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M. Shimmyo, K. Fry, P. S. Hersh, D. Taylor, N. I. Hayashi; Eyes at Risk of Ectasia: Corneal Hysteresis and Corneal Resistance Factor in Keratoconus, Pre-and Post-LASIK Eyes. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2361.
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To evaluate if Reichert’s Ocular Response Analyzer (ORA) provides clinical information on dynamic biomechanical properties of the cornea which can be used in diagnosis of corneal ectasia, as an adjunct to anatomical profiles of corneal thickness and corneal topography.
Clinical, observational chart review. Normal eyes, keratoconus and, pre and post LASIK eyes were included. Corneal Hysteresis (CH) and Corneal Resistance Factor (CRF) were measured in normal eyes, eyes with Keratoconus, and pre and post LASIK eyes. ORA signals, including Goldmann equivalent intraocular pressure (IOP), corneal compensated IOP, CH and CRF values were compared. ORA measures IOP with cornea in motion at inward applanation (P1) and outward applanation (P2). CH (= P1-P2) represents viscous damping of the cornea and CRF (= P1 - 0.70*P2) represents total corneal visco-elastic response to deformation.
Normal eyes (N=202 eyes, male 76, female 126), mean age 38.1 ± 20.3 years, had CH 10.28 ± 1.39 mm Hg, CRF of 10.76 ± 1.25 mm Hg. Keratoconus eyes (N=76 eyes, male 36, female 40), mean age 38.4 ± 11.0 years, had CH of 8.30 ± 2.35 mm Hg and CRF of 7.15 ± 2.41 mm Hg. The differences of CH (-2.47 mm Hg) and CRF (-3.13 mm Hg) between normal and keratoconus eyes were both very significant (p<0.001). The waveform of ORA signals of keratoconus eyes were short in amplitude and had multiphasic waves as compared to smooth and high uniphasic waves of normal cornea. Pre-LASIK eyes (N=98 eyes) had CH of 10.46 ± 1.71 mm Hg, CRF of 10.25 ± 1.88 mm Hg; Post-LASIK eyes (N=98 eyes) had CH of 8.63 ± 1.72 mm Hg and CRF 7.28 ± 1.82 mm Hg. The differences of CH (-1.84 mm Hg) and CRF (-2.97 mm Hg) between pre-LASIK and post-LASIK eyes were very significant (p<0.001). Most post-LASIK eyes has smooth uniphasic waveforms.
Reichert’s ORA provides clinical measurement of corneal viscous damping (CH) and visco-elasticity (CRF) which represent corneal biomechanical properties. These metrics are reduced significantly in keratoconus and following refractive surgery. These metrics as well as characteristic waveforms can distinguish between normal and pathologic corneas and may be able to identify eyes at risk of developing ectasia. Reichert’s ORA may be used in diagnosis of corneal ectasia, as an adjunct to anatomical profiles of corneal thickness and corneal topography.
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