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Evaluation of statistical limits of parametric variability in cross-linked keratoconic corneas: when to define treatment failure/progression after cross-linking?

Poster Details

First Author: G.Prakash UNITED ARAB EMIRATES

Co Author(s):    V. Jhanji   R. Philip   R. Bacero   D. Srivastava           

Abstract Details


A major question faced by keratoconus specialists is to when to consider an apparent worsening of corneal parameters after cross linking as �â�€�œinsufficient / failed�â�€� treatment. Initial scans (upto 6 months) may show apparent worsening, further complicating the interpretation. There is paucity of literature in repeatability data of cross linked corneas after a sufficient time (> 1year) after cross linking. The purpose of this study was to look at keratoconus cases post cross linking (>1year) and evaluate their intersession repeatability to define limits of normal variation and thus define numerical guidelines for considering the initial treatment as insufficient/failed.


Cornea and Refractive Surgery Services, NMC Eye Care, NMC Specialty Hospital, Abu Dhabi, United Arab Emirates


60 patients who had undergone a corneal cross linking for topographically documented keratoconus, at least 1 year ago, were included in this study. All the cases underwent Sirius Scheimpflug Topography System (CSO, Firenze, Italy) by a single user. Three sessions at least 3 days apart each were performed to evaluate inter-session repeatability. Cases having poor quality scans and recent visual deterioration were excluded. The corneal variables measured were thinnest point and apex characteristics (pachymetry, polar coordinates), volume, central simulated keratometry (flat, steep, average, astigmatism), central corneal thickness. Inter-session statistics were computed.


The means for major scheimpflug parameters were: Thinnest corneal pachymetry (TCP): 440.2�Â�µ, Apex Curvature 57.6 D, Corneal volume@central10mm: 55.2 mm3, Steep central keratometry: 50.0 D and Central corneal thickness (CCT) 444.4�Â�µ. The intrameasurement standard deviation (Sw) and Repeatability (2.77xSw) were : TCP: 4.2 &11.6�Â�µ, apex curvature 0.6 &1.6D, corneal volume @central 10mm: 0.7 &2.1 mm3 , steep central keratometry 0.5 &1.2D and CCT 3.6 & 10.0�Â�µ. The intraclass correlations (absolute agreement) were: TCP 0.98, apex curvature 0.98, corneal volume @central 10mm: 0.91, steep central keratometry 0.94 and CCT 0.99..


Based on our inter-session repeatability data, in cross linked keratoconic eyes with at least one year post treatment, a worsening of >12 �Â�µ in TCP, >10�Â�µ in CCT, >1.6 D in apex curvature or >1.25D in steep central keratometry can be considered strongly suggestive (>95% certainty) of progression of the pathology. A biologically plausible combination of these parameters should be used to increase the specificity of interpretation. These guidelines may also be used to re-crosslink a keratoconic cornea if other parameters such as corneal thickness are favorable.

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