Histological changes in human cornea after cross-linking with riboflavin and ultraviolet A

Early findings in a randomised controlled trial on crosslinking protocols using isoosmolar and hypoosmolar riboflavin for the treatment of progressive keratoconus

PURPOSE

To present baseline characteristics and to present the perioperative corneal thickness during corneal crosslinking (CXL) treatment for progressive keratoconus and to describe how the addition of sterile water (SW) efficaciously can maintain the corneal thickness. The treatment efficacy will be evaluated when the 1-year follow-up is complete.

METHODS

A randomised clinical study using epithelium-off CXL with continuous UVA irradiation (9 mW/cm2) and two kinds of riboflavin solutions: (i) isoosmolar dextran-based riboflavin (n = 27) and (ii) hypoosmolar dextran-free riboflavin (n = 27).

INCLUSION CRITERIA

progressive keratoconus with an increase in maximum keratometry value (Kmax) of 1.0 dioptre (12 months) or 0.5 dioptres (6 months). Corneae thinner than 400 μm were also included.

OUTCOME PARAMETERS

Perioperative corneal thickness and the effect of adding SW.

RESULTS

Seventy-four per cent of the patients in the isoosmolar group and 15% in the hypoosmolar group required the addition of SW, which effectively maintained a corneal thickness of 400 μm in all cases during CXL. The addition of SW was primarily needed during the irradiation procedure and not the preoperative soaking period.

CONCLUSIONS

Especially during the CXL irradiation phase, isoosmolar riboflavin causes a significant dehydrating effect leading to corneal thinning during CXL. The customised addition of SW is efficacious in maintaining the corneal thickness during CXL and could increase the safety of the procedure.

Fungal Keratitis: Diagnosis, Management, and Recent Advances

Fungal keratitis is one of the major causes of microbial keratitis that may lead to corneal blindness. Many problems related to diagnosis and therapy are encountered in fungal keratitis, including difficulty in obtaining laboratory diagnoses and the availability and efficacy of antifungal medications. Intensive and prolonged use of antifungal topical preparations may not be enough. The use of antifungal medications is considered the main treatment for fungal keratitis. It is recommended to start antifungal therapy after confirmation of the clinical diagnosis with a smear or positive cultures. Topical application of antifungal medications is a mainstay for the treatment of fungal keratitis; however, systemic, intra-stromal, or intra-cameral routes may be used. Therapeutic keratoplasty is the main surgical procedure approved for the management of fungal keratitis with good success rate. Intrastromal corneal injection of antifungal medications may result in steady-state drug levels within the corneal tissue and prevent intervals of decreased antifungal drug concentration below its therapeutic level. In cases of severe fungal keratitis with deep stromal infiltration not responding to treatment, intracameral injection of antifungal agents may be effective. Collagen cross-linking has been proposed to be beneficial for cases of fungal keratitis as a stand-alone therapy or as an adjunct to antifungal medications. Although collagen cross-linking has been extensively studied in the past few years, its protocol still needs many modifications to optimize UV fluence levels, irradiation time, and concentration of riboflavin to achieve 100% microbial killing.

Diagnosis and management of keratoconus in the paediatric age group: a review of current evidence

The diagnosis and management of keratoconus in the paediatric age group presents additional challenges to those encountered in adults. The most significant of these, encountered in some young patients, are delayed presentation of unilateral disease, more advanced disease at diagnosis, difficulty in obtaining reliable corneal imaging, faster rates of disease progression and challenges in contact lens management. The stabilisation effect of corneal cross-linking (CXL), more extensively studied in adults with randomised trials and long-term follow-up, has been much less rigorously examined in children and adolescents. The high heterogeneity of published studies in younger patients, particularly in the choice of tomography parameters designated as primary outcome measures and the definitions of progression, indicates that improved standardisation for future studies on CXL will be necessary. There is no evidence that corneal transplant outcomes in young patients are poorer than those in adults. This review provides a current perspective on the optimal diagnosis and treatment of keratoconus in children and adolescents.

A review of the epithelial and stromal effects of corneal collagen crosslinking

Induced corneal collagen crosslinking and mechanical stiffening via ultraviolet-A photoactivation of riboflavin (UVA CXL) is now a common treatment for corneal ectasia and Keratoconus. Some effects of the procedure such as induced mechanical stiffening, corneal flattening, and cellular toxicity are well-known, but others remain more controversial. Authors report a variety of contradictory effects, and provide evidence based on individual results and observations. A full understanding of the effects of and mechanisms behind this procedure are essential to predicting its outcome. A growing interest in modifications to the standard UVA CXL protocol, such as transepithelial or accelerated UVA CXL, makes analyzing the literature as a whole more urgent. This review presents an analysis of both the agreed-upon and contradictory results reported and the various methods used to obtain them.

Comparison between three different high fluence UVA levels in corneal collagen cross-linking for treatment of experimentally induced fungal keratitis in rabbits

INTRODUCTION

The aim of this study was to compare the efficacy of Photo-Activated Chromophore for Keratitis - Corneal Collagen Cross-linking (PACK-CXL) of three different total UVA fluence levels and topical voriconazole in treatment of fungal keratitis experimentally induced in rabbits.

METHODS

This is an interventional experimental study including both eyes of 16 rabbits (32 eyes). Fungal keratitis was induced by intrastromal injection of Fusarium Solani into the cornea. The rabbits were then divided into four groups (8 eyes for each) from which group A received Voriconazole eye drops and considered as control group. Group B, C, D received single PACK-CXL session with total fluence levels of 7.2, 10.0 and 15.0 J/cm2 for each respectively. Daily clinical examination was recorded and all corneas were removed for microbiology and histopathology on day ten.

RESULTS

The mean clinical signs score eyes treated with high fluence PACK-CXL showed evident clinical improvement from fourth to tenth day of treatment. This improvement was equivalent to that of Voriconazole treatment. The results showed better improvement with increasing the UVA total fluence levels but this difference was not statistically significant (P < 0.05). Similarly, the median CFU/ml declined on increasing UVA fluence but with no statistically significant values. Histopathological examination revealed better improvement of inflammatory signs on higher fluence levels compared to lower ones.

CONCLUSIONS

High intensity PACK-CXL (30 mW/cm²) was as effective as Voriconazole in the treatment of fungal keratitis in rabbits. Increasing the fluence of UVA was associated with slightly better clinical outcomes with no added risks. More clinical studies are needed to confirm these results.

[Histological changes in keratoconus and wound healing after corneal cross-linking]

BACKGROUND

Keratoconus is classified as a corneal ectasia and is a multifactorial disease. In those affected, mostly adolescent patients visual deterioration occurs due to the development of irregular astigmatism. Treatment by corneal cross-linking (CXL) has been indicated in progressive disease for several years.

OBJECTIVE

To present the pathophysiology and histological changes in keratoconus as well as wound healing processes after CXL and their potential complications.

MATERIAL AND METHODS

Histological changes in keratoconus as well as wound healing processes after CXL and their potential complications are presented based on histological examination of corneal specimens with keratoconus with and without a condition after CXL. Relevant literature and own data are analyzed and discussed.

RESULTS

Besides inflammatory processes, atopic and genetic dispositions play a role in the development of keratoconus. The histological characteristics of keratoconus include changes in the epithelium, Bowman's layer and stroma. Wound healing processes after CXL include healing of the surface epithelium and transient loss of keratocytes and nerve fibers.

CONCLUSION

Keratoconus shows characteristic histopathological changes, such as epithelial irregularities, stromal thinning and breaks of Bowman's layer, whereas the endothelium and Descemet's membrane remain unchanged (apart from cases of corneal hydrops). After CXL wound healing processes can be followed primarily in vivo by confocal microscopy. Complications after CXL are rare. Persistent loss of keratocytes can be clinically manifested as a visually relevant scar.

Outcomes of customized topographic guided epithelial debridement for corneal collagen cross-linking

PURPOSE

To evaluate the outcomes of corneal collagen cross-linking performed with customized epithelial debridement technique in progressive keratoconic corneas.

MATERIALS AND METHODS

Forty eyes of 40 patients were included in the study. We performed an ophthalmologic examination and recorded the uncorrected visual acuity, best corrected visual acuity (BCVA), central corneal thickness at the thinnest point (t-CCT), flat meridian keratometry (K1), steep meridian keratometry (K2), endothelial cell density (ECC), hexagonal cells (HEX), and coefficient of variation of cell areas (CV) measured preoperatively and at the 1st, 3rd, 6th and 12th months postoperatively.

RESULTS

The mean UCVA and BCVA were increased (p < 0.05). The mean flattest and steepest K readings were decreased (p < 0.05). The mean t-CCT decreased in the first months after treatment and increased after 6 months. The mean t-CCT was thicker at 12 months compared to pretreatment status (p < 0.05). The mean ECC and CV were not significantly different between follow-up intervals (p > 0.05). The mean HEX was statistically increased (p > 0.05).

CONCLUSIONS

Corneal collagen cross-linking performed with customized epithelial debridement technique is a successful alternative method for stopping the progression of keratoconus after 12 months of treatment.

Study of the effects of rabbit scleral fibroblasts on cellular biomechanical properties and MMP-2 expression using two modes of riboflavin/ultraviolet A wave collagen cross-linking

OBJECTIVE

The aim of this study is to evaluate the cellular biomechanical properties and MMP-2 expression changes in rabbit scleral fibroblasts using two modes of riboflavin and ultraviolet A (UVA) collagen cross-linking (CXL).

METHODS

Twenty-four New Zealand white rabbits were randomly divided into two groups, A and B. The left eye was chosen for the experimental group and the right eye for the control group. In group A, the eyes were irradiated for 30 min, with a power density of 3.0 mW/cm². In group B, the eyes were irradiated for 9 min, with a power density of 10.0 mW/cm². One week after CXL, full-field electroretinography was performed. Sixty days after CXL, the rabbits were sacrificed, and scleral fibroblasts were extracted from the CXL-treated sclera area and corresponding parts of control sclera and cultured. Cellular biomechanical properties were evaluated using the micropipette aspiration technique, and the MMP-2 protein expression was determined by Western blot analysis.

RESULTS

There was no statistical difference in the amplitude and latency of the dark adaptation 3.0 and light adaptation 3.0 between the CXL and control eyes of groups A and B (P > 0.05). Compared with the control groups, the Young's modulus of the fibroblasts and apparent viscosity of the experimental eyes in groups A and B were increased after CXL (P < 0.05), but there was no significant difference between the two groups under different irradiation modes (P > 0.05). The MMP-2 expression in scleral fibroblasts from experimental eyes was significantly higher than that in scleral fibroblasts from control eyes in groups A and B. Under the two different irradiation modes, the MMP-2 expression in the scleral fibroblasts from experimental eyes in group A was significantly higher than that in the scleral fibroblasts from experimental eyes in group B.

CONCLUSION

The riboflavin-UVA scleral CXL conducted in two different modes produced no significant side effects on the retina and could strengthen the cell biomechanical properties as well as increase the MMP-2 expression of scleral fibroblasts significantly.

Pediatric keratoconus - Current perspectives and clinical challenges

Keratoconus is an ectatic corneal disease characterized by progressive stromal thinning, irregular astigmatism, and defective vision. It can be unilateral or bilateral with asymmetric presentation. It starts at puberty and either progresses rapidly to an advanced stage of the disease or stops in case of delayed onset and slow progression. Pediatric keratoconus is more aggressive than in adults and the management protocols differ because of various rationales such as accelerated progression, advanced stage of disease at the time of diagnosis and co-morbidities. It poses a burden to the society as it affects the quality of life, social, and educational development in children. Hence early diagnosis, recognition of progression, and timely intervention with collagen crosslinking is imperative to arrest the worsening. Association with systemic syndromes and ocular comorbidities can be of concern in pediatric keratoconus. Severe ocular allergy when associated hastens progress and complicates timely intervention of crosslinking treatment and compliance to contact lens wear. Keratoplasty in pediatric keratoconus has good outcomes but can encounter frequent suture-related concerns. This article discusses the epidemiology, etiopathogenesis, clinical challenges, and current perspectives of management of pediatric keratoconus.

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.

Immobilization of Growth Factors to Collagen Surfaces Using Pulsed Visible Light

In the treatment of traumatic injuries, burns, and ulcers of the eye, inadequate epithelial tissue healing remains a major challenge. Wound healing is a complex process involving the temporal and spatial interplay between cells and their extracellular milieu. It can be impaired by a variety of causes including infection, poor circulation, loss of critical cells, and/or proteins, and a deficiency in normal neural signaling (e.g., neurotrophic ulcers). Ocular anatomy is particularly vulnerable to lasting morbidity from delayed healing, whether it be scarring or perforation of the cornea, destruction of the conjunctival mucous membrane, or cicatricial changes to the eyelids and surrounding skin. Therefore, there is a major clinical need for new modalities for controlling and accelerating wound healing, particularly in the eye. Collagen matrices have long been explored as scaffolds to support cell growth as both two-dimensional coatings and substrates, as well as three-dimensional matrices. Meanwhile, the immobilization of growth factors to various substrates has also been extensively studied as a way to promote enhanced cellular adhesion and proliferation. Herein we present a new strategy for photochemically immobilizing growth factors to collagen using riboflavin as a photosensitizer and exposure to visible light (∼458 nm). Epidermal growth factor (EGF) was successfully bound to collagen-coated surfaces as well as directly to endogenous collagen from porcine corneas. The initial concentration of riboflavin and EGF as well as the blue light exposure time were keys to the successful binding of growth factors to these surfaces. The photocrosslinking reaction increased EGF residence time on collagen surfaces over 7 days. EGF activity was maintained after the photocrosslinking reaction with a short duration of pulsed blue light exposure. Bound EGF accelerated in vitro corneal epithelial cell proliferation and migration and maintained normal cell phenotype. Additionally, the treated surfaces were cytocompatible, and the photocrosslinking reaction was proven to be safe, preserving nearly 100% cell viability. These results suggest that this general approach is safe and versatile may be used for targeting and immobilizing bioactive factors onto collagen matrices in a variety of applications, including in the presence of live, seeded cells or in vivo onto endogenous extracellular matrix collagen.

Thinner Corneas Appear to Have More Striking Effects of Corneal Collagen Crosslinking in Patients with Progressive Keratoconus

Purpose. To analyze the outcomes and difference after UVA/riboflavin corneal collagen crosslinking (CXL) in four different corneal thickness groups of patients with progressive keratoconus. Methods. Retrospective study. Eyes with progressive keratoconus after CXL were divided into 4 subgroups as follows: group 1, thinnest corneal thickness (TCT) ≤ 400 µm; group 2, 400 µm < TCT ≤ 450 µm; group 3, 450 µm < TCT ≤ 500 µm; group 4, TCT ≥ 500 µm. Baseline, 6-month, and 12-month visual acuity, corneal topography, TCT, and endothelial cell density were evaluated. Results. The analysis included 123 eyes of 101 patients. At 6 and 12 months after CXL, there was a mean improvement about visual acuity and keratometry values in all patients. There was a reduction in the change of maximum keratometry (K_max) with the increase of TCT. After 1 year of treatment, it was 3.04 ± 0.75 D in group 1, 2.38 ± 0.51 D in group 2, 1.57 ± 0.35 D in group 3, and 0.31 ± 0.20 D in group 4. Conclusion. CXL is successful in halting the progression of keratoconus and there was a negative linear correlation between TCT and K_max. Advanced cases of progressive keratoconus seemed to obtain more benefits from the flatting effects of CXL.

Outcomes of corneal collagen crosslinking using a customized epithelial debridement technique in keratoconic eyes with thin corneas

The purpose of the study was to evaluate the outcomes and possible complications of CXL performed with customized epithelial debridement technique to keratoconic corneas with the thinnest pachymetry values less than 400 µm. Nineteen eyes of 19 patients were included. The uncorrected (UCVA) and best corrected visual acuity (BCVA), flattest and steepest keratometric (K) readings, central corneal thickness at the thinnest point (t-CCT), endothelial cell density (ECD) were assessed before and 12 months after CXL. The mean UCVA was increased (p = 0.001), while the mean BCVA did not show any difference (p > 0.05). The mean flattest and steepest K readings were decreased (p = 0.001). No change was observed in the mean t-CCT (p > 0.05). The mean ECD was decreased (p = 0.001). The mean pre-CXL and post-CXL percentages of polymegathism and pleomorphism did not show any significant difference (p > 0.05). CXL performed with customized epithelial debridement technique is successful in halting the progression of keratoconus in corneas thinner than 400 µm after 12 months of treatment. However, significant endothelial cell loss can occur after this procedure.

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.

Corneal collagen crosslinking in keratoconus and other eye disease

Keratoconus is a condition characterized by biomechanical instability of the cornea, presenting in a progressive, asymmetric and bilateral way. Corneal collagen crosslinking (CXL) with riboflavin and Ultraviolet-A (UVA) is a new technique of corneal tissue strengthening that combines the use of riboflavin as a photo sensitizer and UVA irradiation. Studies showed that CXL was effective in halting the progression of keratoconus over a period of up to four years. The published studies also revealed a reduction of max K readings by more than 2 D, while the postoperative spherical equivalent (SEQ) was reduced by an average of more than 1 D and refractive cylinder decreased by about 1 D. The major indication for the use of CXL is to inhibit the progression of corneal ecstasies, such as keratoconus and pellucid marginal degeneration. CXL may also be effective in the treatment and prophylaxis of iatrogenic keratectasia, resulting from excessively aggressive photo ablation. This treatment has been used to treat infectious corneal ulcers with apparent favorable results. Most recent studies demonstrate the beneficial impact of CXL for iatrogenic ecstasies, pellucid marginal degeneration, infectious keratitis, bullous keratopathy and ulcerative keratitis. Several long-term and short-term complications of CXL have been studied and documented. The possibility of a secondary infection after the procedure exists because the patient is subject to epithelial debridement and the application of a soft contact lens. Formation of temporary corneal haze, permanent scars, endothelial damage, treatment failure, sterile infiltrates, bullous keratopathy and herpes reactivation are the other reported complications of this procedure.

Intraoperative and postoperative corneal thickness change after collagen crosslinking therapy

PURPOSE

To assess intraoperative and postoperative changes in corneal thickness subsequent to riboflavin-UVA (collagen crosslinking [CXL]) treatment.

METHODS

Forty-one eyes of 41 patients (mean age 27.97 ± 6.97 years) were treated with CXL technique. During treatment, isotonic riboflavin was instilled and corneal thickness measurements were obtained at the cornea apex, the thinnest point, and the pupil center at 15 and 30 minutes, then 3 days, 1 week, and 1, 3, 6, and 12 months after surgery using Pentacam HR and an ultrasound pachymeter.

RESULTS

A decrease in corneal thickness was detected 15 minutes intraoperatively with a value of 108.95 ± 48.6 µm, and 112.35 ± 47.3 µm at 30 minutes (p<0.001). Three days after the operation, no deviation was found from the initial values (p = 0.17). No further changes were detected during the follow-up period.

CONCLUSIONS

Isotonic riboflavin solution used during CXL treatment resulted in a significant decrease in corneal thickness, but its effect had disappeared by postoperative day 3.

Corneal crosslinking with riboflavin and ultraviolet A. I. Principles

Changes in the biomechanical properties of the human cornea play an important role in the pathogenesis of corneal ectatic diseases. Biomechanical investigation shows significant differences between human ectatic corneas and normal corneas, including decreased stiffness and reduction of collagen crosslinks in the ectatic cornea. Induction of crosslinks is a well-established procedure in polymer chemistry to increase the elastic modulus of materials. Crosslinking (CXL) in connective tissue can occur during aging and as a side effect of diabetes mellitus. CXL has been used medically to increase stability and reduce the biodegradation of collagen-based biomaterials for bioprostheses. CXL of the cornea using riboflavin and UVA light with a wavelength of 370 nm and a dosage of 5.4 J/cm² is a new approach that increases the mechanical and biochemical stability of stromal tissue. This technique combines the principles of CXL (chemical and nonenzymatic) and the biochemical mechanisms of photo-oxidative CXL with riboflavin as a photosensitizer. In this review, the enrichment of riboflavin in the stroma by standard (epi-off) and transepithelial (epi-on) CXL is discussed. The theoretical and experimental measurements of the absorption of UV light explain the stronger CXL effect in the anterior stroma and its importance for the prevention of damage to the endothelial cells. UV devices are described. Changes of the physical properties after CXL, as well as the cellular changes, are discussed. From these basic investigations, treatment parameters for effective and safe CXL are identified.

The effect of standard and transepithelial ultraviolet collagen cross-linking on human corneal nerves: an ex vivo study

PURPOSE

To evaluate the early effect of standard and transepithelial collagen cross-linking on human corneal nerves in donor eyes by ex vivo confocal microscopy and acetylcholinesterase staining.

DESIGN

Experimental laboratory investigation.

METHODS

Eight human eye bank corneal buttons (mean age, 73.6 years) were included. Ultraviolet A collagen cross-linking was performed postmortem on 3 corneas with the standard protocol involving epithelial debridement and 4 corneas by the transepithelial approach. One cornea served as a control. Corneal nerves were evaluated using confocal microscopy and acetylcholinesterase histology.

RESULTS

Confocal microscopy demonstrated the absence of subbasal nerves in corneas treated by the standard technique. These nerves were preserved in corneas treated by the transepithelial approach. Stromal nerves were visible in both groups. Histology of corneas treated by the standard technique revealed localized swellings of the stromal nerves with disruption of axonal membrane and loss of axonal continuity within the treatment zone. These changes were absent in corneas treated by the transepithelial approach.

CONCLUSIONS

This study highlights the immediate effects of collagen cross-linking on the corneal nerves in an ex vivo model. The absence of subbasal nerves in the early phase of treatment appears to be attributable mainly to mechanical removal of epithelium, rather than ultraviolet light-induced damage. Localized swelling of the stromal nerves was the main difference between the 2 treatment protocols. Further research on laboratory animals would be necessary to verify these changes over a specified time course without the super-addition of postmortem changes.

Collagen cross-linking: current status and future directions

Collagen cross-linking (CXL) using UVA light and riboflavin (vitamin B2) was introduced as a clinical application to stabilize the cornea by inducing cross-links within and between collagen fibers. CXL has been investigated extensively and has been shown clinically to arrest the progression of keratoconic or post-LASIK ectasia. With its minimal cost, simplicity, and proven positive clinical outcome, CXL can be regarded as a useful approach to reduce the number of penetrating keratoplasties performed. Small case series have also indicated that CXL is beneficial in corneal edema by reducing stromal swelling behavior and in keratitis by inhibiting pathogen growth. Despite these encouraging results, CXL remains a relatively new method that is potentially associated with complications. Aspects such as side effects and recurrence rates have still to be elucidated. In light of the growing interest in CXL, our paper summarizes present knowledge about this promising approach. We have intentionally endeavored to include the more relevant studies from the recent literature to provide an overview of the current status of CXL.

Corneal cross-linking and safety issues

Purpose:

To compile the safety aspects of the corneal collagen cross-linking (CXL) by means of the riboflavin/UVA (370 nm) approach.

Materials and Methodology:

Analysis of the current treatment protocol with respect to safety during CXL.

Results:

The currently used UVA dose density of 5.4 J/cm² and the corresponding irradiance of 3 mW/cm² are below the known damage thresholds of UVA for the corneal endothelium, lens, and retina. Regarding the photochemical damages due to the free radicals the damage threshold for endothelial cells is 0.35 mW/cm². In a 400μm thick corneal stroma saturated with riboflavin, the irradiance at the endothelial level is about 0.18 mW/cm², which is a factor of 2 smaller than the damage threshold.

Conclusion:

As long as the corneal stroma treated has a minimal thickness of 400 microns (as recommended), neither corneal endothelium nor deeper structures such as lens and retina will suffer any damages. The light source should provide a homogenous irradiance avoiding hot spots.