Measurement of Corneal Biomechanics
The biomechanical properties of the cornea affect its functional responses and can have a significant impact on vision. The cornea has both elastic and viscous elements, and these play a role in conditions such as keratoconus and ectasia. Corneal hysteresis is defined as the difference between the pressure at which the cornea bends inward during air-jet applanation and the pressure at which it bends out again. This difference, which is measured in millimeters of mercury, gauges elasticity, a biomechanical property of the cornea. Theoretical applications of these measurements might enable detection of corneas at risk for ectasia following corneal refractive surgery.
A commercially available instrument now allows in vivo clinical testing of a cornea’s direct biomechanical properties. The corneal resistance factor (CRF) is calculated based on a mathematical correlation between hysteresis and corneal thickness. CRF values have a normal distribution within the general population but are lower in patients who have undergone LASIK or photorefractive keratectomy or whose corneas have an inherent biomechanical weakness, like forme fruste keratoconus. These values are also lower in patients who have corneal edema secondary to Fuchs endothelial corneal dystrophy. These measurements might also increase the accuracy of IOP measurements, which are affected by corneal properties such as thickness. However, use of the device may not be particularly effective in screening refractive surgery patients for risk of keratectasia or in documenting the increased stiffness associated with corneal crosslinking, aging, and diabetes mellitus, because it measures the viscous and not the elastic properties of the cornea.
Newer technologies for evaluating corneal biomechanics integrate dynamic corneal imaging instruments using Placido disk–based technology, Scheimpflug imaging, or OCT and allow more accurate measurement of the corneal deformation produced by the collimated air puffs. These devices can differentiate the elastic biomechanical properties of normal corneas from those of ectatic corneas. They can also distinguish corneal crosslinking–treated corneas from pretreatment corneas, using variables such as the quantitative amplitude of inward deformation (greater in softer, ectatic corneas) and the area of corneal tissue experiencing inward deformation (less in softer, ectatic corneas). Using these devices, investigators have shown that corneal deformation is influenced by the IOP and corneal thickness as well as the innate elastic biomechanical properties of the cornea.
Dawson DG, Ubels JL, Edelhauser HF. Cornea and sclera. In: Alm A, Kaufman PL, eds. Adler’s Physiology of the Eye. 11th ed. New York: Elsevier; 2011:71–130.
Piñero DP, Alcón N. In vivo characterization of corneal biomechanics. J Cataract Refract Surg. 2014;40(6):870–887.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.