A Critical Assessment of Friedenwald’s Technique for Estimating the Coefficient of Rigidity of the Cornea

Purpose

To determine if Friedenwald's technique for estimating the coefficient of corneal rigidity (Ko, units mmHg/μL), could differentiate between the cornea in keratoconus, normal eyes, and after crosslinking (CXL).

Methods

Two operators (1 and 2) independently measured Ko in three groups (keratoconus, normal, and post-CXL corneas), and repeated the procedure in some where their care remained unchanged and others after routine CXL (>28 days postop, epi-off treatment, 3.0 mW/cm², 30 min). The data were subsequently used to quantify interoperator error, test-retest/intersessional reliability for estimation of Ko, the significance of intergroup differences, and the effect of CXL on Ko.

Results

The major findings were: (i) Ko values were not normally distributed; (ii) mean (±sd, 95% CI) interoperator error was -0.002 (±0.019, −0.006 to 0.003, n = 95) and the limit of agreement between the operators was ±0.039; (iii) RMS differences in the intersessional estimation of Ko values were 0.011 (operator 1) and 0.012 (operator 2); (iv) intergroup differences in Ko were not significant (p > 0.05); (v) intersessional change in Ko (y) was linearly related to Ko estimated (x) at 1^st session (for operator 2 y = 1.187x−0.021, r = 0.755, n = 16, p < 0.01); and (vi) change in Ko (y₁) after CXL was linearly related to Ko (x₁) at preop (for operator 2 y₁ = 0.880x₁−0.016, r = 0.935, n = 20, p < 0.01).

Conclusion

Friedenwald's technique for estimating the Ko is prone to substantial interoperator error and intersessional differences. According to the technique, the change in Ko following CXL is on par with the expected intersessional change observed in controls.

The effect of corneal crosslinking on the rigidity of the cornea estimated using a modified algorithm for the Schiøtz tonometer

Purpose:

The aim of this study was to test a method for estimating corneal rigidity before and after cross-linking (CXL) using a Schiøtz tonometer.

Methods:

The study was performed in the Kyiv City Clinical Ophthalmological Hospital “Eye Microsurgical Center”, Ukraine. This was a prospective, consecutive, randomized, masked, case-by-case, clinical study. Corneal rigidity, indicated by the gradient (G) between lg applied weight and corresponding lg scale reading during Schiøtz tonometry, were obtained by increasing (A-mode) then reducing (D-mode) weights by two operators [A] in keratoconus, post-CXL and control subjects for estimation of (i) interoperator and (ii) intersessional errors, (iii) intergroup differences; [B] before and after CXL. Central corneal thickness CCT was measured by scanning slit pachymetry. ANOVA, t tests, linear regression were the statistical tools used.

Results:

Average interoperator difference (ΔG) was –0.120 (SD = ±0.294, 95%CI = –0.175 to –0.066). A significant correlation between ΔG and the mean of each pair of G values was found (r = –0.196, n = 112, P = 0.038). Intersessional differences in mean G values were insignificant (P > 0.05). There was a significant correlation between G at first session (X₁) and difference between sessions (ΔG) [Operator 1, ΔG = 0.598x₁–0.461, r = 0.601, n = 27, P = 0.009]. Significant intergroup differences in G were found (Operator 1, one-way ANOVA, F = 4.489, P = 0.014). The difference (Δ) between the pre-(X₂) and post-CXL treatment G values was significantly associated with the pre-CXL treatment value (Operator 1, Δ = 1.970x₂-1.622, r = 0.642, n = 18, P = <.001). G values were correlated with CCT in keratoconus and post-CXL.

Conclusion:

Corneal rigidity (G) estimated using the Schiøtz tonometer can be useful for detecting changes after CXL. However, G values are linked to CCT, can vary from time-to-time and the procedure is operator dependent.

Biomechanics of Ophthalmic Crosslinking

Crosslinking involves the formation of bonds between polymer chains, such as proteins. In biological tissues, these bonds tend to stiffen the tissue, making it more resistant to mechanical degradation and deformation. In ophthalmology, the crosslinking phenomenon is being increasingly harnessed and explored as a treatment strategy for treating corneal ectasias, keratitis, degenerative myopia, and glaucoma. This review surveys the multitude of exogenous crosslinking strategies reported in the literature, both "light" (involving light energy) and "dark" (involving non-photic chemical processes), and explores their mechanisms, cytotoxicity, and stage of translational development. The spectrum of ophthalmic applications described in the literature is then discussed, with particular attention to proposed therapeutic mechanisms in the cornea and sclera. The mechanical effects of crosslinking are then discussed in the context of their proposed site and scale of action. Biomechanical characterization of the crosslinking effect is needed to more thoroughly address knowledge gaps in this area, and a review of reported methods for biomechanical characterization is presented with an attempt to assess the sensitivity of each method to crosslinking-mediated changes using data from the experimental and clinical literature. Biomechanical measurement methods differ in spatial resolution, mechanical sensitivity, suitability for detecting crosslinking subtypes, and translational readiness and are central to the effort to understand the mechanistic link between crosslinking methods and clinical outcomes of candidate therapies. Data on differences in the biomechanical effect of different crosslinking protocols and their correspondence to clinical outcomes are reviewed, and strategies for leveraging measurement advances predicting clinical outcomes of crosslinking procedures are discussed. Advancing the understanding of ophthalmic crosslinking, its biomechanical underpinnings, and its applications supports the development of next-generation crosslinking procedures that optimize therapeutic effect while reducing complications.

Accelerated vs. conventional collagen cross-linking for infectious keratitis

Infectious keratitis represents a serious concern for ophthalmologists, with a progressively growing incidence in the last few years. In this prospective comparative study, we evaluated two groups of patients with infectious keratitis or corneal ulcer resistant to antimicrobial and antifungal therapy, treated respectively with conventional and accelerated photoactivated chromophore collagen cross-linking. Eight patients were assigned to each group and they were monitored for 12 months. We investigated the differences between groups, comparing on one side the mean of the quantitative variables using the t-test and on the other side the frequencies of qualitative variables using the Fisher exact test. The time to healing for the group treated with conventional cross-linking was 2 days longer than for the group undergoing accelerated cross-linking (34.9±11.4 vs. 32.9±9.4 days), a difference that did not reach statistical significance (P=0.708). We conclude that the accelerated protocol is as safe and efficient as the classic procedure. The accelerated protocol has an important advantage, both for the doctor and the patient, of being time sparing (the time for accelerated cross-linking is 3 times shorter than in the case of the conventional protocol).

Collagen Crosslinking-assisted Treatment of a Bleb Leak: Enhancement of Vascularization around the Bleb

AIM AND OBJECTIVE

To present a case with bleb leakage treated with collagen crosslinking (CXL).

BACKGROUND

Bleb-related complications can cause serious problems even a long time after trabeculectomy. In this case report, we present a relatively unknown treatment method for bleb leakage which might be one of the long-term complications of trabeculectomy.

CASE DESCRIPTION

A 60-year-old male patient was admitted to our clinic with a decrease in his left vision. The patient had a history of left trabeculectomy. The bleb leakage was observed. Autologous blood was injected into the bleb area as the first line of treatment. Collagen crosslinking was applied to the bleb area upon the recurrence of the leakage. In the 1st week after the CXL, CXL was repeated in the bleb area upon the observation that the leakage recurred. It was observed that the leakage did not recur and bleb vascularization was triggered after CXL.

CONCLUSION

Conjunctival CXL might be a non-invasive, viable method in the management of bleb leakage. It might be applied in conjunction with bandage contact lenses for the treatment of late bleb leaks as an alternative treatment method before going into surgical intervention.

CLINICAL SIGNIFICANCE

As a result of conjunctival CXL application, collagen permeability reduction and neovascularization secondary to CXL application might be effective in preventing bleb leakage. However, further studies are needed to prove this.

HOW TO CITE THIS ARTICLE

Aktas Z, Aribas YK, Bilgihan K, et al. Collagen Crosslinking-assisted Treatment of a Bleb Leak: Enhancement of Vascularization around the Bleb. J Curr Glaucoma Pract 2021;15(1):36-39.

Quantitative Analysis of the Corneal Collagen Distribution after In Vivo Cross-Linking with Second Harmonic Microscopy

Corneal cross-linking (CXL) is a surgical procedure able to modify corneal biomechanics and stabilize keratoconus progression. Although it is known that CXL produces changes in corneal collagen distribution, these are still a topic of discussion. Here we quantitatively compare the corneal stroma architecture between two animal models four weeks after in vivo conventional CXL treatment, with second harmonic generation (SHG) imaging microscopy and the structure tensor (ST). The healing stage and the stroma recovery were also analyzed by means of histological sections. Results show that the CXL effects depend on the initial arrangement of the corneal collagen. While the treatment increases the order in corneas with a low level of initial organization, corneas presenting a fairly regular pattern are hardly affected. Histological samples showed active keratocytes in anterior and middle stroma, what means that the recovery is still in progress. The combination of SHG imaging and the ST is able to objectively discriminate the changes suffered by the collagen arrangement after the CXL treatment, whose effectiveness depends on the initial organization of the collagen fibers within the corneal stroma.

Corneal Collagen Cross-Linking for the Management of Mycotic Keratitis

PURPOSE

To evaluate the efficiency of corneal collagen cross-linking (CXL) in addition to topical voriconazole in cases with mycotic keratitis.

DESIGN

Retrospective case series in a tertiary university hospital.

PARTICIPANTS

CXL was performed on 13 patients with mycotic keratitis who presented poor or no response to topical voriconazole treatment.

METHODS

The clinical features, symptoms, treatment results and complications were recorded retrospectively. The corneal infection was graded according to the depth of infection into the stroma (from grade 1 to grade 3). The visual analogue scale was used to calculate the pain score before and 2 days after surgery.

MAIN OUTCOME MEASURES

Grade of the corneal infection.

RESULTS

Mean age of 13 patients (6 female and 7 male) was 42.4 ± 17.7 years (20-74 years). Fungus was demonstrated in culture (eight patients) or cytological examination (five patients). Seven of the 13 patients (54%) were healed with topical voriconazole and CXL adjuvant treatment in 26 ± 10 days (15-40 days). The remaining six patients did not respond to CXL treatment; they initially presented with higher grade ulcers. Pre- and post-operative pain score values were 8 ± 0.8 and 3.5 ± 1, respectively (p < 0.05).

CONCLUSIONS

The current study suggests that adjunctive CXL treatment is effective in patients with small and superficial mycotic ulcers. These observations require further research by large randomized clinical trials.

Collagen Cross-linking for Late-onset Bleb Leakage: 1-Year Results

PURPOSE

To assess the efficacy of collagen cross-linking in the treatment of late-onset bleb leakage.

METHODS

A retrospective, interventional case-series. Cross-linking was performed for late-onset bleb leakage after failure of standard medical therapy. Primary outcome is measured by complete resolution of bleb leakage. Secondary outcomes including intraocular pressure and corrected distance visual acuity (CDVA) were documented over 1 year and were analyzed using repeated measure ANOVA.

RESULTS

Seven patients underwent cross-linking for bleb leakage between 2012 and 2013. Five (71%) had complete resolution of bleb leakage. Intraocular pressure improved from 3.9±1.0 to 7.4±1.1 mm Hg at 1 to 2 months (P=0.0003) and remained at 9.6±3.0 mm Hg at final visit (P=0.06). The corresponding CDVA also improved from 0.6±0.2 to 0.38±0.37 logMAR (P=0.0069) at 1 to 2 months before stabilizing at 0.38±0.44 (P=0.0021) at 1 year. Two cases (29%) had persistent bleb leakage after treatment. Four patients (57%) had resolved hypotony (≥6 mm Hg) at final visit. No adverse event was reported.

CONCLUSIONS

Collagen cross-linking is a noninvasive treatment offering reasonable success rate for filtering bleb leakage, and may spare patients invasive surgical bleb revisions.

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 cross-linking for infectious keratitis: an update of clinical studies

Collagen cross-linking (CXL) with ultraviolet light-activated riboflavin is a corneal surface procedure developed for the treatment of keratoconus and corneal ectasia. With the known microbicidal and corneal stiffening effects of ultraviolet irradiation and photoactivated riboflavin, it has recently been introduced for the management of infectious keratitis, especially for ulcers resistant to antimicrobial therapy or associated with corneal melting. Various authors have attempted to use CXL as an adjunctive, salvage or even as the sole treatment for infectious corneal ulcers. The aim of this review was to provide a summary of the clinical studies in the literature. It is worth noting that there is still no consensus on the treatment protocol of CXL against infectious keratitis. The disparities in outcome measures, treatment protocol and study design can confound the interpretation and hamper the generalization of the study results. Based on current evidence, the role of CXL in infectious keratitis remained unclear despite the reported success in some clinical cases. Further investigations are warranted concerning the efficacy and safety of treating infectious keratitis with CXL.

Reshaping procedures for the surgical management of corneal ectasia

Corneal ectasia is a progressive, degenerative, and noninflammatory thinning disorder of the cornea. Recently developed corneal reshaping techniques have expanded the treatment armamentarium available to the corneal specialist by offering effective nontransplant options. This review summarizes the current evidence base for corneal collagen crosslinking, topography-guided photorefractive keratectomy, and intrastromal corneal ring segment implantation for the treatment of corneal ectasia by analyzing the data published between the years 2000 and 2014.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

PACK-CXL: Corneal Cross-linking for Treatment of Infectious Keratitis

This article discusses corneal cross-linking (CXL) and how it transitioned from a modality for treating corneal ectatic disorders to an inventive means of treating infectious keratitis. Initially, CXL was successfully developed to halt the progression of ectatic diseases such as keratoconus, using the standard Dresden protocol. Later, indications were extended to treat iatrogenic ectasia developing after laser-assisted in situ keratomileusis (LASIK) and photo-refractive keratectomy (PRK). At the time, it had been postulated that the combination of ultraviolet light with riboflavin could not only biomechanically strengthen the cornea but also was capable of destroying living cells and organisms including keratocytes and pathogens. Thus a new and innovative concept of treatment for infectious keratitis emerged through the use of CXL technology. Initially only advanced infectious melting ulcers resisting standard microbicidal therapy were treated with CXL in addition to standard therapy. In subsequent studies CXL was also used to treat bacterial keratitis as first line therapy without the use of concomitant antibiotic therapy. With the increasing interest in CXL technology to treat infectious keratitis and to clearly separate its use from the treatment of ectatic disorders, a new term was adopted at the 9(th) CXL congress in Dublin for this specific indication: PACK-CXL (photoactivated chromophore for infectious keratitis). PACK-CXL has the potential to eventually become an interesting alternative to standard antibiotic therapy in treating infectious corneal disorders, and may help reduce the global burden of microbial resistance to antibiotics and other therapeutic agents.

Corneal cross-linking

Since its inception in the late 1990s, corneal cross-linking has grown from an interesting concept to a primary treatment for corneal ectatic disease worldwide. Using a combination of ultraviolet-A light and a chromophore (vitamin B2, riboflavin), the cornea can be stiffened, usually with a single application, and progressive thinning diseases such as keratoconus arrested. Despite being in clinical use for many years, some of the underlying processes, such as the role of oxygen and the optimal treatment times, are still being worked out. More than a treatment technique, corneal cross-links represent a physiological principle of connective tissue, which may explain the enormous versatility of the method. We highlight the history of corneal cross-linking, the scientific underpinnings of current techniques, evolving clinical treatment parameters, and the use of cross-linking in combination with refractive surgery and for the treatment of infectious keratitis.

Corneal biomechanical properties in different ocular conditions and new measurement techniques

Several refractive and therapeutic treatments as well as several ocular or systemic diseases might induce changes in the mechanical resistance of the cornea. Furthermore, intraocular pressure measurement, one of the most used clinical tools, is also highly dependent on this characteristic. Corneal biomechanical properties can be measured now in the clinical setting with different instruments. In the present work, we review the potential role of the biomechanical properties of the cornea in different fields of ophthalmology and visual science in light of the definitions of the fundamental properties of matter and the results obtained from the different instruments available. The body of literature published so far provides an insight into how the corneal mechanical properties change in different sight-threatening ocular conditions and after different surgical procedures. The future in this field is very promising with several new technologies being applied to the analysis of the corneal biomechanical properties.

[Effectiveness of initial antibiotic therapy for treatment of contact lens-related bacterial keratitis]

BACKGROUND

Contact lens-related microbial keratitis is a cause of potentially sight-threatening corneal opacification. Effective initial antimicrobial therapy is crucial to prevent long-term complications. This investigation was undertaken to test the effectiveness of current routine empirical antibiotic treatment regimens.

METHODS/PATIENTS

All consecutive cases of contact lens-related keratitis presenting in the outpatient clinic of the Department of Ophthalmology at the Medical University of Innsbruck between January 2010 and April 2012 were retrospectively analyzed.

RESULTS

Cultures were positive in 69 out of the 123 cases included in the study. Culture results identified 59.4 % Gram positive strains, 50.7 % Gram negative strains and 7.2 % fungal strains. Mixed infections accounted for 29 % of cases. The combination of an aminoglycoside and a second generation quinolone antibiotic was the most common initial treatment regimen (87.8 %). In vitro this regimen was less effective compared to combinations of moxifloxacin and ciprofloxacin or moxifloxacin and gentamicin.

CONCLUSION

Empirical combined regimens remain an effective treatment of contact lens-related keratitis. Fluoroquinolones proved to be inadequate for monotherapy.