Purpose

The biomechanical properties of the cornea have an important role in determining the shape of the cornea and visual acuity. Since the cornea is a nonhomogeneous tissue, it is thought that the elastic properties vary throughout the cornea. We aim to measure a map of corneal elasticity across the cornea.

Methods

An acoustic radiation force elasticity microscope (ARFEM) was used to create a map of corneal elasticity in the human cornea. This ARFEM uses a low frequency, high intensity acoustic force to displace a femtosecond laser-generated microbubble, while using a high frequency, low intensity ultrasound to monitor the position of the microbubble within the cornea. From the displacement of the bubble and the magnitude of the acoustic radiation force, the local value of corneal elasticity is calculated in the direction of the displacement. Measurements were conducted at 6 locations, ranging from the central to peripheral cornea at anterior and posterior depths.

Results

The mean anterior elastic moduli were 4.2 ± 1.2, 3.4 ± 0.7, and 1.9 ± 0.7 kPa in the central, mid, and peripheral regions, respectively, while the posterior elastic moduli were 2.3 ± 0.7, 1.6 ± 0.3, and 2.9 ± 1.2 kPa in the same radial locations.

Conclusions

We found that there is a unique distribution of elasticity axially and radially throughout the cornea.

The purpose of this study was to develop and test a nonlinear optical device to photoactivate riboflavin to produce spatially controlled collagen crosslinking and mechanical stiffening within the cornea. A nonlinear optical device using a variable numerical aperture objective was built and coupled to a Chameleon femtosecond laser. Ex vivo rabbit eyes were then saturated with riboflavin and scanned with various scanning parameters over a 4 mm area in the central cornea. Effectiveness of NLO CXL was assessed by evaluating corneal collagen auto fluorescence (CAF). To determine mechanical stiffening effects, corneas were removed from the eye and subjected to indentation testing using a 1 mm diameter probe and force transducer. NLO CXL was also compared to standard UVA CXL. The NLO CXL delivery device was able to induce a significant increase in corneal stiffness, comparable to the increase produced by standard UVA CXL.

The purpose of this study was to measure collagen fiber crimping (CFC) using nonlinear optical imaging of second harmonic generated (SHG) signals to determine the effects of UVA-riboflavin induced corneal collagen crosslinking (UVA CXL) on collagen structure. Two groups, four rabbits each, were treated in the right eye with standard UVA CXL. In vivo confocal microscopy was performed at 1, 2, and 4 weeks after treatment for the first group and up to three months for the second group to measure epithelial/stromal thickness and corneal haze during recovery. Rabbits were sacrificed at one and three months, respectively, and their corneas fixed under pressure. Regions of crosslinking were identified by the presence of collagen autofluorescence (CAF) and then collagen structure was imaged using SHG microscopy. The degree of CFC was determined by measuring the percentage difference between the length of the collagen fiber and the linear distance traveled. CFC was measured in the central anterior and posterior CXL region, the peripheral non-crosslinked region in the same cornea, and the central cornea of the non-crosslinked contralateral eye. No change in corneal thickness was detected after one month, however the stromal thickness surpassed its original baseline thickness at three months by 25.9 μm. Corneal haze peaked at one month and then began to clear. Increased CAF was detected in all CXL corneas, localized to the anterior stroma and extending to 42.4 ± 3.4% and 47.7 ± 7.6% of the corneal thickness at one and three months. There was a significant (P < 0.05) reduction in CFC in the CAF region in all eyes averaging 1.007 ± 0.006 and 1.009 ± 0.005 in one and three month samples compared to 1.017 ± 0.04 and 1.016 ± 0.06 for controls. These results indicate that there is a significant reduction in collagen crimping following UVA CXL of approximately 1%. One possible explanation for this loss of crimping could be shortening of the collagen fibers over the CXL region.

Purpose

The purpose of this study was to investigate femtosecond laser trabeculotomy (FLT) in a clinically relevant manner (i.e., delivering the surgical laser beam through the cornea of the intact, human anterior segment to create channels from the anterior chamber into the Schlemm's canal) and to investigate the effect of this treatment on intraocular pressure in perfused human anterior segments.

Methods

Perfused human anterior segments (15 eyes) received either FLT treatment (n = 8) or a sham-treatment (n = 7). Intraocular pressure (IOP) in the perfused samples was recorded before and after treatment. Spectral domain optical coherence tomography, second harmonic generation imaging, and transmission electron microscopy were used to investigate the FLT channels.

Results

The FLT group (n = 7, 1 eye excluded) had a statistically significant reduction in mean IOP of 20.2% from baseline after treatment (5.06 ± 1.46 mm Hg to 4.04 ± 1.63 mm Hg; P < 0.0005), whereas the control group (n = 7) remained statistically unchanged (7.72 ± 3.45 mm Hg to 7.78 ± 3.51 mm Hg; P < 0.71). Imaging confirmed that the channels traversed the entire trabecular meshwork into the Schlemm's canal.

Conclusions

This study has provided the first direct evidence supporting the feasibility of clinically applicable, noninvasive femtosecond laser trabeculotomy for the treatment of glaucoma. Various imaging modalities revealed minimal collateral damage to adjacent issues.

Translational relevance

This work demonstrates noninvasive femtosecond laser trabeculotomy in a laboratory setting that is clinically relevant.