The Goldmann applanation tonometer (GAT) is the reference instrument for clinical measurement of intraocular pressure (IOP). However, its accuracy is affected by the interindividual variation in corneal geometric properties, such as thickness and curvature. Corneal biomechanical properties such as Young's modulus are also believed to influence GAT readings, but the traditional methods used to determine these properties (strip extensiometry and membrane inflation) cannot be performed in vivo.
Ocular response Analyser has been proposed to characterize corneal biomechanical responses using the noncontact tonometry (NCT) process. An air pulse of increasing force lasting approximately 20 ms is directed onto the eye, causing progressive corneal deformation through a first, inward applanation state (P1) to indentation. A second, outward applanation (P2) is achieved as the cornea returns to its original shape. Infrared light is used to determine the point of corneal flattening by using the cornea as a mirror, as previously described. The air pulse force is noted at P1 and P2 for use in calculating four parameters.
The ORA reports two corneal parameters termed corneal hysteresis (CH) and corneal resistance factor (CRF). The former is intended to quantify the viscoelastic mechanical damping ability of the cornea, whereas the latter is thought to describe its overall viscoelastic resistance. Neither of these variables can be considered corneal properties, because they are responses that are specific to the ORA measurement process. In contrast, true properties such as thickness and Young's modulus are ideally invariant to the measurement technique.
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| Corneal Hysteresis |
Corneal Hysteresis (CH) is the difference in the inward and outward pressure values obtained during the dynamic bi-directional applanation process employed by the Ocular Response Analyzer, as a result of viscous damping in the cornea. It is a characterization of the cornea’s energy
absorption capacity, which is a function of the biomechanical properties of the cornea.
Ocular Response Analyzer is the only instrument in the world capable of measuring corneal
biomechanical properties and Corneal Compensated IOP.
How it Works.
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The Ocular Response Analyzer utilizes a dynamic bi-directional applanation process to measure biomechanical properties of the cornea and the intraocular pressure of the eye. A precisely metered collimated air-pulse causes the cornea to move inwards, past applanation, and into a slight concavity.
Milliseconds after applanation, as the air pulse force decreases, the cornea begins to return to its normal configuration. In the process, it once again passes through an applanated state.
An electro-optical system monitors the curvature of the cornea throughout the deformation process taking 400 data samples during the 20-millisecond measurement. Two independent
pressure values are derived from the inward and outward applanation events. Viscous damping in the cornea results in an offset between the inward and outward pressure values. The difference between these two pressure values is Corneal Hysteresis (CH).
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