Frequently Asked Questions

Measurement Parameters

What is Hysteresis?

What is the difference between viscous and elastic resistance?

How is Corneal Hysteresis measured?

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?

IOPcc (corneal compensated intraocular pressure is a pressure measurement that is less effected by corneal properties than other methods of tonometry.  It compensates for the biomechanical properties of the cornea, NOT just the thickness.  This is an important distinction because "correcting IOP" measurements by using CCT data may result in significant errors in magnitude and direction of the adjusted IOP value.  Thickness is not the same thing as resistance.

IOPcc was developed using pre and post lasik clinical data.  Since the inward and outward applanation events measured by the Ocular Response Analyzer provide independent information about corneal tissue properties, it is possible to mathematically manipulate these values to obtain an IOP measurement that is less influenced by the cornea. 

IOPcc is still strongly correlated with Goldmann-obtained IOP measurements, but has no correlation with CCT and does not change significantly post-lasik.

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?