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

Assessment of Rose Bengal Photodynamic Therapy on Viability and Proliferation of Human Keratolimbal Epithelial and Stromal Cells In Vitro

PURPOSE

To investigate the effect of Rose Bengal photodynamic therapy (RB-PDT) on viability and proliferation of human limbal epithelial stem cells (T-LSCs), human corneal epithelial cells (HCE-T), human limbal fibroblasts (LFCs), and human normal and keratoconus fibroblasts (HCFs and KC-HCFs) in vitro.

METHODS

T-LSCs and HCE-T cell lines were used in this research. LFCs were isolated from healthy donor corneal limbi (n = 5), HCFs from healthy human donor corneas (n = 5), and KC-HCFs from penetrating keratoplasties of keratoconus patients (n = 5). After cell culture, RB-PDT was performed using 0.001% RB concentration and 565 nm wavelength illumination with 0.14 to 0.7 J/cm2 fluence. The XTT and the BrdU assays were used to assess cell viability and proliferation 24 h after RB-PDT.

RESULTS

RB or illumination alone did not change cell viability or proliferation in any of the cell types (p ≥ 0.1). However, following RB-PDT, viability decreased significantly from 0.17 J/cm2 fluence in HCFs (p < 0.001) and KC-HCFs (p < 0.0001), and from 0.35 J/cm2 fluence in T-LSCs (p < 0.001), HCE-T (p < 0.05), and LFCs ((p < 0.0001). Cell proliferation decreased significantly from 0.14 J/cm2 fluence in T-LSCs (p < 0.0001), HCE-T (p < 0.05), and KC-HCFs (p < 0.001) and from 0.17 J/cm2 fluence in HCFs (p < 0.05). Regarding LFCs proliferation, no values could be determined by the BrdU assay.

CONCLUSIONS

Though RB-PDT seems to be a safe and effective treatment method in vivo, its dose-dependent phototoxicity on corneal epithelial and stromal cells has to be respected. The data and experimental parameters applied in this study may provide a reliable reference for future investigations.

Revitalising Riboflavin: Unveiling Its Timeless Significance in Human Physiology and Health

Since the early twentieth century, research on vitamins has revealed their therapeutic potential beyond their role as essential micronutrients. Riboflavin, known as vitamin B2, stands out for its unique characteristics. Despite numerous studies, riboflavin remains vital, with implications for human health. Abundantly present in various foods, riboflavin acts as a coenzyme in numerous enzymatic reactions crucial for human metabolism. Its role in energy production, erythrocyte synthesis, and vitamin metabolism underscores its importance in maintaining homeostasis. The impact of riboflavin extends to neurological function, skin health, and cardiovascular well-being, with adequate levels linked to reduced risks of various ailments. However, inadequate intake or physiological stress can lead to deficiency, a condition that poses serious health risks, including severe complications. This underscores the importance of maintaining sufficient levels of riboflavin for general wellness. The essential role of riboflavin in immune function further emphasises its significance for human health and vitality. This paper examines the diverse effects of riboflavin on health and stresses the importance of maintaining sufficient levels for overall well-being.

Use of photosensitive molecules in the crosslinking of biopolymers: applications and considerations in biomaterials development

The development of diverse types of biomaterials has significantly contributed to bringing new biomedical strategies to treat clinical conditions. Applications of these biomaterials can range from mechanical support and protection of injured tissues to joint replacement, tissue implants, and drug delivery systems. Among the strategies commonly used to prepare biomaterials, the use of electromagnetic radiation to initiate crosslinking stands out. The predominance of photo-induced polymerization methods relies on a fast, efficient, and straightforward process that can be easily adjusted to clinical needs. This strategy consists of irradiating the components that form the material with photons in the near ultraviolet-visible wavelength range (i.e., ∼310 to 750 nm) in the presence of a photoactive molecule. Upon photon absorption, photosensitive molecules can generate excited species that initiate photopolymerization through different reaction mechanisms. However, this process could promote undesired side reactions depending on the target zone or treatment type (e.g., oxidative stress and modification of biomolecules such as proteins and lipids). This review explores the basic concepts behind the photopolymerization process of ex situ and in situ biomaterials. Particular emphasis was put on the photosensitization initiated by the most employed photosensitizers and the photoreactions that they mediate in aqueous media. Finally, the undesired oxidation reactions at the bio-interface and potential solutions are presented.

Long-term outcomes of corneal crosslinking

PURPOSE OF REVIEW

This manuscript summarizes contemporary research from 2018 to 2023 evaluating long-term (≥2 years) outcomes of corneal crosslinking (CXL) for progressive keratoconus (KCN).

RECENT FINDINGS

The standard Dresden protocol (SDP) has been utilized clinically since the early 2000 s to treat ectatic disorders, primarily progressive KCN and postrefractive ectasia. Various modifications have since been introduced including accelerated and transepithelial protocols, which are aimed at improving outcomes or reducing complications. This review summarizes data demonstrating that the SDP halts disease progression and improves various visual and topographic indices (UDVA, CDVA, Kmax, K1, K2) up to 13 years postoperatively. Accelerated and transepithelial protocols have been found to be well tolerated alternatives to SDP with similar efficacy profiles. Studies focusing on pediatric populations identified overall higher progression rates after CXL. All protocols reviewed had excellent safety outcomes in adults and children.

SUMMARY

Recent studies revealed that SDP successfully stabilizes KCN long term, and a variety of newer protocols are also effective. Pediatric patients may exhibit higher progression rates after CXL. Further research is required to enhance the efficacy and ease of these protocols.

Safety and Efficacy of Epithelium-Off Corneal Collagen Cross-Linking for the Treatment of Corneal Ectasia: A Report by the American Academy of Ophthalmology

PURPOSE

To review the evidence on the safety and effectiveness of epithelium-off corneal collagen cross-linking (CXL) for the treatment of progressive corneal ectasia.

METHODS

A literature search of the PubMed database was most recently conducted in March 2024 with no date restrictions and limited to studies published in English. The search identified 359 citations that were reviewed in abstract form, and 43 of these were reviewed in full text. High-quality randomized clinical trials comparing epithelium-off CXL with conservative treatment in patients who have keratoconus (KCN) and post-refractive surgery ectasia were included. The panel deemed 6 articles to be of sufficient relevance for inclusion, and these were assessed for quality by the panel methodologist; 5 were rated level I, and 1 was rated level II. There were no level III studies.

RESULTS

This analysis includes 6 prospective, randomized controlled trials that evaluated the use of epithelium-off CXL to treat progressive KCN (5 studies) and post-laser refractive surgery ectasia (1 study), with a mean postoperative follow-up of 2.4 years (range, 1-5 years). All studies showed a decreased progression rate in treated patients compared with controls. Improvement in the maximum keratometry (Kmax) value, corrected distance visual acuity (CDVA), and uncorrected distance visual acuity (UDVA) was observed in the treatment groups compared with control groups. A decrease in corneal thickness was observed in both groups but was greater in the CXL group. Complications were rare.

CONCLUSIONS

Epithelium-off CXL is effective in reducing the progression of KCN and post-laser refractive surgery ectasia in most treated patients with an acceptable safety profile.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

UV-stimulated riboflavin exerts immunosuppressive effects in BALB/c mice and human PBMCs

Riboflavin (RF) as a photosensitizer has been used in corneal surgery and the inactivation of blood products. However, the effect of RF on immune cells after ultraviolet (UV) light stimulation has not been investigated. This study pioneered a novel application method of RF. Firstly, UV-stimulated RF was co-cultured with human peripheral blood mononuclear cells in vitro, and the apoptosis rate of lymphocyte subsets, cell proliferation inhibition rate and concentrations of IL-1β, IL-6, IL-10, TNF-α were assessed. UV-stimulated RF was then administered intravenously to mice via the tail vein for a consecutive period of 5 days. The levels of immunoglobulin (IgG, IgM, IgA), complement (C3, C4) and cytokines (IFN-γ, IL-4, IL17, TGF-β) were detected by ELISA. Flow cytometry was employed to analyze the populations of CD3+T, CD4+T, CD8+T and CD4+T/CD8+T cells in spleen lymphocytes of mice. The data showed that UV-stimulated RF can effectively induce apoptosis in lymphocytes, and different lymphocyte subtypes exhibited varying degrees of treatment tolerance. Additionally, the proliferative capacity of lymphocytes was suppressed, while their cytokine secretion capability was augmented. The animal experiments demonstrated that UV-stimulated RF led to a significant reduction observed in serum immunoglobulin and complement levels, accompanied by an elevation in IFN-γ, IL-17 and TGF-β levels, as well as a decline in IL-4 level. In summary, the results of both in vitro and in vivo experiments have demonstrated that UV-stimulated RF, exhibits the ability to partially inhibit immune function. This novel approach utilizing RF may offer innovative perspectives for diseases requiring immunosuppressive treatment.

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.

Unilateral progressive keratoconus associated with ipsilateral craniofacial fibrous dysplasia treated with corneal cross-linking

Purpose

The purpose of this study is to describe a case of unilateral keratoconus associated with ipsilateral craniofacial fibrous dysplasia and its subsequent management with corneal collagen cross-linking.

Observations

This is an interventional case report of a 16-year-old male with a history of polyostotic fibrous dysplasia of the left frontal bone and orbital roof status post partial resection six years prior who presented to the pediatric ophthalmology clinic with progressively blurry vision in the left eye. Refraction in this eye revealed an increase in cylinder by > 3D from his last refraction two years prior. Pentacam corneal tomography confirmed the diagnosis of keratoconus in the left eye. The patient underwent corneal collagen cross-linking in the affected eye. Postoperatively, he experienced marked improvement in corrected visual acuity with scleral contact lenses and maintained stable astigmatism and keratometry values on Pentacam corneal tomography at his most recent visit 12 months postoperatively.

Conclusions

While it is otherwise felt to be a bilateral disease, unilateral keratoconus may present in the context of ipsilateral orbital pathology. Corneal collagen cross-linking may be used to successfully prevent keratoconus progression in the setting of stable orbital pathology.

Collagen cross-linking beyond corneal ectasia: A comprehensive review

The history of corneal cross-linking (CXL) dates back to 2003 when some German scientists investigated possible treatments to harden the corneal structure to increase its resistance in ectatic corneal diseases. Nowadays, CXL is considered the most effective therapy in ectatic corneal diseases due to its proven efficacy in hardening the cornea, thus halting the development of the disease. Since 2003, CXL applications have dramatically expanded and have been implemented in several other areas such as infectious keratitis, corneal edema, and before performing keratoplasty for various purposes. Moreover, several irradiation patterns are being studied to correct refractive errors, taking into account the corneal refractive changes that occur after the procedure. Currently, scleral cross-linking is also being investigated as a potential therapy in cases of progressive myopia and glaucoma. In this article, we provide a comprehensive overview of the available applications of cross-linking in nonectatic ocular conditions and highlight the possible future indications of this procedure.

Possible depth-resolved reconstruction of shear moduli in the cornea following collagen crosslinking (CXL) with optical coherence tomography and elastography

Corneal collagen crosslinking (CXL) is commonly used to prevent or treat keratoconus. Although changes in corneal stiffness induced by CXL surgery can be monitored with non-contact dynamic optical coherence elastography (OCE) by tracking mechanical wave propagation, depth dependent changes are still unclear if the cornea is not crosslinked through the whole depth. Here, phase-decorrelation measurements on optical coherence tomography (OCT) structural images are combined with acoustic micro-tapping (AµT) OCE to explore possible reconstruction of depth-dependent stiffness within crosslinked corneas in an ex vivo human cornea sample. Experimental OCT images are analyzed to define the penetration depth of CXL into the cornea. In a representative ex vivo human cornea sample, crosslinking depth varied from ∼100 µm in the periphery to ∼150 µm in the cornea center and exhibited a sharp in-depth transition between crosslinked and untreated areas. This information was used in an analytical two-layer guided wave propagation model to quantify the stiffness of the treated layer. We also discuss how the elastic moduli of partially CXL-treated cornea layers reflect the effective engineering stiffness of the entire cornea to properly quantify corneal deformation.

Causes and Clinical Sequelae of Riboflavin Deficiency

Riboflavin, in its cofactor forms flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), plays fundamental roles in energy metabolism, cellular antioxidant potential, and metabolic interactions with other micronutrients, including iron, vitamin B6, and folate. Severe riboflavin deficiency, largely confined to low-income countries, clinically manifests as cheilosis, angular stomatitis, glossitis, seborrheic dermatitis, and severe anemia with erythroid hypoplasia. Subclinical deficiency may be much more widespread, including in high-income countries, but typically goes undetected because riboflavin biomarkers are rarely measured in human studies. There are adverse health consequences of low and deficient riboflavin status throughout the life cycle, including anemia and hypertension, that could contribute substantially to the global burden of disease. This review considers the available evidence on causes, detection, and consequences of riboflavin deficiency, ranging from clinical deficiency signs to manifestations associated with less severe deficiency, and the related research, public health, and policy priorities.

Analysis of Biomechanical Response After Corneal Crosslinking with Different Fluence Levels in Porcine Corneas

PURPOSE

To evaluate corneal stiffening of porcine corneas induced by corneal crosslinking (CXL) with constant irradiance as a function of total fluence.

METHODS

Ninety corneas from freshly enucleated porcine eyes were divided into five groups of 18 eyes. Groups 1-4 underwent epi-off CXL using a dextran-based riboflavin solution and an irradiance of 18 mW/cm2, group 5 served as the control group. Groups 1 to 4 were treated with a total fluence of 20, 15, 10.8, and 5.4 J/cm2, respectively. Thereafter, biomechanical measurements were performed on 5 mm wide and 6 mm long strips using an uniaxial material tester. Pachymetry measurements were performed on each cornea.

RESULTS

At 10% strain, the stress was 76, 56, 52, and 31% higher in groups 1-4, respectively compared to the control group. The Young's modulus was 2.85 MPa for group 1, 2.53 MPa for group 2, 2.46 MPa for group 3, 2.12 MPa for group 4, and 1.62 MPa for the control group. The difference between groups 1 to 4 and the control group 5 were statistically significant (p = <0.001; p = <0.001; p = <0.001; p = 0.021). In addition, group 1 showed significantly more stiffening than group 4 (p = <0.001), no other significant differences were found. Pachymetry measurements revealed no statistically significant differences among the five groups.

CONCLUSION

Additional mechanical stiffening can be achieved by increasing the fluence of the CXL. There was no threshold detected up to 20 J/cm2. A higher fluence could compensate the weaker effect of accelerated or epi-on CXL procedures.

Comparison Between Pulsed and Continuous Accelerated Corneal Cross-Linking Protocols

Purpose

To compare between two accelerated corneal cross-linking (A-CXL) protocols in the management of keratoconus (KC) as regard to the extent of corneal treatment.

Methods

This retrospective, comparative study included patients having mild to moderate, progressive KC. The study population was divided into two groups; group 1 enrolled 103 eyes of 62 patients who received pulsed light A-CXL (pl-CXL) at a power of 30 mW/cm² with an irradiation time of 4 minutes, while group 2 comprised 87 eyes of 51 patients who received continuous light A-CXL (cl-CXL) at a power of 12 mW/cm² with an irradiation time of 10 minutes. Recordings of the central and peripheral demarcation line depths (DD), and the maximum (DDmax) and minimum (DDmin) DD, using anterior segment optical coherence tomography, were compared between the two studied groups one month after the treatment protocol. Treatment stability was also evaluated pre and postoperatively (one year following surgery) by comparing the refractive and keratometric outcomes in both groups.

Results

The differences between the preoperative corneal thickness (minimum and central) and the epithelial thickness measurements between both groups were not statistically significant. Although group 1 had slightly larger central DD (223.4 ± 62.3 um), DDmax (240.4 ± 61.8 um), and DDmin (201 ± 54 um) than those of group 2 (221.8 ± 37 um, 229.1 ± 38.4 um, and 212 ± 37.2 um, respectively), the differences between both groups' measurements were not statistically significant. Also, the two groups showed statistically insignificant differences regarding the subjective refraction and the average and maximum keratometry pre and postoperatively, denoting visual, refractive, and keratometric stability in both groups.

Conclusion

Longer duration cl-CXL seems to be as effective as pl-CXL regarding both postoperative stability and the extent of corneal tissue penetration by the ultraviolet treatment.

Hemocompatibility tuning of an innovative glutaraldehyde-free preparation strategy using riboflavin/UV crosslinking and electron irradiation of bovine pericardium for cardiac substitutes

Hemocompatibility tuning was adopted to explore and refine an innovative, GA-free preparation strategy combining decellularization, riboflavin/UV crosslinking, and low-energy electron irradiation (SULEEI) procedure. A SULEEI-protocol was established to avoid GA-dependent deterioration that results in insufficient long-term aortic valve bioprosthesis durability. Final SULEEI-pericardium, intermediate steps and GA-fixed reference pericardium were exposed in vitro to fresh human whole blood to elucidate effects of preparation parameters on coagulation and inflammation activation and tissue histology. The riboflavin/UV crosslinking step showed to be less efficient in inactivating extracellular matrix (ECM) protein activity than the GA fixation, leading to tissue-factor mediated blood clotting. Intensifying the riboflavin/UV crosslinking with elevated riboflavin concentration and dextran caused an enhanced activation of the complement system. Yet activation processes induced by the previous protocol steps were quenched with the final electron beam treatment step. An optimized SULEEI protocol was developed using an intense and extended, trypsin-containing decellularization step to inactivate tissue factor and a dextran-free, low riboflavin, high UV crosslinking step. The innovative and improved GA-free SULEEI-preparation protocol results in low coagulant and low inflammatory bovine pericardium for surgical application.

The research progress on the molecular mechanism of corneal cross-linking in keratoconus treatment

Keratoconus (KC) is a corneal anomaly that is manifested in a limited cone-like bulge with corneal thinning. Many molecules in the cornea change during the development of KC, including various components of the extracellular matrix, cytokines, cell connection, and cell adhesion-related proteins. Several treatment options are available, with corneal cross-linking (CXL) being the treatment of choice for early KC. However, postoperative complications have been reported in some CXL patients, mainly caused by corneal epithelial resection. Despite the fact that some novel approaches have helped to reduce some of the initial post-operative issues, their effectiveness seems to be inferior to that of the original CXL. To keep effectiveness while avoiding these negative effects, it is necessary to study the mechanism of CXL in KC treatment at the molecular level. This article provides a review of the molecular mechanism of CXL in the treatment of KC from four aspects: enzyme activity, signal transduction pathway, corneal-related proteins, and other KC-related molecules, further confirming the feasibility of CXL treatment of KC, providing new ideas for improving CXL.

Efficacy and Safety of Standard Corneal Cross-Linking Procedures Performed With Short Versus Standard Riboflavin Induction: A Save Sight Keratoconus Registry Study

PURPOSE

The objective of this study was to compare the effectiveness and safety of short versus standard riboflavin induction times in cross-linking (CXL) for keratoconus.

METHODS

A retrospective comparative study was conducted with data from the Save Sight Keratoconus Registry. Inclusion criteria were epithelium-off technique, standard UVA CXL protocol (3 mW/cm 2 for 30 minutes), riboflavin induction for 15 minutes (short) or 30 minutes (standard), and 1 year of follow-up data after CXL. Outcome measures included changes in best-corrected visual acuity (BCVA), keratometry in the steepest meridian (K2), maximum keratometry (Kmax), thinnest pachymetry (TCT), and adverse events. Analysis was conducted using mixed-effects regression models adjusted for age, sex, visual acuity, keratometry, pachymetry, practice, and eye laterality.

RESULTS

Two hundred eighty eyes (237 patients; mean, 27.3 ± 10.5 years old; 30% female) were included. The riboflavin induction time was short in 102 eyes (82 patients) and standard in 178 eyes (155 patients). The baseline characteristics (sex, mean age, BCVA, keratometry, and pachymetry [TCT]) were similar between the groups. At the 1-year follow-up visit, no statistically significant differences were observed in flattening in K2 and improvement in BCVA. Greater Kmax flattening [-1.5 diopters (D) vs. -0.5D, P = 0.031] and a greater proportion of >2% increase in TCT (23.5 vs. 11.3, P = 0.034) and haze (29 vs. 15, P = 0.005) were observed with short riboflavin induction.

CONCLUSIONS

Short and standard riboflavin induction times achieved similar degrees of flattening in K2 and improvement in vision. Greater improvements in Kmax and TCT were seen with short riboflavin times; however, this group had higher rates of haze.

The Use of Collagen-Based Materials in Bone Tissue Engineering

Synthetic bone substitute materials (BSMs) are becoming the general trend, replacing autologous grafting for bone tissue engineering (BTE) in orthopedic research and clinical practice. As the main component of bone matrix, collagen type I has played a critical role in the construction of ideal synthetic BSMs for decades. Significant strides have been made in the field of collagen research, including the exploration of various collagen types, structures, and sources, the optimization of preparation techniques, modification technologies, and the manufacture of various collagen-based materials. However, the poor mechanical properties, fast degradation, and lack of osteoconductive activity of collagen-based materials caused inefficient bone replacement and limited their translation into clinical reality. In the area of BTE, so far, attempts have focused on the preparation of collagen-based biomimetic BSMs, along with other inorganic materials and bioactive substances. By reviewing the approved products on the market, this manuscript updates the latest applications of collagen-based materials in bone regeneration and highlights the potential for further development in the field of BTE over the next ten years.

Role of Corneal Epithelial Measurements in Differentiating Eyes with Stable Keratoconus from Eyes that Are Progressing

Purpose

To evaluate measures of corneal epithelium in eyes that showed documented signs of keratoconus (KC) progression and compare with stable eyes and healthy controls. Also, to determine the correlation of these epithelial parameters with maximum keratometry (K max) and pachymetry.

Design

Prospective, observational, comparative study.

Participants

One-hundred and fifty eyes from 150 patients. The study included 50 eyes from patients with documented KC progression, 50 eyes with stable KC, and 50 clinically normal eyes to serve as controls.

Methods

A spectral-domain (SD)-OCT imaging was obtained in all eyes, and mean values were compared between the groups. The correlation of epithelial parameters with K max and thinnest pachymetry was also investigated.

Main Outcome Measures

For the purposes of this study, the epithelial measures maximum, minimum, superior, and inferior values as well as the difference between the minimum and maximum (min-max) and epithelial standard deviation were considered, obtained from SD-OCT and compared between groups. Measurements of the thinnest point and min-max in pachymetry were also recorded.

Results

The only epithelial parameter that presented a statistically significant difference between stable and progressive KC was epithelium min-max. Although stable KC presented epithelium min-max mean values of -18.2 ± 6.6, progressive KC eyes presented mean values of -23.4 ± 10.3 (P < 0.0001). Epithelial maximum (P = 0.16), minimum (P = 0.25), superior (P = 0.28), inferior (P = 0.23), and standard deviation (P = 0.25) values were not significantly different between stable and progressive eyes. Difference min-max pachymetry points in stable (-108.3 ± 33.5) and progressive KC (-115.2 ± 56.0) were not significantly different (P = 0.723). There was no significant correlation between epithelium min-max with corneal thinning (P = 0.39) or K max (P = 0.09) regardless of disease progression.

Conclusions

Epithelial measures are useful to identify KC eyes that are progressing; the parameters that measure the difference between min-max epithelium points were significantly different between stable and progressive groups, unlike this difference in pachymetry. Finally, this epithelial parameter seems to be independent of corneal thinning and K max.

Financial Disclosures

Proprietary or commercial disclosure may be found after the references.

Acoustic Micro-Tapping Optical Coherence Elastography to Quantify Corneal Collagen Cross-Linking: An Ex Vivo Human Study

Purpose

To evaluate changes in the anisotropic elastic properties of ex vivo human cornea treated with ultraviolet cross-linking (CXL) using noncontact acoustic micro-tapping optical coherence elastography (AμT-OCE).

Design

Acoustic micro-tapping OCE was performed on normal and CXL human donor cornea in an ex vivo laboratory study.

Subjects

Normal human donor cornea (n = 22) divided into 4 subgroups. All samples were stored in optisol.

Methods

Elastic properties (in-plane Young's, E, and out-of-plane, G, shear modulus) of normal and ultraviolet CXL-treated human corneas were quantified using noncontact AμT-OCE. A nearly incompressible transverse isotropic model was used to reconstruct moduli from AμT-OCE data. Independently, cornea elastic moduli were also measured with destructive mechanical tests (tensile extensometry and shear rheometry).

Main Outcome Measures

Corneal elastic moduli (in-plane Young's modulus, E, in-plane, μ, and out-of-plane, G, shear moduli) can be evaluated in both normal and CXL treated tissues, as well as monitored during the CXL procedure using noncontact AμT-OCE.

Results

Cross-linking induced a significant increase in both in-plane and out-of-plane elastic moduli in human cornea. The statistical mean in the paired study (presurgery and postsurgery, n = 7) of the in-plane Young's modulus, E = 3 μ , increased from 19 MPa to 43 MPa, while the out-of-plane shear modulus, G, increased from 188 kPa to 673 kPa. Mechanical tests in a separate subgroup support CXL-induced cornea moduli changes and generally agree with noncontact AμT-OCE measurements.

Conclusions

The human cornea is a highly anisotropic material where in-plane mechanical properties are very different from those out-of-plane. Noncontact AμT-OCE can measure changes in the anisotropic elastic properties in human cornea as a result of ultraviolet CXL.

Oxygen-supplemented transepithelial-accelerated corneal crosslinking with pulsed irradiation for progressive keratoconus: 1 year outcomes

PURPOSE

To investigate the effects of combining oxygen supplementation with enhanced UV-A light and increased riboflavin permeability in improving the efficacy of epithelium-on crosslinking (epi-on CXL).

SETTING

Private eye clinic in Brisbane, Queensland, Australia.

DESIGN

Retrospective single-center nonrandomized uncontrolled longitudinal cohort case series.

METHODS

Transepithelial CXL was performed on keratoconic eyes. Applications of an oxygen goggle and pulsed UV-A irradiation (1 second on, 1 second off) were used to enhance oxygen kinetics during epi-on CXL. Additional procedural modifications included the use of benzalkonium chloride and high UV-A irradiance level (30 mW/cm 2 ) to improve the stromal bioavailability of riboflavin and UV-A. The main efficacy outcomes were the changes in mean corrected distance visual acuity (CDVA) and safety over 12 months. Additional refractive and keratometry (K) outcomes were also observed.

RESULTS

53 eyes (38 patients) were included in this study. 12 months postoperatively, mean CDVA improved from a mean of 0.18 ± 0.2 at baseline to 0.07 ± 0.1 logMAR ( P < .0001). No statistically significant change was observed in maximum K (Kmax) and mean K, which were respectively 51.7 ± 5.8 diopters (D) and 46.4 ± 3.85 D at baseline and 51.2 ± 5.7 D ( P = .152) and 46.0 ± 3.84 D ( P = .06) 12 months postoperatively. Only 3 eyes experienced an increase of more than 2 D in Kmax; however, none of these eyes experienced a CDVA loss. There were no reported infections, corneal scarring, or other severe adverse effects.

CONCLUSIONS

Performing supplemental oxygen epi-on CXL with accelerated, pulsed UV-A irradiation in conjunction with riboflavin permeability enhancers resulted in improved CDVA ( P < .0001) and stable keratometry up to 12 months postoperatively with a good safety profile.

Corneal Crosslinking: Present and Future

Keratoconus is a progressive corneal thinning disorder that can lead to vision loss. In the last 2 decades, corneal crosslinking (CXL) has emerged as an effective method to halt the progression of keratoconus and reduce the number of patients requiring keratoplasty. The procedure has been adopted globally and has evolved to become a part of combination treatments to regularize the cornea and improve visual outcomes. CXL has even been extrapolated in managing other ocular pathologies such as progressive myopia, infectious keratitis, and bullous keratopathy. This review aims to summarize the current role of CXL in keratoconus and its alternative uses, and provide insights into future developments in this fast-developing field.

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.

Late onset of a persistent, deep stromal and endothelial scarring after corneal collagen crosslinking for keratoconus: A case report

INTRODUCTION

To report a late onset, deep stromal and endothelial corneal scar in a keratoconus patient after corneal collagen cross-linking (CXL).

CASE DESCRIPTION

Observational case report. A 29-year-old man with bilateral keratoconus received an accelerated (A-CXL 10*9) epithelium-off CXL procedure in the left eye.6-months postoperatively, a 2.2 × 1.2 mm inferocentral corneal scar was detected, which was located in the posterior stroma ranging from approximately 350 µm until the endothelium, therefore was situated below the demarcation line. A topical corticosteroid treatment did not influence the magnitude or configuration of the scar. Visual acuity was never affected, which includes the examination 12 months postoperatively.

CONCLUSIONS

We report a case of a late onset deep stromal and endothelial corneal scar 6 months after accelerated CXL as postoperative complication without affecting visual acuity.

Association of Novel Loci With Keratoconus Susceptibility in a Multitrait Genome-Wide Association Study of the UK Biobank Database and Canadian Longitudinal Study on Aging

Importance

Keratoconus can be a debilitating corneal ectasia in which the cornea thins, bulges, and steepens into a conical shape. Early features of keratoconus include myopia and irregular astigmatism, which affect vision and can be treated with contact lenses, collagen cross-linking, or, in advanced cases, corneal transplant. Recent estimates of the prevalence of keratoconus based on results of Scheimpflug imaging in young adults are as high as 1.2%. However, obtaining very large keratoconus data sets for a genome-wide association study (GWAS) is problematic because few population studies include Scheimpflug imaging and because severe keratoconus is relatively rare.

Objective

To identify novel keratoconus loci using corneal resistance factor (CRF) and central corneal thickness (CCT).

Design, Setting, and Participants

This multitrait GWAS used European ancestry CRF data from UK Biobank (UKB) (n = 105 427) and the Canadian Longitudinal Study on Aging (CLSA) (n = 18 307) and European ancestry CCT data from the International Glaucoma Genetics Consortium (IGGC) (n = 17 803). The CRF and CCT variants in published keratoconus data sets (4669 cases and 116 547 controls) were compared. The data set from UKB was compiled March 24, 2020; data were released from the CLSA in July 2020; and IGGC data were available from May 1, 2018.

Main Outcomes and Measures

Association of CRF and CCT variants with keratoconus risk.

Results

The GWAS included 4 cohorts: 105 427 UKB European ancestry (56 134 women [53.2%] and 49 293 men [46.7%]; mean [SD] age, 57 [8] years), 5029 UKB South Asian ancestry (2368 women [47.1%] and 2661 men [52.9%]; mean [SD] age, 54 [8] years), 902 UKB East Asian ancestry (622 women [68.9%] and 280 men [31.0%]; mean [SD] age, 53 [8] years), and 18 307 CLSA European ancestry (9260 women [50.6%] and 9047 men [49.4%]; mean [SD] age, 63 [10] years) participants. A total of 369 CRF and 233 CCT loci were identified, including 36 novel CRF loci and 114 novel CCT loci. Twenty-nine CRF loci and 24 CCT loci were associated with keratoconus. Polygenic risk scores (PRS) were constructed using CRF- and CCT-associated variants and published keratoconus variants. The PRS result showed that adding a CRF- or CCT-based PRS to the keratoconus PRS from previously published variants improved the prediction area under the receiver operating characteristic curve (from 0.705 to 0.756 for CRF and from 0.715 to 0.755 for CCT).

Conclusions and Relevance

These findings support the use of multitrait modeling of corneal parameters in a relatively large data set to identify new keratoconus risk loci and enhance polygenic risk score models.

Effect of Ultraviolet-A and Riboflavin treatment on the architecture of the center and periphery of normal rat cornea: 7 days post treatment

Corneal collagen cross-linking (CXL) is a treatment that is widely applied to halt the progression of ectatic diseases such as keratoconus by creating biomechanical strength in the cornea. Most of the studies assessed the effect of the CXL on the cornea without any differentiation of its effect between periphery and the center of the untreated control cornea especially after the 7 days of CXL application. We investigate the ultrastructural changes in the architecture of the center and periphery of rat corneas, 7 days after standard CXL application. Five Wistar rats (10 corneas) were used in the present study. The left eye corneas (5 mm area) were de-epithelialized and irradiated with standard CXL application using riboflavin and Ultraviolet-A (UVA) (3 mW/cm² for 30 min). The right eye corneas were used as a control. The sclera-cornea button was removed and processed for electron microscopy. Digital images were captured with a bottom mounted Quemesa camera and analyzed using the iTEM software. The ultrastructure of epithelium, hemi-desmosomes, Bowman's layer and stroma were organized in both untreated control and CXL rat cornea in both untreated control and CXL rat cornea. Within the same CXL cornea, both the collagen fibril (CF) diameter and interfibrillar spacing at the center were significantly smaller compared to the peripheral diameter and spacing of the cornea. When comparing the untreated control and CXL cornea, the central interfibrillar spacing of the CXL cornea was significantly smaller than the central spacing the untreated control cornea. In the CXL cornea the peripheral spacing was significantly higher compared to the peripheral interfibrillar spacing of the untreated control cornea. Within the CXL cornea, the proteoglycans (PGs) area and density of the periphery was significantly higher compared to the area and density of the center of the cornea. It suggests that CXL was more effective at the periphery of the cornea. This could be due to the higher amount of leucine rich PG lumican and higher diffusion of oxygen and riboflavin at the periphery cornea.

A Photosensitized Singlet Oxygen (1O2) Toolbox for Bio-Organic Applications: Tailoring 1O2 Generation for DNA and Protein Labelling, Targeting and Biosensing

Singlet oxygen (1O2) is the excited state of ground, triplet state, molecular oxygen (O2). Photosensitized 1O2 has been extensively studied as one of the reactive oxygen species (ROS), responsible for damage of cellular components (protein, DNA, lipids). On the other hand, its generation has been exploited in organic synthesis, as well as in photodynamic therapy for the treatment of various forms of cancer. The aim of this review is to highlight the versatility of 1O2, discussing the main bioorganic applications reported over the past decades, which rely on its production. After a brief introduction on the photosensitized production of 1O2, we will describe the main aspects involving the biologically relevant damage that can accompany an uncontrolled, aspecific generation of this ROS. We then discuss in more detail a series of biological applications featuring 1O2 generation, including protein and DNA labelling, cross-linking and biosensing. Finally, we will highlight the methodologies available to tailor 1O2 generation, in order to accomplish the proposed bioorganic transformations while avoiding, at the same time, collateral damage related to an untamed production of this reactive species.

Keratoconus: An updated review

Keratoconus is a bilateral and asymmetric disease which results in progressive thinning and steeping of the cornea leading to irregular astigmatism and decreased visual acuity. Traditionally, the condition has been described as a noninflammatory disease; however, more recently it has been associated with ocular inflammation. Keratoconus normally develops in the second and third decades of life and progresses until the fourth decade. The condition affects all ethnicities and both sexes. The prevalence and incidence rates of keratoconus have been estimated to be between 0.2 and 4,790 per 100,000 persons and 1.5 and 25 cases per 100,000 persons/year, respectively, with highest rates typically occurring in 20- to 30-year-olds and Middle Eastern and Asian ethnicities. Progressive stromal thinning, rupture of the anterior limiting membrane, and subsequent ectasia of the central/paracentral cornea are the most commonly observed histopathological findings. A family history of keratoconus, eye rubbing, eczema, asthma, and allergy are risk factors for developing keratoconus. Detecting keratoconus in its earliest stages remains a challenge. Corneal topography is the primary diagnostic tool for keratoconus detection. In incipient cases, however, the use of a single parameter to diagnose keratoconus is insufficient, and in addition to corneal topography, corneal pachymetry and higher order aberration data are now commonly used. Keratoconus severity and progression may be classified based on morphological features and disease evolution, ocular signs, and index-based systems. Keratoconus treatment varies depending on disease severity and progression. Mild cases are typically treated with spectacles, moderate cases with contact lenses, while severe cases that cannot be managed with scleral contact lenses may require corneal surgery. Mild to moderate cases of progressive keratoconus may also be treated surgically, most commonly with corneal cross-linking. This article provides an updated review on the definition, epidemiology, histopathology, aetiology and pathogenesis, clinical features, detection, classification, and management and treatment strategies for keratoconus.

Recent Advances in Photodynamic Therapy against Fungal Keratitis

Fungal keratitis is a serious clinical infection on the cornea caused by fungi and is one of the leading causes of blindness in Asian countries. The treatment options are currently limited to a few antifungal agents. With the increasing incidence of drug-resistant infections, many patients fail to respond to antibiotics. Riboflavin-mediated corneal crosslinking (similar to photodynamic therapy (PDT)) for corneal ectasia was approved in the US in the early 2000s. Current evidence suggests that PDT could have the potential to inhibit fungal biofilm formation and overcome drug resistance by using riboflavin and rose bengal as photosensitizers. However, only a few clinical trials have been initiated in anti-fungal keratitis PDT treatment. Moreover, the removal of the corneal epithelium and repeated application of riboflavin and rose bengal are required to improve drug penetration before and during PDT. Thus, an improvement in trans-corneal drug delivery is mandatory for a successful and efficient treatment. In this article, we review the studies published to date using PDT against fungal keratitis and aim to enhance the understanding and awareness of this research area. The potential of modifying photosensitizers using nanotechnology to improve the efficacy of PDT on fungal keratitis is also briefly reviewed.

Biomechanical Response After Corneal Cross-linking With Riboflavin Dissolved in Dextran Solution Versus Hydroxypropyl Methylcellulose

PURPOSE

To evaluate corneal stiffening in porcine eyes induced by corneal cross-linking (CXL) using riboflavin dissolved in either aqueous dextran or hydroxypropyl methylcellulose (HPMC) solution.

METHODS

Fifty-one porcine corneas were divided into three groups of 17 each. After deepithelialization, the first (Dresden) group was treated for 30 minutes with 0.1% riboflavin (riboflavin-5-monophosphate in 0.9% NaCl) dissolved in hypertonic 20% dextran and the second (HPMC) group for 30 minutes with isotonic solution containing 0.1% riboflavin and 1.1% HPMC. Thereafter, corneas of both groups were irradiated using 5.4 J/cm² (irradiance of 9 mW/cm² for 10 minutes; 10*9). After CXL, all corneas were kept in an isotonic 16% dextran bath for 2 hours to obtain an equal hydration state. The third group served as the control group. Stress-strain measurements were performed on 5-mm-wide strips. Corneal thickness was monitored throughout the entire course of the experiments.

RESULTS

The required stress for a 10% strain was increased by 83% in the Dresden group and 35% in the HPMC group compared to the control group. Resultant Young's modulus (at 10% strain) was 2.53 ± 0.73, 1.87 ± 0.50, and 1.47 ± 0.44 Pa for the Dresden, HPMC, and control groups, respectively. The differences between the Dresden and HPMC groups (P = .006), the Dresden and control groups (P < .001), and the HPMC and control groups (P = .014) were statistically significant. Pachymetry measurements showed a significantly increased corneal thickness after application of HPMC compared with the Dresden group (P = .002) and control group (P = .041).

CONCLUSIONS

The biomechanical stiffening of the cornea by CXL can be achieved using dextran- and HPMC-based riboflavin solutions in porcine corneas with an application time of 30 minutes. Dextran-based riboflavin solutions seem to induce a slightly stronger biomechanical response in this setting. HPMC solutions induce less thinning than dextran solutions. [J Refract Surg. 2021;37(9):631-635.].