Accelerated cross-linking technique using a protective corneal donor flap in the treatment of progressive keratoconus on «thin» corneas

Purpose. To develop a method of accelerated ultraviolet crosslinking with an initial corneal thickness equal to or less than 400 microns using a protective donor corneal flap. Definition of safety and effectiveness of this method in the treatment of progressive keratoconus. Material and methods. 20 patients (20 eyes) with a diagnosis of progressive keratoconus 2–3 stages were included in the study. The accelerated crosslinking was carried out with an IROC-VX-2000 device (Switzerland). A protective donor corneal flap was cut out by using a Femto LDV Z8 femtosecond laser (Ziemer, Switzerland). The thickness of the protective flap of the donor cornea was determined as the difference between 450 μm and the obtained value of the patient's pachymetry in 30 minutes of saturation with Dextralink in μm. Results. Indicators of uncorrected visual acuity and corrected visual acuity returned to the level of preoperative values and did not tend to decrease until the end of the observation period for 6 months. The average keratometry indices gradually decreased: after 3 months – 50.3 ± 1.5 μm, after 6 months – 48.9 ± 1.8 μm, by 12 months – 46.7 ± 2.1 μm. A decrease in elevation indicators of the anterior and posterior surfaces was marked on elevation maps throughout the observation period. Conclusion. The proposed ultraviolet crosslinking technique using a protective donor corneal flap in patients with corneal thickness 400 μm or less is reproducible. This technique proves its effectiveness and safety and allows to stabilize the condition in patients with progressive keratoconus with adequate visual function. This makes this technique necessary, taking into account the lack of adequate cross-linking technology with a thin cornea. Keywords: progressive keratoconus, thin cornea, ultraviolet crosslinking, protective donor corneal flap.

Up-Dated the Critical Issues of Corneal Cross-Linking (Type-I and II): Safety Dose for Ultra-Thin Cornea, Role of Oxygen and Initiator Regeneration

Aims:To update analytic formulas for the overall efficacy of corneal collagen crosslinking (CXL) including both type-I and oxygen-mediated type-II mechanisms, the role of oxygen and the initiator regeneration. Study Design:modeling the kinetics of CXL in UV light and using riboflavin as the photosensitizer.Place and Duration of Study:New Taipei City, Taiwan, between June, 2021 and July, 2021.Methodology:Coupled kinetic equations are derived under the quasi-steady state condition for the 2-pathway mechanisms of CXL. For type-I CXL, the riboflavin (RF) triplet state [T] may interact directly with the stroma collagen substrate [A] to form radical (R) and regenerate initiator. For type-II process, [T] interacts with oxygen to form a singlet oxygen [1O2]. Both reactive radical (R) and [1O2], can relax to their ground state, or interact with the substrate [A]) for crosslinking. Based on a safety dose, the minimum corneal thickness formula is derived. Results:Our updated theory/modeling showed that oxygen plays a limited and transient role in the process, in consistent with that of Kamave [2]. In contrary, Kling et al [3] believed that type-II is the predominant mechanism, which however conflicting with the epi-on CXL results. For both type-I and type-II, a transient state conversion (crosslink) efficacy in an increasing function of light intensity (or dose), whereas, its steady state efficacy is a deceasing function of light intensity. RF depletion in type-I is compensated by the RF regeneration term (RGE) which is a decreasing function of oxygen. For the case of perfect regeneration case (or when oxygen=0), RF is a constant due to the catalytic cycle. Unlike the conventional Dresden rule of 400 um thickness, thin cornea CXL is still safe as far as the dose is under a threshold dose (E*), based on our minimum thickness formula (Z*). Our formula for thin cornea is also clinically shown by Hafez et al forultra thin (214 nm) CXL. Conclusion: For both type-I and type-II, a transient state conversion (crosslink) efficacy in an increasing function of light intensity (or dose), whereas, its steady state efficacy is a deceasing function of light intensity. Ultra thin cornea is still safe as far as it is under a threshold dose (E*), based on our minimum thickness formula.

Application of the method of accelerated ultraviolet crosslinking of corneal collagen using a customized protective contact lens without an ultraviolet filter in a patient with secondary keratoectasia after Lasik (a clinical case)

Purpose. To evaluate the clinical and functional results of the technique of ultraviolet crosslinking on the thin cornea using a customized soft contact lens without an ultraviolet filter in a patient with secondary corneal ectasia. Material and methods. Under observation was patient S., 32 years old, diagnosed with secondary keratoectasia of the left eye after LASIK surgery, performed in 2008 for moderate myopia. To stabilize the keratectatic process, complicated by the presence of a thin cornea with a thickness of less than 400 microns, an operation was performed: ultraviolet crosslinking of corneal collagen using a customized protective lens without an ultraviolet filter. Results. There were no complications during and after the operation. According to keratorefractometry and keratotopography, there was no progression of the disease after surgery, and no loss of endothelial cells was detected. Conclusion. Thus, conducting ultraviolet crosslinking of corneal collagen using a protective customized lens without an ultraviolet filter is a promising technique that allows you to achieve stabilization of the keratectatic process. Key words: corneal ectasia, thin cornea, UV crosslinking, customized lens.

Relationship between optic nerve head topography and nerve fiber layer thickness with central corneal thickness in patients with primary open-angle glaucoma

Introduction/Objective. In patients with primary open-angle glaucoma (POAG) we explored the relationship between optic nerve head (ONH) topography parameters and retinal nerve fiber layer (RNFL) thickness with central corneal thickness (CCT). Methods. This retrospective study included 97 patients (97 eyes) with primary open-angle glaucoma. Patients were divided into a thin CCT<540?m (45 eyes) and a thick CCT?540?m (52 eyes) group, using ultrasonic pachymeter. Topographic measurements of the ONH parameters and RNFL thickness was performed using optical coherence tomography (OCT). The outcomes were compared with the thin and thick CCT and correlated with the thin CCT of the subjects. Results. There were significantly lower mean intraocular pressure (IOP) (p<0.0001) and CCT (p<0.0001) in patients with thin CCT compared to patients with thick CCT. Statistically significant differences of ONH parameters were found in thin cornea group compared to thick cornea group in: cup/disc area ratio (p<0.03), vertical cup/disc ratio (p<0.01) and rim volume (p<0.01). Statistically significant differences of RNFL thickness were found in thin cornea group compared to thick cornea group in: average (p<0.001), superior (p<0.03), inferior (p<0.03) and nasal (p<0.01). Significant positive correlation was found between thin CCT and OCT parameters in: optic disc area (r=0.429, p=0.003), cup/disc area ratio (r=0.287, p=0.05), horizontal cup/disc ratio (r=0.472, p<0.001), vertical cup/disc ratio (r=0.578, p<0.001), average RNFL (r=0.796, p<0.001), superior RNFL (r=0.665, p<0.001), inferior RNFL (r=0.650, p<0.001), nasal RNFL (r=0.611, p<0.001) and temporal RNFL thickness (r=0.601, p<0.001). Conclusion. POAG patients with thin cornea will probably develop larger glaucoma changes than those with a thicker cornea. Ultrasonic pachymetry measurements of CCT and OCT analysis of ONH topography parameters and RNFL thickness provide significant information in early diagnosis and monitoring progression of POAG.