An in vitro investigation into the impact of corneal rinsing on riboflavin/UVA corneal cross-linking.

BACKGROUND: Corneal cross-linking (CXL) using riboflavin and ultraviolet-A light (UVA) is a treatment used to prevent progression of keratoconus. This ex vivo study assesses the impact on CXL effectiveness, as measured by tissue enzymatic resistance and confocal microscopy, of including a pre-UVA corneal surface rinse with balanced salt solution (BSS) as part of the epithelium-off treatment protocol. METHODS: Sixty-eight porcine eyes, after epithelial debridement, were assigned to six groups in three experimental runs. Group 1 remained untreated. Groups 2-6 received a 16-min application of 0.1% riboflavin/Hydroxypropyl methylcellulose (HPMC) drops, after which Group 3 was exposed to 9 mW/cm2 UVA for 10 min, and Groups 4-6 underwent corneal surface rinsing with 0.25 mL, 1 mL or 10 mL BSS followed by 9 mW/cm2 UVA exposure for 10 min. Central corneal thickness (CCT) was recorded at each stage. Central 8.0 mm corneal buttons from all eyes were subjected to 0.3% collagenase digestion at 37 °C and the time required for complete digestion determined. A further 15 eyes underwent fluorescence confocal microscopy to assess the impact of rinsing on stromal riboflavin concentration. RESULTS: Application of riboflavin/HPMC solution led to an increase in CCT of 73 ± 14 µm (P < 0.01) after 16 min. All CXL-treated corneas displayed a 2-4 fold greater resistance to collagenase digestion than non-irradiated corneas. There was no difference in resistance between corneas that received no BSS rinse and those that received a 0.25 mL or 1 mL pre-UVA rinse, but each showed a greater level of resistance than those that received a 10 mL pre-UVA rinse (P < 0.05). Confocal microscopy demonstrated reduced stromal riboflavin fluorescence after rinsing. CONCLUSIONS: All protocols, with and without rinsing, were effective at enhancing the resistance to collagenase digestion, although resistance was significantly decreased, and stromal riboflavin fluorescence reduced with a 10 mL rinse. This suggests that a 10 mL surface rinse can reduce the efficacy of CXL through the dilution of the stromal riboflavin concentration.

Structural control of corneal transparency, refractive power and dynamics.

The cornea needs to be transparent to visible light and precisely curved to provide the correct refractive power. Both properties are governed by its structure. Corneal transparency arises from constructive interference of visible light due to the relatively ordered arrangement of collagen fibrils in the corneal stroma. The arrangement is controlled by the negatively charged proteoglycans surrounding the fibrils. Small changes in fibril organisation can be tolerated but larger changes cause light scattering. Corneal keratocytes do not scatter light because their refractive index matches that of the surrounding matrix. When activated, however, they become fibroblasts that have a lower refractive index. Modelling shows that this change in refractive index significantly increases light scatter. At the microscopic level, the corneal stroma has a lamellar structure, the parallel collagen fibrils within each lamella making a large angle with those of adjacent lamellae. X-ray scattering has shown that the lamellae have preferred orientations in the human cornea: inferior-superior and nasal-temporal in the central cornea and circumferential at the limbus. The directions at the centre of the cornea may help withstand the pull of the extraocular muscles whereas the pseudo-circular arrangement at the limbus supports the change in curvature between the cornea and sclera. Elastic fibres are also present; in the limbus they contain fibrillin microfibrils surrounding an elastin core, whereas at the centre of the cornea, they exist as thin bundles of fibrillin-rich microfibrils. We present a model based on the structure described above that may explain how the cornea withstands repeated pressure changes due to the ocular pulse.

Membrane structures and functional correlates in the bi-segmented eye lens of the cephalopod.

The cephalopod eye lens is unique because it has evolved as a compound structure with two physiologically distinct segments. However, the detailed ultrastructure of this lens and precise optical role of each segment are far from clear. To help elucidate structure-function relationships in the cephalopod lens we conducted multiple structural investigations on squid. Synchrotron x-ray scattering and transmission electron microscopy disclose that an extensive network of structural features that resemble cell membrane complexes form a substantial component of both anterior and posterior lens segments. Optically, the segments are distinct, however, and Talbot interferometry indicates that the posterior segment possesses a noticeably higher refractive index gradient. We propose that the hitherto unrecognised network of membrane structures in the cephalopod lens has evolved to act as an essential conduit for the internal passage of ions and other metabolic agents through what is otherwise a highly dense structure owing to a very high protein concentration.

Development of graphitic carbon nitride quantum dots-based oxygen self-sufficient platforms for enhanced corneal crosslinking.

Keratoconus, a disorder characterized by corneal thinning and weakening, results in vision loss. Corneal crosslinking (CXL) can halt the progression of keratoconus. The development of accelerated corneal crosslinking (A-CXL) protocols to shorten the treatment time has been hampered by the rapid depletion of stromal oxygen when higher UVA intensities are used, resulting in a reduced cross-linking effect. It is therefore imperative to develop better methods to increase the oxygen concentration within the corneal stroma during the A-CXL process. Photocatalytic oxygen-generating nanomaterials are promising candidates to solve the hypoxia problem during A-CXL. Biocompatible graphitic carbon nitride (g-C3N4) quantum dots (QDs)-based oxygen self-sufficient platforms including g-C3N4 QDs and riboflavin/g-C3N4 QDs composites (RF@g-C3N4 QDs) have been developed in this study. Both display excellent photocatalytic oxygen generation ability, high reactive oxygen species (ROS) yield, and excellent biosafety. More importantly, the A-CXL effect of the g-C3N4 QDs or RF@g-C3N4 QDs composite on male New Zealand white rabbits is better than that of the riboflavin 5'-phosphate sodium (RF) A-CXL protocol under the same conditions, indicating excellent strengthening of the cornea after A-CXL treatments. These lead us to suggest the potential application of g-C3N4 QDs in A-CXL for corneal ectasias and other corneal diseases.