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D. H. Glass, C. J. Roberts, A. S. Litsky, P. A. Weber, R. G. Lembach; Evaluation of the Deformation Response to an Air Puff in Healthy and Diseased in vivo Human Corneas. Invest. Ophthalmol. Vis. Sci. 2008;49(13):646.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the deformation response using a Reichert Ocular Response Analyzer (ORA), and to examine how responses differ between keratoconic corneas and normal corneas.
The ORA detects applanation by reflecting infrared light (IR) from the surface of the cornea to an infrared detector. As the cornea flattens the light becomes aligned on the detector, with peak IR intensity occurring during applanation. There are two applanation events during an ORA measurement with corresponding peaks in IR intensity, Peak1 and Peak 2. In 4 normal subjects and 3 subjects previously diagnosed with keratoconus, high speed photography was used to capture the deformation of the cornea over time during an ORA measurement. Two cameras were used, aligned from the temporal and inferior directions. Four measurements were filmed for each subject. The images were acquired at the rate of 1 per 2ms, for a total of 15 images per acquisition. The displacements of the corneal surface were extracted and the shape of the deformation was determined for both groups.The ORA IR signal peak heights were statistically analyzed using a t test, and then compared with the characteristics of the deformation.
The heights of the IR signal peaks 1 and 2 were statistically different by group (P=0.0026 and P=0.0005, respectively). A low peak 1 and peak 2 were associated with keratoconic corneas relative to normal corneas. This indicates that less light is reaching the detector in keratoconic corneas. Two possible explanations for this phenomenon are that the keratoconic cornea has a smaller applanation area, or that the plane of the applanation is tilted causing misalignment of the IR light source, corneal surface and IR detector. Preliminary image analysis suggests that both of these alterations in applanation geometry play a role in IR signal intensity reduction in keratoconic corneas.
Keratoconic corneas deform differently than normal corneas when subjected to an external air pressure, which affects the heights of the IR signal peaks acquired during an ORA measurement. The peak heights may provide additional information about the fundamental biomechanical properties of the cornea.
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