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Berthold Seitz, Larissa Haefner, Achim Langenbucher, Adam Bekhit, Elias Flockerzi; Repeatability of Corvis® ST measurements in different keratoconus stages. Invest. Ophthalmol. Vis. Sci. 2020;61(7):5209.
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© ARVO (1962-2015); The Authors (2016-present)
The aim of this study was to evaluate repeatability and reliability of the corneal biomechanical response measured with the Corneal Visualisation Scheimpflug Technology noncontact tonometer (Corvis® ST, Oculus Optikgeräte, Wetzlar, Germany) in different keratoconus (KC) stages.
A total of 146 eyes including 30 controls and KC stages 1, 2, 3 and 4 (n=25|32|30|29) according to the topographic KC classification (TKC) provided by Pentacam® software (Oculus) were examined. A sequence of five consecutive Corvis® ST measurements was performed, each after repositioning of the patient’s head in the chinrest and re-adjusting the device. Main outcome measures included (1) stiffness parameter A1 (SPA1), (2) DA ratio 1mm, (3) DA ratio 2mm, (4) A1 velocity, (5) maximal deformation amplitude, and (6) Ambrosio relational thickness horizontal (ARTh). The Corvis Biomechanical Index (CBI) is reported to be extracted out of these six measures. Mean values and standard deviations of the five measurements in a sequence were calculated and data were analyzed for each KC stage. Cronbach’s α was calculated as a measure for reliability of the measurements in the different KC stages.
ARTh and SPA1 were significantly higher in controls (535|123) compared to TKC1 (382|93), TKC2 (227|65), TKC3 (160|54) and TKC4 (137|50; p<0.001). The other parameters included in the CBI were similar in controls and TKC1 (DA ratio 1mm: 1.5|1.6; DA ratio 2mm: 3.7|4.5; A1 velocity: 0.148|0.147m/s; max. deformation amplitude: 1.1|1.1mm), but significantly higher in TKC stages 2 to 4 (DA ratio 1mm: 1.7|1.75|1.77; DA ratio 2mm: 5.5|6.3|7.0; A1 velocity: 0.17|0.18|0.19m/s; max. deformation amplitude: 1.2|1.26|1.3mm; p≤0.013). Standard deviation (SD) of the measurement sequences increased from TKC1 to TKC4 for all parameters except ARTh and SPA1. All parameters proved to be highly reliable (Cronbach’s α ≥0.91).
The CBI (consisting of SPA1, ARTh, DA ratio 1 and 2mm, A1 velocity and max. deformation amplitude) discriminates between controls and KC stages 1-4, but the individual parameters differ stage-dependently with decreased repeatability in advanced stages. The measurement of the biomechanical properties in KC using the Corvis® ST proved to be highly reliable in KC except in stage TKC4, which could be related to biomechanical changes due to stromal scarring of the thinned corneal tissue.
This is a 2020 ARVO Annual Meeting abstract.
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