Just What Do We Know About Corneal Collagen Turnover?

Late corneal ectasia after penetrating and deep anterior lamellar keratoplasty for keratoconus

AIM

To investigate tomographic features of late corneal ectasia after keratoplasty for keratoconus and compare penetrating keratoplasty (PK) and deep anterior lamellar keratoplasty (DALK) in terms of incidence, time of onset and risk factors of corneal ectasia.

METHODS

Sixty eyes with PK and 30 eyes with DALK operated between 1999 and 2021 were analyzed. Final Pentacam scans were evaluated together with vision and previous topographies. Main outcome measures were vision, K values, apparent thinning on graft-host cornea and the difference between opposing quadrants in the thinnest point measurements. Anterior segment optic coherence tomography was performed for further evaluation.

RESULTS

Mean follow-up was 127.2mo (24-282mo) in PK, and 64.3mo (24-144mo) in DALK. K max was higher in DALK (60.6 vs 56.7 D, P=0.012). Inferior recipient was thinner (595.9 µm) in PK than DALK (662.2 µm, P=0.021), due to longer follow-up. Overall corneal ectasia rate was 20.0% within 24y. Ectasia rate was the same (6.7%) in DALK 2/30 and in PK 4/60 in 10y and 13.3% in 12y (4/30 and 8/60, respectively). It increased to 23.3% (14/60) in PK over 24y. While ectasia was not seen before 7y in PK, it could be seen in DALK starting from the 5th year. The intervals between keratoplasty and ectasia were 144.5mo in PK and 99mo in DALK. Inferior recipient was significantly thinner in 18 eyes with ectasia (502.7 µm) compared to 76 non-ectasia (649.1 µm, P=0.000). Inferior graft was thinner (561.0 vs 620.4 µm, P=0.006), K max (63.3 vs 56.5 D, P=0.000), and anterior elevation was higher in ectasia (89.1 vs 48.6 µm, P=0.002). Accelerated crosslinking was performed on 5 eyes.

CONCLUSION

Inferior-superior recipient and inferior graft thinning on tomography, with high K max and anterior elevation emerge as the most reliable criteria for the diagnosis of late ectasia. The incidence of corneal ectasia increases with the time.

Cell therapy in the cornea: The emerging role of microenvironment

The cornea is an ideal testing field for cell therapies. Its highly ordered structure, where specific cell populations are sequestered in different layers, together with its accessibility, has allowed the development of the first stem cell-based therapy approved by the European Medicine Agency. Today, different techniques have been proposed for autologous and allogeneic limbal and non-limbal cell transplantation. Cell replacement has also been attempted in cases of endothelial cell decompensation as it occurs in Fuchs dystrophy: injection of cultivated allogeneic endothelial cells is now in advanced phases of clinical development. Recently, stromal substitutes have been developed with excellent integration capability and transparency. Finally, cell-derived products, such as exosomes obtained from different sources, have been investigated for the treatment of severe corneal diseases with encouraging results. Optimization of the success rate of cell therapies obviously requires high-quality cultured cells/products, but the role of the surrounding microenvironment is equally important to allow engraftment of transplanted cells, to preserve their functions and, ultimately, lead to restoration of tissue integrity and transparency of the cornea.

Determination of efficacy of repeated CXL and probable risk factors in patients with progressive keratoconus

PURPOSE

To evaluate the efficacy of Repeated CXL (Re-CXL) and determine probable risk factors that lead to Re-CXL in patients with progressive keratoconus.

METHOD

In this retrospective study, the medical records of patients who had been re-operated in our center between 2014 to 2020 due to progressive keratoconus were evaluated; seven eyes of seven patients had undergone Re-CXL procedure. Pre- and post-treatment variables were recorded and analyzed using IBM SPSS Statistics software.

RESULTS

The mean interval between the 1st and 2nd CXL was 49.71 months (range 12-72 months). Out of 7 patients for whom Re-CXL was considered necessary, eye rubbing was detected in 6 patients. Six patients were very young with a mean age of 13 years at primary CXL and 16.83 years at Re-CXL. Visual acuity and astigmatism did not change significantly after the Re-CXL procedure (p-values = 0.18, 0.91, respectively). When measurements of these indices prior to Re-CXL and post Re-CXL were compared, K1 (p-value = 0.01), K2 (p-value = 0.01), Kmean (p-value = 0.01), and Kmax (p-value = 0.008) changed significantly. As to pachymetry (p-value = 0.46), it did not change significantly. Kmax value regressed in all eyes after Re-CXL.

CONCLUSION

Re-CXL procedure was effective in halting the progression of disease. As to the risk factors, eye rubbed-related mechanism like eye rubbing and VKC, lower age, and pre-operative Kmax value > 58 D are the risk factors of Re-CXL procedure.

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.

Current clinical practice in corneal crosslinking for treatment of progressive keratoconus in four Nordic countries

PURPOSE

To evaluate clinical practice in the diagnosis and treatment of progressive keratoconus with corneal crosslinking (CXL) in four Nordic countries.

METHODS

A questionnaire was sent to all centres at which keratoconus patients are evaluated and CXL is performed in Sweden, Denmark, Norway and Iceland. Nineteen of 20 centres participated.

RESULTS

CXL is performed approximately 1300 times per year in these four Nordic countries with a population of around 21.7 million (2019). In most cases, progression is evaluated using the Pentacam HR, and the maximum keratometry reading (K_max ) is considered the most important parameter. The most frequently used treatment protocol in Scandinavia is the 9 mW/cm² epi-off protocol, using hydroxylpropyl methylcellulose riboflavin (HPMC-riboflavin). The participants deemed the following areas to be in most need of improvement: adaptation of the CXL protocol to individual patients (5/19), the development of effective epi-on treatment protocols (4/19), optimal performance of CXL in thin corneas (4/19), improvement of the definition of progression (2/19), and diagnosis of the need for re-treatment (2/19).

CONCLUSIONS

We concluded that the diagnosis of progressive keratoconus and the diagnostic equipment used are similar. Treatment strategies are also similar but are suitably different to provide an interesting basis for the comparison of treatment outcomes. The high degree of participation in this survey indicates the possibility of future scientific collaboration on CXL focusing on the areas deemed to need improvement. It would also be of interest to evaluate the possibility of creating a Nordic CXL Registry. The high number of CXL treatments performed ensures sufficient statistical power to solve many questions. Such a registry could be an important contribution to evidence-based care and would allow for longitudinal evaluation.

Corneal Regeneration Using Adipose-Derived Mesenchymal Stem Cells

Adipose-derived stem cells are a subtype of mesenchymal stem cell that offers the important advantage of being easily obtained (in an autologous manner) from low invasive procedures, rendering a high number of multipotent stem cells with the potential to differentiate into several cellular lineages, to show immunomodulatory properties, and to promote tissue regeneration by a paracrine action through the secretion of extracellular vesicles containing trophic factors. This secretome is currently being investigated as a potential source for a cell-free based regenerative therapy for human tissues, which would significantly reduce the involved costs, risks and law regulations, allowing for a broader application in real clinical practice. In the current article, we will review the existing preclinical and human clinical evidence regarding the use of such adipose-derived mesenchymal stem cells for the regeneration of the three main layers of the human cornea: the epithelium (derived from the surface ectoderm), the stroma (derived from the neural crest mesenchyme), and the endothelium (derived from the neural crest cells).

ROCK 1 and 2 affect the spatial architecture of 3D spheroids derived from human corneal stromal fibroblasts in different manners

The objective of the current study was to examine the roles of ROCK1 and 2 on the spatial architecture of human corneal stroma. We examined the effects of a pan-ROCK inhibitor (pan-ROCK-i), ripasudil, and a ROCK2 inhibitor (ROCK2-i), KD025 on the expression of genes that encode for ECM proteins including collagen (COL) 1, 4, 6, and fibronectin (FN), their regulators, a tissue inhibitor of metalloproteinase (TIMP) 1-4, matrix metalloproteinase (MMP) 2, 9 and 14, and ER stress-related factors of two- and three-dimensional (2D and 3D) cultures of human corneal stroma fibroblasts (HCSFs), and the physical properties of 3D HCSF spheroids. A gene expression analysis using ROCK-is indicated that KD025 (ROCK2 selective ROCK inhibitor) induced more significant changes than Rip (ripasudil, pan-ROCK inhibitor), suggesting that ROCK2 might be more extensively involved in the metabolism of ECM proteins and cell architectures of the 2D cultured HCSFs than ROCK1. In terms of the physical properties, size and stiffness of the 3D HCSFs spheroids, Rip caused a significant enlargement and this enhancement was concentration-dependent while KD025 also exerted a similar but less pronounced effect. In contrast, Rip and KD025 modulated physical stiffness differently, in that Rip caused a substantial decrease and KD025 caused an increase. Such diverse effects between Rip and KD025 were also observed for the gene expressions of ECM proteins, their regulators, and ER-stress related factors. The findings presented herein suggest that the ROCK1 and 2 influence the spatial architecture of 3D HCFS spheroids in different manners.

Mechanical Properties of Bioengineered Corneal Stroma

For the majority of patients with severe corneal injury or disease, corneal transplantation is the only suitable treatment option. Unfortunately, the demand for donor corneas greatly exceeds the availability. To overcome shortage issues, a myriad of bioengineered constructs have been developed as mimetics of the corneal stroma over the last few decades. Despite the sheer number of bioengineered stromas developed , these implants fail clinical trials exhibiting poor tissue integration and adverse effects in vivo. Such shortcomings can partially be ascribed to poor biomechanical performance. In this review, existing approaches for bioengineering corneal stromal constructs and their mechanical properties are described. The information collected in this review can be used to critically analyze the biomechanical properties of future stromal constructs, which are often overlooked, but can determine the failure or success of corresponding implants.

Collagen Mimetic Peptides Promote Corneal Epithelial Cell Regeneration

The cornea of the eye is at risk for injury through constant exposure to the extraocular environment. A highly collagenous structure, the cornea contains several different types distributed across multiple layers. The anterior-most layer contains non-keratinized epithelial cells that serve as a barrier to environmental, microbial, and other insults. Renewal and migration of basal epithelial cells from the limbus involve critical interactions between secreted basement membranes, composed primarily of type IV collagen, and underlying Bowman's and stromal layers, which contain primarily type I collagen. This process is challenged in many diseases and conditions that insult the ocular surface and damage underlying collagen. We investigated the capacity of a collagen mimetic peptide (CMP), representing a fraction of a single strand of the damaged triple helix human type I collagen, to promote epithelial healing following an acute corneal wound. In vitro, the collagen mimetic peptide promoted the realignment of collagen damaged by enzymic digestion. In an in vivo mouse model, topical application of a CMP-containing formulation following a 360° lamellar keratectomy targeting the corneal epithelial layer accelerated wound closure during a 24 h period, compared to vehicle. We found that the CMP increased adherence of the basal epithelium to the underlying substrate and enhanced density of epithelial cells, while reducing variability in the regenerating layer. These results suggest that CMPs may represent a novel therapeutic to heal corneal tissue by repairing underlying collagen in conditions that damage the ocular surface.

The Need for Improved Therapeutic Approaches to Protect the Cornea Against Chemotoxic Injuries

Cornea, a highly specialized transparent tissue, is the major refractive element of the eye. The cornea is highly susceptible to chemotoxic injury through topical exposure to vapors, microparticles, and aqueous drops, as well as through systemically absorbed chemicals that access the cornea via tear film, aqueous humor, and limbal vasculature. Corneal injury activates a carefully orchestrated series of repair processes capable of resolving minor lesions over time, but it often fails to overcome the menace of moderate, severe, and chronic injuries and secondary pathophysiologies that permanently impair vision. The most serious complications of chemical injuries-persistent corneal edema, neovascularization, scarring/haze, limbal stem cell deficiency, and corneal melting-often manifest over months to years, suggesting that a better understanding of endogenous regenerative mechanisms of corneal repair can lead to the development of improved treatments that may attenuate or prevent corneal defects and protect vision.

Corneal stroma regeneration: Preclinical studies

Corneal grafting is one of the most common and successful forms of human tissue transplantation in the world, but the need for corneal grafting is growing and availability of human corneal donor tissue to fulfill this increasing demand is not assured worldwide. The stroma is responsible for many features of the cornea, including its strength, refractive power and transparency, so enormous efforts have been put into replicating the corneal stroma in the laboratory to find an alternative to classical corneal transplantation. Unfortunately this has not been yet accomplished due to the extreme difficulty in mimicking the highly complex ultrastructure of the corneal stroma, and none of the obtained substitutes that have been assayed has been able to replicate this complexity yet. In general, they can neither match the mechanical properties nor recreate the local nanoscale organization and thus the transparency and optical properties of a normal cornea. In this context, there is an increasing interest in cellular therapy of the corneal stroma using Induced Pluripotent Stem Cells (iPSCs) or mesenchymal stem cells (MSCs) from either ocular or extraocular sources, as they have proven to be capable of producing new collagen within the host stroma, modulate preexisting scars and enhance transparency by corneal stroma remodeling. Despite some early clinical data is already available, in the current article we will summary the available preclinical evidence about the topic corneal stroma regeneration. Both, in vitro and in vivo experiments in the animal model will be shown.

Type I Collagen-Fibrin Mixed Hydrogels: Preparation, Properties and Biomedical Applications

Type I collagen and fibrin are two essential proteins in tissue regeneration and have been widely used for the design of biomaterials. While they both form hydrogels via fibrillogenesis, they have distinct biochemical features, structural properties and biological functions which make their combination of high interest. A number of protocols to obtain such mixed gels have been described in the literature that differ in the sequence of mixing/addition of the various reagents. Experimental and modelling studies have suggested that such co-gels consist of an interpenetrated structure where the two proteins networks have local interactions only. Evidences have been accumulated that immobilized cells respond not only to the overall structure of the co-gels but can also exhibit responses specific to each of the proteins. Among the many biomedical applications of such type I collagen-fibrin mixed gels, those requiring the co-culture of two cell types with distinct affinity for these proteins, such as vascularization of tissue engineering constructs, appear particularly promising.

Khalil A, Tawfik M, Said A, Amer I, Nooreldin A, Said O, Reffat M, Anwar S, Badawi A. Standard cross-linking protocol versus accelerated and transepithelial cross-linking protocols for treatment of paediatric keratoconus: a 2-year comparative study

PURPOSE

To compare the efficacy, safety and stability of standard epithelium-off cross-linking (SCXL) versus accelerated epithelium-off cross-linking (ACXL) and transepithelial epithelium-on cross-linking (TCXL) in the treatment of progressive keratoconus (KC) in children.

METHODS

This prospective multicentre controlled trial included 271 eyes (136 children) with grade 1-3 progressive KC who were randomized to undergo SCXL (n = 91, as a control group), ACXL (n = 92) or TCXL (n = 88). Uncorrected and corrected distance visual acuity, subjective refraction, pachymetry, keratometry and corneal topography measurements were recorded preoperatively and 6, 12 and 24 months postoperatively.

RESULTS

At 1 year, there was no significant difference in uncorrected distance visual acuity, refractive sphere, cylinder, spherical equivalent or Kmax between the ACXL and SCXL groups; however, during year 2, ACXL regressed while SCXL continued to improve. After 2 years, there were significant differences in all visual, refractive and keratometric components between SCXL and both ACXL and TCXL (p < 0.0001) and between ACXL and TCXL (p < 0.0001). KC progressed in 5.4% of patients who had ACXL and 28.4% of those who had TCXL but in none of those who had SCXL. Vernal keratoconjunctivitis was documented in 43.3% of eyes that progressed postoperatively.

CONCLUSION

SCXL was more effective for paediatric KC and achieved greater stability than either ACXL or TCXL, and ACXL was superior to TCXL. SCXL also achieved marked improvement in both myopia and spherical equivalent; however, these refractive outcomes were unpredictable and uncontrollable. TCXL had a 28.4% failure rate within 2 years. SCXL is preferable for management of paediatric KC.

Peripheral Cornea Crosslinking Before Deep Anterior Lamellar Keratoplasty

Since Cornea crosslinking (CXL) has been proven to halt progression and biomechanically stabilize keratoconus, we hypothesized that CXL of the corneal periphery 3 months prior to corneal transplantation can reduce the incidence of recurrent ectasia by strengthening the peripheral corneal tissue and causing apoptosis of diseased peripheral host keratocytes. Thus, the aim of this case-report was to propose a novel peripheral CXL technique prior to keratoplasty and evaluate its safety. A 22-year-old woman was admitted with advanced right keratoconus and corrected distance visual acuities of 20/30 in the right eye and 20/200 in the left eye with a manifest refraction of -3.00D/ -8.00D × 36° and -17.00D/ -11.50D × 90°, respectively. The proposed treatment involved crosslinking of peripheral corneal tissue (6.5-9.5mm), sparing the central cornea and limbus, three months prior to corneal transplantation as a means of biomechanically strengthening the peripheral cornea tissue. This procedure was feasible and safe with repopulation of the peripheral cornea with keratocytes, no significant endothelial cell loss and a routine postoperative course following CXL and DALK. This method might reduce or eliminate the need for repeat corneal transplantation in patients with recurrent ectasia. Further studies are needed to confirm the results.

Corneal Stromal Regeneration: Current Status and Future Therapeutic Potential

The corneal stroma comprises 90% of the corneal thickness and is critical for the cornea's transparency and refractive function necessary for vision. When the corneal stroma is altered by disease, injury, or scarring, however, an irreversible loss of transparency can occur. Corneal stromal pathology is the cause of millions of cases of blindness globally, and although corneal transplantation is the standard therapy, a severe global deficit of donor corneal tissue and eye banking infrastructure exists, and is unable to meet the overwhelming need. An alternative approach is to harness the endogenous regenerative ability of the corneal stroma, which exhibits self-renewal of the collagenous extracellular matrix under appropriate conditions. To mimic endogenous stromal regeneration, however, is a challenge. Unlike the corneal epithelium and endothelium, the corneal stroma is an exquisitely organized extracellular matrix containing stromal cells, proteoglycans and corneal nerves that is difficult to recapitulate in vitro. Nevertheless, much progress has recently been made in developing stromal equivalents, and in this review the most recent approaches to stromal regeneration therapy are described and discussed. Novel approaches for stromal regeneration include human or animal corneal and/or non-corneal tissue that is acellular or is decellularized and/or re-cellularized, acellular bioengineered stromal scaffolds, tissue adhesives, 3D bioprinting and stromal stem cell therapy. This review highlights the techniques and advances that have achieved first clinical use or are close to translation for eventual therapeutic application in repairing and regenerating the corneal stroma, while the potential of these novel therapies for achieving effective stromal regeneration is discussed.