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Cynthia J. Roberts, Ashraf M. Mahmoud, Isaac Ramos, Diogo Caldas, Renata Siqueira da Silva, Renato Ambrósio, Jr.; Factors Influencing Corneal Deformation and Estimation of Intraocular Pressure. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4384.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the relative influence of intraocular pressure (IOP), biomechanical properties, thickness and curvature on corneal deformation characteristics.
The new CorVis ST (Oculus) was used to acquire video images of the cornea at 4,300 frames per second while deforming under a collimated air puff in both normal (N) (n = 39) and Keratoconus (KCN) (n= 29) subjects. Deformation amplitude (Da), pachymetry (Pach), and uncorrected IOP (IOPu) were recorded from the CorVis ST. The Ocular Response Analyzer (ORA - Reichert) was used for IOP with corneal compensation (IOPcc) and corneal hysteresis (CH). The Pentacam (Oculus) was used for central corneal thickness (CCT) at the apex, minimum thickness (CMT), and curvature (K), defined as average SimK. Two stepwise regressions were performed in SAS by group to determine predictors of Da, the first with IOPu and second with IOPcc, as well as the following parameters: Pach, CCT, CMT, K, and CH. Results are compared to theory.
For all 4 stepwise regressions, IOP was the strongest predictor of Da. With IOPu (p<0.0001) in the N group, Pach (p=0.0472) and CCT (p=0.0649) were also selected (model R2 = 0.5029). With IOPu (p=0.005) in the KCN group, K (p=0.0133) and Pach (p=0.0251) and CMT (p=0.1230) were selected (model R2 = 0.6080). With IOPcc (p<0.0143) in N, none of the other parameters were selected as significant predictors (model R2 = 0.1515). With IOPcc (p=0.0002) in the KCN group, only K (p=0.0143) was selected as an additional significant predictor (model R2 = 0.4994).
IOP has the greatest influence on corneal deformation amplitude, followed by thickness only in normals. In Keratoconus, curvature is the second strongest predictor, followed by thickness. If compensated IOP is used, then all thickness values cease to be predictors. In Keratoconus, however, curvature remains a significant predictor. Curvature is strongly associated with disease severity, and likely biomechanical properties in the form of elastic modulus. This highlights the importance of elastic properties in determining corneal deformation, with greater influence than thickness in Keratoconus. This is consistent with theory. Work is underway to calculate elastic modulus from the deformation images of the CorVis.
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