Characterization of the biomechanical properties of the in vivo human cornea

Purpose. To investigate corneal hysteresis as measured by the Reichert Ocular Response Analyzer (ORA) and to develop new methods for evaluating the biomechanical properties of the human cornea through the analysis of its deformation in response to an air pulse.;Methods. A standard three-element spring and dashpot model was used to represent the viscoelastic behavior of the cornea during an ORA measurement. The model was used to drive a computer simulation using Matlab 7.3.0 with Simulink. To investigate the effect of changing values for elasticity and viscosity on corneal hysteresis, values for elasticity and viscosity were varied while a sinusoidal stress was applied to the model.;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: peak 1 and peak 2. 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. The diameter of the deformation was measured from each view and used to calculate the deformation area and eccentricity. The deformation areas and eccentricities were statistically compared between keratoconic and normal subjects to investigate size and symmetry differences between the groups using a t test. Additionally, the ORA signal peak heights and deformation areas were compared between groups to investigate the relationship between them and the presence of keratoconus. The correlation was calculated between peak height and deformation area for both peak 1 and peak 2.;Using the values for the air pressure applied to the cornea by the ORA, the mechanical model, and the deformations measured with the high speed photography, a method for analyzing the viscosity and elasticity of the in vivo human cornea was demonstrated in normal and glaucomatous subjects.;Results. The model reproduced the expected viscoelastic responses to step and sinusoidal loadings. Ophthalmologically relevant findings included confirmation of a direct relationship between the viscosity of the dashpot element and hysteresis. An inverse relationship was found between the stiffness of the spring in parallel with the dashpot and hysteresis. The opposite behavior was observed in the purely elastic element---that is, as the spring constant was increased, hysteresis increased. If both elastic elements are varied together, hysteresis peaks as a function of viscosity. Below the peak value, lower values of elasticity are associated with higher levels of hysteresis. Above the peak value, higher values of elasticity are associated with higher levels of hysteresis.;The heights of the ORA IR signal peaks 1 and 2 were significantly shorter for keratoconic versus 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. Image analysis demonstrated that decreased deformation area plays a role in the reduction of IR signal intensity for keratoconic corneas.;A non-contact method for measuring viscosity and elasticity of the in vivo cornea was introduced and used to measure the biomechanical properties of normal and glaucomatous corneas. The elasticity of glaucomatous corneas was found to be significantly higher than normals, but no significant relationship was found between glaucoma and viscosity.;Conclusion. Clinically, hysteresis has been shown to be an independent, but weak, risk factor for glaucomatous damage. In addition, hysteresis has been shown to be low in keratoconus and to increase after stiffening the cornea via cross-linking techniques. This model illustrates how changing viscosity and elasticity affects the hysteresis measurement in various ways. It also allows viscosity and elasticity measurements to be calculated from hysteresis, using clinical data of position, time, and stress from ORA signal analysis.;The high speed photography with normal and keratoconic subjects demonstrated that there is additional information about the biomechanical state of the cornea in the IR signal peak heights. This is data that many clinicians are already collecting but not using and thus may offer useful information that could help with disease detection and diagnosis.;High speed photography and the spring and dashpot model were both utilized to measure the elasticity and viscosity of the in vivo human cornea. The values that were calculated for elasticity are within the range of values that have previously been measured for ex vivo human corneas. Both glaucomatous and normal corneas were measured. Glaucomatous corneas were found to be significantly stiffer than normal corneas; however, no significant relationship was found between the disease and viscosity.