Theranostic-Guided UV-A Light Corneal Wavefront Photo-Reshaping for Presbyopia Correction: A Preclinical Study

This study investigated the effect of a theranostic-guided UV-A light corneal photo-reshaping technique on corneal elevation and wavefront aberration (WA) in human donor eyes. A specialized platform, combining UV-A light with corneal iontophoresis for controlled, patterned, riboflavin delivery, was used for both distribution assessment and concentration-driven photopolymerization of corneal proteins. In all cases, a consistent riboflavin concentration gradient, with lower levels in the central prepupillary zone, was recorded. Corneal topography and WA measurements showed significant corneal steepening and smooth wavefront shaping, respectively, with a delay in the central 2.0 mm of the WA and advancement in the surrounding zone, as well as a 50% reduction in corneal spherical aberration over a 5.0 mm pupil size. Notably, the corneal optical quality, measured via modulation transfer function (MTF), remained stable. This incision-free approach demonstrated the potential to extend focal range without compromising distance vision, presenting a new solution for presbyopia correction.

Spatial targeted delivery of riboflavin with a controlled corneal iontophoresis delivery system in theranostic-guided UV-A light photo-therapy

Seven human donor eye globes underwent corneal cross-linking using theranostic UV-A device with accessory corneal iontophoresis system for patterned delivery of a 0.22% riboflavin solution. Theranostic-guided UV-A light illumination assessed riboflavin distribution and treated corneas at 10 mW/cm2 for 9 min with a 5.0-mm beam size. Corneal topography maps were taken at baseline and 2-h post-treatment. Analysis utilized corneal topography elevation data, with results showing controlled riboflavin delivery led to a consistent gradient, with 40% higher levels centrally (248 ± 79 μg/cm3) than peripherally (180 ± 72 μg/cm3 at ±2.5 mm from the center). Theranostic-guided UV-A light irradiation resulted in significant changes in corneal topography, with a decrease in best-fit sphere value (-0.7 ± 0.2 D; p < 0.001) and consistent downward shift in corneal elevation map (-11.7 ± 3.7 μm). The coefficient of variation was 2.5%, indicating high procedure performance in achieving significant and reliable corneal flattening.

Computational modeling of corneal and scleral collagen photocrosslinking

Scleral photocrosslinking is increasingly investigated for treatment of myopia and glaucoma. In this study a computational model was developed to predict crosslinking efficiency of visible/near infrared photosensitizers in the sclera. Photocrosslinking was validated against riboflavin corneal crosslinking experimental studies and subsequently modeled for the sensitizer, methylene blue, administered by retrobulbar injection to the posterior sclera and irradiated with a transpupillary light beam. Optimal ranges were determined for treatment parameters including light intensity, methylene blue concentration, injection volume, and inspired oxygen concentration. Additionally, sensitivity of crosslinking to various parameters was quantified. The most sensitive parameters (in order of greatest to least sensitive) were tissue parameters (including scleral thickness and choroidal melanin concentration), treatment parameters (including treatment duration and inspired oxygen concentration), and sensitizer parameters (including triplet quantum yield).

A Critical Review for Synergic Kinetics and Strategies for Enhanced Photopolymerizations for 3D-Printing and Additive Manufacturing

The synergic features and enhancing strategies for various photopolymerization systems are reviewed by kinetic schemes and the associated measurements. The important topics include (i) photo crosslinking of corneas for the treatment of corneal diseases using UVA-light (365 nm) light and riboflavin as the photosensitizer; (ii) synergic effects by a dual-function enhancer in a three-initiator system; (iii) synergic effects by a three-initiator C/B/A system, with electron-transfer and oxygen-mediated energy-transfer pathways; (iv) copper-complex (G1) photoredox catalyst in G1/Iod/NVK systems for free radical (FRP) and cationic photopolymerization (CP); (v) radical-mediated thiol-ene (TE) photopolymerizations; (vi) superbase photogenerator based-catalyzed thiol-acrylate Michael (TM) addition reaction; and the combined system of TE and TM using dual wavelength; (vii) dual-wavelength (UV and blue) controlled photopolymerization confinement (PC); (viii) dual-wavelength (UV and red) selectively controlled 3D printing; and (ix) three-wavelength selectively controlled in 3D printing and additive manufacturing (AM). With minimum mathematics, we present (for the first time) the synergic features and enhancing strategies for various systems of multi-components, initiators, monomers, and under one-, two-, and three-wavelength light. Therefore, this review provides not only the bridging between modeling and measurements, but also guidance for further experimental studies and new applications in 3D printings and additive manufacturing (AM), based on the innovative concepts (kinetics/schemes).

Multi-physics modeling and finite element formulation of corneal UV cross-linking

The UV cross-linking technique applied to the cornea is a popular and effective therapy for eye diseases such as keratoconus and ectatic disorders. The treatment strengthens the cornea by forming new cross-links via photochemical reactions and, in turn, prevents the disease from further developing. To better understand and capture the underlying mechanisms, we develop a multi-physics model that considers the migration of the riboflavin (i.e., the photo-initializer), UV light absorption, the photochemical reaction that forms the cross-links, and biomechanical changes caused by changes to the microstructure. Our model is calibrated to a set of nanoindentation tests on UV cross-linked corneas from the literature. Additionally, we implement our multi-physics model numerically into a commercial finite element software. We also compare our simulation against a set of inflation tests from the literature. The simulation capability allows us to make quantitative predictions of a therapy's outcomes in full 3-D, based on the actual corneal geometry; it also helps medical practitioners with surgical planning.

Photochemical Collagen Cross-Linking Reverses Elastase-Induced Mechanical Degradation of Upper Eyelid Tarsus

INTRODUCTION

The floppy eyelid syndrome describes an eyelid disorder characterized by floppy tarsal plates that may be caused by a loss of elastin. The authors attempted to create floppy eyelids by digesting elastin from cadaveric tarsus and then treated them with cross-linking using ultraviolet A and riboflavin.

METHODS

Nine right and 9 left upper eyelids were excised from cadavers. Four vertical strips of central tarsus were removed from each eyelid. One strip of tarsus from each eyelid was treated with 10 units/ml of elastase for 2 hours. Another tarsal strip from each eyelid was immersed in normal saline for 2 hours (control). A third strip from the same eyelid was cross-linked using ultraviolet A at 6 mW/cm for 18 minutes. Finally, a fourth strip of tarsus was cross-linked in the same manner following treatment with elastase for 2 hours. A microtensile load cell was used to measure the Young modulus (stiffness) of each tissue.

RESULTS

Mean (standard deviation) Young modulus for controls (18.9 ± 3.6 MPa) was significantly higher than samples treated with elastase alone (6.6 ± 3.8 MPa, p <0.01). Samples that were treated with cross-linking after elastase had a mean (standard deviation) Young modulus of 26 ± 2.3 MPa, while those treated with cross-linking alone had a mean (standard deviation) Young modulus of 34 ± 0.15 MPa. The differences in stiffness between all groups were significant (p <0.01).

DISCUSSION

Treatment with elastase significantly reduces the stiffness of tarsal plates. This effect is reversed by cross-linking, raising the possibility of using this modality for the treatment of FES.

Modeling the Kinetics, Curing Depth, and Efficacy of Radical-Mediated Photopolymerization: The Role of Oxygen Inhibition, Viscosity, and Dynamic Light Intensity

Kinetic equations for a modeling system with type-I radical-mediated and type-II oxygen-mediated pathways are derived and numerically solved for the photopolymerization efficacy and curing depth, under the quasi-steady state assumption, and bimolecular termination. We show that photopolymerization efficacy is an increasing function of photosensitizer (PS) concentration (C ₀) and the light dose at transient state, but it is a decreasing function of the light intensity, scaled by [C ₀/I ₀]^0.5 at steady state. The curing (or cross-link) depth is an increasing function of C ₀ and light dose (time × intensity), but it is a decreasing function of the oxygen concentration, viscosity effect, and oxygen external supply rate. Higher intensity results in a faster depletion of PS and oxygen. For optically thick polymers (>100 um), light intensity is an increasing function of time due to PS depletion, which cannot be neglected. With oxygen inhibition effect, the efficacy temporal profile has an induction time defined by the oxygen depletion rate. Efficacy is also an increasing function of the effective rate constant, K = k'/ k T 0 . 5 , defined by the radical producing rate (k') and the bimolecular termination rate (k _T). In conclusion, the curing depth has a non-linear dependence on the PS concentration, light intensity, and dose and a decreasing function of the oxygen inhibition effect. Efficacy is scaled by [C ₀/I ₀]^0.5 at steady state. Analytic formulas for the efficacy and curing depth are derived, for the first time, and utilized to analyze the measured pillar height in microfabrication. Finally, various strategies for improved efficacy and curing depth are discussed.

A Review of Structural and Biomechanical Changes in the Cornea in Aging, Disease, and Photochemical Crosslinking

The study of corneal biomechanics is motivated by the tight relationship between biomechanical properties and visual function within the ocular system. For instance, variation in collagen fibril alignment and non-enzymatic crosslinks rank high among structural factors which give rise to the cornea's particular shape and ability to properly focus light. Gradation in these and other factors engender biomechanical changes which can be quantified by a wide variety of techniques. This review summarizes what is known about both the changes in corneal structure and associated changes in corneal biomechanical properties in aging, keratoconic, and photochemically crosslinked corneas. In addition, methods for measuring corneal biomechanics are discussed and the topics are related to both clinical studies and biomechanical modeling simulations.

Customized Corneal Cross-linking Using Different UVA Beam Profiles

PURPOSE

To evaluate the performance of different customized corneal cross-linking (CXL) methods.

METHODS

This was a single-center interventional, prospective, longitudinal case series. Four different customized CXL methods were evaluated in keratoconic eyes: (1) uniform (uniform intensity ultraviolet-A [UVA] beam [9 mW/cm²] for 10 minutes) (n = 12 eyes); (2) sector axial map (sector-based UVA irradiation) (n = 12 eyes); (3) ring axial map (concentric rings of UVA beam intensity centered at the steepest curvature of the anterior axial map) (n = 12 eyes); and ring tangential map (same as the ring axial map but centered at the steepest curvature of the anterior tangential map) (n = 14 eyes). Peak UVA energy density in the sector and ring axial map (and ring tangential map) protocols did not exceed 15.0 and 10.8 J/cm², respectively. A 0.1% riboflavin solution was applied after epithelium removal. Corneal tomography and visual acuity were assessed before and 6 months after CXL.

RESULTS

Average and peak energy density was lowest in the ring tangential protocol and highest in the sector axial map group (P < .001). Treated area was lowest in the ring tangential map group and highest in the uniform group (P < .001). Decrease in curvature was similar among the uniform, sector axial map, and ring axial map groups (P < .05). The ring tangential map group had the greatest decrease in curvature per unit energy dose to the cornea (P < .05). Improvement in uncorrected (0.081 ± 0.056 logMAR) and corrected (0.041 ± 0.026 logMAR) distance visual acuity per unit energy density was greatest in the ring tangential map group (P > .05).

CONCLUSIONS

When normalized to the average energy density, the ring tangential map protocol appeared to provide maximum flattening and improvement in visual acuity. Further studies with larger sample sizes are needed to validate the findings of this pilot study. [J Refract Surg. 2017;33(10):676-682.].

Modeling the efficacy profiles of UV-light activated corneal collagen crosslinking

Objective

Analysis of the crosslink time, depth and efficacy profiles of UV-light-activated corneal collagen crosslinking (CXL).

Methods

A modeling system described by a coupled dynamic equations are numerically solved and analytic formulas are derived for the crosslinking time (T*) and crosslinking depth (z*). The z-dependence of the CXL efficacy is numerically produced to show the factors characterizing the profiles.

Results

Optimal crosslink depth (z*) and maximal CXL efficacy (Ceff) have opposite trend with respective to the UV light intensity and RF concentration, where z* is a decreasing function of the riboflavin concentration (C₀). In comparison, Ceff is an increasing function of C₀ and the UV exposure time (for a fixed UV dose), but it is a decreasing function of the UV light intensity. CXL efficacy is a nonlinear increasing function of [C₀/I₀]^-0.5 and more accurate than that of the linear theory of Bunsen Roscoe law. Depending on the UV exposure time and depth, the optimal intensity ranges from 3 to 30 mW/cm² for maximal CXL efficacy. For steady state (with long exposure time), low intensity always achieves high efficacy than that of high intensity, when same dose is applied on the cornea.

Conclusions

The crosslinking depth (z*) and the crosslinking time (T*) have nonlinear dependence on the UV light dose and the efficacy of corneal collagen crosslinking should be characterized by both z* and the efficacy profiles. A nonlinear scaling law is needed for more accurate protocol.