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Frequently Asked Questions
Measurement Parameters
- What is Hysteresis?
- What is the difference between viscous and elastic resistance?
- How is Corneal Hysteresis measured?
- Corneal Hysteresis is the difference in the inward and outward pressure values obtained during the dynamic bi-directional applanation process employed in the Ocular Response Analyzer, as a result of viscous damping in the cornea.
A precisely-metered collimated-air-pulse causes the cornea to move inwards, past applanation, and into a slight concavity. Milliseconds after applanation, the air pump shuts off and as the pressure decreases, the cornea begins to return to its normal configuration. In the process, it once again passes through an applanated state.
An electro-optical applanation detection system
monitors the shape of the cornea throughout the entire process, and two independent pressure values are derived from the inward and outward applanation events.
Due to the dynamic nature of the measurement, viscous damping in the cornea causes delays
in the inward and outward applanation events, resulting in a discrepancy between the two pressure values. The difference between these two pressure values is Corneal Hysteresis (CH).
The ability to measure this effect is the key to understanding the biomechanical properties of the cornea and their influence on the IOP measurement process.
- What is Corneal Hysteresis
- What is IOPg?
- Is IOPg different from the IOP value presented by a traditional Non-Contact Tonometer (NCT)?
- Is there a massage effect caused by repeated measurements with the Ocular Response Analyzer?
- What is IOPcc?
- How is IOPcc measured?
- What is CRF?
- What is the range of Corneal Hysteresis in normal Subjects?
- What is the range of Corneal Resistance Factor (CRF) in normal subjects
- What is the average value of Corneal Hysteresis in normal subjects?
- What is the average value of Corneal Resistance factor (CRF) in normal subjects?
- Are CH and CRF Related to the “Modulus of Elasticity” of the cornea?
- What is the acceptable repeat measurement variability for the corneal hysteresis measurement?
- What is the acceptable repeat measurement variability for the corneal resistance factor (CRF) measurement?
- Why is IOPg higher/lower than my Goldmann IOP value?
- How do Corneal Hysteresis (CH) and Corneal Reistance Factor (CRF) differ?
- Is Corneal Hysteresis (CH) related to Central Corneal Thickness (CCT)?
Interpreting results
- What is the average value of Corneal Hysteresis in normal subjects?
- What is the acceptable repeat measurement variability for the corneal hysteresis measurement?
- What is the acceptable repeat measurement variability for the corneal resistance factor (CRF) measurement?
- Why is IOPg higher/lower than my Goldmann IOP value?
- What are the characteristics of a "good" measurement signal for a normal subject?
- How can I tell a "bad signal" from a "good signal"?
Using the instrument
- What is the acceptable repeat measurement variability for the corneal hysteresis measurement?
- What is the acceptable repeat measurement variability for the corneal resistance factor (CRF) measurement?
- Why is IOPg higher/lower than my Goldmann IOP value?
- Why are the average values in the software different from the true mathematical average?
- Where are the results saved when I use the pachymeter?
- Should I use anesthesia before taking an Ocular Response Analyzer measurement?
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