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Cynthia J Roberts, Ashraf M Mahmoud, Jeffrey Bons, Arif Hossain, Ahmed Elsheikh, Riccardo Vinciguerra, paolo vinciguerra, Renato Ambrosio; A New Stiffness Parameter in Air Puff Induced Corneal Deformation Analysis. Invest. Ophthalmol. Vis. Sci. 2016;57(12):2390.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate a new stiffness parameter in corneal deformation analysis and compare responses in normal (NL) and keratoconic (KC) subjects, matched for intraocular pressure (IOP).
A new stiffness parameter (SP) is defined as the resultant pressure at inward applanation, divided by corneal deflection amplitude at highest concavity (HCDeflectAmp). The spatial and temporal profiles of the Corvis ST air puff (Oculus, Wetzlar, Germany) were characterized using hot wire anemometry from 0 to 16mm from the nozzle. Measured velocity was correlated with the pressure profile exported by the Corvis ST, measured within the nozzle. The z position of the cornea at the time of inward applanation was used to calculate an adjusted air pressure value (adjAP1) at the time and position of first applanation. An algorithm to correct IOP estimation based on finite element modeling, termed IOPfem, was used for the equation: SP = (adjAP1 - IOPfem)/ HCDeflectAmp. Linear regression analyses between corneal deformation parameters (CDP’s) and SP were performed on a retrospective dataset of 258 KC eyes and 460 NL eyes. CDP’s from a subset of 119 eyes of 119 subjects in each group were matched for IOPfem and compared using t-tests. Significance threshold was p < 0.05.
All CDP’s evaluated showed a significant difference between NL and KC, except peak distance. (Table 1) The KC group had lower SP values, thinner pachymetry, shorter applanation lengths, greater absolute values of applanation velocities, earlier first applanation times and later second applanation times, greater HC deformation and HC deflection amplitudes, and lower HC radius of concave curvature (greater concave curvature). All CDP’s evaluated showed a significant relationship with SP in both groups. Stiffer eyes were associated with greater pachymetry, longer applanation lengths, lower absolute value of applanation velocities, later first applanation times, earlier second applanation times, lower HC deformation and HC deflection amplitudes, shorter peak distances, greater HC radius of concave curvatures (flatter), and higher values of IOPfem.
Keratoconic eyes demonstrated less resistance to deformation than normal eyes with similar IOP. All of the deformation parameters investigated showed a significant relationship with the new stiffness parameter. This may be useful in future biomechanical studies comparing populations without having to match IOP.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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