Pharmacological Modification of the Epithelial Permeability by Benzalkonium Chloride in UVA/Riboflavin Corneal Collagen Cross-Linking

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.

A Novel Riboflavin Formulation for Corneal Delivery Without Damaging Epithelial Cells

Purpose

This study aimed to evaluate the trans-epithelial permeability enhancement and cell damage caused by a novel riboflavin composition for corneal delivery.

Methods

We developed a trans-epithelial formulation of riboflavin for corneal delivery using 1,2-dioleoyl-3-dimethylammonium-propane (DODAP) and isostearic acid (ISA). The permeation enhancement was evaluated using an in vitro corneal epithelial cell culture system by measuring the amount of transferred riboflavin with high-performance liquid chromatography. Riboflavin permeation of MedioCROSS TE, a commercially available riboflavin formulation containing benzalkonium chloride, was also evaluated and compared to that of the DODAP/ISA formulation by changing the riboflavin concentration. The trans-epithelial electrical resistance (TEER) was measured after exposure to the samples in an in vitro corneal epithelial cell culture system to assess cytotoxicity.

Results

The DODAP/ISA formulation demonstrated greater permeation when used together than when each component was used individually. The permeation enhancement effect of the DODAP/ISA formulation was almost the same as that of MedioCROSS TE. However, when a 10-fold higher riboflavin concentration was used in the DODAP/ISA formulation, the permeation enhancement effect surpassed that of MedioCROSS TE. After 24 hours of exposure, the TEER of the DODAP/ISA formulation was higher than that of MedioCROSS TE, indicating that the DODAP/ISA formulation was less cytotoxic than MedioCROSS TE.

Conclusions

This study indicated that the DODAP/ISA formulation could serve as a less cytotoxic alternative to MedioCROSS TE. Further studies are required to determine the clinical efficacy and safety of the DODAP/ISA formulation in vivo.

Translational Relevance

This study may provide alternative procedures for corneal collagen crosslinking with less of a cytotoxic effect on corneal epithelial cells.

A fluorophotometric comparison of anterior chamber riboflavin delivery via corneal application and fornix instillation

INTRODUCTION AND OBJECTIVES

We investigated whether riboflavin self-administration by patients could be a feasible option for corneal cross-linking, given the considerable resources required to impregnate the cornea with riboflavin. We analysed whether administering riboflavin in the inferior fornix (the site of self-administration) results in non-inferior riboflavin concentrations as when applied directly on the cornea (the site of administration by medical personnel).

PATIENTS AND METHODS

We conducted a prospective study to evaluate riboflavin concentrations at six time-points (baseline, 5, 15, 30, 45 and 60min) in 18 healthy volunteers for each of two application sites: cornea and fornix. Anterior chamber riboflavin (Peschke® TE 0.25%) concentrations were measured by fluorophotometry (Fluorotron™ Master FM-2).

RESULTS

For the two application sites cornea and fornix, participants did not differ in terms of age and sex. At baseline, the autofluorescence in the anterior chamber was 16.7ng/mL (SD 5.5) and 14.6ng/mL (SD 4.6) (P=.221). After 30min, anterior chamber fluorescein concentrations had risen to 55.1ng/mL (SD 25.5) and 46.1ng/mL (SD 25.1) (P=.293) without a further relevant increase by 60min.

CONCLUSIONS

This study found that applying riboflavin drops in the inferior fornix was non-inferior to applying it directly to the cornea, based on fluorophotometric measurements of anterior chamber riboflavin concentrations. This suggests that self-application of riboflavin is feasible in terms of corneal riboflavin impregnation.

Recent advances in medicinal compounds related to corneal crosslinking

Corneal crosslinking (CXL) is the recognized technique to strengthen corneal collagen fibers through photodynamic reaction, aiming to halt progressive and irregular changes in corneal shape. CXL has greatly changed the treatment for keratoconus (KCN) since it was introduced in the late 1990's. Numerous improvements of CXL have been made during its developing course of more than 20 years. CXL involves quite a lot of materials, including crosslinking agents, enhancers, and supplements. A general summary of existing common crosslinking agents, enhancers, and supplements helps give a more comprehensive picture of CXL. Either innovative use of existing materials or research and development of new materials will further improve the safety, effectiveness, stability, and general applicability of CXL, and finally benefit the patients.

Comparison of topographic outcomes between HPMC based and vitamin E TPGS based riboflavin solutions after corneal cross-linking

PURPOSE

To compare the visual and topographic results between patients who underwent epithelium-off cross-linking using riboflavin solutions compounds hydroxypropyl methylcellulose (HPMC) 1.1% and D-alpha-tocopheryl polyethylene-glycol 1000 succinate (VE-TPGS).

METHODS

In this study, 37 eyes treated with HPMC and 29 eyes treated with VE-TPGS were evaluated retrospectively. Spherical equivalent (SE), refractive cylinder, corrected distance visual acuity tests (CDVA), corneal topography indices (flat and steep meridians' keratometry (K1 and K2)), maximum keratometry (K max), central, thinnest, and apical corneal thicknesses, the front and back keratoconus vertex index (KVf, KVb), and the surface asymmetry index of the front and back surface (SIf, SIb), and endothelial cell density were compared at baseline and postoperative follow-up visits (1, 3, 6, and 12months).

RESULTS

At the end of the 12th-month, K1, K2, and Kmax were decreased in both groups. In comparison to baseline, there was a decline in the HPMC group in the 3rd- month Kmax change, an increase was observed in the VE-TPGS group. In the 12th-month KVb change, an increase was observed in the HPMC group compared to the baseline, while a decrease was observed in the VE-TPGS group. The other parameters did not show a statistically significant difference between the groups (p > 0.05).

CONCLUSION

At the end of 12 months, both riboflavins were effective in stopping the progression of keratoconus and were safe for endothelium. Although both riboflavins provide a decrease in keratometry values, it can be said that VE-TPGS is superior to HPMC in correcting the ectasia on the posterior corneal surface.

Engineering Hibiscus-Like Riboflavin/ZIF-8 Microsphere Composites to Enhance Transepithelial Corneal Cross-Linking

Riboflavin-5-phosphate (RF) is the most commonly used photosensitizer in corneal cross-linking (CXL), but its hydrophilicity and negative charge limit its penetration through the corneal epithelium into the stroma. To enhance the corneal permeability of RF and promote its efficacy in the treatment of keratoconus, novel hibiscus-like RF@ZIF-8 microsphere composites [6RF@ZIF-8 NF (nanoflake)] are prepared using ZIF-8 nanomaterials as carriers, which are characterized by their hydrophobicity, positive potential, biocompatibility, high loading capacities, and large surface areas. Both hematoxylin and eosin endothelial staining and TUNEL assays demonstrate excellent biocompatibility of 6RF@ZIF-8 NF. In in vivo studies, the 6RF@ZIF-8 NF displayed excellent corneal permeation, and outstanding transepithelial CXL (TE-CXL) efficacy, slightly better than the conventional CXL protocol. Furthermore, the special hibiscus-like structures of 6RF@ZIF-8 NF meant that it has better TE-CXL efficacy than that of 6RF@ZIF-8 NP (nanoparticles) due to the larger contact area with the epithelium and the shorter RF release passage. These results suggest that the 6RF@ZIF-8 NF are promising for transepithelial corneal cross-linking, avoiding the need for epithelial debridement.

[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.

A New Surgical Technique to Deliver Riboflavin Beneath Corneal Epithelium: The Corneal Cross-Linking Epi-Pocket

PURPOSE

To describe a new technique to deliver riboflavin into the corneal stroma during Corneal Cross-Linking (CXL) without the removal of corneal epithelium.

METHODS

Keratoconus patients underwent CXL for progressive keratoconus. Riboflavin was delivered by manually creating an epithelial pocket (CXL Epi-Pocket). Verbal rating scale was recorded postoperatively. Best-corrected visual acuity, keratometric indices, corneal thickness and corneal densitometry were recorded at baseline and at 12-month follow-up.

RESULTS

Eighteen eyes of 18 patients were included in the study. At a 12-month follow-up, best-corrected visual acuity, K1, K2 and densitometry values were stable. Maximum keratometry (Kmax) reduced from 55.31 ± 6.21 (SD) to 52.34d ± 4.12d (SD) (P value = 0.032). the thinnest point went from 441 ± 21.18 (SD) to 425.4 ± 19.02 (SD) um (P value = 0.041). The verbal rating scale at 1, 2 and 3 days postoperatively were 1.76 ± 0.19 (SD), 1.02 ± 0.51 (SD) and 0.28 ± 0.14 (SD).

CONCLUSIONS

CXL Epi-Pocket is able to deliver riboflavin to halt the progression of keratoconus at a 12-month follow-up.

Transepithelial Corneal Crosslinking Using a Novel Ultraviolet Light-Emitting Contact Lens Device: A Pilot Study

Purpose

To evaluate the feasibility of a novel, on-eye UVA light-emitting contact lens device driven by fiber optics for the corneal crosslinking (CXL) of patients with keratoconus.

Methods

In nine corneal transplant candidates with advanced keratoconus a scleral contact lens reservoir containing 0.007% benzalkonium chloride preserved with 0.25% riboflavin-monophosphate was placed on the eye for 30 minutes. The reservoir lens was removed and replaced with the CXLens UVA light-emitting contact lens. A 375-nm UVA light at 4 mW/cm² intensity was delivered for 30 minutes for a dose of 7.2 J/cm². A one-sided paired t-test was used to evaluate mean differences in maximum keratometry, thinnest corneal thickness, and endothelial cell density between screening and 6 months after CXL. A two-sided paired t-test was used to evaluate differences in best-corrected distance visual acuity between screening and 6 months after CXL.

Results

All patients received the treatment as per protocol and adhered to follow-up testing. At 6 months after CXL, treated eyes had an average −1.0 ± 1.6 diopters decrease in the maximum keratometry (P = 0.049), a nonsignificant 2.3 ± 7.5 letter improvement in best-corrected distance visual acuity (P = 0.19), a nonsignificant −17 ± 14 µm decrease in thinnest corneal thickness (P < 0.01), and a nonsignificant −86 ± 266 cells/mm² decrease in endothelial cell density (P = 0.20).

Conclusions

Our pilot study demonstrated the feasibility of the novel CXL device for the treatment of keratoconus and indicates the device is ready for larger scale studies with longer follow-up periods.

Translational Relevance

The novel CXLens on-eye UVA light-emitting contact lens device offers the potential for efficient, high-throughput transepithelial corneal CXL.

The Effect of Sodium Iodide on Stromal Loading, Distribution and Degradation of Riboflavin in a Rabbit Model of Transepithelial Corneal Crosslinking

Purpose

To evaluate effects of sodium iodide (NaI) on riboflavin concentration in corneal stroma before and during ultraviolet A (UVA) light exposure using a novel transepithelial corneal collagen crosslinking (CXL) procedure (EpiSmart CXL system, CXL Ophthalmics, Encinitas CA).

Methods

Riboflavin solutions with NaI (Ribostat, CXL Ophthalmics, Encinitas CA) and without NaI were used for CXL in rabbits using EpiSmart. A pilot study determined sufficient riboflavin loading time. Four rabbits were dosed and monitored. Riboflavin fluorescence intensity was assessed from masked slit-lamp photos. A 12 min loading time was selected. Sixteen additional rabbits received the two formulae in contralateral eyes for CXL. Riboflavin uptake was assessed at 0, 10, 15, 20, 25, and 30 min of UVA exposure using a scale for riboflavin fluorescence previously validated against stromal concentration. Post sacrifice, corneal stromal samples were analyzed for concentrations of riboflavin and riboflavin 5ʹ-phosphate.

Results

Eyes dosed with NaI riboflavin had higher riboflavin grades compared to eyes dosed with the NaI-free riboflavin formulation immediately after riboflavin loading and persisting throughout UVA exposure, with significantly higher (P < 0.01 to < 0.05) riboflavin grades from 15 through 25 min of UVA exposure. Riboflavin grades decreased more slowly in eyes dosed with NaI riboflavin through 25 minutes of UVA exposure. Minor conjunctival irritation was noted with or without NaI.

Conclusion

The addition of NaI to riboflavin solution is associated with increased riboflavin concentration in corneal stroma throughout a clinically relevant time course of UVA exposure. This effect may be a combination of enhanced epithelial penetration and reduced riboflavin photodegradation and should enhance intrastromal crosslinking.

Crosslinking und Keratokonus

Ein Keratokonus führt zu einer progressiven Vorwölbung und Verdünnung der Hornhaut. Um dies aufzuhalten, kann ein Crosslinking durchgeführt werden. Dabei ist eine Behandlung nach dem „Dresdener Protokoll“ eine effektive und sichere Behandlungsmöglichkeit, aber auch zahlreiche neue Anwendungsprotokolle (akzeleriertes Crosslinking, transepitheliales Crosslinking) und sogar weitere Indikationen (refraktive Eingriffe, infektiöse Keratitis) wurden in den letzten Jahren veröffentlicht.

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.

[Crosslinking and Keratoconus]

Keratoconus leads to a progressive protrusion and thinning of the cornea. In order to stop this, corneal crosslinking can be performed if the progression of the disease is proven. Crosslinking according to the "Dresden protocol" includes abrasion of the corneal epithelium, application of riboflavin eye drops and irradiation with UV-A light of an intensity of 3 mW/cm² for 30 minutes. The efficacy has been shown in several prospective randomized studies. One of the more recent developments is accelerated crosslinking, which allows a shorter irradiation time. On the other hand, the possibility of transepithelial crosslinking was presented, which does not require an abrasion of the cornea. This should reduce the occurrence of postoperative pain. The range of indications has also been expanded. Corneal crosslinking is used for post-LASIK keratectasia as well. It is also being considered for use in infectious keratitis. Topographically controlled crosslinking can likewise be used to try to positively influence the refractive power of the cornea. The risks of crosslinking include the occurrence of pain, haze or scarring, endothelial cell damage and, rarely, the occurrence of keratitis.

Design of ocular drug delivery platforms and in vitro - in vivo evaluation of riboflavin to the cornea by non-interventional (epi-on) technique for keratoconus treatment

AIM

Keratoconus is a common and progressive eye disease characterized by thinning and tapering of the cornea. This degenerative eye disease is currently treated in the clinic with an interventional technique ("epi-off") that can cause serious side effects as a result of the surgical procedure. The aim of this project is to design innovative formulations for the development of a riboflavin-containing medicinal product to develop a non-invasive ("epi-on") keratoconus treatment as an alternative to current treatment modalities.

METHODS

Nanostructured lipid carriers (NLCs) were successfully loaded with either riboflavin base of riboflavin-5-phosphate sodium and designed with either Stearylamine (positive charge) or Trancutol P (permeation enhancer). In vitro characterization studies, cytotoxicity and permeability studies were performed. Selected formulations and commercial preparations were applied and compared in ex-vivo corneal drug accumulation and transition studies. Furthermore, in vivo studies were performed to assess drug accumulation in the rat cornea and the corneal stability after NLC treatment was investigated via a biomechanical study on isolated rabbit corneas.

RESULTS

Both in vitro and ex-vivo as well as in vivo data showed that from the prepared NLC formulations, the most effective formulation was riboflavin-5-phosphate sodium containing NLC with Transcutol P as permeation enhancer. It possessed the highest drug loading content, low accumulation in the cornea but high permeability through the cornea as well as the highest functional performance in corneal crosslinking.

CONCLUSION

Topical application of riboflavin-5-phosphate sodium loaded NLC systems designed with permeation enhancer Transcutol P may act as a potential alternative for non-invasive keratoconus treatments.

Effects of collagen crosslinking on porcine and human tarsal plate

BACKGROUND

Floppy eyelid syndrome is a disorder in which the tarsal plate is easily distensible and is currently treated with conservative or surgical measures. Human tarsal plate contains type I collagen, which is crosslinked in corneal tissue as a treatment for keratoconus. We hypothesized that collagen crosslinking would similarly stiffen tarsal plate tissue and investigated this in porcine and human tarsal plate specimens.

METHODS

Riboflavin-sensitized porcine and human tarsus samples were irradiated with ultraviolet-A light. Porcine experiments were analyzed with gross photographs, anterior segment optical computed tomography (AS-OCT) imaging, and tensile testing. A prospective study of human tarsus was performed on samples from patients undergoing wedge resection for floppy eyelid syndrome and was analyzed with AS-OCT and tensile testing.

RESULTS

73 porcine adnexa and 9 patients (16 eyelids) who underwent wedge excision were included in the study. Grossly, greater stiffness was observed in crosslinked porcine tissue. AS-OCT imaging in porcine tissue showed a distinct hyperreflective band in crosslinked specimens whose area and intensity increased with longer treatment time (P = 0.003); this band was also visible in crosslinked human specimens. Tensile testing was performed, but results were not statistically significant.

CONCLUSIONS

AS-OCT imaging, which has not been previously described for tarsal plate, showed a characteristic change in crosslinked porcine and human specimens. Tissue stiffness was increased grossly, but changes in tensile properties were not statistically significant. Further study is warranted to determine relevance as a potential treatment for floppy eyelid syndrome.

Corneal stromal demarcation line after 4 protocols of corneal crosslinking in keratoconus determined with anterior segment optical coherence tomography

PURPOSE

To use anterior segment optical coherence tomography (AS-OCT) to compare corneal stromal demarcation line depth after 4 treatment protocols of corneal crosslinking (CXL).

SETTING

Eye Clinic, Sapienza University of Rome, Terracina (Latina), Italy.

DESIGN

Prospective case series.

METHODS

Patients with progressive keratoconus were delegated to one of the following CXL treatments: (1) conventional epithelium (epi)-off 3 mW/cm² according to the standard Dresden protocol (C-CXL group), (2) accelerated epi-off 10 mW/cm² (A-CXL group), (3) transepithelial epi-on 3 mW/cm² (TE-CXL group), or (4) transepithelial epi-on by iontophoresis 10 mW/cm² (I-CXL group). Two independent observers measured the corneal stromal demarcation line using AS-OCT.

RESULTS

The study comprised 70 patients (120 eyes, 30 eyes in each group). The corneal stromal demarcation line was identified on AS-OCT scans in 109 eyes (90.8%). One month after the treatment, the mean stromal demarcation line depth was 275.05 μm ± 41.83 (SD) in the C-CXL group, 279.35 ± 33.07 μm in the A-CXL group, 132.60 ± 22.14 μm in the TE-CXL group, and 235.40 ± 37.08 μm in the I-CXL group. The difference in stromal demarcation line depth was not statistically significant between the C-CXL and A-CXL group, but it was statistically significant (P < .05) between the epi-off and epi-on CXL groups and between the 2 epi-on groups, where the demarcation line was significantly deeper in the I-CXL group than in the TE-CXL group.

CONCLUSION

The corneal stromal demarcation line was significantly deeper after epi-off 30-minute standard CXL treatment and after epi-off 9-minute accelerated CXL with high-intensity ultraviolet-A irradiation.

Transepithelial corneal crosslinking for keratoconus

PURPOSE

To evaluate outcomes of corneal crosslinking (CXL) using a transepithelial technique for the treatment of keratoconus.

SETTING

Cornea and refractive surgery subspecialty practice.

DESIGN

Prospective case series.

METHODS

Transepithelial CXL was performed in keratoconic eyes using riboflavin 0.1% and topical anesthetic containing benzalkonium chloride to facilitate riboflavin diffusion through the epithelium. Eyes were randomized to receive riboflavin administration either every 1 minute or every 2 minutes during ultraviolet-A exposure at 3mW/cm². The principal outcome was change in maximum keratometry (K) and secondary outcomes included uncorrected (UDVA) and corrected (CDVA) distance visual acuities, mean K, and comparison of randomized groups.

RESULTS

Eighty-two eyes of 56 patients were treated. At 1 year, maximum K decreased significantly by 0.45 diopters (D) ± 1.94 (SD); it improved by 2.0 D or more in 11 eyes (13%) and worsened by 2.0 D or more in 4 eyes (5%). The mean UDVA significantly improved by 0.7 lines, whereas the CDVA improved by 0.2 lines. Two eyes showed both continued progression with loss of CDVA. Only the 1-minute subgroup showed significant improvements in maximum K (-0.73 D) and UDVA. Transient corneal erosion and epitheliopathy were reported in 21% of eyes.

CONCLUSIONS

Transepithelial CXL resulted in significant improvements in maximum K and UDVA over 1 year. There was a suggestion that increased riboflavin dosing might improve procedure outcomes. Further study is required to determine the relative advantages and disadvantages of different transepithelial approaches to the standard CXL protocol with epithelial removal.

Corneal Cross-Linking: The Science Beyond the Myths and Misconceptions

PURPOSE

There has been a recent explosion in the variety of techniques used to accomplish corneal cross-linking (CXL) for the treatment of ectatic corneal diseases. To understand the success or failure of various techniques, we review the physicochemical basis of corneal CXL and re-evaluate the current principles and long-standing conventional wisdom in the light of recent, compelling, and sometimes contradictory research.

METHODS

Two clinicians and a medicinal chemist developed a list of current key topics, controversies, and questions in the field of corneal CXL based on information from current literature, medical conferences, and discussions with international practitioners of CXL.

RESULTS

Standard corneal CXL with removal of the corneal epithelium is a safe and efficacious procedure for the treatment of corneal ectasias. However, the necessity of epithelium removal is painful for patients, involves risk and requires significant recovery time. Attempts to move to transepithelial corneal CXL have been hindered by the lack of a coherent understanding of the physicochemistry of corneal CXL. Misconceptions about the applicability of the Bunsen-Roscoe law of reciprocity and the Lambert-Beer law in CXL hamper the ability to predict the effect of ultraviolet A energy during CXL. Improved understanding of CXL may also expand the treatment group for corneal ectasia to those with thinner corneas. Finally, it is essential to understand the role of oxygen in successful CXL.

CONCLUSIONS

Improved understanding of the complex interactions of riboflavin, ultraviolet A energy and oxygen in corneal CXL may provide a successful route to transepithelial corneal CXL.

Recent advances in corneal collagen cross-linking

Corneal collagen cross-linking has become the preferred modality of treatment for corneal ectasia since its inception in late 1990s. Numerous studies have demonstrated the safety and efficacy of the conventional protocol. Our understanding of the cross-linking process is ever evolving, with its wide implications in the form of accelerated and pulsed protocols. Newer advancements in technology include various riboflavin formulations and the ability to deliver higher fluence protocols with customised irradiation patterns. A greater degree of customisation is likely the path forward, which will aim at achieving refractive improvements along with disease stability. The use of cross-linking for myopic correction is another avenue under exploration. Combination of half fluence cross-linking with refractive correction for high errors to prevent post LASIK regression is gaining interest. This review aims to highlight the various advancements in the cross-linking technology and its clinical applications.

Trans-epithelial accelerated corneal cross-linking for keratoconus in children

The aim of the study is to evaluate the safety and efficacy of trans-epithelial accelerated corneal cross-linking (TE-ACXL) in children with progressive keratoconus. Retrospective, case-series of 23 eyes of 14 children who underwent TE-ACXL. Evaluations were performed at baseline and 1, 3, 6, 12 and 18mo postoperatively. Mean follow-up time of 23.82±3.15mo and mean age was 13.7±1.4y (range 11 to 16y). Mean preoperative uncorrected distance visual acuity changed from 0.92±0.45 logMAR (20/160) to 0.71±0.40 logMAR (20/100) (P=0.001). Mean keratometry (Km) changed from 53.87± 6.03 to 53.00±5.81 (P=0.001). Pachymetry did not have significant changes at last follow-up (P=0.30). The mean preoperative sphere was -5.58±2.48 and -4.89±4.66 D (P=0.11) at last follow-up; refractive cylinder from -5.58±2.48 to -5.02±2.23 (P=0.046). In conclusion, tomographic and refractive stability are shown in over 91% of eyes with pediatric progressive keratoconus who underwent TE-ACXL.

Keratoconus Treatment Algorithm

Keratoconus management has significantly changed over the last two decades. The advent of new interventions such as cornea cross-linking, intrastromal corneal ring segments, and combined treatments provide corneal clinicians a variety of treatment options for the visual rehabilitation of keratoconus patients. This review summarizes current evidence for these treatments and highlights their place in keratoconus management while new promising emerging therapies are being investigated.

Transepithelial corneal crosslinking in treatment of progressive keratoconus: 12 months' clinical results

OBJECTIVE

The purpose of this study was to evaluate the safety and efficacy of transepithelial corneal collagen cross linking (TE-CXL) with modified riboflavin and accelerated UVA irradiance in thin corneas with pachymetry less than 400 microns at thinnest point, untreatable by epithelium off corneal collagen cross linking (CXL) in adult Pakistani population with progressive keratoconus.

METHODS

This quasi experimental study included twenty six eyes of 26 patients with progressive keratoconus who underwent accelerated transepithelial CXL in Armed forced institute of ophthalmology with 12 months follow up. Modified riboflavin, ParaCel ((riboflavin 0.25%, Benzalkonium chloride, EDTA, Trometamol, hydroxypropyl methylcellulose) and vibeX Xtra (riboflavin 0.25%) (Avedro, USA)) were applied to cornea in two stages. Uncorrected and Corrected Distant Visual Acuities (UDVA, CDVA), spherical equivalent (SE), astigmatism, pachymetry at thinnest point (Pachy thin), apex keratometry (Kmax), simulated and steep keratometry (Sim K, steep K) were measured at baseline and at 3, 6 and 12 months post operatively. The cornea was then exposed to accelerated UVA irradiance of 9mW/cm² for 10 min (total dose 30 mW/cm²).

RESULTS

The mean age of the patient was 24.54±5.16 years. UDVA, CDVA, SE, astigmatism significantly improved at all postoperative test points (p=0.000, 0.004, 0.000, 0.004 respectively). Kmax and pachy thin were significantly reduced over baseline at 1 year (p=0.000, 0.004 respectively). Topographic indices Sim K and steep K did not show significant changes. No intra or post-operative complications were reported.

CONCLUSION

Transepithelial accelerated CXL with modified riboflavin is a safe and effective procedure which halt disease progression in thin corneas with progressive keratoconus.

Efficacy of transepithelial corneal collagen crosslinking for keratoconus: 12-month follow-up

Purpose

To evaluate the efficacy of transepithelial corneal collagen crosslinking (TE-CXL) in patients with progressive keratoconus.

Patients and methods

This is a prospective interventional consecutive study carried out on 30 eyes of 18 patients with progressive keratoconus who underwent TE-CLX using both ParaCel™ (riboflavin 0.25%, hydroxy propyl methyl cellulose, NaCl, ethylenediaminetetraacetic acid [EDTA], Tris, and benzalkonium chloride) and vibeX-Xtra (riboflavin 0.22%, phosphate-buffered saline solution). The procedure was carried out at Ebsar Eye Center in Egypt in the period from 2012 to 2014. The follow-up visits were scheduled on days 1, 3, 6, and 12 months after treatment.

Results

There were statistically significant improvements (P<0.001) in the mean best-corrected visual acuity (0.54±0.22 preoperatively vs 0.61±0.19 at 12 months postoperatively), the mean manifest refraction spherical equivalent (MRSE; −6.16±3.90 diopters [D] preoperatively and −5.91±3.72 D at 12 months postoperatively), and the mean preoperative corneal astigmatism (−3.39±2.11 D preoperatively and −2.46±2.60 D at 12 months postoperatively).

Conclusion

TE-CXL could halt the progression of keratoconus in adult patients. TE-CXL resulted in a statistically significant improvement in best-corrected visual acuity, manifest refraction, refractive and corneal astigmatism and K values in keratoconus patients at the 12-month follow-up. Larger sample sizes and longer follow-ups are required in order to make meaningful conclusions.

Corneal collagen crosslinking for corneal ectasias: a review

PURPOSE

To review the published literature on corneal collagen crosslinking (CXL).

METHODS

Importance has been placed on seminal publications, systemic reviews, meta-analyses, and randomized controlled clinical trials. Where such evidence was not available, cohort studies, case-controlled studies, and case series with follow-up greater than 12 months were examined.

RESULTS

Corneal collagen crosslinking with riboflavin and ultraviolet A (UVA) 370 nm radiation appears to be capable of arresting the progression of ectatic corneal disorders, with most studies reporting significant improvements in visual, keratometric, and topographic measurements. Its mode of action at the molecular level is undetermined. Follow-up is limited to 5-10 years but suggests sustained stability and enhancement in corneal shape with time. Nearly all published long-term data and comparative studies are with epithelium-off techniques. Epithelium-on investigations suggest some efficacy but less than with epithelium-off treatments and long-term data are unavailable. Accelerated techniques with higher UVA fluencies and shorter treatments times, delivering the same UVA energy dosage, are the subject of recent investigation, with some laboratory and clinical studies suggesting reduced efficacy compared to the standard 3 mW/cm2 for 30 minutes irradiation procedure. Combined methodologies of CXL with techniques such as photorefractive keratectomy and intrastromal rings show promise but long-term follow-up is indicated. Sight-threatening complications of CXL are rare.

CONCLUSIONS

Studies of epithelium-off CXL with irradiation at 3 mW/cm2 for 30 minutes support its efficacy. Refinement in techniques may allow for safer and more rapid procedures with less patient discomfort but require further investigation.

Matrix-Based Regenerating Agent for Corneal Wound Healing After Collagen Cross-Linking

PURPOSE

Corneal collagen cross-linking (CXL) is associated with massive cellular damage, the long-term clinical consequences of which have still to be elucidated. In this study, we seek to determine whether matrix-based regenerating agent (RGTA) therapy has a positive influence on stromal regeneration after CXL.

METHODS

Twelve New Zealand white rabbits were randomly divided into 2 groups of 6. CXL of the right cornea was performed in both groups, followed by topical corneal application of one drop of vehicle or one drop of RGTA. In vivo confocal laser-scanning microscopy was used to monitor corneal changes over time. At the end of the experiment, 2 weeks postoperatively, the corneas were excised and processed for histology.

RESULTS

Epithelial healing was noted in both groups 3 days after CXL. One week after treatment, in vivo confocal laser-scanning microscopy in both groups revealed a significantly thicker acellular stromal zone in the vehicle group compared with the RGTA group (P = 0.0165). This difference persisted up to 2 weeks postoperatively, when acellular zone thickness was 96 ± 10 µm in the RGTA group and 146 ± 34 µm in the vehicle group (P = 0.0058). These findings were confirmed by histology, which also revealed earlier cell repopulation in the RGTA group.

CONCLUSIONS

The anterior stroma of cross-linked corneas exhibited cell loss after CXL. Cell repopulation in the anterior corneal stroma occurred earlier when CXL was followed by topical RGTA treatment. Further studies are required to examine the impact of this ophthalmic RGTA on corneal nerve regeneration after CXL.

An investigation into corneal enzymatic resistance following epithelium-off and epithelium-on corneal cross-linking protocols

The aim of this study was to investigate corneal enzymatic resistance following epithelium off and on riboflavin/UVA cross-linking (CXL). One hundred and fourteen porcine eyes were divided into four non-irradiated control groups and seven CXL groups. The latter comprised; (i) epithelium-off, 0.1% iso-osmolar riboflavin, 9 mW UVA irradiation for 10 min, (ii) disrupted epithelium, 0.1% hypo-osmolar riboflavin, 9 mW UVA for 10 min, (iii) epithelium-on, 0.25% hypo-osmolar riboflavin with 0.01% benzylalkonium chloride (BACS), 9 mW UVA for 10 min, (iv) epithelium-on, 5 min iontophoresis at 0.1 mA for 5 min with 0.1% riboflavin solution, 9 mW UVA for 10 min or (v) 12.5 min, (vi) epithelium-on, prolonged iontophoresis protocol of 25 min with 1.0 mA for 5 min and 0.5 mA for 5 min with 0.25% riboflavin with 0.01% BACS, 9 mW UVA for 10 min or (vii) 12.5 min. Enzymatic resistance was assessed by daily measurement of a corneal button placed in pepsin solution and measurement of corneal button dry weight after 11 days of digestion. This study revealed that the enzymatic resistance was greater in CXL corneas than non-irradiated corneas (p < 0.0001). Epithelium-off CXL showed the greatest enzymatic resistance (p < 0.0001). The prolonged iontophoresis protocol was found to be superior to all other trans-epithelial protocols (p < 0.0001). A 25% increase in UVA radiance significantly increased corneal enzymatic resistance (p < 0.0001). In conclusion, although epithelium-on CXL appears to be inferior to epithelium-off CXL in terms of enzymatic resistance to pepsin digestion, the outcome of epithelium-on CXL may be significantly improved through the use of higher concentrations of riboflavin solution, a longer duration of iontophoresis and an increase in UVA radiance.

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Epi-off CXL is superior to epi-on CXL in terms of enzymatic resistance.

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Epi-on CXL effectiveness can be improved by using a prolonged iontophoresis protocol.

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Higher riboflavin concentrations and increased UVA radiance may also enhance epi-on CXL.

One-year Outcomes of Pachymetry and Epithelium Thicknesses after Accelerated (45 mW/cm(2)) Transepithelial Corneal Collagen Cross-linking for Keratoconus Patients

The thickness of corneal pachymetry and the epithelium after accelerated (45 mW/cm(2)) transepithelial corneal collagen cross-linking (CXL) for keratoconus were assessed in this prospective case series study. Twenty-eight patients were treated for keratoconus. The mean Kmax was 56.18 ± 7.90. The thinnest point, as assessed by optical coherence tomography (OCT), was 443.18 ± 39.75 μm. Accelerated transepithelial CXL was performed, and corrected distance visual acuity (CDVA), corneal topography, and OCT were recorded at 1 week postoperatively as well as at 1, 3, 6, and 12 months. The surgery was uneventful in all eyes. Postoperative epithelial edema was observed and faded in 3 days. The postoperative Kmax was 54.56 ± 8.81, 55.78 ± 8.11, 56.37 ± 8.71, 55.80 ± 7.92, and 55.47 ± 8.24 at 1 week, 1 month, 3 months, 6 months, and 12 months, respectively (all, P > 0.05). The thinnest postoperative corneal point, 439.04 ± 44.99 μm, was observed at 12 months (P = 0.109). The epithelial thickness decreased during the first postoperative week then showed a gradual recovery. Postoperative pachymetry thickness showed no significant changes for up to 12 months. Postoperative epithelial thickness decreased temporarily, then stabilized at month 12. Accelerated transepithelial CXL was shown to be effective and safe for the treatment of keratoconus.

Transepithelial Corneal Cross-Linking With Vitamin E-Enhanced Riboflavin Solution and Abbreviated, Low-Dose UV-A: 24-Month Clinical Outcomes

Purpose:

To report the clinical outcomes with 24-month follow-up of transepithelial cross-linking using a combination of a d-alpha-tocopheryl polyethylene-glycol 1000 succinate (vitamin E-TPGS)-enhanced riboflavin solution and abbreviated low fluence UV-A treatment.

Methods:

In a nonrandomized clinical trial, 25 corneas of 19 patients with topographically proven, progressive, mild to moderate keratoconus over the previous 6 months were cross-linked, and all patients were examined at 1, 3, 6, 12, and 24 months. The treatments were performed using a patented solution of riboflavin and vitamin E-TPGS, topically applied for 15 minutes, followed by two 5-minute UV-A treatments with separate doses both at fluence below 3 mW/cm² that were based on preoperative central pachymetry.

Results:

During the 6-month pretreatment observation, the average Kmax increased by +1.99 ± 0.29 D (diopter). Postoperatively, the average Kmax decreased, changing by −0.55 ± 0.94 D, by −0.88 ± 1.02 D and by −1.01 ± 1.22 D at 6, 12, and 24 months. Postoperatively, Kmax decreased in 19, 20, and 20 of the 25 eyes at 6 months, 12 months, and 24 months, respectively. Refractive cylinder was decreased by 3 months postoperatively and afterward, changing by −1.35 ± 0.69 D at 24 months. Best spectacle-corrected visual acuity (BSCVA) improved at 6, 12, and 24 months, including an improvement of −0.19 ± 0.13 logarithm of the minimum angle of resolution units at 24 months. There was no reduction in endothelial cell count. No corneal abrasions occurred, and no bandage contact lenses or prescription analgesics were used during postoperative recovery.

Conclusions:

Transepithelial cross-linking using the riboflavin-vitamin E solution and brief, low-dose, pachymetry-dependent UV-A treatment safely stopped keratoconus progression.

Ultraviolet A: Visible spectral absorbance of the human cornea after transepithelial soaking with dextran-enriched and dextran-free riboflavin 0.1% ophthalmic solutions

PURPOSE

To evaluate the stromal concentration of 2 commercially available transepithelial riboflavin 0.1% solutions in human donor corneas with the use of spectrophotometry.

SETTING

University of Calabria, Rende, Italy.

DESIGN

Experimental study.

METHODS

The absorbance spectra of 12 corneal tissues were measured in the 330 to 700 nm wavelength range using a purpose-designed spectrophotometry setup before and after transepithelial corneal soaking with a 15% dextran-enriched riboflavin 0.1% solution (n = 6) or a hypotonic dextran-free riboflavin 0.1% solution (n = 6). Both ophthalmic solutions contained ethylenediaminetetraacetic acid and trometamol as enhancers. In addition, 4 deepithelialized corneal tissues underwent stromal soaking with a 20% dextran-enriched riboflavin 0.1% solution and were used as controls. All the riboflavin solutions were applied topically for 30 minutes. The stromal concentration of riboflavin was quantified by analysis of absorbance spectra of the cornea collected before and after application of each solution.

RESULTS

The mean stromal riboflavin concentration was 0.012% ± 0.003% (SD), 0.0005% ± 0.0003% (P < .001), and 0.004% ± 0.001% (P < .01) in tissues soaked with 20% dextran-enriched, 15% dextran-enriched, and hypotonic dextran-free solutions, respectively. The difference of stromal riboflavin concentration between the 2 transepithelial solutions was statistically significant (P < .01).

CONCLUSIONS

Dextran-enriched solutions required complete corneal deepithelialization to permit effective stromal soaking with riboflavin. Nevertheless, riboflavin in hypotonic dextran-free solution with enhancers permeates across stroma through an intact epithelium.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Standard versus accelerated riboflavin-ultraviolet corneal collagen crosslinking: Resistance against enzymatic digestion

PURPOSE

To examine the effect of standard and accelerated corneal collagen crosslinking (CXL) on corneal enzymatic resistance.

SETTING

School of Optometry and Vision Sciences, Cardiff University, Cardiff, United Kingdom.

DESIGN

Experimental study.

METHODS

Sixty-six enucleated porcine eyes (with corneal epithelium removed) were assigned to 6 groups. Group 1 remained untreated, group 2 received dextran eyedrops, and groups 3 to 6 received riboflavin/dextran eyedrops. Group 4 had standard CXL (3 mW/cm(2) ultraviolet-A for 30 minutes), whereas groups 5 and 6 received accelerated CXL (9 mW/cm(2) for 10 minutes and 18 mW/cm(2) for 5 minutes, respectively). Trephined central 8.0 mm buttons from each cornea underwent pepsin digestion. Corneal diameter was measured daily, and the dry weight of 5 samples from each group was recorded after 12 days of digestion.

RESULTS

All CXL groups (4 to 6) took longer to digest and had a greater dry weight at 12 days (P < .0001) than the nonirradiated groups (1 to 3) (P < .0001). The time taken for complete digestion to occur did not differ between the standard and accelerated CXL groups, but the dry weights at 12 days showed significant differences between treatments: standard CXL 3 mW > accelerated CXL 9 mW > accelerated CXL 18 mW (P < .0001).

CONCLUSIONS

Standard and accelerated CXL both increased corneal enzymatic resistance; however, the amount of CXL might be less when accelerated CXL is used. The precise amount of CXL needed to prevent disease progression is not yet known.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Rate of riboflavin diffusion from intrastromal channels before corneal crosslinking

PURPOSE

To determine the diffusion of riboflavin from intrastromal channels through the effective diffusion coefficients compared with traditional axial diffusion with epithelium on or off.

SETTING

Advanced Optical Imaging Laboratory, University College Dublin, and Wellington Eye Clinic, Sandyford, Dublin, Ireland.

DESIGN

Experimental study.

METHODS

The rate of diffusion in whole-mounted porcine eyes was monitored for a 30 minutes using an optical setup with a charge-coupled device camera and a bandpass filter (central wavelength 550 nm and 40 nm bandpass) to image the fluorescence under ultraviolet illumination (365 nm wavelength). For comparison, an isotropic corneal stroma with an annular channel was modeled numerically for different diffusion constants and boundary conditions.

RESULTS

Numerical and experimental results were compared, allowing determination of the effective diffusion coefficient for each case. Experimental results for 6 different riboflavin solutions were in all cases found to be higher than for the common crosslinking (CXL) riboflavin protocol, where the diffusion constant is D0 = 6.5 × 10(-5) mm(2)/sec. For the intrastromal channel, 2 isotonic solutions containing riboflavin 0.1% correlated with a diffusion constant of 5D0 = 32.5 × 10(-5) mm(2)/sec. Hypotonic solutions and transepithelium had a higher diffusion coefficient approaching 10D0 = 65.0 × 10(-5) mm(2)/sec, which is an order-of-magnitude increase compared with the typical diffusion coefficient found in standard CXL.

CONCLUSIONS

In this study, riboflavin had a faster stromal diffusion when injected into a corneal channel than when applied as drops to the anterior corneal surface. Further numerical modeling might allow optimization of the channel structure for any specific choice of riboflavin.

Keratoconus and Other Corneal Diseases: Pharmacologic Cross-Linking and Future Therapy

The ability to cross-link collagen fibers and use this technique to strengthen the cornea has become of great interest to ophthalmologists in the last decade. For progressive diseases such as keratoconus, collagen cross-linking confers the possibility of halting progression and stabilizing the cornea, a benefit that is not observed with any other current treatment. Collagen cross-linking uses riboflavin combined with ultraviolet A light to induce the formation of bonds between collagen fibrils that strengthen the cornea. This chapter will discuss the theory, technique, indications, and complications of corneal cross-linking. Much of what will be discussed is in areas of active research that will likely be further clarified as more experience is gained with this procedure.

Two-photon microscopy of a Flt1 peptide-hyaluronate conjugate

AIM

Two-photon microscopy was performed to visualize ocular distribution of Flt1 peptide-hyaluronate (HA) conjugate micelles for eye drop treatment of corneal neovascularization.

MATERIALS & METHODS

Flt1 peptide-HA conjugate micelles were topically administered to the eye for two-photon microscopy and antiangiogenic effect assessment after silver nitrate cauterization.

RESULTS

In vivo two-photon microscopy revealed that Flt1 peptide-HA conjugate micelles were absorbed and remained on the corneal epithelia with an increased residence time, facilitating the corneal delivery of carboxyfluorescein succinimidyl ester (CFSE) as a model drug. Furthermore, repeated eye drops of Flt1 peptide-HA conjugate micelles showed comparable therapeutic effect to the subconjunctival injection on the corneal neovascularization.

DISCUSSION & CONCLUSION

We confirmed the feasibility of Flt1 peptide-HA conjugate micelles for eye drop treatment of corneal neovascularization.

Can the effect of transepithelial corneal collagen cross-linking be improved by increasing the duration of topical riboflavin application? An in vivo confocal microscopy study

OBJECTIVE

To evaluate the effect of transepithelial corneal collagen cross-linking (CXL) with prolonged riboflavin application by in vivo confocal microscopy and to compare this effect with that of standard CXL with complete epithelial debridement.

METHODS

In eyes with progressive keratoconus, CXL procedure was performed with standard technique and transepithelial technique after prolonged riboflavin drop application for 2 hr. Patients were evaluated with in vivo confocal microscopic examination preoperatively and at postoperative months 1 and 6.

RESULTS

The depth of CXL effect was similar in both groups (i.e., 380.86 ± 103.23 μm in standard CXL group and 342.2 ± 68.6 μm in transepithelial CXL group) (P=0.4). The endothelial cell counts and morphological parameters (i.e., pleomorphism and polymegathism) were not significantly affected in both groups (P>0.05 for all). In the standard CXL group, in vivo confocal microscopy revealed anterior stromal acellular hyperreflective honeycomb edema with posteriorly gradually decreasing reflectivity and increasing number of keratocytes and some sheets of longitudinally aligned filamentary deposits. The keratocytes were seen to repopulate in the posterior-to-anterior direction. In transepithelial CXL group, although the depth of CXL effect was similar, less pronounced keratocyte damage, extracellular matrix hyperreflectivity, and sheets of filamentary deposits at the posterior stroma was observed.

CONCLUSIONS

Transepithelial CXL with prolonged peroperative riboflavin application can achieve similar depth of effect in the stroma with less pronounced confocal microscopic changes as compared with the standard CXL with complete epithelial debridement.

Riboflavin concentration in corneal stroma after intracameral injection

AIM

To evaluate the enrichment of riboflavin in the corneal stroma after intracameral injection to research the barrier ability of the corneal endothelium to riboflavin in vivo.

METHODS

The right eyes of 30 New Zealand white rabbits were divided into three groups. Different concentrations riboflavin-balanced salt solutions (BSS) were injected into the anterior chamber (10 with 0.5%, 10 with 1%, and 10 with 2%). Eight corneal buttons of 8.5 mm in diameter from each group were dissected at 30min after injection and the riboflavin concentrations in the corneal stroma were determined using high-performance liquid chromatography (HPLC) after removing the epithelium and endothelium. The other two rabbits in every group were observed for 24h and sacrificed. As a comparison, the riboflavin concentrations from 16 corneal stromal samples were determined using HPLC after instillation of 0.1% riboflavin-BSS solution for 30min on the corneal surface (8 without epithelium and 8 with intact epithelium).

RESULTS

The mean riboflavin concentrations were 11.19, 18.97, 25.08, 20.18, and 1.13 µg/g for 0.5%, 1%, 2%, de-epithelialzed samples, and the transepithelial groups, respectively. The color change of the corneal stroma and the HPLC results showed that enrichment with riboflavin similar to classical de-epithelialized corneal collagen crosslinking (CXL) could be achieved by intracameral 1% riboflavin-BSS solution after 30min; the effect appeared to be continuous for at least 30min.

CONCLUSION

Riboflavin can effectively penetrate the corneal stroma through the endothelium after an intracameral injection in vivo, so it could be an enhancing method that could improve the corneal riboflavin concentration in transepithelial CXL.

Corneal cross-linking

Since its inception in the late 1990s, corneal cross-linking has grown from an interesting concept to a primary treatment for corneal ectatic disease worldwide. Using a combination of ultraviolet-A light and a chromophore (vitamin B2, riboflavin), the cornea can be stiffened, usually with a single application, and progressive thinning diseases such as keratoconus arrested. Despite being in clinical use for many years, some of the underlying processes, such as the role of oxygen and the optimal treatment times, are still being worked out. More than a treatment technique, corneal cross-links represent a physiological principle of connective tissue, which may explain the enormous versatility of the method. We highlight the history of corneal cross-linking, the scientific underpinnings of current techniques, evolving clinical treatment parameters, and the use of cross-linking in combination with refractive surgery and for the treatment of infectious keratitis.

A review of collagen cross-linking in cornea and sclera

Riboflavin/UVA cross-linking is a technique introduced in the past decades for the treatment of keratoconus, keratectasia, and infectious keratitis. Its efficacy and safety have been investigated with clinical and laboratory studies since its first clinical application by Wollensak for the treatment of keratoconus. Although its complications are encountered during clinical practice, such as infection inducing risk, minimal invasion merits a further investigation on its future application in clinical practice. Recently, collagen cross-linking in sclera shows a promising prospect. In present study, we summarized the representative studies describing the clinical and laboratory application of collagen cross-linking published in past decades and provided our opinion on the positive and negative results of cross-linking in the treatment of ophthalmic disorders.

Interaction of ultraviolet light with the cornea: clinical implications for corneal crosslinking

Understanding ultraviolet (UV) interaction with the human corneal tissue is of interest among corneal specialists given the widespread application of corneal crosslinking. This article reviews the current knowledge of light interaction with the cornea in the UV wavelength range. It also uses a novel experimental study to illustrate the role of 2 important corneal properties that have not yet been clarified: the epithelial contribution to overall UVA corneal absorbance and the regional anisotropy of UVA light transmittance. Finally, it presents the most recent insights into how different methods of UVA light irradiation and corneal soaking with riboflavin influence the outcome of corneal crosslinking.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Safety and efficacy of epithelium removal and transepithelial corneal collagen crosslinking for keratoconus

This review aims to assess the efficacy and safety of epithelial removal (ER) and transepithelial (TE) corneal collagen crosslinking (CXL) for the treatment of keratoconus. We used MEDLINE to identify all ER and TE CXL studies on keratoconic eyes (n≥20, follow-up ≥12 months). Ex vivo and studies for non-keratoconus indications or in conjunction with other procedures were excluded. Data on uncorrected (UDVA) and corrected (CDVA) distance visual acuity, refractive cylinder, maximum keratometry (Kmax), and adverse events were collected at the latest follow-up and 1 year. Only one randomised controlled trial (RCT) qualified inclusion. Forty-four ER and five TE studies were included. For logMAR UDVA, CDVA, mean spherical equivalent, refractive cylinder and Kmax, at latest follow-up 81, 85, 93, 62, and 93% ER studies vs 66.7, 80, 75, 33, and 40% TE studies reported improvement, respectively. Whereas at 1 year, 90, 59, and 91% ER studies vs 80, 50, and 25% TE studies reported improvement, respectively. The majority of studies showed reduced pachymetry in both groups. Treatment failure, retreatment rates, and conversion to transplantation were reported to be up to 33, 8.6, and 6.25%, respectively, in ER studies only. Stromal oedema, haze, keratitis, and scarring were only reported in ER studies, whereas endothelial cell counts remained variable in both groups. Both ER and TE studies showed improvement in visual acuity, refractive cylinder but Kmax worsened in most TE studies. Adverse events were reported more with ER studies. This review calls for more high quality ER and TE studies with comparable parameters for further assessment of safety and efficacy.

Evaluation of epithelial integrity with various transepithelial corneal cross-linking protocols for treatment of keratoconus

Purpose. Corneal collagen cross-linking (CXL) has been demonstrated to stiffen cornea and halt progression of ectasia. The original protocol requires debridement of central corneal epithelium to facilitate diffusion of a riboflavin solution to stroma. Recently, transepithelial CXL has been proposed to reduce risk of complications associated with epithelial removal. Aim of the study is to evaluate the impact of various transepithelial riboflavin delivery protocols on corneal epithelium in regard to pain and epithelial integrity in the early postoperative period. Methods. One hundred and sixty six eyes of 104 subjects affected by progressive keratoconus underwent transepithelial CXL using 6 different riboflavin application protocols. Postoperatively, epithelial integrity was evaluated at slit lamp and patients were queried regarding their ocular pain level. Results. One eye had a corneal infection associated with an epithelial defect. No other adverse event including endothelial decompensation or endothelial damage was observed, except for epithelial damages. Incidence of epithelial defects varied from 0 to 63%. Incidence of reported pain varied from 0 to 83%. Conclusion. Different transepithelial cross-linking protocols have varying impacts on epithelial integrity. At present, it seems impossible to have sufficient riboflavin penetration without any epithelial disruption. A compromise between efficacy and epithelial integrity has to be found.

Acoustic radiation force for noninvasive evaluation of corneal biomechanical changes induced by cross-linking therapy

OBJECTIVES

To noninvasively measure changes in corneal biomechanical properties induced by ultraviolet-activated riboflavin cross-linking therapy using acoustic radiation force (ARF).

METHODS

Cross-linking was performed on the right eyes of 6 rabbits, with the left eyes serving as controls. Acoustic radiation force was used to assess corneal stiffness before treatment and weekly for 4 weeks after treatment. Acoustic power levels were within US Food and Drug Administration guidelines for ophthalmic safety. Strain, determined from ARF-induced displacement of the front and back surfaces of the cornea, was fit to the Kelvin-Voigt model to determine the elastic modulus (E) and coefficient of viscosity (η). The stiffness factor, the ratio of E after treatment to E before treatment, was calculated for treated and control eyes. At the end of 4 weeks, ex vivo thermal shrinkage temperature analysis was performed for comparison with in vivo stiffness measurements. One-way analysis of variance and Student t tests were performed to test for differences in E, η, the stiffness factor, and corneal thickness.

RESULTS

Biomechanical stiffening was immediately evident in cross-linking-treated corneas. At 4 weeks after treatment, treated corneas were 1.3 times stiffer and showed significant changes in E (P= .006) and η (P= .007), with no significant effect in controls. Corneal thickness increased immediately after treatment but did not differ significantly from the pretreatment value at 4 weeks.

CONCLUSIONS

Our findings demonstrate a statistically significant increase in stiffness in cross-linking-treated rabbit corneas based on in vivo axial stress/strain measurements obtained using ARF. The capacity to noninvasively monitor corneal stiffness offers the potential for clinical monitoring of cross-linking therapy.