-Meier survival plots for both standard and adaptive thresholds. The primary outcome was keratoconus progression five years after a baseline visit 9-15 months following CXL. Results: Rates of progression were 8% with a standard (≥1.5D) threshold for K2, or 6% with the static multi-parameter definition. With a ≥1D threshold for Kmax, the progression was significantly higher at 29%. With adaptive Kmax or K2 measurements the progression rates were similar (20%), but less than with the adaptive multi-parameter method (22%). Conclusions: Estimates of progression of keratoconus progression following CXL vary widely according to the reference criteria. Using new adaptive thresholds to define the repeatability of keratometry (LoA) gives estimates for progression markedly higher than the standard multi-parameter method.