Changes in Corneal Biomechanical Properties With Different Corneal Cross-linking Irradiances

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.

High-Fluence Epithelium-off Accelerated Pulsed Corneal Cross-linking (15 mW/cm2; 7.2 J/cm2) for Pediatric Keratoconus: A 3-Year Retrospective Analysis

PURPOSE

To assess the safety and efficacy of treatment and secondarily determine the topographic changes, visual outcomes, and demarcation line depth after high-fluence pulsed light accelerated cross-linking (ACXL) in pediatric patients (younger than 18 years) with progressive keratoconus.

METHODS

This retrospective analysis included 32 eyes (25 children, aged 11 to 18 years), with progressive keratoconus treated with high-energy epithelium-off pulsed light ACXL (7.2 J/cm2, 15 mW/cm2, 12 minutes, 2 seconds on/1 second off). Corrected distance visual acuity (CDVA), Scheimpflug tomography, and anterior optical coherence tomography measurements were recorded preoperatively and 1, 2, and 3 years postoperatively.

RESULTS

A total of 32 eyes were included. Significant CDVA improvement, pachymetry, and maximum keratometry reduction were found at all follow-up visits. Mean keratometric values remained stable, and astigmatism showed a mild worsening (< 0.25 D) with statistical significance at 1 and 3 years. Total aberration showed discordant results and coma aberration had a slight improvement without statistical significance. The demarcation line depth was 265 ± 26 μm. Three patients developed mild haze without visual acuity loss. None of the patients underwent a second CXL procedure.

CONCLUSIONS

In pediatric patients, high-fluence epithelium-off pulsed light ACXL appears to be a safe and effective procedure to halt the progression of keratoconus, slightly improving the CDVA and keratometric values. [J Refract Surg. 2024;40(3):e148-e155.].

Accelerated corneal cross-linking (18mW/cm2 for 5 min) with HPMC-riboflavin in progressive keratoconus - 5 years follow-up

PURPOSE

To evaluate long-term results of accelerated corneal cross-linking (ACXL) in patients with progressive keratoconus, seventy-four eyes of 53 patients with progressive keratoconus (documented Kmax progression > 1D/a) who underwent ACXL (18mW/cm2 for 5 min) were included in a retrospective observational clinical study. The investigation focused on tomographic and keratometric parameters, refractive data, and visual outcomes at 5 years follow-ups.

METHODS

Corrected distance visual acuity (CDVA), slit lamp, and Pentacam® examinations were conducted, including assessments of thinnest corneal point (TP), minimum radius (Rmin), corneal astigmatism, and maximum anterior keratometry (Kmax). These examinations were performed two weeks before the surgery and, on average, 56 months after the surgery. In a subgroup of 24 eyes, Pentacam® examination data from an intermediate visit at 12 months until the final visit was evaluated to confirm continuous stability. The ACXL protocol included corneal abrasion, hydroxypropylmethylcellulose (HPMC)-riboflavin eye drops administered every 5 min for a total duration of 30 min, and irradiation with 18mW/cm2 for 5 min using riboflavin eye drops applied every minute during the irradiation process. Intraoperatively, minimal corneal pachymetry of > 400 µm was ensured in every patient.

RESULTS

After 56 months, all values exhibited statistically significant changes (paired t-test; CDVA p = 0.002; Kmax p < 0.001; Rmin p < 0.001; astigmatism p = 0.03; TP p < 0.001). In the subgroup analysis of 24 eyes, which included tomographical and keratometric parameters, no statistically significant changes were observed during the last 12 months of observation (paired t-test; Kmax p = 0.72; Rmin p = 0.67; astigmatism p = 0.72). Treatment failure was strictly defined as an increase in Kmax (> 1D) during the 5-year follow-up and was observed in only 3 eyes (4%).

CONCLUSIONS

ACXL is an effective and safe treatment for patients with progressive keratoconus. Our results demonstrate improvements in functional and tomographical outcomes even after high-energy ACXL (18mW/cm2 for 5 min) over a long-term period of 56 months. Our analysis indicates stable conditions in previously progressive keratoconus, particularly during the final year of the observation period. The treatment failure rate was 4%.

Evaluation of Biomechanical Changes After Accelerated Cross-Linking in Progressive Keratoconus: A Prospective Follow-Up Study

PURPOSE

The aim of this study was to analyze the biomechanical effect of accelerated corneal cross-linking (9*10) in progressive keratoconus (KC) in comparison to untreated fellow eyes using Scheimpflug-based tonometry (Corvis ST, CVS).

METHODS

Forty-three eyes of 43 patients with KC showed progressive KC and were treated using accelerated corneal cross-linking. Twenty-five untreated fellow eyes were used as the control group. All eyes were examined biomechanically (CVS) and tomographically (Pentacam) at baseline, after 1-month, 6-month, and 12-month follow-up. Statistical analysis was performed using a linear mixed model. A logistic regression was performed to attribute the effects of changes in each parameter to treatment status (treated or untreated).

RESULTS

Maximum keratometry values decreased statistically significantly at 12 months by -1.1 D (95 confidence interval: -2.0 to -0.1, P = 0.025) compared with baseline. Thinnest corneal thickness decreased significantly after 1 month ( P < 0.001) and recovered to baseline after 12 months ( P = 0.752). In the corneal cross-linking (CXL) group, biomechanical changes were observed by an increased bIOP, a shorter A2 time, and a lower integrated radius after 1 month (all P < 0.05). No biomechanical and tomographical changes were observed in the control group (all P > 0.05). Logistic regression pointed out that treated eyes can be separated from untreated eyes by differences in bIOP, corneal thickness, A1 velocity, integrated radius, and Kc mean at 1, 6, and 12 months.

CONCLUSIONS

The alterations in biomechanical parameters indicated a corneal stiffening effect after CXL treatment, which was mostly detectable 1 month after treatment, although corneal thickness was reduced. The logistic regression model showed an adequate separation between CXL-treated and untreated eyes.

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.

Analysis of riboflavin/ultraviolet a corneal cross-linking by molecular spectroscopy

Corneal cross-linking (CXL) with riboflavin and ultraviolet A light is a therapeutic procedure to restore the mechanical stability of corneal tissue. The treatment method is applied to pathological tissue, such as keratoconus and induces the formation of new cross-links. At present, the molecular mechanisms of induced cross-linking are still not known exactly. In this study, we investigated molecular alterations within porcine cornea tissue after treatment with riboflavin and ultraviolet A light by surface enhanced Raman spectroscopy (SERS). For that purpose, after CXL treatment a thin silver layer was vapor-deposited onto cornea flaps. To explore molecular alterations induced by the photochemical process hierarchical cluster analysis (HCA) was used. The detailed analysis of SERS spectra reveals that there is no general change in collagen secondary structure while modifications on amino acid side chains are the most dominant outcome. The formation of secondary and aromatic amine groups as well as methylene and carbonyl groups were observed. Even though successful cross-linking could not be registered in all treated samples, Raman signals of newly formed chemical groups are already present in riboflavin only treated corneas.

Carbon Nanostructures for Ocular Tissue Reinforcement

Purpose

The purpose of this study was to improve the biomechanical properties of the cornea through the incorporation of carbon nanostructures.

Methods

Healthy Japanese rabbits were used to evaluate the effect of carbon nanostructures’ incorporation in the cornea. Rabbits were divided in two groups A and B. In each of these groups, the corneas were divided in (i) corneas not submitted to any treatment (the control group), (ii) corneas modified either with carbon nanostructures (group A), or with the traditional cross-linking technology (group B). After modification, rabbits were euthanized at different time intervals. The biomechanical properties of the treated corneas were evaluated using the inflation method.

Results

Biomechanical tests based on the inflation method show that the incorporation of carbon nanostructures to the cornea and their proper distribution within it gives rise to a large improvement in the mechanical properties and tangential elastic modulus (up to 155%). These results anticipate that this novel and easy approach based on nanotechnology is able to compete with the actual cross-linking technology applied in clinical ophthalmology using a photosensitive molecule, such as riboflavin and unpleasant UV-A radiation.

Conclusions

The incorporation of carbon nanostructures (single-walled carbon nanotubes and graphene) in corneal stroma is proposed as a promising alternative to improve the mechanical properties in the treated eyes. The proper dispersion of the carbon nanostructures a few days after implementation (down to 60 micrometers depth) explains the successful results achieved.

Translational Relevance

Nanotechnology applied to the eye constitutes a promising approach for ocular tissue reinforcement.

Effectiveness of collagen cross-linking induced by two-photon absorption properties of a femtosecond laser in ex vivo human corneal stroma

This study aimed to investigate the effectiveness of two-photon induced collagen cross-linking (CXL) using femtosecond lasers in human corneal stroma. An 800-nm femtosecond laser optical path for CXL was established. Corneal samples that received two-photon induced CXL and ultraviolet-A (UVA) CXL underwent uniaxial stretching experiments, proteolytic resistance assays and observation of collagen fiber structure changes. Two-photon induced CXL can achieve corneal stiffening effects comparable to UVA CXL and showed better advantages at low strains. The cornea after two-photon induced CXL exhibited high enzymatic resistance and tight collagen fiber arrangement. Two-photon induced CXL promises to be a new option for keratoconus.

The bactericidal effect of two photoactivated chromophore for keratitis-corneal crosslinking protocols (standard vs. accelerated) on bacterial isolates associated with infectious keratitis in companion animals

Background

Bacterial corneal infections are common and potentially blinding diseases in all species. As antibiotic resistance is a growing concern, alternative treatment methods are an important focus of research. Photoactivated chromophore for keratitis-corneal crosslinking (PACK-CXL) is a promising oxygen radical-mediated alternative to antibiotic treatment. The main goal of this study was to assess the anti-bactericidal efficacy on clinical bacterial isolates of the current standard and an accelerated PACK-CXL treatment protocol delivering the same energy dose (5.4 J/cm²).

Methods

Clinical bacterial isolates from 11 dogs, five horses, one cat and one guinea pig were cultured, brought into suspension with 0.1% riboflavin and subsequently irradiated. Irradiation was performed with a 365 nm UVA light source for 30 min at 3mW/cm² (standard protocol) or for 5 min at 18mW/cm² (accelerated protocol), respectively. After treatment, the samples were cultured and colony forming units (CFU’s) were counted and the weighted average mean of CFU’s per μl was calculated. Results were statistically compared between treated and control samples using a linear mixed effects model.

Results

Both PACK-CXL protocols demonstrated a significant bactericidal effect on all tested isolates when compared to untreated controls. No efficacy difference between the two PACK-CXL protocols was observed.

Conclusion

The accelerated PACK-CXL protocol can be recommended for empirical use in the treatment of bacterial corneal infections in veterinary patients while awaiting culture results. This will facilitate immediate treatment, the delivery of higher fluence PACK-CXL treatment within a reasonable time, and minimize the required anesthetic time or even obviate the need for general anesthesia.

Long-term Effects of Riboflavin Ultraviolet-A-Induced CXL With Different Irradiances on the Biomechanics of In Vivo Rabbit Corneas

PURPOSE

To evaluate the long-term effects of ultraviolet-A corneal cross-linking (CXL) with different irrandiances on the biomechanical properties of rabbit corneas and the corresponding changes in stromal microstructure.

METHODS

The study involved the left eyes of 85 healthy white Japanese rabbits, randomly divided into five groups (n = 16 to 18 each). After removing the epithelium, the first four groups were exposed to riboflavin (0.22% concentration by volume) and ultraviolet-A (370 nm) at different CXL irradiations but with the same total dose (5.4 J/cm2). The four groups were defined as standard CXL (SCXL; 3 mW/cm2 for 30 minutes, n = 17), accelerated CXL1 (ACXL1; 9 mW/cm2 for 10 minutes, n = 16), accelerated CXL2 (ACXL2; 18 mW/cm2 for 5 minutes, n = 17), and accelerated CXL3 (ACXL3; 30 mW/cm2 for 3 minutes, n = 17). The control group (n = 18) was treated with riboflavin without ultraviolet-A exposure. Nine months after CXL, 10 corneas from each group were tested ex vivo under inflation, and the tangent modulus (Et) was estimated using an inverse analysis process. The remaining six to eight specimens in each group were examined by electron microscopy to determine the mean fibril diameter and interfibrillar spacing.

RESULTS

The SCXL and ACXL1 groups showed statistically significant differences in Et at all stresses (0.005, 0.010, and 0.015 MPa) analyzed compared to the control group (all P < .01), but the differences were non-significant in the ACXL3 group (P = 1.000, .785, and .679, respectively). For the ACXL2 group, there was no statistical difference in Et under the low stress of 0.005 MPa (P = .155), but the differences became significant at 0.010 and 0.015 MPa when compared with the control group (all P < .05).

CONCLUSIONS

CXL had a significant effect on corneal biomechanics in both standard and accelerated procedures. However, standard CXL was the most effective, and this effectiveness decreased gradually with increasing ultraviolet-A power intensity. [J Refract Surg. 2022;38(6):389-397.].

Pediatric Crosslinking: Current Protocols and Approach

Keratoconus (KC) is likely to be more aggressive in the pediatric population, with a higher risk of progression and visual loss. Several techniques have been proposed for corneal crosslinking (CXL) so far. The standard CXL (SCXL) technique, or the Dresden Protocol, originally developed by Wollensak et al., has been shown to be safe and effective in the pediatric KC group. With similar efficacy to the conventional method, the accelerated CXL (ACXL) protocols proposed a reduced UVA exposure time by increasing the intensity of UVA irradiation. Transepithelial CXL (TCXL), considered an "epithelium-on" method, emerged as a strategy to improve safety and reduce postoperative complications and discomfort. For thinner corneas, we can highlight the use of hypoosmolar riboflavin and new studies, such as contact lens-assisted CXL (CACXL), the epithelial-island CXL (EI-CXL), and the Sub400 protocol. In addition to the different protocols used, another factor that changes CXL results is the type of carrier used: dextran-based or hydroxypropyl methylcellulose-based (HPMC) riboflavin solutions. There are several ways to perform a CXL surgery, and it is still unclear which method is the safest and most effective in the pediatric group. This review of the literature in English, available in PubMed, provides an update on corneal CXL in the pediatric KC group, exploring the data on the techniques currently used and under investigation, including their advantages, efficacy, safety profiles, risks, and cost analyses.

Statistical Evaluation of Correlated Measurement Data in Longitudinal Setting Based on Bilateral Corneal Cross-Linking

PURPOSE

In ophthalmology, data from both eyes of a person are frequently included in the statistical evaluation. This violates the requirement of data independence for classical statistical tests (e.g. t-Test or analysis of variance (ANOVA)) because it is correlated data. Linear mixed models (LMM) were used as a possibility to include the data of both eyes in the statistical evaluation.

METHODS

The LMM is available for a variety of statistical software such as SPSS or R. The application was applied to a retrospective longitudinal analysis of an accelerated corneal cross-linking (ACXL (9*10)) treatment in progressive keratoconus (KC) with a follow-up period of 36 months. Forty eyes of 20 patients were included, whereas sequential bilateral CXL treatment was performed within 12 months. LMM and ANOVA for repeated measurements were used for statistical evaluation of topographical and tomographical data measured by Pentacam (Oculus, Wetzlar, Germany).

RESULTS

Both eyes were classified into a worse and better eye concerning corneal topography. Visual acuity, keratometric values and minimal corneal thickness were statistically significant between them at baseline (p < 0.05). A significant correlation between worse and better eye was shown (p < 0.05). Therefore, analyzing the data at each follow-up visit using ANOVA partially led to an overestimation of the statistical effect that could be avoided by using LMM. After 36 months, ACXL has significantly improved BCVA and flattened the cornea.

CONCLUSION

The evaluation of data of both eyes without considering their correlation using classical statistical tests leads to an overestimation of the statistical effect, which can be avoided by using the LMM.

Spatial Assessment of Heterogeneous Tissue Natural Frequency Using Micro-Force Optical Coherence Elastography

Analysis of corneal tissue natural frequency was recently proposed as a biomarker for corneal biomechanics and has been performed using high-resolution optical coherence tomography (OCT)-based elastography (OCE). However, it remains unknown whether natural frequency analysis can resolve local variations in tissue structure. We measured heterogeneous samples to evaluate the correspondence between natural frequency distributions and regional structural variations. Sub-micrometer sample oscillations were induced point-wise by microliter air pulses (60–85 Pa, 3 ms) and detected correspondingly at each point using a 1,300 nm spectral domain common path OCT system with 0.44 nm phase detection sensitivity. The resulting oscillation frequency features were analyzed via fast Fourier transform and natural frequency was characterized using a single degree of freedom (SDOF) model. Oscillation features at each measurement point showed a complex frequency response with multiple frequency components that corresponded with global structural features; while the variation of frequency magnitude at each location reflected the local sample features. Silicone blocks (255.1 ± 11.0 Hz and 249.0 ± 4.6 Hz) embedded in an agar base (355.6 ± 0.8 Hz and 361.3 ± 5.5 Hz) were clearly distinguishable by natural frequency. In a beef shank sample, central fat and connective tissues had lower natural frequencies (91.7 ± 58.2 Hz) than muscle tissue (left side: 252.6 ± 52.3 Hz; right side: 161.5 ± 35.8 Hz). As a first step, we have shown the possibility of natural frequency OCE methods to characterize global and local features of heterogeneous samples. This method can provide additional information on corneal properties, complementary to current clinical biomechanical assessments, and could become a useful tool for clinical detection of ocular disease and evaluation of medical or surgical treatment outcomes.

Effect of travoprost, latanoprost and bimatoprost PGF2α treatments on the biomechanical properties of in-vivo rabbit cornea

Prostaglandin F2α analogues (PGF2α), one of the most commonly prescribed classes of hypotensive agents, could decrease collagen fibril density and remodel the extracellular matrix in cornea. We hypothesized that PGF2α's would induce corneal softening, reduce the accuracy of intraocular pressure (IOP) measurement and lead to uncertainty in the effectiveness of the therapy. We determined the stress-strain behavior of rabbit cornea after PGF2α usage and evaluated the effect of biomechanical changes associated with PGF2α treatment on IOP measurements by Goldmann Applanation Tonometry (GAT). The tangent modulus decreased after PGF2α treatment, while the stromal interfibrillar spacing increased. PGF2α was shown to also affect the lateral eye with lower effect, which did not undergo direct eyedrop treatment. Significant decreases in the numerical predictions of GAT-IOP were predicted in all treated groups relative to control groups. Different PGF2α's (travoprost, latanoprost and bimatoprost) were associated with different extents of reduction in tissue stiffness and changes in corneal microstructure. PGF2α-induced changes in corneal mechanical properties could reduce the accuracy of IOP measurement and may cause an overestimation of the effect of PGF2α in lowering IOP, possibly leading to uncertainties in glaucoma management.

[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.

Exploring the Biomechanical Properties of the Human Cornea In Vivo Based on Corvis ST

Purpose: The aim of this study was to provide a method to determine corneal nonlinear viscoelastic properties based on the output data of corneal visualization Scheimpflug technology (Corvis ST).

Methods: The Corvis ST data from 18 eyes of 12 healthy humans were collected. Based on the air-puff pressure and the corneal displacement from the Corvis ST test of normal human eyes, the work done by the air-puff attaining the whole corneal displacement was obtained. By applying a visco-hyperelastic strain energy density function of the cornea, in which the first-order Prony relaxation function and the first-order Ogden strain energy were employed, the corneal strain energy during the Corvis ST test was calculated. Then the work done by the air-puff attaining the whole corneal displacement was completely regarded as the strain energy of the cornea. The identification of the nonlinear viscoelastic parameters was carried out by optimizing the sum of difference squares of the work and the strain energy using the genetic algorithm.

Results: The visco-hyperelastic model gave a good fit to the data of corneal strain energy with time during the Corvis ST test (R² > 0.95). The determined Ogden model parameter μ ranged from 0.42 to 0.74 MPa, and α ranged from 32.76 to 55.63. The parameters A and τ in the first-order Prony function were 0.09–0.36 and 1.21–1.95 ms, respectively.

Conclusion: It is feasible to determine the corneal nonlinear viscoelastic properties based on the corneal contour information and air-puff pressure of the Corvis ST test.

Changes in Corneal Biomechanical Properties in PRK Followed by Two Accelerated CXL Energy Doses in Rabbit Eyes

PURPOSE

To evaluate whether photorefractive keratectomy (PRK) combined with the two commonly delivered energy doses in accelerated corneal cross-linking (A-CXL) could help the cornea maintain its preoperative stiffness level.

METHODS

A total of 72 corneas of 36 healthy white Japanese rabbits were randomly divided into four equal groups. The groups included an untreated control group and three that had undergone PRK. After tissue ablation, one of the latter three groups (PRK group) was left untreated, whereas the other two were exposed to riboflavin (0.22% concentration by volume) and ultraviolet-A (370 nm) with the same irradiation (30 mW/cm2) but different CXL energy doses of 1.8 J/cm2 (PXL group) and 2.7 J/cm2 (PXH group). Dynamic Scheimpflug analyzer (Corvis ST; Oculus Optikgeräte GmbH) measurements of stiffness parameter at first applanation (SP-A1), Stress-Strain Index (SSI), and other dynamic corneal response parameters were taken 3 days preoperatively and 1 month postoperatively. Subsequently, ex vivo inflation testing was performed and the tangent modulus of each specimen was estimated using an inverse analysis process.

RESULTS

In comparison to the control group, the tangent modulus at a stress of 10 kPa decreased by 8.9% in the PRK group and increased by 10.6% and 22.4% in the PXL and PXH groups, respectively. SP-A1 decreased postoperatively in the PRK group (P < .05), indicating an overall stiffness reduction of -7.4, -3.5, and -5.3 mm Hg/mm in PRK, PXL, and PXH groups, respectively. The material stiffness parameter SSI remained almost unchanged in the PRK group (P = .989), increased slightly in the PXL group (8.3%, P = .077), and increased significantly in the PXH group (11.1%) (P < .05).

CONCLUSIONS

Biomechanical deterioration following PRK was significant and could not be fully compensated for by ACXL with either 1.8 or 2.7 J/cm2 doses. The increased value of corneal overall stiffness was higher in A-CXL with 2.7 J/cm2 energy than with 1.8 J/cm2 energy. [J Refract Surg. 2021;37(12):853-860.].

Singlet oxygen formation during accelerated and hyperaccelerated corneal cross-linking: in vitro study

BACKGROUND

To evaluate the singlet oxygen (¹O₂) production of oxygen assisted %0.1 riboflavin and ultraviolet-A (UVA) crosslinking therapy (with and without oxygen assistance), in combination with standard, accelerated and hyper-accelerated procedures via an important quantitive marker of ¹O₂ which is the photo-oxidation of 1,3 diphenylisobenzofuran (DPBF).

METHODS

%0.1 riboflavin-containing wells were irradiated with UVA light (365-nm wavelength) with or without 2-4-6-8 L/min oxygen flow assistance. Measurements of decrease in absorbance of DPBF were made in 30 mW (hyper-accelerated), 9 mW (accelerated), and 3 mW UV-A (standard) applications, and with additional 2-4-6-8 L/min oxygen flow in 30 mW and 2 L/min oxygen flow in 9 mW. A total of 8 different UV-A irradiance with and without oxygen supplementation groups were formed.

RESULTS

2 L/min oxygen assisted accelerated UV-A irradiance group has shown a greater decrease in DPBF absorbance compared to Dresden protocol. (p = 0.014) Also, Dresden protocol has shown a greater decrease in DPBF compared to all groups except accelerated crosslinking with 2 L/min oxygen. (p < 0.001) Oxygen assisted hyper-accelerated crosslinking groups were showed greater reduction in DPBF absorbance compared to standard crosslinking without oxygen groups. (p < 0.001).

CONCLUSION

Oxygen supplementation may increase the singlet oxygen generation to the similar levels of Dresden Protocol's in accelerated group. Also, more singlet oxygen generation with oxygen supplementation compared to standard UV-A application might be considered to be promising in terms of shortening the crosslinking therapy.

Comparison of pulsed and continuous accelerated corneal crosslinking for keratoconus: 1-year results at a single center

PURPOSE

To compare the clinical outcomes between pulsed and continuous accelerated crosslinking (CXL) for keratoconus.

SETTING

Hospital.

DESIGN

Retrospective comparison study.

METHODS

Korean patients who were treated for keratoconus between September 2015 and January 2018 at Seoul St. Mary's Hospital were included. Eyes were subjected to pulsed accelerated crosslinking (30 mW/cm2 for 8 minutes, 1 second on/1 second off) or continuous accelerated crosslinking (30 mW/cm2 for 4 minutes; delivering 7.2 J/cm2). Outcomes were evaluated after 1 year.

RESULTS

At 1 year, the 2 groups did not exhibit changes in their corrected and uncorrected distance visual acuity values. The pulsed group (25 eyes in 25 patients) exhibited significantly improved values for sphere (P = .009) and spherical equivalent (P = .033), although no statistically significant difference was observed in the continuous group (20 eyes in 20 patients). All keratometry (k)values (SimKf, SimKs, SimKmean, and Kmax) improved in both groups (all P < .05), although the pulsed group had significantly greater changes in the SimKmean value (P = .036) and the Kmax value (P = .03). Both groups had significantly decreased central and thinnest corneal thicknesses (all P < .001), although the pulsed group had a substantially lower thinnest corneal thickness (P = .017). Corneal densitometry measured using the Pentacam device increased in both groups (all P < .001), with a higher densitometry value in the pulsed group (P = .013). Furthermore, the depth of the demarcation line was deeper in the pulsed group (P = .015).

CONCLUSIONS

Pulsed accelerated crosslinking might provide better postcrosslinking effects than continuous accelerated crosslinking.

Long term results of accelerated 9 mW corneal crosslinking for early progressive keratoconus: the Siena Eye-Cross Study 2

Purpose

To assess clinical results of the 9 mW/5.4 J/cm² accelerated crosslinking (ACXL) in the treatment of progressive keratoconus (KC) over a span of 5 years.

Methods

The prospective open non-randomized interventional study (Siena Eye-Cross Study 2) included 156 eyes of 112 patients with early progressive KC undergoing the Epi-Off 9 mW/5.4 J/cm² ACXL at the Siena Crosslinking Centre, Italy. The mean age was 18.05 ± 5.6 years. The 20-min treatments were performed using the New KXL I (Avedro, Waltham, USA), 10 min of 0.1% HPMC Riboflavin soaking (VibeX Rapid, Avedro, Waltham, USA) and 10 min of continuous-light UV-A irradiation. Uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), Kmax, coma, minimum corneal thickness (MCT), surface asymmetry index (SAI), endothelial cell count (ECC) were measured, and corneal OCT performed.

Results

UDVA and CDVA improved significantly at the 3rd (P = 0.028), Δ + 0.17 Snellen lines and 6th postoperative month, respectively (P < 0.001), Δ + 0.23 Snellen lines. Kmax improved at the 6th postoperative month (P = 0.03), Δ − 1.49 diopters from the baseline value. Also, coma aberration value improved significantly (P = 0.004). A mild temporary haze was recorded in 14.77% of patients without affecting visual acuity and without persistent complications. Corneal OCT revealed a mean demarcation line depth at 332.6 ± 33.6 μm.

Conclusion

The 5-year results of Epi-Off 9 mW/5.4 J/cm² ACXL demonstrated statistically significant improvements in UCVA and CDVA, corneal curvature and corneal higher-order aberrations which confers a long-term stability for progressive ectasia. Based on the results of the Siena Eye-Cross Study 2, the 9 mW/5.4 J/cm² ACXL is a candidate to be  the natural evolution of Epi-Off CXL treatment for the management of early progressive corneal ectasia, and thus optimize clinic workflow.

Experimental in-vitro investigation on Epi-Off-Crosslinking on porcine corneas

Aim

To evaluate quantitatively the effects of the Epi-Off-CXL irradiance dose on the stromal stiffening of pig corneas.

Setting

Laboratory of Biological structures (LaBS), Politecnico di Milano, Milano, Italy.

Methods

Inflation tests have been carried on 90 excised and de-epithelized pig corneas, monitoring the change of configuration of the corneal dome at specific pressures. Test have been carried out twice on each cornea, once before and once after Epi-Off-CXL performed at a constant irradiance of 9 mW/cm² and variable UV-A exposure times. Corneas were grouped according to the exposure time (2.5, 5, 10, 15 and 20 min), proportional to the irradiation dose (1.35, 2.7, 5.4, 8.1, and 10.8 J/cm²). A theoretical model based on linearized shell theory has been used to estimate the increment of the corneal stiffness.

Results

The linearized shell theory allowed to establish a quantitative relation between the increment of the stiffness parameters and the irradiation dose. Relative to the pre-treatment values, in all experiments the post-treatment corneal stiffness revealed a pronounced increase. In general, the stiffness gain increased with the exposure time. No significant differences in stiffening was observed between tests conducted at 2.5, 5, and 10 min exposure.

Conclusions

Qualitatively, the effectiveness of accelerated CXL treatments observed in pig corneas complies very well with in-vivo clinical results in humans, suggesting that experimental data in pigs can be very useful for the design of the procedure in humans. A larger irradiation dose provides a larger increment of the corneal stiffness. Due to the biological variability of the tissues, however, it is difficult to distinguish quantitatively the level of the reinforcement induced by accelerated protocols (low doses with < = 10 min exposure), less prone to induce damage in the corneal tissue. Therefore, the definition of personalized treatments must be related to the actual biomechanics of the cornea.

Corneal Cross-Linking for Keratoconus: Current Knowledge and Practice and Future Trends

Corneal collagen cross-linking (CXL) with riboflavin is an accepted universal standard of care for our keratoconus patients with progressive disease. It has been a game changer in how we manage keratoconus. Early diagnosis and treatment is essential in paediatric patients as younger patients progress more rapidly and have poorer transplant outcomes. There is an ongoing debate around standard, accelerated, and transepithelial protocols of CXL, the role of CXL, and the combination of laser refractive surgery. Future developments will improve CXL safety and efficacy and the scope of utilization, but we must be careful not to leap too far ahead with clinical applications before publication of basic science research and good clinical results with standardized protocols.

The biology of corneal cross-linking derived from ultraviolet light and riboflavin

Over the past 20 years, corneal crosslinking (CXL) has been used by surgeons to halt progression in eyes with keratoconus. We reviewed the literature regarding the mechanism of action of CXL, the role of each of its components the strong biologic reaction, and their effects on cell interaction, proteins involved, wound healing, and cytotoxic reaction. CXL surgery involves a photochemical response in which ultraviolet light at a given wavelength and riboflavin participate. The combination of irradiation with UVA light and riboflavin leads to an intense process of apoptosis of keratocytes in the anterior stroma. Differences in light irradiation, as well as the importance of riboflavin and its vehicle, were also detailed. The surgery creates additional chemical bonds between the amino terminals of the collagen side chains and the proteoglycans of the extracellular matrix. A photosensitization reaction catalyzed by riboflavin classically involves the production of singlet oxygen. Microstructure studies show changes in the size of the fibril and potentially in the interfibrillar space, that the most significant changes related to the stiffening effect of CXL occur in the anterior third of the cornea and that short irradiation times, especially below 5 min, may not have the same biological effect. Changes in the riboflavin vehicle, with the incorporation of Hydroxypropyl methylcellulose as a carrier, can lead to faster diffusion and a more intense photochemical reaction. These are findings that can impact the optimal adjustment of irradiation time according to the riboflavin (and its carrier) used. Many studies have suggested that CXL is safe and effective in the standard and accelerated protocols that have been used by surgeons. After the initial depletion of anterior keratocytes, keratocyte density seems to return to average 6-12 months after surgery when corneas are examined with the confocal microscope.

Biomechanical Effects of Two Forms of PGF2α on Ex-vivo Rabbit Cornea

PURPOSE

To investigate the biomechanical effects of two synthetic prostaglandin F2α analogues (PGF2α), namely Travoprost and Tafluprost, on the ex-vivo rabbit cornea.

MATERIALS AND METHODS

Ninety-six eyes of 48 Japanese white rabbits were divided into 3 equal groups randomly; the Travoprost treated group (Tra), the Tafluprost treated group (Taf) and the control group (Co). Eyes in Tra and Taf groups were preserved in storage medium for 10 days with 1:10 Travoprost and Tafluprost diluents, respectively; while the Co eyes were preserved in a similar but PGF2α-free medium. Twenty-four corneas of each group were tested under inflation conditions with up to 30 mmHg posterior pressure. The pressure-deformation data obtained experimentally were used in an inverse analysis process to derive the stress-strain behavior of the tissue, using which the tangent modulus, a direct measure of the tissue's material stiffness, was calculated. The remaining eight specimens of each group were analyzed using electron microscopy for fibril diameter and interfibrillar spacing.

RESULTS

Although the central corneal thickness increased significantly in the three groups after storage (p < .01), it was similar in all groups both before (p = .598) and after storage (p = .181). After treatment with Travoprost and Tafluprost, the corneas exhibited lower tangent modulus (by 29.2% and 29.8%, respectively, at 6 kPa stress) and larger stromal interfibril spacing (by 21.9% and 23.6%) compared with the control group. There was no significant change in fibril diameter with either Travoprost or Tafluprost treatment (p = .769).

CONCLUSIONS

The results demonstrated significant reductions in tangent modulus and increases in interfibrillar spacing, which were of similar magnitudes, with the application of two different forms of PGF2α.

Changes and quantitative characterization of hyper-viscoelastic biomechanical properties for young corneal stroma after standard corneal cross-linking treatment with different ultraviolet-A energies

Corneal collagen cross-linking (CXL) treatment can restore vision in patients suffering from keratoconus and corneal injury, by improving the mechanical properties of the cornea. The correlation between ultraviolet-A (UVA) irradiant energies of standard CXL (SCXL) and corneal visco-hyperelastic mechanical behavior remains unknown. In this study, SCXL with four different UVA irradiant energy doses (0-5.4 J/cm²) were administered as part of quantitative treatments of corneal stromal lenticules extracted from young myopic patients via small incision lenticule extraction (SMILE) corneal refractive surgery. Double-strip samples with symmetric geometries were cut simultaneously for SCXL treatment and non-treated control. First, 40 pairs of strips were loaded to failure to assess the mechanical parameters of the material. Then, another 40 pairs were tested using a special uniaxial tensile test including quasi-static loading-unloading, instantaneous loading, and stress relaxation, to determine the visco-hyperelastic mechanical behavior. Upon combining the collagen fibril crimping constitutive model with the quasi-linear viscoelastic model, it was observed that with increasing UVA energy dose, the corneal strength and hyperelastic stiffness were significantly enhanced, while the maximum stretch and viscosity of the cornea were significantly reduced. Considering the quantitative analysis of SCXL and the rehabilitation prediction of keratoconus treatment, the results clarify the biomechanical behavior of human corneal stroma in SCXL clinical surgery. STATEMENT OF SIGNIFICANCE: This study quantitatively analyzes the improvement in the biomechanical properties of young central corneal stroma, due to SCXL treatment with different energies. Furthermore, the correlation between the hyper-viscoelastic mechanical parameters and UVA irradiant energy doses of SCXL are clarified. The contribution of this study fills the knowledge gap of the CXL on corneal biomechanics. It can not only clarify this mechanism better but also assist with guiding SCXL surgery for individualized patient corneas.

Comparison of waveform-derived corneal stiffness and stress-strain extensometry-derived corneal stiffness using different cross-linking irradiances: an experimental study with air-puff applanation of ex vivo porcine eyes

Purpose

To assess corneal stiffening of standard (S-CXL) and accelerated (A-CXL) cross-linking protocols by dynamic corneal response parameters and corneal bending stiffness (Kc[mean/linear]) derived from Corvis (CVS) Scheimpflug-based tonometry. These investigations were validated by corneal tensile stiffness (K[ts]), derived from stress-strain extensometry in ex vivo porcine eyes.

Methods

Seventy-two fresh-enucleated and de-epithelized porcine eyes were soaked in 0.1% riboflavin solution including 10% dextran for 10 min. The eyes were separated into four groups: controls (n = 18), S-CXL (intensity in mW/cm²*time in min; 3*30) (n = 18), A-CXL (9*10) (n = 18), and A-CXL (18*5) (n = 18), respectively. CXL was performed using CCL Vario. CVS measurements were performed on all eyes. Subsequently, corneal strips were extracted by a double-bladed scalpel and used for stress-strain measurements. K[ts] was calculated from a force-displacement curve. Mean corneal stiffness (Kc[mean]) and constant corneal stiffness (Kc[linear]) were calculated from raw CVS data.

Results

In CVS, biomechanical effects of cross-linking were shown to have a significantly decreased deflection amplitude as well as integrated radius, an increased IOP, and SP A1 (P < 0.05). Kc[mean]/Kc[linear] were significantly increased after CXL (P < 0.05). In the range from 2 to 6% strain, K[ts] was significantly higher in S-CXL (3*30) compared to A-CXL (9*10), A-CXL (18*5), and controls (P < 0.05). At 8% to 10% strain, all protocols induced a higher stiffness than controls (P < 0.05).

Conclusion

Several CVS parameters and Kc[mean] as well as Kc[linear] verify corneal stiffening effect after CXL on porcine eyes. S-CXL seems to have a higher tendency of stiffening than A-CXL protocols have, which was demonstrated by Scheimpflug-based tonometry and stress-strain extensometry.

In vivo evaluation of corneal biomechanical properties by optical coherence elastography at different cross-linking irradiances

Corneal collagen cross-linking (CXL) strengthens the biomechanical properties of damaged corneas. Quantifying the changes of stiffness due to different CXL protocols is difficult, especially in vivo. A noninvasive elastic wave-based optical coherence elastography system was developed to construct in vivo corneal elasticity maps by excitation of air puff. Biomechanical differences were compared for rabbit corneas given three different CXL protocols while keeping the total energy delivered constant. The Young’s modulus was weaker in corneas treated with higher irradiance levels over shorter durations, and a slight increase of Young’s modulus was present in all groups one week after the recovery process. Due to the noninvasive nature and minimal force to generate corneal elastic waves, this technique has the potential for early detection and treatment of corneal diseases in clinic.

Correlation between corneal stromal demarcation line depth and topographic outcomes after two pulsed-light-accelerated crosslinking protocols

Purpose

To report the visual and topographic outcomes of two pulsed-light-accelerated CXL (A-CXL) protocols at a 12-month follow-up and their correlation with the corneal stromal demarcation line (DL) depth.

Patients and methods

Retrospective comparative cohort of patients with documented progressive keratoconus were included. Two epi-off pulsed-light [1s on–1s off] A-CXL protocols were compared: irradiance 30*8 and 45*5:20 (fluence 7.2 J/cm²). UDVA, CDVA, spherical equivalent (SE), topographic astigmatism, K_min, K_max, K_m, central corneal thickness (CCT), thinnest pachymetry (TCT) and endothelial cell density (ECD) were measured preoperatively and months 1, 3, 6 and 12 postoperative. Corneal DL was measured 1 month postoperatively using anterior segment optical coherence tomography.

Results

Fifty eyes (27 patients): 22 eyes in group A-CXL (30*8), 28 eyes in group A-CXL (45*5:20). Mean age (years) was 19.04±4.71 and 20.32±4.57. DL depth (µm) at month 1 was 200.63±10.01 µm and 184.53±19.68 µm for group A-CXL (30*8) and group A-CXL (45*5:20), respectively (p<0.001). Significant improvement in CDVA, topographic astigmatism, K_min, K_max and K_m was observed in both groups (no significant difference between groups) and no significant changes were observed in CCT, TCT and ECD with regard to baseline. Over 85% of the eyes in both protocols achieved stabilization or improvement in maximum K at the end of the follow-up. No significant correlations between DL and any visual or topographic outcomes were observed at 12 months.

Conclusion

No correlation between DL depth and visual or topographic outcomes was observed on either protocol. Although significant improvement on CDVA, topographic astigmatism, K_min, K_max and K_m was observed in both groups at 12 months, further research is needed to assure safety and effectiveness at stabilizing keratoconus progression.

Standard cross-linking versus photorefractive keratectomy combined with accelerated cross-linking for keratoconus management: a comparative study

Purpose

To compare the safety and efficacy of standard 30 min epithelium‐off cross‐linking (CXL) versus photorefractive keratectomy (PRK) combined with accelerated epithelium‐off cross‐linking (AXL) for the treatment of progressive keratoconus (CXL‐Plus).

Methods

This study was a prospective multicentre comparative clinical study. A total of 125 eyes of 75 patients with grade 1 keratoconus and documented progression were divided into two groups. Group A included 58 eyes treated with standard CXL. Group B included 67 eyes treated with combined PRK and AXL. The recorded data included UDVA, CDVA, subjective and objective refraction, keratometry and pachymetry using corneal topographies preoperatively and postoperatively at 3, 6, 12 and 24 months of follow‐up.

Results

In group A, at 24 months of UDVA and CDVA were improved from 1.12 ± 0.38 and 0.58 ± 0.42 to 0.66 ± 0.20 and 0.20 ± 0.12 (LogMAR±SD). The spherical equivalent was reduced from 4.03 ± 1.18 to 1.78 ± 1.04 D. The cylinder reduction was 0.32 ± 0.19 D. In group B, at 24 months of UDVA and CDVA were improved from 1.26 ± 0.52 and 0.68 ± 0.36 to 0.58 ± 0.28 and 0.20 ± 0.16 (LogMAR ± SD). The spherical equivalent was reduced from 4.23 ± 0.95 to 1.92 ± 0.74 D. The cylinder reduction was ±1.76 D.

Conclusion

Surprisingly, standard CXL showed close results to CXL‐Plus at the 24th follow‐up month. Standard CXL acted as a stabilizing procedure associated with a late myopic component reduction. CXL‐Plus acted as a refractive and stabilizing procedure with an early effect on both the myopic and the astigmatic component but no later improvements. Standard CXL seems to be more powerful than AXL in its long‐term effect. Therefore, in the future, we want to test the combination of PRK with standard CXL.

Comparative Functional Outcomes After Corneal Crosslinking Using Standard, Accelerated, and Accelerated With Higher Total Fluence Protocols

PURPOSE

To compare the relative 12-month corneal crosslinking (CXL) functional outcomes using standard protocol and accelerated protocols in patients with progressive keratoconus.

METHODS

CXL was performed using 3 epithelium-off protocols: standard [3 mW/cm for 30 minutes, 5.4 J/cm (S3/30-CXL)], accelerated with equivalent total irradiance [9 mW/cm for 10 minutes, 5.4 J/cm (A9/10-CXL)], and accelerated with increased total irradiance [30 mW/cm for 4 minutes, 7.2 J/cm (A30/4-CXL)]. Efficacy measurements were evaluated 12 months after treatment with Scheimpflug imaging (Pentacam HR) and included change in maximum keratometry (K Max), corrected distance visual acuity (CDVA), other keratometric variables, pachymetry, keratoconus indices, astigmatism, asphericity, manifest refraction, and higher order aberrations.

RESULTS

Ninety-three eyes (67 patients) were evaluated: 35 eyes (26 patients) with S3/30-CXL, 29 eyes (19 patients) with A9/10-CXL, and 29 eyes (22 patients) with A30/4-CXL. Mean [INCREMENT]K Max was -1.53 ± 2.1 diopter (D) for S3/30-CXL, -0.71 ± 1.3 D for A9/10-CXL, and -0.70 ± 2.3 D for A30/4-CXL (P = 0.37). Mean [INCREMENT]CDVA(logMAR) was -0.18 ± 0.2 for S3/30-CXL, -0.13 ± 0.2 for A9/10-CXL, and -0.18 ± 0.2 for A30/4-CXL (P = 0.79). [INCREMENT]K Mean (r = -0.29 to -0.46), anterior asphericity (r = -0.34 to -0.40), and central keratoconus index (r = -0.18 to -0.38) best correlated with [INCREMENT]CDVA. S3/30-CXL had greater changes in index of surface variance, index of vertical asymmetry, keratoconus index, and regularization index compared to A9/10-CXL and A30/4-CXL. There were no other differences between protocols.

CONCLUSIONS

All 3 protocols showed improvements in K Max, CDVA, and other variables, with similar functional outcomes for each despite greater change in keratoconus indices after S3/30-CXL. Correlations between change in measured variables and CDVA were poor overall; however, K Mean, central keratoconus index, and anterior asphericity were better correlated with CDVA than K Max.

Biomechanical efficacy of contact lens-assisted collagen cross-linking in porcine eyes

PURPOSE

Contact lens-assisted corneal cross-linking (CACXL) has been proposed for the cross-linking treatment in thin corneas. The aim of this study was to assess the biomechanical efficacy of this treatment.

METHODS

Post-mortem porcine eyes were treated with standard cross-linking and with cross-linking placing a contact lens soaked with isoosmolar riboflavin solution on the debrided cornea with or without an adherent precorneal riboflavin film of up to 100 μm thickness. Three soft contact lenses (Air Optix Aqua, SofLens and Galifa) with different degrees of hydrophilic properties were tested. After cross-linking with a surface UVA irradiance of 3 mW/cm² for 30 min (fluence 5.4 J/cm²), a 400 μm deep anterior corneal flap was created using a lamellar rotating microkeratome. Biomechanical stress-strain measurements and thermal shrinkage tests were performed.

RESULTS

In the Air Optix Aqua group (30% hydration) without riboflavin film, Young's modulus and stress at 8% strain were increased significantly versus untreated controls and the effect was 92.4% respectively 86.35% of the standard CXL value. In the SofLens group (59% hydration) without riboflavin film, Young's modulus and stress at 8% strain were increased significantly versus untreated controls and the effect was 67.04% respectively 65.28% of the standard CXL value. In the Galifa group (72% hydration) without riboflavin film, Young's modulus and stress at 8% strain were increased significantly versus untreated controls and the effect was about 68.48% respectively 75.52% of the standard CXL value. In all samples with a precorneal riboflavin film under the contact lens, there was no significant biomechanical effect compared to the untreated controls. Similarly, in the hydrothermal experiments at 70°C, there was a typical mushroom pattern with increased resistance to thermal shrinkage in the anterior stroma after standard CXL, a markedly reduced mushroom effect using a riboflavin-soaked contact lens only and no effect with the use of a riboflavin-soaked contact lens plus a precorneal riboflavin film.

CONCLUSION

The biomechanical effect of CACXL in porcine corneas is about one-third less than after standard CXL. The efficacy of CACXL might be improved by reducing or omitting the riboflavin film on the contact lens. Further risk assessment studies are necessary.