Corneal Biomechanical Evaluation After Conventional Corneal Crosslinking With Oxygen Enrichment

Hyperbaric Oxygenation Maintains Elevated Stromal Oxygen Availability During Corneal Collagen Crosslinking with and Without Epithelial Removal

PURPOSE

Corneal collagen cross-linking (CXL) can halt corneal ectasia. Leaving corneal epithelium intact during treatment may reduce the incidence of complications. However, it is under debate whether this reduces efficacy and if oxygen supplementation may be necessary to optimize the cross-linking effect. This study aimed to investigate the impact of hyperbaric oxygenation (HBO) on intracorneal oxygen concentrations during epi-off and epi-on CXL.

METHODS

CXL was performed using riboflavin and ultraviolet-A (UV-A) irradiance (3 mW/cm2 for 30 min) on porcine corneas under normobaric and hyperbaric conditions, with and without supplemented oxygen, with and without epithelium. Intracorneal oxygen concentrations were continuously monitored before and during irradiation. Biomechanical properties were assessed through tensile strength testing.

RESULTS

HBO alone did not cause perceivable changes in stromal oxygen concentrations. Oxygen supplementation resulted in higher oxygen concentration in corneal stroma during CXL. HBO may cause a further increase in oxygen levels, although this was not statistically significant in this study. Notably, a tendency of oxygen levels to rise continuously during UV-irradiation was observed using HBO. Biomechanical properties showend no statistically significant differences between any groups.

CONCLUSIONS

In this ex-vivo model, HBO increased stromal oxygen levels during CXL, regardless of the presence of corneal epithelium. The dynamics in oxygen concentrations in corneal stromal tissue during CXL suggest that time is an important factor to consider in modifications of established protocols. Also, we hypothesize that stromal levels of riboflavin and UV-A irradiance may be more critical to the CXL effect when oxygen is supplemented and epithelium is not removed.

Thermographic analysis of the corneal surface in epi-on and epi-off corneal crosslinking for keratoconus

PURPOSE

To analyse the temperature of the corneal surface in keratoconus during corneal customized crosslinking (CXL) with a preserved epithelium (epi-on) under oxygen flow, and epi-off CXL in room air, and to assess the effect of pre-heating the oxygen.

METHODS

This masked, intra-individual comparing randomized study included 14 participants with bilateral progressive keratoconus treated with bilateral CXL: one eye with epi-on CXL under a flow of 2.5 L/min oxygen; the fellow eye with epi-off CXL in room air. In a second setting involving 12 healthy participants, room-tempered oxygen was flushed over one eye and oxygen pre-heated to 37°C over the fellow eye. The corneal surface temperature was assessed with infrared photography.

RESULTS

A reduction in corneal surface temperature was seen from the pre-treatment application of topical riboflavin in the epi-off group (-1.1 ± 1.0°C, p < 0.001). The temperature increased during the first half of the CXL treatment in both groups (+0.7 ± 1.2°C, p = 0.041 for epi-on; +0.7 ± 0.9°C, p = 0.023 for epi-off CXL, respectively). In epi-on CXL an overall temperature increase was seen during the treatment (+0.8 ± 1.2°C, p = 0.016). In the second setting, pre-heating the oxygen rendered a surface temperature increase of +1.8 ± 0.2°C (p < 0.001).

CONCLUSION

In epi-off CXL, the application of topical room-tempered riboflavin decreases the corneal surface temperature, likely due to increased evaporation. A slight temperature increase is seen during CXL with both epi-on and epi-off CXL, albeit far below the corneal safety limit. The corneal temperature can, however, be increased by applying pre-heated oxygen, a possible approach to modify or augment the treatment effect in CXL.

Squishy matters - Corneal mechanobiology in health and disease

The cornea, as a dynamic and responsive tissue, constantly interacts with mechanical forces in order to maintain its structural integrity, barrier function, transparency and refractive power. Cells within the cornea sense and respond to various mechanical forces that fundamentally regulate their morphology and fate in development, homeostasis and pathophysiology. Corneal cells also dynamically regulate their extracellular matrix (ECM) with ensuing cell-ECM crosstalk as the matrix serves as a dynamic signaling reservoir providing biophysical and biochemical cues to corneal cells. Here we provide an overview of mechanotransduction signaling pathways then delve into the recent advances in corneal mechanobiology, focusing on the interplay between mechanical forces and responses of the corneal epithelial, stromal, and endothelial cells. We also identify species-specific differences in corneal biomechanics and mechanotransduction to facilitate identification of optimal animal models to study corneal wound healing, disease, and novel therapeutic interventions. Finally, we identify key knowledge gaps and therapeutic opportunities in corneal mechanobiology that are pressing for the research community to address especially pertinent within the domains of limbal stem cell deficiency, keratoconus and Fuchs' endothelial corneal dystrophy. By furthering our understanding corneal mechanobiology, we can contextualize discoveries regarding corneal diseases as well as innovative treatments for them.

Excited-state intramolecular proton transfer (ESIPT)-based fluorescent probes for biomarker detection: design, mechanism, and application

Biomarkers are essential in biology, physiology, and pharmacology; thus, their detection is of extensive importance. Fluorescent probes provide effective tools for detecting biomarkers exactly. Excited state intramolecular proton transfer (ESIPT), one of the significant photophysical processes that possesses specific photoisomerization between Keto and Enol forms, can effectively avoid annoying interference from the background with a large Stokes shift. Hence, ESIPT is an excellent choice for biomarker monitoring. Based on the ESIPT process, abundant probes were designed and synthesized using three major design methods. In this review, we conclude probes for 14 kinds of biomarkers based on ESIPT explored in the past five years, summarize these general design methods, and highlight their application for biomarker detection in vitro or in vivo.

Assessment of Efficacy of a Novel Crosslinking Protocol with Intracameral Oxygen (Bubble-CXL) in Increasing the Corneal Stiffness Using Atomic Force Microscopy

The environmental oxygen level plays a critical role in corneal crosslinking (CXL), a treatment method to increase corneal biomechanical stability. In this study, we introduce a new CXL method (Bubble-CXL), in which intracameral oxygen serves as an additional oxygen source during eye treatment. The efficiency of this new method was compared with the efficiency of the standard CXL method. Three fresh porcine eye pairs were included in this study. One eye of each pair was treated with standard CXL, whereas in the partner eye, intracameral oxygen was injected prior to CXL and removed at the end of the procedure. The Young's modulus of each cornea was measured using atomic force microscopy. All analyzed corneas treated with intracameral oxygen showed significantly higher Young's modulus and thus an increased stiffness compared to the cornea of the partner eye treated with the standard protocol. Using intracameral oxygen in CXL therapy may increase crosslinking efficiency and improve biomechanical corneal properties.

Oxygen in Corneal Collagen Crosslinking to Treat Keratoconus: A Systematic Review and Meta-Analysis

PURPOSE

Keratoconus is a disorder that results in visual loss from increased corneal high-order aberrations and irregular astigmatism and reduces quality of life. The primary treatment for progressive keratoconus is crosslinking (CXL). Recently, it has been suggested that oxygen enhances the type II photodynamic reaction of CXL that is oxygen dependent. Our study investigated the effect of increased oxygen availability in epithelium-on CXL on visual acuity and corneal curvature.

METHODS

We searched PubMed, EMBASE, Medline, Web of Science, and Scopus databases on November 3, 2021. We included studies that reported increased oxygen availability during CXL in patients with keratoconus published within the last 10 years. A meta-analysis on the primary outcomes, maximum keratometry, and corrected distance visual acuity, was conducted.

RESULTS

The search yielded 108 publications which were screened and assessed for eligibility. Six studies were included in the systematic review and 5 studies were included in our meta-analysis of the outcomes of increased oxygen availability in accelerated CXL. The meta-analysis on data after 6 months of follow-up found a significant decrease in mean maximum keratometry of 1.2 diopter (95% confidence interval: 0.2-2.3; P =0.02) and an improvement in mean corrected distance visual acuity by 0.08 logMAR (95% confidence interval, 0.02-0.13; P =0.01). There were no serious adverse events reported.

CONCLUSIONS

Increasing oxygen during epithelium-on CXL improved visual acuity and produced corneal flattening without any serious adverse events in patients with keratoconus. The demarcation line depth was significantly higher with oxygen compared to the control group. Further data are required with a control group and long-term follow-up across a range of CXL protocols for implementation into standard clinical practice.

Corneal Collagen Cross-Linking Inhibits Corneal Blood and Lymphatic Vessels Temporarily in Alkali-Burned Rabbits

PURPOSE

This study aimed to explore whether corneal cross-linking (CXL) could regress corneal blood vessels (CBV) and corneal lymphatic vessels (CLV) in alkali-burned rabbits.

METHODS

A total of 80 rabbits 2-3 months old weighing 1.5-2.0 kg were randomly divided into four groups: CXL7 group; CTL7 group; CXL14 group; and CTL14 group. Then, 3% sodium pentobarbital 1 ml/kg and tetracaine eye drop 5 g/L were administered before surgery. NaOH 2 mol/L was topically applied to the central cornea to establish the alkali burning model. Then CXL was administered within 2 h in groups CXL7 and CXL14. Corneal opacity and edema, CBV and CLV volume, cluster differentiation 31 (CD31), and lymphatic vessel endothelial receptor 1 (LYVE-1) expression levels were analyzed on days 7 and 14.

RESULTS

CXL reduced cornea opacity, CNV, and CLV volumes on day 7 in alkali-burned rabbits. However, CNV and CLV volumes were increased on day 14. CXL also showed down- and upregulation of CD31 and LYVE-1 expression levels on days 7 and 14, respectively.

CONCLUSIONS

CXL effectively regulated CBV and CLV in alkali-burned rabbits. The transient angioregression and lymphangioregression induced by CXL may be potentially helpful in vascularized high-risk eyes.

Corneal Biomechanics and Intraocular Pressure Following Scleral Lens Wear in Penetrating Keratoplasty and Keratoconus

OBJECTIVE

To compare corneal biomechanics and intraocular pressure (IOP) in keratoconus and penetrating keratoplasty eyes before and after nonfenestrated scleral lens wear.

METHODS

Twenty-three participants were enrolled, and 37 eyes were included in the analysis (11 penetrating keratoplasty and 26 keratoconus). A range of corneal biomechanical parameters and IOP were measured using the CORVIS ST before and after 8 hr of nonfenestrated scleral lens wear (Keracare, Acculens, Denver, CO).

RESULTS

Before lens wear, penetrating keratoplasty eyes displayed significantly greater median values for central corneal thickness (97 μm thicker, P=0.02), IOP (3.89 mm Hg higher, P=0.01), and biomechanical parameter A2 length (0.48 mm longer, P=0.003) compared with keratoconic eyes. No significant changes in corneal biomechanical parameters or IOP were observed after scleral lens wear in either group (all P>0.05).

CONCLUSION

Although nonfenestrated scleral contact lenses can induce a subatmospheric pressure after lens settling and compress tissue surrounding the limbus, no significant changes were detected in the corneal biomechanical parameters studied using CORVIS ST after scleral lens wear in eyes with penetrating keratoplasty and keratoconus.

The efficiency and safety of oxygen-supplemented accelerated transepithelial corneal cross-linking

PURPOSE

To investigate the impact of oxygen delivery on the clinical outcomes of accelerated transepithelial corneal cross-linking (A-TE CXL).

METHODS

Fifty-seven eyes of 44 progressive keratoconus (KCN) patients were randomly separated into two age-sex-matched groups. Twenty-nine eyes of 23 KCN patients that underwent oxygen-supplemented A-TE CXL formed the study group and 28 eyes of 21 patients treated with the same procedure but under room air conditions formed the control group. All patients were examined preoperatively, one, six and twelve months after the procedure. The logMAR spectacle-corrected distance visual acuity (CDVA), maximum keratometry (Kmax), mean keratometry, apical posterior keratometry, cylindrical power, minimum central corneal thickness, keratoconus vertex front and back, ocular aberrations, endothelial cell density (ECD), demarcation line depth (DLD) and proportion measures were recorded for statistical analysis.

RESULTS

The preoperative, 1st, 6th and 12th months mean Kmax values of the study group were 55.14 ± 3.99D, 54.85 ± 3.82D, 54.37 ± 3.84D and 54.40 ± 3.86, respectively, and 54.47 ± 3.17D, 54.52 ± 2.97D, 54.25 ± 2.95D and 54.20 ± 2.97 in the control group. The mean Kmax value was decreased significantly more in the oxygen-supplemented group after 12 months compared to the control group (p = 0.019). The mean DLD was also significantly deeper in the study group (320 ± 17 µm) compared to the control group (269 ± 19 µm). There was no significant difference between the two groups in terms of ECD alterations at any of the time intervals (p > 0.05).

CONCLUSION

Keratoconus progression was significantly halted in both groups 12 months after the treatment. In addition, oxygen supplementation during A-TE CXL further significantly increased clinical outcomes compared to room air conditions without any significant change in ECD measures.

Oxygen Kinetics During Corneal Cross-linking With and Without Supplementary Oxygen

PURPOSE

To measure and simulate oxygen kinetics during corneal cross-linking at different irradiances with and without supplementary oxygen.

DESIGN

Experimental, laboratory study.

METHODS

In de-epithelialized porcine eyes, a femtosecond-laser-generated tunnel was used to place a fiber probe in corneal depths of 100, 200, and 300 μm to measure the local oxygen concentration. After riboflavin imbibition, the corneas were irradiated at 3, 9, 18, and 30 mW/cm² while the oxygen concentration was measured. All experiments were performed under normoxic (21%) and hyperoxic (>95%) conditions. The obtained data were used to identify parameters of a numerical model for oxygen consumption and diffusion.

RESULTS

The equilibrium stromal oxygen concentration under atmospheric oxygen at 3 mW/cm² was 2.3% in 100 μm decreasing to <1% in 300 μm. With 9, 18, and 30 mW/cm², no oxygen was available in 200 μm, respectively, 100 μm or deeper. Using a hyperoxic environment, the concentration was 50% using 3 mW/cm² in 100 μm, decreasing to 40% in 300 μm. At 9 mW/cm², the concentrations were 5%, 3%, and 1% in 100, 200 and 300 μm, respectively. Using 18 and 30 mW/cm², all oxygen was depleted at 100 μm; however, oxygen half-lives were longer at 18 mW/cm² than at 30 mW/cm². The oxygen model was able to reproduce the experiments and indicated an exponential decay with increasing distance to the anterior surface.

CONCLUSION

Supplementary oxygen increases the oxygen availability during corneal cross-linking. At higher irradiances, supplementary oxygen is beneficial and eliminates the bottleneck of oxygen allowing a potentially more efficient cross-linking. The calibrated numerical model can quantify the spatial oxygen concentration related to different scenarios such as irradiance or environmental oxygen concentration.

Effects of hydrogen peroxide solution on the biomechanics of porcine cornea

OBJECTIVES

To determine the biomechanical changes of porcine corneas after the application of hydrogen peroxide(H₂O₂) solution.

METHODS

Fifty-five porcine eyeballs with similar sizes were divided into 11 groups based on the H₂O₂ application. The eyeballs were treated with the following concentrations of H₂O₂ solution: 1 mol/L, 500 mmol/L, 250 mmol/L, 125 mmol/L, 62.5 mmol/L, 31.25 mmol/L, 15.63 mmol/L, 7.81 mmol/L, 3.91 mmol/L, 0.9% saline, or blank. The eyeballs were immersed into the solution for 30 min. The biomechanics of each cornea in the different groups was determined soon after the indentation and tensile tests. We calculated the average Young's modulus of the different groups to determine the effects of H₂O₂ solution on porcine corneas. The comparison between the groups was conducted using ANOVA analysis. Moreover, the safety of each concentration of H₂O₂ solution on the corneal tissues was determined by histopathological examination.

RESULTS

The Young's modulus was significantly different among all the groups (p = 0.003). The modulus was the highest in the group treated with 3.91 mmol/L H₂O₂ and it was significantly different from that in the group treated with 0.9% saline or the blank group, for both the indentation and tensile tests. Histopathological examination showed that H₂O₂ at a concentration of ⩾62.5 mmol/L damaged the epithelium, stroma, or both, while H₂O₂ at a concentration ⩽31.25 mmol/L did not change the morphology of the epithelium or stroma.

CONCLUSIONS

Treatment with 3.91 mmol/L H₂O₂ solution can safely and effectively increase the biomechanical strength of the cornea.