Depth- and direction-dependent changes in solute transport following cross-linking with riboflavin and UVA light in ex vivo porcine cornea

Crosslinking with UV-A and riboflavin in progressive keratoconus: From laboratory to clinical practice - Developments over 25 years

Changes in the biomechanical and biochemical properties of the human cornea play an important role in the pathogenesis of ectatic diseases. A number of conditions in primarily acquired (keratoconus or pellucid marginal degeneration) or secondarily induced (iatrogenic keratectasia after refractive laser surgeries) ectatic disorders lead to decreased biomechanical stability. Corneal collagen cross-linking (CXL) represents a technique to slow or even halt the progression of ectatic pathologies. In this procedure, riboflavin is applied in combination with ultraviolet A radiation. This interaction induces the production of reactive oxygen species, which leads to the formation of additional covalent bonds between collagen molecules and subsequent biomechanical corneal strengthening. This procedure is so far the only method that partially interferes etiopathogenetically in the treatment of ectatic diseases that slows or stops the process of corneal destabilization, otherwise leading to the need for corneal transplantation. Besides, CXL process increases markedly resistance of collagenous matrix against digesting enzymes supporting its use in the treatment of corneal ulcers. Since the discovery of this therapeutic procedure and the first laboratory experiments, which confirmed the validity of this method, and the first clinical studies that proved the effectiveness and safety of the technique, it has been spread and adopted worldwide, even with further modifications. Making use of the Bunsen-Roscoe photochemical law it was possible to shorten the duration of this procedure in accelerated CXL and thus improve the clinical workflow and patient compliance while maintaining the efficacy and safety of the procedure. The indication spectrum of CXL can be further expanded by combining it with other vision-enhancing procedures such as individualized topographically-guided excimer ablation. Complementing both techniques will allow a patient with a biomechanically stable cornea to regularize it and improve visual acuity without the need for tissue transplantation, leading to a long-term improvement in quality of life.

Use of Nanoindentation in Determination of Regional Biomechanical Properties of Rabbit Cornea After UVA Cross-Linking

Purpose

To evaluate the regional effects of different corneal cross-linking (CXL) protocols on corneal biomechanical properties.

Methods

The study involved both eyes of 50 rabbits, and the left eyes were randomized to the five intervention groups, which included the standard CXL group (SCXL), which was exposed to 3-mW/cm2 irradiation, and three accelerated CXL groups (ACXL1-3), which were exposed to ultraviolet-A at irradiations of 9 mW/cm2, 18 mW/cm2, and 30 mW/cm2, respectively, but with the same total dose (5.4 J/cm2). A control (CO) group was not exposed to ultraviolet-A. No surgery was done on the contralateral eyes. The corneas of each group were evaluated by the effective elastic modulus (Eeff) and the hydraulic conductivity (K) within a 7.5-mm radius using nanoindentation measurements.

Results

Compared with the CO group, Eeff (in regions with radii of 0-1.5 mm, 1.5-3.0 mm, and 3.0-4.5 mm) significantly increased by 309%, 276%, and 226%, respectively, with SCXL; by 222%, 209%, and 173%, respectively, with ACXL1; by 111%, 109%, and 94%, respectively, with ACXL2; and by 59%, 41%, and 37%, respectively, with ACXL3 (all P < 0.05). K was also significantly reduced by 84%, 81%, and 78%, respectively, with SCXL; by 75%, 74%, and 70%, respectively, with ACXL1; by 64%, 62%, and 61%, respectively, with ACXL2; and by 33%, 36%, and 32%, respectively, with ACXL3 (all P < 0.05). For the other regions(with radii between 4.5 and 7.5 mm), the SCXL and ACXL1 groups (but not the ACXL2 and ACXL3 groups) still showed significant changes in Eeff and K.

Conclusions

CXL had a significant effect on corneal biomechanics in both standard and accelerated procedures that may go beyond the irradiated area. The effect of CXL in stiffening the tissue and reducing permeability consistently decreased with reducing the irradiance duration.

Effect of corneal collagen crosslinking on viscoelastic shear properties of the cornea

The cornea is responsible for most of the refractive power in the eye and acts as a protective layer for internal contents of the eye. The cornea requires mechanical strength for maintaining its precise shape and for withstanding external and internal forces. Corneal collagen crosslinking (CXL) is a treatment option to improve corneal mechanical properties. The primary objective of this study was to characterize CXL effects on viscoelastic shear properties of the porcine cornea as a function of compressive strain. For this purpose, corneal buttons were prepared and divided into three groups: control group (n = 5), pseudo-crosslinked group (n = 5), and crosslinked group (n = 5). A rheometer was used to perform dynamics torsional shear experiments on corneal disks at different levels of compressive strain (0%-40%). Specifically, strain sweep experiments and frequency sweep tests were done in order to determine the range of linear viscoelasticity and frequency dependent shear properties, respectively. It was found that the shear properties of all samples were dependent on the shear strain magnitude, loading frequency, and compressive strain. With increasing the applied shear strain, all samples showed a nonlinear viscoelastic response. Furthermore, the shear modulus of samples increased with increasing the frequency of the applied shear strain and/or increasing the compressive strain. Finally, the CXL treatment significantly increased the shear storage and loss moduli when the compressive strain was varied from 0% to 30% (p < 0.05); larger shear moduli were observed at compressive 40% strain but the difference was not significant (P = 0.12).

Changes in Rat Scleral Collagen Structure Induced by UVA-Riboflavin Crosslinking at Various Tissue Depths in Whole Globe Versus Scleral Patch

Purpose

To investigate structural changes in scleral collagen fibers at various tissue depths before and after photosensitized crosslinking (CXL) both isolated scleral patch versus whole globe using second-harmonic generation (SHG) imaging.

Methods

Scleral tissues were harvested from Sprague-Dawley rats and separated into three groups: untreated sclera (control), full-thickness scleral patch for CXL (Free Scleral CXL group), and sclera in intact globe for CXL (Globe CXL group). The CXL groups were soaked in 0.1% riboflavin and irradiated with 365 nm ultraviolet-A light (power, 0.45 mW/cm2) for 30 minutes. SHG images were acquired every 5 µm between 10 and 60 µm from the outer scleral surface. Collagen fiber waviness was calculated as the ratio of the total length of a traced fiber and the length of a straight path between the fiber ends.

Results

In the Free Scleral CXL group, collagen waviness was significantly increased compared to the control group at 35 to 50 µm (P < 0.05). In the Globe CXL group, collagen waviness was decreased compared to control at all depths with statistical significance (P < 0.05) achieved from 10 to 45 µm.

Conclusions

Depending upon its initial state (i.e., free scleral patch versus mechanically loaded intact globe under pressure), collagen may experience different structural changes after CXL. In addition, the extent of the CXL effects may vary at different depths away from the surface.

Translational Relevance

Understanding the CXL effects on collagen structure may be important in optimizing the scleral crosslinking protocol for future clinical applications such as preventing myopic progression.

Properties of the acellular porcine cornea crosslinked with UVA/riboflavin as scaffolds for Boston Keratoprosthesis

The Boston Keratoprosthesis type I (B-KPro) is widely used in the world, but the lack of donor corneas limits its application. This study aims to prepare the acellular porcine cornea (APC) crosslinked with ultraviolet A (UVA)/riboflavin instead of donor corneas as the scaffold for B-KPro. Decellularization of freeze-thaw combined with biological enzymes resulted in approximately 5 ng/mg DNA residue, the a-Gal removal rate of 99%, and glycosaminoglycans retention at a high level of 46.66 ± 2.59 mg/mg. UVA/ riboflavin cross-linking was adopted to induce the formation of new chemical bonds between adjacent collagen chains in the corneal stroma to improve the mechanical properties and resistance to enzymatic hydrolysis. Through comprehensive analysis of the biomechanics, enzyme degradation, immunogenicity and histological structure of the APC crosslinked at different times, CL3 (irradiation conditions, 365 nm, 3 mW/cm, 80 min, both sides) was selected and transplanted into the rabbit cornea model through interlamellar keratoplasty and penetrating keratoplasty as the scaffold of the B-KPro. Compared with the native porcine cornea (NPC) and APC, the experiment of interlamellar pocket indicated that the structure of CL3 was homogeneous without degradation and vascularization in vivo at 12 weeks after surgery. Simultaneously, the results of transplantation of B-KPro showed complete epithelialization of CL3 within 1 week, and neovascularization of the cornea indicated rejection but could be controlled with immunosuppressants. At 3 months postoperatively, the lens of B-KPro remained transparent, and the structure of CL3 was compact and uniform, accompanied by the migration and proliferation of a large number of stromal cells without degradation, suggesting the CL3 could be a promising corneal substitute.

Computational modeling of corneal and scleral collagen photocrosslinking

Scleral photocrosslinking is increasingly investigated for treatment of myopia and glaucoma. In this study a computational model was developed to predict crosslinking efficiency of visible/near infrared photosensitizers in the sclera. Photocrosslinking was validated against riboflavin corneal crosslinking experimental studies and subsequently modeled for the sensitizer, methylene blue, administered by retrobulbar injection to the posterior sclera and irradiated with a transpupillary light beam. Optimal ranges were determined for treatment parameters including light intensity, methylene blue concentration, injection volume, and inspired oxygen concentration. Additionally, sensitivity of crosslinking to various parameters was quantified. The most sensitive parameters (in order of greatest to least sensitive) were tissue parameters (including scleral thickness and choroidal melanin concentration), treatment parameters (including treatment duration and inspired oxygen concentration), and sensitizer parameters (including triplet quantum yield).

[Principles of corneal cross-linking : Presentation based on the development of the various treatment protocols]

BACKGROUND

Corneal cross-linking (CXL) is used to treat corneal ectatic diseases. The aim is to improve the reduced consolidation of the cornea in order to halt further corneal protrusion and therefore subsequent deterioration of the optical imaging proportions.

MATERIAL AND METHODS

In this article the principles of corneal cross-linking based on riboflavin and UV light are presented including recent research results. Furthermore, the most important treatment protocols including standard CXL (S-CXL), accelerated CXL (A-CXL), transepithelial CXL (TE-CXL) and the approach of the CXL procedure for thin corneas are explained.

RESULTS

The CXL method depends on four major components, the riboflavin solution, oxygen, UV light and the availability of cross-linking sites on the collagen tissue. According to the present state of knowledge, the photochemical process of the CXL method induces covalent bonds between the fibrils and proteoglycans and thus stabilizes the collagen fibers, resulting in corneal consolidation. In addition to the S‑CXL, which has proven its effectiveness and safety in a large number of studies, there are other treatment protocols that have been developed based on the Bunsen-Roscoe law of reciprocity. The A‑CXL protocol has the advantage of having a shorter irradiation time but it seems to be less effective than the S‑CXL protocol concerning the increase in corneal stiffness. The use of TE-CXL has so far not yet gained acceptance in the clinical practice.

CONCLUSION

The CXL procedures primarily aim to stabilize the cornea. In the future, in addition to stabilization of the cornea, simultaneous improvement of visual acuity will be the main focus.