June 2021
Volume 62, Issue 8
Open Access
ARVO Annual Meeting Abstract  |   June 2021
Theoretical Analysis of an Unknown Change in Corneal Refractive Index Following Corneal Cross-Linking Introduces Artifact in Optical Measurements of Central Corneal Thickness
Author Affiliations & Notes
  • Austin James DeGroff
    The Ohio State University College of Optometry, Columbus, Ohio, United States
  • Phillip Thomas Yuhas
    The Ohio State University College of Optometry, Columbus, Ohio, United States
  • Cynthia J Roberts
    Ophthalmology and Visual Sciences, The Ohio State University College of Medicine, Columbus, Ohio, United States
    Biomedical Engineering, The Ohio State University College of Engineering, Columbus, Ohio, United States
  • Footnotes
    Commercial Relationships   Austin DeGroff, None; Phillip Yuhas, None; Cynthia Roberts, Oculus Optikgeräte GmbH (C), Optimo Medical AG (C), Ziemer Ophthalmic Systems AG (C)
  • Footnotes
    Support  None
Investigative Ophthalmology & Visual Science June 2021, Vol.62, 2016. doi:
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      Austin James DeGroff, Phillip Thomas Yuhas, Cynthia J Roberts; Theoretical Analysis of an Unknown Change in Corneal Refractive Index Following Corneal Cross-Linking Introduces Artifact in Optical Measurements of Central Corneal Thickness. Invest. Ophthalmol. Vis. Sci. 2021;62(8):2016.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose : Central corneal thickness (CCT) measurements vary following corneal cross-linking (CXL), depending on the instruments used. We sought to explain this discrepancy by investigating the consequences of a proposed unexpected change in corneal refractive index (nc) on Scheimpflug imaging and optical coherence tomography (OCT) measurements of CCT.

Methods : We developed two theoretical models to test the effect of a change in nc on CCT. Our Scheimpflug model used existing equations that utilize a side viewing angle of an optical section of cornea illuminated by a central slit to predict CTT. Similarly, our OCT model used existing equations that describe optical path length (OPL) to predict CCT. Predicted CCT was calculated for a range of nc values in both models. CCT measurement error was defined as the difference between the CCT measurement at the instrument-assumed index of 1.376 (CCT1.376) and the predicted CCT at other indices (CCTn). This error was plotted versus nc to analyze the effect a change in nc would have on predicted CCT for both models. To quantify the effect, we calculated the percent error in predicted CCT measurements and plotted it versus percent increase of nc for both models.

Results : Our Scheimpflug model produced a positive association between predicted CCT and nc. It also suggested a negative association between CCT measurement error(CCT1.376-CCTn) and nc, which was quantified as a 0.822% reduction in CCT for every 1% increase in nc (Fig1). In contrast, our OCT model produced a negative association between predicted CCT and nc with a positive association between CCT measurement error and nc, resulting in a 0.983% increase in CCT for every 1% increase in nc (Fig2).

Conclusions : Our models suggest that an unknown increase in nc results in underestimation of CCT from Scheimpflug-based devices, and, conversely, overestimation of CCT from OCT devices. These results suggest that a change in nc after CXL introduces artifact in optical CCT measurement techniques. Our predictions are consistent with literature reports of CCT 2 weeks after CXL that show a 9.7μm increase when measured with OCT yet a 16μm decrease when measured by Scheimpflug (Antonios et al. 2016).

This is a 2021 ARVO Annual Meeting abstract.

 

For every 1% increase in ncornea we see a 0.822% decrease in CCT in Scheimpflug

For every 1% increase in ncornea we see a 0.822% decrease in CCT in Scheimpflug

 

For every 1% increase in ncornea we see a 0.923% increase in CCT in OCT

For every 1% increase in ncornea we see a 0.923% increase in CCT in OCT

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