A new look at corneal biomechanics

Take-home

Highest concavity time may be another important parameter in describing corneal biomechanics as well as a potential risk factor for glaucoma.

Dr. Chang

By Lynda Charters; Reviewed by Robert T. Chang, MD

Palo Alto, CA-A novel corneal biomechanical parameter-highest concavity time-may be a possible risk factor for glaucoma, according to data obtained from non-contact tonometry.

Results of a prospective, comparative cohort study conducted in Hong Kong suggest that corneas in eyes with glaucoma may be weaker and more deformable than corneas in normal eyes.

“This is important, because corneal thickness and hysteresis may not explain all of the corneal biomechanical differences between eyes with and without glaucoma,” said Robert T. Chang, MD, assistant professor, Stanford University, Palo Alto, CA.

New biomechanical parameters

The time to highest concavity is the point at which the cornea reaches the maximum deformation as the corneal curvature change is continuosly tracked (16.77 ± 0.63 ms versus 17.01 ± 0.60 ms for glaucoma versus normal, p = 0.00110). (Figure courtesy of Robert T. Chang, MD)A novel non-contact tonometer (Corvis ST, Oculus) was used to measure IOP, central corneal thickness (CCT), and corneal biomechanics.

Dr. Chang and colleagues compared 102 patients with glaucoma with 66 normal controls. The highest concavity time differed between the two groups-16.77 seconds in the group with glaucoma compared with 17.01 seconds in the group of normal controls (p = 0.0011).

The investigators used the device-which combines a pneumotonometer with a high-speed Scheimpflug camera that collects images at 4,330 frames per second-to visualize corneal deformation during an entire cycle after an air pulse in real time. This makes the device different from another instrument (Ocular Response Analyzer, Reichert Technologies), Dr. Chang noted.

Inward and outward applanation

Corvis ST automatically calculates the time and length of the cornea as well as the instantaneous velocity of corneal movement at the inward and outward applanation states based on extremely high-speed video capture of the entire corneal depression and reformation cycle. (Figure courtesy of Oculus)In the ingoing phase, video captures the exact time and velocity to cornea flattening (the applanation phase).

In the outgoing phase, the cornea undergoes another applanation state before returning to its normal state, investigators explained.

The device is able to measure IOP, CCT, and various novel biomechanical parameters, including corneal deformation amplitude and inward and outward applanation length/velocity, as well as the highest concavity time.

The device is currently only approved by the FDA to measure tonometry and pachymetry, Dr. Chang noted.

The authors found a strong correlation (R = 0.79) between IOP measured by the device and a non-contact pneumotonometer. There also was a strong correlation (R = 0.89) between CCT measured by the device and ultrasound pachymetry.

Linear regression was used to analyze the correlation among age, IOP, CCT, and the various corneal parameters including highest concavity time.

Dr. Chang and his team found no correlation between age, IOP, and CCT with the highest concavity time. This suggested the new corneal biomechanical parameter might be worth studying further as a possible independent risk factor for glaucomatous disease.

Robert T. Chang, MD

E: viroptic@gmail.com

Dr. Chang did not indicate any proprietary interest in the subject matter.

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