Understanding corneal hysteresis fundamental in grasping biomechanics concepts

Key Points

Recently, clinicians gained the opportunity to measure corneal biomechanical properties through the commercial availability of the Ocular Response Analyzer (ORA, Reichert). In addition to its role as a more accurate tool for IOP determination, the ORA generates biomechanics measurements known as the corneal hysteresis (CH) and corneal resistance factor.

What is corneal hysteresis?

"Now that I understand corneal hysteresis better, I am intrigued and motivated to go back and analyze the information obtained with the ORA in more depth. I believe corneal hysteresis may be part of the puzzle for understanding corneal biomechanics, but that it is likely the tip of the iceberg, and we still have a lot more to learn," said Dr. Durrie, professor of ophthalmology, University of Kansas Medical Center, and president, Durrie Vision, Kansas City, Kansas.

The first step to understanding what CH is for led Dr. Durrie to look up the dictionary definition of hysteresis. According to the Merriam-Webster Dictionary, hysteresis is defined as a temporary resistance to change from a condition previously induced. CH represents a measurement of the viscous damping of the corneal tissue and can be defined as its "energy absorption capability." Dr. Durrie used an analogy of an automotive suspension strut assembly to facilitate his explanation.

"The suspension strut assembly is composed of two parts-a spring and a shock absorber. For the spring, which is an elastic material, the amount of deformation or strain induced by a force relates to the amount of pressure applied, but is not influenced by how fast the force was applied or how long it is held. In contrast, the strain induced on the shock absorber, which provides viscous damping, depends on how hard and how fast it is hit as well as the duration of the exposure to pressure," he said.

"Like the suspension strut assembly, the cornea is also made up of elastic and viscous materials that react to a force in the same manner as the spring and the shock absorber," he said.

Measuring corneal hysteresis

Understanding CH also requires an understanding of how the ORA operates and the features of its "signal plot." The ORA is an air-puff tonometer that induces bi-directional applanation of the cornea and measures pressure values associated with the inward and outward applanation events. In response to a precisely metered collimated air pulse emitted by the ORA, the cornea moves inward to form a concavity and then returns to its normal shape. An integrated electro-optical system monitors the curvature of the cornea and the pressure values during applanation. However, viscous damping in the cornea causes delays in the inward and outward applanation events and a discrepancy between the two pressure values.

The signal plot graphs pressure/signal amplitude against time, and the two peaks in the plot represent the pressure at the inward and outward applanation points, respectively. CH is simply the difference between the two applanation pressures, Dr. Durrie explained.

Studies to date that evaluated CH in various groups of eyes suggest it may have diagnostic significance. Those investigations have shown that there are differences in CH comparing normal subjects against eyes that have suspected keratoconus, that have true keratoconus, and that are postLASIK.

"However, closer examination of the signal plot shows there are differences between these groups in the sizes of the individual applanation peaks as well as in their relative difference. This observation leads me to wonder if there is more information we can be getting out of our measurements with the ORA," Dr. Durrie said.