Instrument analyzes biomechanical function of tissue

August 15, 2008

Proprietary instrumentation (Ocular Response Analyzer, Reichert Ophthalmic Instruments) is useful in ophthalmology, but the current generation of the instrument may have some limitations, according to one physician.

Key Points

Bordeaux, France-Proprietary instrumentation (Ocular Response Analyzer [ORA], Reichert Ophthalmic Instruments) has numerous applications, said David Touboul, MD, discussing the use of this technology in keratoconus, refractive surgery, and glaucoma.

"Analyzing the biomechanical function of tissue in a dynamic fashion is a new concept in medicine," said Dr. Touboul, of the Department of Ophthalmology, Hôpital Universitaire, Bordeaux, France. "This is now possible with the analyzer."

The concept of the analyzer is rather simple, he said, in that the instrument focuses on two specific pressure values: the inward and outward corneal surface displacement.

Factors that define the speed of movement are not currently available with the analyzer, he said, but are important to consider because in cases of rigidity or IOP fluctuations, those corneas that move faster than others may have different biomechanical properties.

"The corneal resistance factor [CRF] may help to compensate for that problem now," Dr. Touboul said.

In the measurement of corneal hysteresis, there are two peaks, 1 and 2; corneal hysteresis is determined by the difference in pressure between the two peaks. CRF is more correlated to peak 1 than is corneal hysteresis. The corneal-compensated IOP is a viscoelasticity-free IOP (IOP cc), and the Goldmann-correlated IOP (IOP GA) is the mean of peaks 1 and 2, according to Dr. Touboul.

In patients with keratoconus, the corneal hysteresis decreases with increasing severity of the disease but was much more correlated with pachymetry than with keratometry, he said. Dr. Touboul and colleagues conducted a study in which they compared keratoconic eyes with normal eyes that had the same pachymetric range from 500 to 600 mm.

"We learned two things, the first being that a nomogram pachymetry adjustment is needed for normal eyes for corneal hysteresis and CRF," he said. "The second was that the corneal hysteresis was lower than normal in keratoconic eyes."

When trying to determine the value of the analyzer for determining the risk factors for keratoconus, the investigators found that a CRF value lower than 8 had greater sensitivity than corneal hysteresis value lower than 8. A corneal hysteresis value higher than the CRF value was the more specific index, Dr. Touboul said.

Corneal changes

Dr. Touboul and colleagues also evaluated changes in the cornea following implantation of corneal inserts (Intacs, Addition Technology).

"Interestingly, we found that only the corneal shape changed and not the biomechanical properties of the cornea, which is noteworthy in that field of surgery," he said.

When they evaluated the effects of LASIK and PRK, the biomechanical profiles were found to be very similar to the profiles in patients with keratoconus.

"Both the corneal hysteresis and CRF values were lower than normal, and the corneal hysteresis values were higher than the CRF values in most of the cases," Dr. Touboul said.

In patients with keratoconus, the corneal hysteresis values were higher than the CRF values after LASIK but not after PRK.

In patients with glaucoma, the conventional IOP cc was higher than IOP GA in every cornea, he said. The difference was in the weakest corneas with the lowest corneal hysteresis values.

"One millimeter of change in IOP Goldmann applanation was equivalent to 1.6 mm of change in IOP cc," Dr. Touboul said. "Interestingly, we found that the corneal hysteresis value was lower than normal in patients with glaucoma; however, the CRF stayed the same."

One possible hypothesis for this finding is that a decrease in corneal hysteresis may predict the progression of glaucoma because of an abnormal corneal response to mechanical stress, he said.

Dr. Touboul emphasized that the analyzer has two limitations, namely, a dependency on pachymetry and the necessity of a nomogram for corneal hysteresis and CRF and not for IOP cc.

"There is always the possibility for confusion between the surface and biomechanical properties," he said. "Curve analysis is currently not user-friendly, and the speed of movement is not currently analyzed. [This analyzer] is the first device to quantify abnormal biomechanical properties, with good sensitivity but low specificity. Curve analysis and refinements of the system are needed in the future."