Intrastromal lenticule extraction for refractive correction: Can it raise the tide for refractive surgery?

Corneal biomechanics: Measurement and structural correlations

The characterization of corneal biomechanical properties has important implications for the management of ocular disease and prediction of surgical responses. Corneal refractive surgery outcomes, progression or stabilization of ectatic disease, and intraocular pressure determination are just examples of the many key clinical problems that depend highly upon corneal biomechanical characteristics. However, to date there is no gold standard measurement technique. Since the advent of a 1-dimensional (1D) air-puff based technique for measuring the corneal surface response in 2005, advances in clinical imaging technology have yielded increasingly sophisticated approaches to characterizing the biomechanical properties of the cornea. Novel analyses of 1D responses are expanding the clinical utility of commercially-available air-puff-based instruments, and other imaging modalities-including optical coherence elastography (OCE), Brillouin microscopy and phase-decorrelation ocular coherence tomography (PhD-OCT)-offer new opportunities for probing local biomechanical behavior in 3-dimensional space and drawing new inferences about the relationships between corneal structure, mechanical behavior, and corneal refractive function. These advances are likely to drive greater clinical adoption of in vivo biomechanical analysis and to support more personalized medical and surgical decision-making.

Depth-Dependent Corneal Biomechanical Properties in Normal and Keratoconic Subjects by Optical Coherence Elastography

Purpose

Compare depth-resolved biomechanical properties in normal and keratoconic corneas in live human subjects using optical coherence elastography (OCE).

Methods

In a prospective series of normal and keratoconus (KC) eyes, a corneal perturbation was applied by a custom swept-source OCE system using a transparent flat lens coupled to force transducers. Cross-correlation was applied to track frame-by-frame OCT speckle displacement. Regional displacements for the anterior and posterior stroma were plotted in force versus displacement (k) graphs. A spatial biomechanical property ratio (k_a/k_p ) was defined by dividing the maximum total displacement by the maximum force for the anterior (k_a ) and posterior cornea (k_p) and was compared between normal and KC groups with the Mann-Whitney U test. Area under the receiver operating characteristics curve (AUROC) for differentiating normal and KC eyes was calculated for k_a/k_p , k_max, and thinnest point of corneal thickness (TPCT).

Results

Thirty-six eyes were analyzed (21 eyes of 12 normal subjects and 15 KC eyes of 12 subjects). The k_a/k_p for the normal group was 1.135 ± 0.07 (mean ± standard deviation) and 1.02 ± 0.08 for the KC group (P < 0.001), indicating a relative deficit in anterior stromal stiffness in KC eyes. AUROC was 0.91 for k_a /k_p , 0.95 for k_max, and 1 for TPCT.

Conclusions

Significant differences in depth-dependent corneal biomechanical properties were observed between normal and KC subjects.

Translational Relevance

OCE was applied for the first time to human KC subjects and revealed alterations in the normal anterior-to-posterior stromal stiffness gradient, a novel and clinically accessible disease biomarker.