Corneal collagen crosslinking in patients treated with dextran versus isotonic hydroxypropyl methylcellulose (HPMC) riboflavin solution: a retrospective analysis

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.

Analysis of Biomechanical Response After Corneal Crosslinking with Different Fluence Levels in Porcine Corneas

PURPOSE

To evaluate corneal stiffening of porcine corneas induced by corneal crosslinking (CXL) with constant irradiance as a function of total fluence.

METHODS

Ninety corneas from freshly enucleated porcine eyes were divided into five groups of 18 eyes. Groups 1-4 underwent epi-off CXL using a dextran-based riboflavin solution and an irradiance of 18 mW/cm2, group 5 served as the control group. Groups 1 to 4 were treated with a total fluence of 20, 15, 10.8, and 5.4 J/cm2, respectively. Thereafter, biomechanical measurements were performed on 5 mm wide and 6 mm long strips using an uniaxial material tester. Pachymetry measurements were performed on each cornea.

RESULTS

At 10% strain, the stress was 76, 56, 52, and 31% higher in groups 1-4, respectively compared to the control group. The Young's modulus was 2.85 MPa for group 1, 2.53 MPa for group 2, 2.46 MPa for group 3, 2.12 MPa for group 4, and 1.62 MPa for the control group. The difference between groups 1 to 4 and the control group 5 were statistically significant (p = <0.001; p = <0.001; p = <0.001; p = 0.021). In addition, group 1 showed significantly more stiffening than group 4 (p = <0.001), no other significant differences were found. Pachymetry measurements revealed no statistically significant differences among the five groups.

CONCLUSION

Additional mechanical stiffening can be achieved by increasing the fluence of the CXL. There was no threshold detected up to 20 J/cm2. A higher fluence could compensate the weaker effect of accelerated or epi-on CXL procedures.

Innovations in Corneal Crosslinking

PURPOSE

Corneal Crosslinking (CXL) strengthens the keratoconus cornea and prevents further disease progression. Modified crosslinking protocols and different riboflavin solutions have been proposed to optimize the procedure and improve treatment success.

METHODS

PubMed research of relevant publications and report of own experiences with different CXL protocols.

RESULTS

Accelerated CXL shows comparable efficiency with shorter surgery time and similar complication rates. Customized CXL provides improved results with faster epithelial healing. CXL in a hyperoxic environment seems to be a safe and effective transepithelial alternative with presumably less complications and fewer side effects. Thin corneas (<400 µm) can be treated safely by corneal swelling using hypoosmolar riboflavin solutions and reducing the applied UV-energy. The combination of CXL with photorefractive keratectomy (PRK) can be considered in patients with contact lens intolerance improving visual acuity, however, with increased risk of visual loss compared to CXL alone. Two-Photon (2Ph) CXL is a promising new technology enabling three-dimensional CXL.

DISCUSSION

Recently developed CXL protocols offer advantages over the standard "Dresden-protocol" and should be considered in patients with progressive keratectasia.

The Impact of Different Rose Bengal Formulations on Corneal Thickness and the Efficacy of Rose Bengal/Green Light Corneal Cross-linking in the Rabbit Eye

PURPOSE

To examine central corneal thickness (CCT) changes during in vivo rose bengal-green light corneal cross-linking (RG-CXL) and compare the CXL efficacy of different rose bengal formulations.

METHODS

After epithelium removal, the right eyes of rabbits were immersed in rose bengal solution prepared by different solvents (water, phosphate buffered saline, dextran, and hydroxypropyl methylcellulos [HPMC]) for 2 or 20 minutes, then the rose bengal distribution in the corneal stroma was analyzed by confocal fluorescence detection. During the RG-CXL process, the CCT was measured at seven time points. The left eyes served as the untreated control group. Corneal enzymatic resistance and corneal biomechanics were tested to compare the RG-CXL efficacy.

RESULTS

The rose bengal infiltration depths were 120 and 200 µm for the 2- and 20-minute groups, respectively. CCT increased significantly after infiltration, then decreased significantly in the first 200 seconds of irradiation and decreased slowly for the next 400 seconds. The CCT of the 20-minute groups was significantly thicker than that of the 2-minute groups (P < .0001). All RG-CXL treatments improved the corneal enzymatic resistance and corneal biomechanics, with the effects being greater in the 20-minute groups. The inclusion of 1.1% HPMC in the rose bengal formulation helped to maintain CCT during irradiation while not affecting either the infiltration of rose bengal or the efficacy of RG-CXL.

CONCLUSIONS

Within the range studied, RG-CXL efficacy increased with infiltration time. The incorporation of a 20-minute infiltration of 0.1% rose bengal-1.1% HPMC into the RG-CXL procedure may further improve the safety of the treatment and its prospects for clinical use. [J Refract Surg. 2022;38(7):450-458.].

Short- and long-term safety and efficacy of corneal collagen cross-linking in progressive keratoconus: A systematic review and meta-analysis of randomized controlled trials

PURPOSE

The purpose of the study is to evaluate the safety and outcomes of corneal collagen cross-linking (CXL) and different CXL protocols in progressive keratoconus (PK) population at short and long-term.

MATERIALS AND METHODS

A systematic review and meta-analysis was conducted. A total of eight literature databases were searched (up to February 15, 2022). Randomized controlled trials (RCTs) comparing CXL versus placebo/control or comparing different CXL protocols in the PK population were included. The primary objective was assessment of outcomes of CXL versus placebo and comparison of different CXL protocols in terms of maximum keratometry (Kmax) or Kmax change from baseline (Δ), spherical equivalent, best corrected visual acuity (BCVA), and central corneal thickness (CCT) in both at short term (6 months) and long term (1^st, 2^nd, and 3^rd year or more). The secondary objective was comparative evaluation of safety. For the meta-analysis, the RevMan5.3 software was used.

RESULTS

A total of 48 RCTs were included. Compared to control, CXL was associated with improvement in Δ Kmax at 1 year (4 RCTs, mean difference [MD], -1.78 [-2.71, -0.86], P = 0.0002) and 2 and 3 years (1 RCT); ΔBCVA at 1 year (7 RCTs, -0.10 [-0.14, -0.06], P < 0.00001); and Δ CCT at 1 year (2 RCTs) and 3 years (1 RCT). Compared to conventional CXL (C-CXL), deterioration in Δ Kmax, ΔBCVA and endothelial cell density was seen at long term in the transepithelial CXL (TE-CXL, chemical enhancer). Up to 2 years, there was no difference between TE-CXL using iontophoresis (T-ionto) and C-CXL. At 2 and 4 years, C-CXL performed better compared to accelerated CXL (A-CXL) in terms of improving Kmax. Although CCT was higher in the A-CXL arm at 2 years, there was no difference at 4 years. While exploring heterogeneity among studies, selection of control eye (fellow eye of the same patient vs. eye of different patient) and baseline difference in Kmax were important sources of heterogeneity.

CONCLUSION

CXL outperforms placebo/control in terms of enhancing Kmax and CCT, as well as slowing disease progression over time (till 3 years). T-ionto protocol, on the other hand, performed similarly to C-CXL protocol up to 2 years.

A Bioprinted Tubular Intestine Model Using a Colon-Specific Extracellular Matrix Bioink

Tremendous advances have been made toward accurate recapitulation of the human intestinal system in vitro to understand its developmental process, and disease progression. However, current in vitro models are often confined to 2D or 2.5D microarchitectures, which is difficult to mimic the systemic level of complexity of the native tissue. To overcome this problem, physiologically relevant intestinal models are developed with a 3D hollow tubular structure using 3D bioprinting strategy. A tissue-specific biomaterial, colon-derived decellularized extracellular matrix (Colon dECM) is developed and it provides significant maturation-guiding potential to human intestinal cells. To fabricate a perfusable tubular model, a simultaneous printing process of multiple materials through concentrically assembled nozzles is developed and a light-activated Colon dECM bioink is employed by supplementing with ruthenium/sodium persulfate as a photoinitiator. The bioprinted intestinal tissue models show spontaneous 3D morphogenesis of the human intestinal epithelium without any external stimuli. In consequence, the printed cells form multicellular aggregates and cysts and then differentiate into several types of enterocytes, building junctional networks. This system can serve as a platform to evaluate the effects of potential drug-induced toxicity on the human intestinal tissue and create a coculture model with commensal microbes and immune cells for future therapeutics.

Biomechanical Response After Corneal Cross-linking With Riboflavin Dissolved in Dextran Solution Versus Hydroxypropyl Methylcellulose

PURPOSE

To evaluate corneal stiffening in porcine eyes induced by corneal cross-linking (CXL) using riboflavin dissolved in either aqueous dextran or hydroxypropyl methylcellulose (HPMC) solution.

METHODS

Fifty-one porcine corneas were divided into three groups of 17 each. After deepithelialization, the first (Dresden) group was treated for 30 minutes with 0.1% riboflavin (riboflavin-5-monophosphate in 0.9% NaCl) dissolved in hypertonic 20% dextran and the second (HPMC) group for 30 minutes with isotonic solution containing 0.1% riboflavin and 1.1% HPMC. Thereafter, corneas of both groups were irradiated using 5.4 J/cm² (irradiance of 9 mW/cm² for 10 minutes; 10*9). After CXL, all corneas were kept in an isotonic 16% dextran bath for 2 hours to obtain an equal hydration state. The third group served as the control group. Stress-strain measurements were performed on 5-mm-wide strips. Corneal thickness was monitored throughout the entire course of the experiments.

RESULTS

The required stress for a 10% strain was increased by 83% in the Dresden group and 35% in the HPMC group compared to the control group. Resultant Young's modulus (at 10% strain) was 2.53 ± 0.73, 1.87 ± 0.50, and 1.47 ± 0.44 Pa for the Dresden, HPMC, and control groups, respectively. The differences between the Dresden and HPMC groups (P = .006), the Dresden and control groups (P < .001), and the HPMC and control groups (P = .014) were statistically significant. Pachymetry measurements showed a significantly increased corneal thickness after application of HPMC compared with the Dresden group (P = .002) and control group (P = .041).

CONCLUSIONS

The biomechanical stiffening of the cornea by CXL can be achieved using dextran- and HPMC-based riboflavin solutions in porcine corneas with an application time of 30 minutes. Dextran-based riboflavin solutions seem to induce a slightly stronger biomechanical response in this setting. HPMC solutions induce less thinning than dextran solutions. [J Refract Surg. 2021;37(9):631-635.].

The intra-operative corneal pachymetry changes during accelerated corneal cross-linking in progressive keratoconus patients with thin corneas

Purpose

To report the intra-operative corneal pachymetry changes during accelerated corneal cross-linking (A-CXL) in progressive keratoconus patients with thin corneas.

Methods

Thirty-six eyes (mean age: 22.26 ± 4.02 years) with progressive keratoconic thin corneas (< 400 µm without epithelium) who underwent A- CXL with ultraviolet A (9 mW/cm2) using isotonic RF-HPMC (riboflavin5-phosphate 0.1% with 1.1% hydroxypropyl methycellulose/Mediocross M®) were included in this retrospective study. Intra-operative corneal pachymetric changes were noted before the procedure, after removal of epithelium, after RF-HPMC instillation, before and after UV irradiation. The mean of corneal pachymetric values were compared statistically.

Results

The mean corneal pachymetry reduced from 415.72±29.66 µm to 369.50±23.45 µm after removal of the epithelium (p<0.05). After the application of RF-HPMC solution the mean TCP increased to 412.89±26.94 µm. Statistically significant increase was observed in TCP after saturation with RF-HPMC (p=0.001). The mean corneal pachymetry was before and after UV-A irradiation respectively 419.86±10.41 µm, 417.47±8.25 µm (p>0.05).

Conclusion

Isotonic RF-HPMC lead to a significant increase in intra-operative mean TCP. RF-HPMC seems to be a favorable riboflavin option in keratoconus patients with thin corneas.

Corneal Cross-Linking for Keratoconus: Current Knowledge and Practice and Future Trends

Corneal collagen cross-linking (CXL) with riboflavin is an accepted universal standard of care for our keratoconus patients with progressive disease. It has been a game changer in how we manage keratoconus. Early diagnosis and treatment is essential in paediatric patients as younger patients progress more rapidly and have poorer transplant outcomes. There is an ongoing debate around standard, accelerated, and transepithelial protocols of CXL, the role of CXL, and the combination of laser refractive surgery. Future developments will improve CXL safety and efficacy and the scope of utilization, but we must be careful not to leap too far ahead with clinical applications before publication of basic science research and good clinical results with standardized protocols.