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To correlate the measured variables and waveform analysis of in vivo corneal deformation using CorVis ST with the severity of keratoconus

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Session Details

Session Title: Corneal Biomechanics

Session Date/Time: Monday 15/09/2014 | 17:00-18:30

Paper Time: 18:00

Venue: Capital Hall A

First Author: : P.Srivatsa INDIA

Co Author(s): :    R. Shetty   A. Sinha Roy   K. Shetty        

Abstract Details


To study the Biomechanical response of the Cornea to a high pressure air pulse, in Normal and Keratoconus eyes and to correlate the waveform and measured variables thus obtained from CorVis ST with the severity of Keratoconus.


Tertiary Eye Care Center, Research and Post-graduate Teaching Institute, Bangalore, India.


Retrospective observational cross-sectional analysis of 27 eyes of normal individuals and 88 eyes diagnosed with keratoconus (Grade 1 – 36, Grade 2 – 29, Grade 3 – 8, Grade 4 – 15) on the basis of clinical features and tomographic evidence, who underwent a complete ophthalmic examination including Corneal Tomography (Pentacam; OCULUS, Wetzlar, Germany) and evaluation of Corneal deformation process in response to a Non-Contact tonometry using CorVis ST ( Corneal Visualization – Scheimpflug Technology; Oculus, Wetzlar, Germany). The eyes diagnosed to have keratoconus were classified on the basis of severity using the Amsler Krumeich Keratoconus Severity Scoring System. The measured variables were: The time (in ms) from starting until the first applanation (A1); the time (in ms) from starting until the second applanation (A2); the difference in time (in ms) between the first and the second applanation (A2-A1 = A2 minus A1); Maximum Amplitude (in mm) at the highest concavity – Deformation Amplitude (DA). In addition, the corneal deformation waveform was also analyzed using Fourier transform to calculate the Root-Mean-Square (RMS) of the Fourier co-efficient and the Area under the corneal deformation curve (AUC) All variables of corneal biomechanics measured were evaluated against the keratoconus severity.


The mean for A1, A2, A2-A1 and DA was 7.43 ± 0.07, 21.47 ± 0.08, 14.05 ± 0.07 and 1.07 ± 0.02 for controls and 7.05 ± 0.03, 21.68 ± 0.04, 14.60 ± 0.06 and 1.28 ± 0.02 for the Keratoconus group. The mean for an-RMS, bn-RMS and AUC was 0.098 ± 0.002, 0.02 ± 0.00 and 13.65 ± 0.61 for controls and 0.12 ± 0.002, 0.03 ± 0.00 and 20.20 ± 0.68 for the keratoconus group. A1 shows a negative correlation with the severity of keratoconus (-0.31) and was significantly reduced in grade 4 Keratoconus as compared to grade 1 (p<0.05). While A2, A2 – A1 and DA, an-RMS, bn-RMS and AUC showed a positive correlation with the severity of keratoconus (0.39, 0.38, 0.69, 0.70, 0.36, 0.70). A2 and A2-A1 were significantly more in grade 4 as compared to grade 1 and grade 2. DA, an-RMS and AUC were significantly more in grade 4 keratoconus as compared to all the other grades. While, bn-RMS was significantly different only between grades 1 and 4 of keratoconus (p<0.05). The trend-line linearity analysis shows a nonlinear trend in the parameters when plotted as per the grading system.


Increasingly weaker Corneal Biomechanics is seen with increasing severity of keratoconus. However, linear gradation in severity of Keratoconus using corneal tomography did not result in linear biomechanical response of the cornea in the Keratoconus eyes. This behavior should be considered while developing any gradation system based on in-vivo biomechanical response of the cornea in eyes with Keratoconus.

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