Abstract
Objective To define if keratoconus progression estimates following collagen cross-linkage (CXL) vary according to the parameter used to identify changes in corneal shape.
Methods We estimated progression following CXL in 1,677 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 pachymetry from the 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 6,463 paired readings from keratoconus eyes, with a variation of Bland-Altman to determine the 95% regression-based limits of agreement (LoA). We created Kaplan-Meier survival plots for standard and adaptive thresholds. The primary outcome was keratoconus progression five years after a reference 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 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 keratometry method.
