In Vivo Optical Coherence Elastography Unveils Spatial Variation of Human Corneal Stiffness

OBJECTIVE

The mechanical properties of corneal tissues play a crucial role in determining corneal shape and have significant implications in vision care. This study aimed to address the challenge of obtaining accurate in vivo data for the human cornea.

METHODS

We have developed a high-frequency optical coherence elastography (OCE) technique using shear-like antisymmetric (A0)-mode Lamb waves at frequencies above 10 kHz.

RESULTS

By incorporating an anisotropic, nonlinear constitutive model and utilizing the acoustoelastic theory, we gained quantitative insights into the influence of corneal tension on wave speeds and elastic moduli. Our study revealed significant spatial variations in the shear modulus of the corneal stroma on healthy subjects for the first time. Over an age span from 21 to 34 (N = 6), the central corneas exhibited a mean shear modulus of 87 kPa, while the corneal periphery showed a significant decrease to 44 kPa. The central cornea's shear modulus decreases with age with a slope of -19 +/- 8 kPa per decade, whereas the periphery showed non-significant age dependence. The limbus demonstrated an increased shear modulus exceeding 100 kPa. We obtained wave displacement profiles that are consistent with highly anisotropic corneal tissues.

CONCLUSION

Our approach enabled precise measurement of corneal tissue elastic moduli in situ with high precision (<7%) and high spatial resolution (<1 mm). Our results revealed significant stiffness variation from the central to peripheral corneas.

SIGNIFICANCE

The high-frequency OCE technique holds promise for biomechanical evaluation in clinical settings, providing valuable information for refractive surgeries, degenerative disorder diagnoses, and intraocular pressure assessments.

Insulin eye drops improve corneal wound healing in STZ-induced diabetic mice by regulating corneal inflammation and neuropeptide release

INTRODUCTION

In recent years, insulin eye drops have attracted increasing attention from researchers and ophthalmologists. The aim of this study was to investigate the efficacy and possible mechanism of action of insulin eye drops in diabetic mice with corneal wounds.

METHODS

A type 1 diabetes model was induced, and a corneal epithelial injury model of 2.5 mm was established. We used corneal fluorescein staining, hematoxylin-eosin (H-E) staining and the Cochet-Bonnet esthesiometer to examine the process of wound healing. Subsequently, the expression levels of Ki-67, IL-1β, β3-tubulin and neuropeptides, including substance P (SP) and calcitonin gene-related peptide (CGRP), were examined at 72 h after corneal injury.

RESULTS

Fluorescein staining demonstrated an acceleration of the recovery of corneal epithelial injury in diabetic mice compared with the saline treatment, which was further evidenced by the overexpression of Ki-67. Moreover, 72 h of insulin application attenuated the expression of inflammatory cytokines and neutrophil infiltration. Remarkably, the results demonstrated that topical insulin treatment enhanced the density of corneal epithelial nerves, as well as neuropeptide SP and CGRP release, in the healing cornea via immunofluorescence staining.

CONCLUSIONS

Our results indicated that insulin eye drops may accelerate corneal wound healing and decrease inflammatory responses in diabetic mice by promoting nerve regeneration and increasing levels of neuropeptides SP and CGRP.

The influence of the tear film on the intraocular pressure and the corneal biomechanical properties analyzed with the Ocular Response Analyzer

PURPOSE

As ocular dryness and glaucoma are more prevalent with increasing age, understanding how the tear film affects tonometry is important. The present study aims to understand the impact that changes in the tear film have on intraocular pressure (IOP), corneal hysteresis, and corneal resistance factor measurements.

METHODS

Cross-sectional research was conducted and 37 patients were assessed. The tear film lipid layer and the non-invasive break-up time (NIBUT) were evaluated using the Tearscope Plus (Keeler, Windsor, UK). Dry eye symptoms were evaluated using the Ocular Surface Disease Index (OSDI) questionnaire. IOP was measured using rebound tonometry and the Ocular Response Analyzer (ORA, Reichert). Corneal biomechanical properties were measured using ORA.

RESULTS

It was found that an increase in the IOP measured with the iCare was directly correlated with the subclass that evaluated symptomatology associated with environmental factors (r = 0.414, p<0.05, Spearman). Goldmann-correlated IOP (IOPg) and Corneal-compensated IOP (IOPcc) values were statistically significantly different between the various interferometric patterns (p<0.05). It was also found that an increase in the corneal biomechanical properties measured with ORA was directly correlated with the overall scores obtained when using the OSDI and some of its subclasses.

CONCLUSIONS

Tear film interferometric patterns were shown to have some impact on the IOP measured using ORA. The IOP measured with iCare seems to be related to the symptomatology obtained from OSDI. Corneal biomechanical properties were related to the OSDI total score and some of its subclasses. An increase in symptomatology was associated with an increase in the measured biomechanical properties of the cornea.

Trigeminal Sensitization in a Closed Head Model for Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) is often accompanied by neurological and ocular symptoms that involve trigeminal nerve pathways. Laser-induced shock wave (LISW) was applied to the skull of male rats as a model for mTBI, while behavioral and neural recording methods were used to assess trigeminal function. The LISW caused greater eye wiping behavior to ocular instillation of hypertonic saline (Sham = 4.83 ± 0.65 wipes/5 min, LISW = 12.71 ± 1.89 wipes/5 min, p < 0.01) and a marked reduction in the time spent in bright light consistent with enhanced periocular and intraocular hypersensitivity, respectively (Sham = 16.3 ± 5.6 s, LISW = 115.5 ± 27.3 s, p < 0.01). To address the early neural mechanisms of mTBI, single trigeminal brainstem neurons, identified by activation to corneal or dural mechanical stimulation, were recorded in trigeminal subnucleus interpolaris/caudalis (Vi/Vc) and trigeminal subnucleus caudalis/upper cervical cord (Vc/C1) regions. The LISW caused marked sensitization to hypertonic saline and to exposure to bright light in neurons of both regions (p < 0.05). Laser speckle imaging revealed an increase in meningeal arterial blood flow to bright light after LISW (Sham = 4.7 ± 2.0 s, LISW = 469.0 ± 37.9 s, p < 0.001). Local inhibition of synaptic activity at Vi/Vc, but not at Vc/C1, by microinjection of CoCl2, prevented light-evoked increases in meningeal blood flow in LISW-treated rats. By contrast, topical meningeal application of phenylephrine significantly reduced light-evoked responses of Vi/Vc and Vc/C1 neurons. These data suggested that neurons in both regions became sensitized after LISW and were responsive to changes in meningeal blood flow. Neurons at the Vi/Vc transition and at Vc/C1, however, likely serve different roles in mediating the neurovascular and sensory aspects of mTBI.

Reliability of Measurements Using Ocular Response Analyzer as a Screening Tonometer and Corneal Hysteresis Values in the Presence or Absence of Glaucomatous Changes in Fundus

PRCIS

Use of the Ocular Response Analyzer (ORA) as a screening tonometer in clinical practice yielded reliable measurements in over 80% of eyes screened. Including corneal hysteresis (CH) data in screening may improve the accuracy of glaucoma detection.

PURPOSE

To examine measurement reliability when the ORA is used as a screening tonometer, and to compare CH measurements in eyes with and those without glaucomatous changes in the fundus.

PATIENTS AND METHODS

1488 eyes of 747 patients (mean age: 53.5 ± 20.4 y, range: 6-94 y) underwent intraocular pressure (IOP) measurement using ORA as screening. The percentage of eyes with a waveform score ≥6, the recommended threshold indicating reliability, was calculated. Eyes that had waveform score ≥6 and had undergone fundus photography and optical coherence tomography were assessed for the presence or absence of glaucomatous changes in fundus from optical coherence tomography and fundus images, and CH was compared between the 2 groups.

RESULTS

Mean ± SD (range) of ORA measurements were: Goldmann-correlated IOP 14.9 ± 4.8 (1.0-63.2) mm Hg, corneal-compensated IOP 16.2 ± 4.7 (3.2-73.6) mm Hg, CH 9.7 ± 1.5 (0.0-20.6) mm Hg, and waveform score 7.3 ± 1.5 (0.1-9.7). Eighty-four percent of eyes had a waveform score ≥6. Among 192 eyes (127 patients, aged 53.5 ± 18.0 y) with waveform score ≥6 and evaluable for glaucomatous changes in the fundus, 53 eyes were determined as positive and 139 eyes as negative. CH was 9.6 ± 1.4 (6.8-13.3) mm Hg in the positive group and 10.2 ± 1.2 (6.9-13.3) mm Hg in the negative group, and was significantly lower in the positive group ( P =0.003).

CONCLUSION

When using ORA as a screening tonometer, reliable results were obtained in ~80% of the eyes. CH was lower in the glaucomatous change-positive group compared with the glaucomatous change-negative group, but the ranges overlapped between the 2 groups.

Squishy matters - Corneal mechanobiology in health and disease

The cornea, as a dynamic and responsive tissue, constantly interacts with mechanical forces in order to maintain its structural integrity, barrier function, transparency and refractive power. Cells within the cornea sense and respond to various mechanical forces that fundamentally regulate their morphology and fate in development, homeostasis and pathophysiology. Corneal cells also dynamically regulate their extracellular matrix (ECM) with ensuing cell-ECM crosstalk as the matrix serves as a dynamic signaling reservoir providing biophysical and biochemical cues to corneal cells. Here we provide an overview of mechanotransduction signaling pathways then delve into the recent advances in corneal mechanobiology, focusing on the interplay between mechanical forces and responses of the corneal epithelial, stromal, and endothelial cells. We also identify species-specific differences in corneal biomechanics and mechanotransduction to facilitate identification of optimal animal models to study corneal wound healing, disease, and novel therapeutic interventions. Finally, we identify key knowledge gaps and therapeutic opportunities in corneal mechanobiology that are pressing for the research community to address especially pertinent within the domains of limbal stem cell deficiency, keratoconus and Fuchs' endothelial corneal dystrophy. By furthering our understanding corneal mechanobiology, we can contextualize discoveries regarding corneal diseases as well as innovative treatments for them.

In vivo assessment of the ocular biomechanical properties in patients with idiopathic normal pressure hydrocephalus

PURPOSE

Idiopathic normal pressure hydrocephalus (iNPH) is associated with an increased prevalence of open-angle glaucoma, attributed to variations of the pressure gradient between intraocular and intracranial compartments at the level of the lamina cribrosa (LC). As ocular biomechanics influence the behavior of the LC, and a lower corneal hysteresis (CH) has been associated to a higher risk of glaucomatous optic nerve damage, in this study we compared ocular biomechanics of iNPH patients with healthy subjects.

METHODS

 Twenty-four eyes of 24 non-shunted iNPH patients were prospectively recruited. Ocular biomechanical properties were investigated using the ocular response analyzer (Reichert Instruments) for the calculation of the CH, corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg), and corneal-compensated intraocular pressure (IOPcc). Results were compared with those of 25 eyes of 25 healthy subjects.

RESULTS

 In iNPH eyes, the median CH value and interquartile range (IQR) were 9.7 mmHg (7.8-10) and 10.6 mmHg (9.3-11.3) in healthy controls (p = 0.015). No significant differences were found in IOPcc [18.1 mmHg (14.72-19.92) vs. 16.4 mmHg (13.05-19.6)], IOPg [15.4 mmHg (12.82-19.7) vs. 15.3 mmHg (12.55-17.35)], and CRF [9.65 mmHg (8.07-11.65) vs. 10.3 mmHg (9.3-11.5)] between iNPH patients and controls.

CONCLUSIONS

 In iNPH patients, the CH was significantly lower compared to healthy subjects. This result suggests that ocular biomechanical properties may potentially contribute to the risk of development of glaucomatous optic nerve damage in iNPH patients.

A new R,R-RvD6 isomer with protective actions following corneal nerve injury

The transparent cornea is the most densely innervated tissue in the body, primarily by sensory nerves originating from the trigeminal ganglia (TG). Damage to corneal nerves reduces sensitivity and tear secretion and results in dry eye. Consequently, ocular pain, for which no satisfactory therapies exist, arises in many cases. Treatment of injured corneas with pigment epithelium-derived factor (PEDF) combined with docosahexaenoic acid (DHA) stimulates nerve regeneration in models of refractive surgery, which damages nerves. The mechanism involves the synthesis of a stereoisomer of resolvin D6 (R,R-RvD6) formed after incorporating DHA into membrane lipids. Activation of a PEDF receptor (PEDF-R) with phospholipase activity releases DHA to synthesize the new resolvin isomer, which is secreted via tears. Topical treatment of mice corneas with R,R-RvD6 shows higher bioactivity in regenerating nerves and increasing sensitivity compared to PEDF+DHA. It also stimulates a transcriptome in the TG that modulates genes involved in ocular pain. Our studies suggest an important therapeutic role for R,R-RvD6 in regenerating corneal nerves and decreasing pain resulting from dry eye.

Smoking, Corneal Biomechanics, and Glaucoma: Results From Two Large Population-Based Cohorts

Purpose

Smoking may influence measured IOP through an effect on corneal biomechanics, but it is unclear whether this factor translates into an increased risk for glaucoma. This study aimed to examine the association of cigarette smoking with corneal biomechanical properties and glaucoma-related traits, and to probe potential causal effects using Mendelian randomization (MR).

Methods

Cross-sectional analyses within the UK Biobank (UKB) and Canadian Longitudinal Study on Aging (CLSA) cohorts. Multivariable linear and logistic regression models were used to assess associations of smoking (status, intensity, and duration) with corneal hysteresis (CH), corneal resistance factor, IOP, inner retinal thicknesses, and glaucoma. Two-sample MR analyses were performed.

Results

Overall, 68,738 UKB (mean age, 56.7 years; 54.7% women) and 22 845 CLSA (mean age, 62.7 years; 49.1% women) participants were included. Compared with nonsmokers, smokers had a higher CH (UKB, +0.48 mm Hg; CLSA, +0.57 mm Hg; P < 0.001) and corneal resistance factor (UKB, +0.47 mm Hg; CLSA, +0.60 mm Hg; P < 0.001) with evidence of a dose-response effect in both studies. Differential associations with Goldmann-correlated IOP (UKB, +0.25 mm Hg; CLSA, +0.36 mm Hg; P < 0.001) and corneal-compensated IOP (UKB, -0.28 mm Hg; CLSA, -0.32 mm Hg; P ≤ 0.001) were observed. Smoking was not associated with inner retinal thicknesses or glaucoma status in either study. MR provided evidence for a causal effect of smoking on corneal biomechanics, especially higher CH.

Conclusions

Cigarette smoking seems to increase corneal biomechanical resistance to deformation, but there was little evidence to support a relationship with glaucoma. This outcome may result in an artefactual association with measured IOP and could account for discordant results with glaucoma in previous epidemiological studies.

Depth-dependent mechanical properties of the human cornea by uniaxial extension

The purpose of this study was to investigate the depth-dependent biomechanical properties of the human corneal stroma under uniaxial tensile loading. Human stroma samples were obtained after the removal of Descemet's membrane in the course of Descemet's membrane endothelial keratoplasty (DMEK) transplantation. Uniaxial tensile tests were performed at three different depths: anterior, central, and posterior on 2 × 6 × 0.15 mm strips taken from the central DMEK graft. The measured force-displacement data were used to calculate stress-strain curves and to derive the tangent modulus. The study showed that mechanical strength decreased significantly with depth. The anterior cornea appeared to be the stiffest, with a stiffness approximately 18% higher than that of the central cornea and approximately 38% higher than that of the posterior layer. Larger variations in mechanical response were observed in the posterior group, probably due to the higher degree of alignment of the collagen fibers in the posterior sections of the cornea. This study contributes to a better understanding of the biomechanical tensile properties of the cornea, which has important implications for the development of new treatment strategies for corneal diseases. Accurate quantification of tensile strength as a function of depth is critical information that is lacking in human corneal biomechanics to develop numerical models and new treatment methods.

Prospective assessment of corneal biomechanical properties and intraocular pressure after scleral lens wear: A 12-month follow-up study

PURPOSE

To investigate the long-term influence of scleral lens (SL) wear on corneal biomechanical properties and intraocular pressure (IOP) in irregular and regular corneas. Secondary goal comprised evaluate the fluid reservoir (FR) thickness overtime and correlate it with the changes in corneal biomechanical parameters and IOP.

METHODS

Seventy (70) eyes with irregular corneas (IC Group) and 21 eyes with regular corneas (RC Group) were fitted with 16.4 mm SL and wore the lenses for 12 months. Corrected IOP (IOPcc), Goldmann equivalent IOP (IOPg) and corneal biomechanical parameters (Corneal Hysteresis (CH) and Corneal Resistance Factor (CRF)) were measured with Ocular Response Analyzer. Slit lamp images were analyzed with ImageJ software to assess FR thickness overtime. Measurements were taken at lens dispensing visit prior lens wear (LDV1) and after 60 min of lens wear (LDV2) and at 1, 6 and 12-month follow-up visits. Measurements were done immediately after lens removal.

RESULTS

There were no statistically significant differences on IOPcc, IOPg, CRF and CH over the follow-up visits in both groups. Mean IOPcc and IOPg fluctuations overtime were clinically insignificant and below 1 mmHg in both groups. IOPg, CH and CRF were significantly lower on IC Group (p < 0.001), although no statistically significant differences were found between groups for IOPcc. Regarding FR thickness, statistically significant differences were found over the follow-up on both groups, with a mean decrease of 186.29 µm on IC Group and 175.32 µm on RC Group (p < 0.001). Statistically significant moderate to high negative correlations between FR and IOPg, CRF and CH were found only in the RC Group.

CONCLUSIONS

Long-term SL wear was not associated to changes in corneal biomechanical parameters neither on IOP as measured after lens removal. Besides IOP measurement without SL removal, more studies are needed to investigate the potential relationship with SL fitting characteristics (namely FR thickness).

Functional Changes of the Ocular Surface Sensory Nerves Due to Contact Lens Use in Young Symptomatic and Asymptomatic Users

Purpose

The purpose of this study was to analyze the differences in corneal sensory nerve functionality in young asymptomatic (CL-A) and symptomatic (CL-S) contact lens (CL) users.

Methods

CL wearers (23.8 ± 1.0 years, n = 31) were classified as CL-S with an Ocular Surface Disease Index (OSDI) ≥ 13 (n = 14) or CL-A. Users of eye glasses (EG; 24.5 ± 0.8 years, n = 29) with OSDI < 13 participated as controls. The sensations evoked by mechanical, chemical (gas esthesiometer), and cold (4°C saline drops) stimuli were measured using the Visual Analogue Scales (VASs). Moreover, tear volume, tear break up time (TBUT), blinking frequency (BF), and ocular surface temperature (OST; IR thermography) were also measured.

Results

Mechanical and chemical stimuli produced similar scores in the CL-A and EG participants, although the CL-A subjects referred to stronger irritation (p < 0.05). Likewise, the VAS intensity in response to cold stimuli did not differ between CL-A and EG subjects, while the ability to detect cold was significantly worse in CL-S users (p < 0.05). CL-A users had a similar tear volume, a higher BF (p < 0.01) and shorter TBUT (p < 0.001) to EG wearers, and blinking and TBUT were also altered significantly in CL-S users (p < 0.01). Interestingly, the OST was significantly lower in CL-A users (p < 0.05) than in EG wearers, but not in CL-S users.

Conclusions

Using CLs modifies corneal sensitivity, blinking and tearing in young volunteers. Even if they have yet to develop clinical signs of inflammation, they display changes in corneal sensitivity consistent with the sensitization of corneal nociceptors and the inhibition cold thermoreceptors, phenomena that occur under inflammatory conditions. The differences in corneal sensitivity and OST between CL-A and CL-S users could reflect the extent of nerve damage and inflammation at the ocular surface.

Development of a Corneal and Eye Protection Strategy in Domestic Swine

Large animal models are essential to research in facial paralysis, face transplant, craniofacial surgery, and ophthalmology. Pigs are a well-studied species with high similarity to human anatomy and physiology for these research areas. However, in contrast to cats and dogs protecting the cornea and eye is difficult in swine due to the inability to use an Elizabethan collar (E-collar) and the complexity of placing and maintaining a temporary tarsorrhaphy for corneal protection due to the strength of the pig levator muscle. This study presents an effective method to provide corneal and eye protection in the domestic swine for at least 50 d. Furthermore, protection of the eye and face is achieved through the innovative use of a modified ophthalmologic face shield. The findings from this study will advance large animal research in these fields, enabling innovation in surgery and tissue engineering in areas of both craniofacial and ophthalmologic research.

Intereye Differences in the Clinical Assessment of Intraocular Pressure and Ocular Biomechanics

SIGNIFICANCE

Clinicians and researchers will have evidence whether intereye differences confound clinical measurements of intraocular pressure or of ocular biomechanical parameters.

PURPOSE

The purpose of this study was to determine whether intraocular pressure and biomechanical parameters, as measured by the Ocular Response Analyzer (ORA) and by Cornea Visualization with Scheimpflug Technology (CorVis ST), are different between the first and second eye measured.

METHODS

Intraocular pressure and biomechanical parameters were collected from both eyes of healthy participants (N = 139). The ORA measured corneal-compensated intraocular pressure, Goldmann-correlated intraocular pressure, and corneal hysteresis. The CorVis ST measured biomechanically corrected intraocular pressure, stiffness parameter at first applanation, and stiffness parameter at highest concavity. For each measurement, a paired t test compared the value of the first eye measured against that of the second eye measured.

RESULTS

For the ORA, Goldmann-correlated intraocular pressure was significantly higher ( P = .001) in the first eye (14.8 [3.45] mmHg) than in the second eye (14.3 [3.63] mmHg). For the CorVis ST, biomechanically corrected intraocular pressure was significantly higher ( P < .001) in the second eye (14.7 [2.14] mmHg) than in the first eye (14.3 [2.11] mmHg). Stiffness parameter at first applanation (intereye difference, 6.85 [9.54] mmHg/mm) was significantly ( P < .001) higher in the first eye than in the second eye. Stiffness parameter at highest concavity was significantly higher ( P = .01) in the second eye (14.3 [3.18] mmHg/mm) than in the first eye (14.0 [3.13] mmHg/mm).

CONCLUSIONS

Although there were statistically significant intereye differences in intraocular pressure and in biomechanical parameters for both devices, the variations were small and thus unlikely to affect clinical outcomes.

Smoking, alcohol consumption and corneal biomechanical parameters among Chinese university students

BACKGROUND/OBJECTIVES

Smoking and alcohol consumption are important risk factors for several ocular disorders, but their effects on corneal biomechanics remain unclear. Our study aims to explore the association between smoking and alcohol consumption with corneal biomechanical parameters measured by Corvis-ST (CST) among university students.

SUBJECTS/METHODS

A total of 1645 healthy university students from a university-based study were included, and all participants underwent corneal biomechanical parameters measurement by CST. We selected 10 reliable parameters that can reflect the corneal deformation response. All participants had a standardised interview to determine their smoking and alcohol consumption status.

RESULTS

The mean age of the participants was 19.0 ± 0.9 years, and 1132 (68.8%) were women. Smoking was significantly associated with stiffer corneas. Smokers showed significantly slower second applanation velocity (A2v) (β = 0.007 m/s, 95% confidence interval 0.001 to 0.014, P = 0.032) and lower integrated radius (IR) (β = -0.214 mm-1, 95% confidence interval -0.420 to -0.007, P = 0.043) than non-smokers after adjusting for age, gender, eye-rubbing, myopia, and body mass index (BMI). Smokers with BMI no less than 24.0 showed slower A2v and lower IR. Alcohol consumption and passive smoking were found no significant association with corneal biomechanics.

CONCLUSION

Smokers had less deformable corneas, especially those with BMI no less than 24.0. Our findings provide new evidence for the association between smoking and ocular disorders associated with corneal biomechanics like glaucoma.

Corneal regeneration strategies: From stem cell therapy to tissue engineered stem cell scaffolds

Corneal epithelial defects and excessive wound healing might lead to severe complications. As stem cells can self-renew infinitely, they are a promising solution for regenerating the corneal epithelium and treating severe corneal epithelial injury. The chemical and biophysical properties of biological scaffolds, such as the amniotic membrane, fibrin, and hydrogels, can provide the necessary signals for stem cell proliferation and differentiation. Multiple researchers have conducted investigations on these scaffolds and evaluated them as potential therapeutic interventions for corneal disorders. These studies have identified various inherent benefits and drawbacks associated with these scaffolds. In this study, we provided a comprehensive overview of the history and use of various stem cells in corneal repair. We mainly discussed biological scaffolds that are used in stem cell transplantation and innovative materials that are under investigation.

Corneal biomechanics after small incision lenticule extraction and femtosecond laser in situ keratomileusis: A systematic review and meta-analysis

BACKGROUND

Small incision lenticule extraction (SMILE) and femtosecond laser in situ keratomileusis (FS-LASIK) have been extensively studied as the main surgical methods for corneal refractive surgery. However, there is no consensus on whether SMILE is superior to FS-LASIK in corneal biomechanics. Therefore, this systematic review and meta-analysis used the results of ocular response analyzer and corvis ST to explore whether SMILE is superior to FS-LASIK in corneal biomechanics.

METHODS

The literature was searched in PubMed, EMBASE, and Controlled Trials Register databases. The Cochrane Collaboration's "risk of bias" tool was used to evaluate the quality of the included randomized clinical trials, and the Newcastle-Ottawa Scale was used to evaluate the included non-randomized controlled trials. The results were analyzed using Revman 5.3.

RESULTS

Sixteen studies (3 randomized clinical trials and 13 non-randomized controlled trials) were included in this meta-analysis. There was no statistical difference in corneal biomechanics between SMILE and FS-LASIK in corneal hysteresis [mean difference (MD), 0.20; 95% confidence interval (CI): -0.09, 0.49; P = .18] and corneal resistant factor (MD, 0.31; 95% CI: -0.09, 0.71; P = .13), A1 time (MD, -0.02; 95% CI: -0.11, 0.07; P = .66), A1 length (MD, 0.01; 95% CI: -0.01, 0.03; P = .42), A1 velocity (MD, 0.00; 95% CI: -0.01, 0.01; P = .85), A2 velocity (MD, -0.01; 95% CI: -0.11, 0.09; P = .86), HC time (MD, 0.12; 95% CI: -0.13, 0.38; P = .35), The stiffness parameter at first applanation (MD, -7.91; 95% CI: -17.96, 2.14; P = .12), The ratio between the deformation amplitude 2 mm away from apex and the apical deformation (MD, 0.01; 95% CI: -0.26, 0.27; P = .96).

CONCLUSION

A comprehensive assessment of the parameters of ocular response analyzer and corvis ST showed that SMILE is not superior to LASIK in corneal biomechanics 3 months post-surgery.

Interobserver agreement between central topography and Scheimpflug tomography for premium intraocular lens selection

PURPOSE

To assess the inter-observer agreement and reliability of central topography (CT) of IOL Master 700 with that of Pentacam HR, as a screening tool for determining the suitability for premium (toric, monofocal and presbyopia correcting) intraocular lens (IOL) implants.

SETTING

Nethradhama Superspeciality Eye Hospital, Bangalore, India.

METHOD

200 eyes of eligible patients undergoing unilateral or bilateral phacoemulsification with premium IOL's were included in the study. Preoperatively, IOL Master 700 CT and Pentacam scans were performed for corneal topography evaluation. 2 experienced clinicians were randomised to interpret the axial map of either of the scans at a time and their responses were then enclosed in an envelope, and forwarded to a third observer for analysis. If there was a conflict, final decision was based on the interpretation of the Pentacam scan.

RESULTS

Of the total scans interpreted by the 2 observers, 77.5% (n = 155) showed agreement whereas 22.5% (n = 45) showed disagreement. The sensitivity, specificity, positive predictive value and negative predictive value (NPV) of IOL Master 700 CT in diagnosing irregular corneas when compared with Pentacam HR, was 72.73%, 78.09%, 29.09% and 95.86% respectively. The Cohen's κ inter-observer agreement value was 0.44, signifying a moderate degree of agreement, which was statistically significant (P < .00).

CONCLUSIONS

High NPV of IOL Master CT suggested that the device efficiently ruled out irregular corneas, while any suspicious scan should be confirmed with Pentacam HR or a similar device for appropriate decision-making regarding premium IOL selection.

TRIAL REGISTRATION NUMBER

CTRI/2021/11/038053.

Corneal Biomechanical Properties of Various Types of Glaucoma and Their Impact on Measurement of Intraocular Pressure

INTRODUCTION

Corneal biomechanical properties could affect intraocular pressure (IOP) measurement. The aim of this study was to evaluate the differences in corneal biomechanical properties of various types of glaucoma, assess their effect on IOP measurements.

METHODS

This is an observational clinical study of 486 subjects including 102 normal subjects, 104 ocular hypertension (OHT), 89 normal tension glaucoma (NTG), and 191 high tension glaucoma (HTG). Corneal biomechanical parameters were measured using an ocular response analyzer. The main parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated pressure measurement (IOPg), and corneal-compensated intraocular pressure (IOPcc). Ultrasound pachymetry was used to measure central corneal thickness (CCT). IOP was measured by a Goldmann applanation tonometer (GAT) and a noncontact tonometer (NCT). Visual field (VF) and refractive status were also recorded. Results were analyzed by one-way analysis of variance, univariate and multivariate linear regression analyses, and Bland-Altman plots.

RESULTS

Multiple comparison by analysis of variance showed significantly lower CH and CRF in NTG compared to HTG, OHT, and normal subjects (CH: 0.011, 0.015, and 0.033; CRF: 0.001, <0.001, and 0.042, respectively). CRF and CH associated with IOP were measured using either GAT, NCT and IOPcc-GAT, IOPcc-NCT, yet CCT was not. GAT correlated strongly with IOPg (r = 0.79; p < 0.001) and IOPcc (r = 0.77; p < 0.001), but limits of agreement between the measurements were poor. CH and CRF were both negatively correlated with VF change (p < 0.01).

CONCLUSION

CH and CRF affect the measurement of IOP and were related to types of glaucoma or severity of glaucoma. Pure CCT should not be used to correct IOP values or estimate the risk of disease.

Agreement of ocular response analyzer cornea compensated IOP with corvis ST biomechanical IOP following Femtosecond Laser-assisted LASIK

OBJECTIVES

To compare intraocular pressure (IOP) measurement by ORA-IOPcc and Corvis-bIOP after femtosecond laser-assisted LASIK (FS-LASIK).

METHODS

In this prospective cohort study, 56 eyes from 56 consecutive patients scheduled for FS-LASIK were enrolled. All patients had IOP measurement with ORA and Corvis ST by two blinded independent expert examiners. IOP examinations were conducted between 8 and 11 A.M. Data were collected at baseline and 3 months after FS-LASIK.

RESULTS

The mean age of the participants was 29.1 ± 6.3 years, and 42 (75%) were female. The average of central corneal thickness (CCT) decreased from 537 ± 23 µm at baseline to 458 ± 31 µm after FS-LASIK. The mean postoperative change of IOP was 0.0 ± 2.1 for bIOP and -2.5 ± 3.2 mmHg for IOPcc. The corresponding 95% limits of agreement (LoA) was -4.1 to 4.1 mmHg and -3.8 to 8.8 mmHg, respectively. Both methods showed no significant correlation between ∆IOP and ∆CCT. The 95% LoA between bIOP and IOPcc after FS-LASIK was -4.8 to 9.1 mmHg.

CONCLUSIONS

Compared to the ORA-IOPcc, the Corvis-bIOP showed less variation after FS-LASIK and might be a more appropriate choice for measuring IOP in this condition. The agreement of bIOP vs. IOPcc after FS-LASIK is below the clinically acceptable level, and the two methods could not be regarded as interchangeable.

Autophagy in the normal and diseased cornea

The cornea and covering tear film are together the 'objective lens' of the eye through which 80% of light is refracted. Despite exposure to a physically harsh and at times infectious or toxic environment, transparency essential for sight is in most cases maintained. Such resiliency makes the avascular cornea a superb model for the exploration of autophagy in the regulation of homeostasis with relevancy to all organs. Nonetheless, missense mutations and inflammation respectively clog or apparently overwhelm autophagic flux to create dystrophies much like in neurodegenerative diseases or further exacerbate inflammation. Here there is opportunity to generate novel topical therapies towards the restoration of homeostasis with potential broad application.

Assessing Corneal Endothelial Damage Using Terahertz Time-Domain Spectroscopy and Support Vector Machines

The endothelial layer of the cornea plays a critical role in regulating its hydration by actively controlling fluid intake in the tissue via transporting the excess fluid out to the aqueous humor. A damaged corneal endothelial layer leads to perturbations in tissue hydration and edema, which can impact corneal transparency and visual acuity. We utilized a non-contact terahertz (THz) scanner designed for imaging spherical targets to discriminate between ex vivo corneal samples with intact and damaged endothelial layers. To create varying grades of corneal edema, the intraocular pressures of the whole porcine eye globe samples (n = 19) were increased to either 25, 35 or 45 mmHg for 4 h before returning to normal pressure levels at 15 mmHg for the remaining 4 h. Changes in tissue hydration were assessed by differences in spectral slopes between 0.4 and 0.8 THz. Our results indicate that the THz response of the corneal samples can vary according to the differences in the endothelial cell density, as determined by SEM imaging. We show that this spectroscopic difference is statistically significant and can be used to assess the intactness of the endothelial layer. These results demonstrate that THz can noninvasively assess the corneal endothelium and provide valuable complimentary information for the study and diagnosis of corneal diseases that perturb the tissue hydration.

Effect of corneal collagen crosslinking on viscoelastic shear properties of the cornea

The cornea is responsible for most of the refractive power in the eye and acts as a protective layer for internal contents of the eye. The cornea requires mechanical strength for maintaining its precise shape and for withstanding external and internal forces. Corneal collagen crosslinking (CXL) is a treatment option to improve corneal mechanical properties. The primary objective of this study was to characterize CXL effects on viscoelastic shear properties of the porcine cornea as a function of compressive strain. For this purpose, corneal buttons were prepared and divided into three groups: control group (n = 5), pseudo-crosslinked group (n = 5), and crosslinked group (n = 5). A rheometer was used to perform dynamics torsional shear experiments on corneal disks at different levels of compressive strain (0%-40%). Specifically, strain sweep experiments and frequency sweep tests were done in order to determine the range of linear viscoelasticity and frequency dependent shear properties, respectively. It was found that the shear properties of all samples were dependent on the shear strain magnitude, loading frequency, and compressive strain. With increasing the applied shear strain, all samples showed a nonlinear viscoelastic response. Furthermore, the shear modulus of samples increased with increasing the frequency of the applied shear strain and/or increasing the compressive strain. Finally, the CXL treatment significantly increased the shear storage and loss moduli when the compressive strain was varied from 0% to 30% (p < 0.05); larger shear moduli were observed at compressive 40% strain but the difference was not significant (P = 0.12).

Bactericidal Efficacy of High Irradiance Ultraviolet A Photoactivation of Riboflavin Versus Standard Corneal Cross-Linking Protocol In Vitro

PURPOSE

The purpose of this study was to compare the efficacy of high ultraviolet A (UVA) irradiance photoactivation of riboflavin (vitamin B2) versus the standard corneal cross-linking protocol on bacterial viability.

METHODS

Methicillin-sensitive Staphylococcus aureus (MSSA) Newman strain and methicillin-resistant multidrug-resistant S. aureus (MDR-MRSA) USA300, CA409, CA127, GA656, and NY315 strains were exposed to a UVA energy dose of 5.4 to 6 J/cm 2 by 2 high irradiance regimens: A) 30 mW/cm 2 for 3 minutes and B) 10 mW/cm 2 for 10 minutes with B2 0.1%. Control groups included B2/UVA alone, CA409 exposed to standard B2 0.1% + UVA (3 mW/cm 2 for 30 minutes), and an untreated sample. Cell viability was assessed. Triplicate values were obtained. The Mann-Whitney test and Student t test were used for statistical analysis.

RESULTS

There was no difference comparing the median bacterial load (log CFU/mL) of the untreated samples versus regimen A: Newman P = 0.7, CA409 P = 0.3, USA300 P = 0.5, CA127 P = 0.6, GA656 P = 0.1, and NY315 P = 0.2 ( P ≥ 0.1); and B: Newman P = 0.1, CA409 P = 0.3, USA300 P = 0.4, CA127 P = 0.6, GA656 P = 0.1, and NY315 P = 0.3 ( P ≥ 0.1). Standard regimen killed 100% of CA409.

CONCLUSIONS

Photoactivation of B2 by high UVA irradiance does not seem to be effective for bacterial eradication in this study.

[Effects of SMILE with different residual stromal thicknesses on corneal biomechanical properties of rabbits in vivo]

Femtosecond laser small incision lenticule extraction (SMILE) with different residual stromal thicknesses (RST) is set to investigate its effect on corneal biomechanical properties of rabbits in vivo. In this study, 24 healthy adult Japanese rabbits were randomly divided into group A and B. The RST of group A was set 30% of the corneal central thickness (CCT), and the RST of group B was 50% of the CCT. The thickness of the corneal cap in both groups was set one third of CCT. Corneal visualization Scheimpflug technology (Corvis ST) and Pentacam three-dimensional anterior segment analyzer were used to determine corneal biomechanical and morphological parameters before surgery, and 1 week, 1 month and 3 months after surgery. Pearson correlation analysis was used to analyze factors affecting corneal biomechanical parameters after SMILE. The results showed that the corneal stiffness of group A was significantly higher than that of group B at 1 week and 1 month after surgery, and most biomechanical parameters returned to preoperative levels at 3 months postoperatively. The results of correlation analysis showed that postoperative CCT and RST were the main factors affecting corneal biomechanical parameters after SMILE. There was no significant difference in corneal posterior surface height (PE) between 3 months after surgery and before surgery in both two groups. It indicates that although the ability to resist deformation of cornea decreases in SMILE with thicker corneal cap and less RST, there is no tendency to keratoconus, which may be related to the preservation of more anterior stromal layer.

Non-immune and immune functions of interleukin-36γ suppress epithelial repair at the ocular surface

Regulation of innate inflammation is critical for maintaining tissue homeostasis and barrier function, especially in those interfacing the external environments such as the skin and cornea. Expression of pro-inflammatory cytokines by injured tissues has been shown to exacerbate the inflammatory cascade, causing tissue damage. Interleukin 36, a subfamily of the IL-1 superfamily, consists of three pro-inflammatory agonists-IL36α, IL36β, and IL36γ and an IL36 receptor antagonist (IL36Ra). The current investigation, for the first time, reports that IL36γ is the primary agonist expressed by the corneal epithelium, which is significantly upregulated following corneal injury. The function of IL36γ on non-immune cells, in addition to innate inflammatory cells, in regulating tissue homeostasis has not been well investigated. Using a loss-of-function approach via neutralizing antibody treatment, our data demonstrate that blocking endogenously expressed IL36γ in epithelial cells promotes rapid re-epithelialization in in vitro wound closure assay. Finally, by utilizing a naturally occurring antagonist IL36Ra in a well-established murine model of ocular injury, our study demonstrates that inhibition of IL36γ accelerates epithelial regeneration and suppresses tissue inflammation. Given rapid wound healing is critical for re-establishing normal tissue structure and function, our investigation on the function of IL36γ provides evidence for the development of novel IL36γ-targeting strategies to promote tissue repair.

Longitudinal assessment of the effect of alkali burns on corneal biomechanical properties using optical coherence elastography

Eye injury due to alkali burn is a severe ocular trauma that can profoundly affect corneal structure and function, including its biomechanical properties. Here, we assess the changes in the mechanical behavior of mouse corneas in response to alkali-induced injury by conducting longitudinal measurements using optical coherence elastography (OCE). A non-contact air-coupled ultrasound transducer was used to induce elastic waves in control and alkali-injured mouse corneas in vivo, which were imaged with phase-sensitive optical coherence tomography. Corneal mechanical properties were estimated using a modified Rayleigh-Lamb wave model, and results show that Young's modulus of alkali-burned corneas were significantly greater than that of their healthy counterparts on days 7 (p = 0.029) and 14 (p = 0.026) after injury. These findings, together with the changes in the shear viscosity coefficient postburn, indicate that the mechanical properties of the alkali-burned cornea are significantly modulated during the wound healing process.

Corneal biomechanical changes after Descemet stripping automated endothelial keratoplasty, penetrating keratoplasty, and phacoemulsification

PURPOSE

To evaluate corneal biomechanical changes after Descemet stripping automated endothelial keratoplasty (DSAEK), penetrating keratoplasty (PK), and phacoemulsification (PE).

METHODS

This prospective study included 138 eyes which underwent PK (26 eyes), DSAEK (26 eyes), PE (57 eyes), and 29 normal eyes. Intraocular pressure (IOP) was measured by Goldmann applanation tonometer (GAT), and central corneal thickness (CCT) and axial length by ultrasound. The ocular response analyzer was used to measure corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-related IOP (IOPg), and cornea-compensated IOP (IOPcc) preoperatively and 1, 3, and 6 months postoperatively.

RESULTS

At baseline, PK group had the lowest CH and CRF. There was a significant increase in CH and CRF to normal values in PK (P = 0.015 and 0.006) and PE (P = 0.005 and 0.0001) groups over the study period. At 6 months, CH and CRF increased and reached normal values in the PK group; increased to a lower level than normal in the DSAEK group; and, after an initial reduction, increased to normal values in the PE group. At 6 months, DSAEK group had the lowest CH and CRF. There was a significant positive correlation between CRF and GAT (r = 0.281, P = 0.009), IOPg and GAT (r = 0.335, P = 0.001), and IOPcc and GAT (r = 0.282, P = 0.001). CH was negatively correlated with age (r = - 0.189, P = 0.04).

CONCLUSION

Corneal biomechanical factors increase after DSAEK and PK. At post-operative month six, they reach normal values in PK group, but are lower than normal in DSAEK group.

A stochastic approach to estimate intraocular pressure and dynamic corneal responses of the cornea

IntraOcular Pressure (IOP) is one of the most informative factors for monitoring the eye-health. This is usually measured by tonometers. However, the outputs of the tonometers depend on the physical and geometrical properties of the cornea. Therefore, the common practice is to develop a numerical model to generate some correction factors. The main challenge here is the accuracy and efficiency of a numerical model in predicting the IOP and Dynamic Corneal Response (DCR) of each patient. This study addresses this issue by developing a two-step surrogate model based on adaptive sparse Polynomial Chaos Expansion (PCE) for fast and accurate prediction of the IOP. In this regard, first, an FE model of the cornea has been developed to predict the DCR parameters. This FE model has been replaced with a PCE-based surrogate model to speed up the simulation step. The uncertainties in the geometry and material model of the cornea have been propagated through the surrogate model to estimate the distributions of the DCR parameters. In the second step, the combination of DCR parameters and the input parameters provide a proper parameter space for developing an efficient data-driven PCE model to predict the IOP. Moreover, sensitivity analysis by using PCE-based Sobol indices has been performed. The results demonstrate the accuracy and efficiency of the proposed method in predicting the IOP. Sensitivity analysis revealed that IOP measurement was influenced mostly by deflection amplitude and applanation time. The analysis indicates the importance of the interactions between the parameters.

Effects of Corneal Biomechanical Properties on Rebound Tonometry (Icare200) and Applanation Tonometry (Perkins) Readings in Patients With Primary Congenital Glaucoma

OBJECTIVE

The aim was to assess the influence of corneal biomechanics on intraocular pressure (IOP) measurements made with the Icare200 (IC200) rebound tonometer and the Perkins handheld applanation tonometer in patients with primary congenital glaucoma (PCG).

MATERIALS AND METHODS

A total of 40 PCG patients and 40 healthy controls, age, and sex-matched, were recruited. IOP was measured with the Ocular Response Analyzer (IOPc, IOPg), IC200 and Perkins. The variables age, IOP, corneal hysteresis (CH), corneal resistance factor (CRF), central corneal thickness (CCT), best-corrected visual acuity, spherical equivalent, medications, and glaucoma surgeries were recorded for each subject. Univariate and multivariate analysis were used to detect effects of variables on IOP measurements.

RESULTS

Mean CCT was 545.65±71.88 μm in PCG versus 558.78±27.58 μm in controls (P=0.284). CH and CRF were significantly lower in PCG group than in control group: mean CH 8.11±1.69 versus 11.15±1.63 mm Hg (P<0.001), and mean CRF 9.27±2.35 versus 10.71±1.75 mm Hg (P=0.002). Mean differences between IOP IC200-Perkins were 0.79±0.53 mm Hg in PCG versus 0.80±0.23 mm Hg in controls (P<0.001) and mean differences IC200-IOPc were -0.89±5.15 mm Hg in PCG (P<0.001) versus 1.60±3.03 mm Hg in controls (all P<0.009). Through multivariate analysis, CRF showed positive association and CH negative association with IOP measured with Perkins or IC200 in both subject groups. No association was detected for CCT, age, or sex.

CONCLUSION

CH and CRF were identified as the main factors interfering with IOP measurements made with both tonometers in patients with PCG and healthy controls.

Clinical evaluation of flat peripheral curve design with aspherical-curve and multi-curve hard contact lenses for keratoconus

Aspherical- and multi-curve rigid gas-permeable hard contact lenses (HCLs) have a flattened curve in the peripheral zone and are mostly used for patients with keratoconus who cannot wear glasses, soft contact lenses, or spherical HCLs. In this retrospective study, a total of 95 eyes of 77 patients who used aspherical- or multi-curve HCLs (mean age: 40.0 ± 11.0 years) were evaluated. This study examined the types of aspherical- and multi-curve HCLs, best-corrected visual acuity (BCVA) values before and after wearing HCLs, the association with the Amsler-Krumeich classification, duration of wear, corneal/conjunctival disorder, and the frequency of changing HCLs. There were 78 eyes that used aspherical-curve HCLs and 17 that used multi-curve HCLs. BCVA significantly improved from 0.42 logMAR to 0.06 logMAR after wearing either form of HCL. The Amsler-Krumeich classification showed that aspherical-curve HCLs were commonly used for patients with stage 2 keratoconus, and multi-curve HCLs were commonly used for stage 4 patients. The BCVA values were worse when the disease stage was more severe (stages 3 and 4) regardless of HCL type. The mean base curve of the lenses was steeper in multi-curve HCLs than in aspherical-curve HCLs. The more severe the disease stage, the steeper the base curve in both aspherical- and multi-curve HCLs. The duration of wear significantly improved from 2.1 h to 10.2 h, and corneal/conjunctival disorder similarly improved. The mean frequency of changing HCL types was 1.1 times. This study suggests that a flat peripheral curve design with aspherical- and multi-curve HCLs is useful for patients with keratoconus.

Corneal Nerve Parameter Reference Values for Chinese Adults Assessed by Corneal Confocal Microscopy

BACKGROUND

Confocal corneal microscopy is an excellent new noninvasive tool for assessing diabetic peripheral neuropathy. We aimed to investigate the clinical variables associated with corneal nerve parameters and establish reference values for clinical use in healthy Chinese adults.

METHODS

The study enlisted 257 healthy volunteers (137 females and 120 males) from two clinical academic centers in China. Two experts captured and selected images of the central corneal subbasal nerve plexus at each center using the same corneal confocal microscopy instrument according to a commonly adopted protocol. Corneal nerve fiber density (CNFD), corneal nerve branch density (CNBD), and corneal nerve fiber length (CNFL) were measured using fully automated software (ACCMetrics). The correlation between clinical indicators and confocal corneal microscopy measures was determined using partial correlation. Quantile regression was used to calculate reference values and estimate the effects of clinical factors on the normative values of confocal corneal microscopy measures.

RESULTS

Females had significantly higher CNFD, CNBD, and CNFL than males. There was no correlation between age, glycated hemoglobin (HbA1c), height, weight, body mass index (BMI), and any corneal nerve fiber parameter in both sexes. In either sex, age, weight, height, BMI, and HbA1c did not affect the 0.05^th quantile values of any corneal nerve parameter.

CONCLUSIONS

This study establishes sex-adjusted reference values for corneal confocal microscopy measures in Chinese adults and provides a reference for clinical practice and research with this technique.

The utility of measures of anterior segment parameters of a Pentacam Scheimpflug tomographer in discriminating high myopic astigmatism from keratoconus

The study aimed to evaluate and compare anterior segment parameters between keratoconic eyes and eyes with high myopic astigmatism using Pentacam Scheimpflug tomography. This was a retrospective cross-sectional study that included sixty keratoconic eyes (thirty-two persons) and seventy-three eyes (forty-six persons) with high myopic astigmatism with mean ages 24.72 ± 11.65years and 26.60 ± 10.69years, respectively. Twenty-three parameters from the topographic map and fifteen parameters from the Belin-Ambrosió enhanced ectasia display map of the printouts of a Scheimpflug principle-based Pentacam tomographer were evaluated for their diagnostic accuracy using Receiver Operating Characteristic (ROC) curve. All parameters except cornea volume, anterior chamber volume, and anterior chamber angle indicated a significant difference between high myopic astigmatism and keratoconic eyes. The area under the receiver operating characteristic (AUROC) of eighteen Pentacam parameters was excellent (0.9–1.0) in discriminating keratoconus from high myopic astigmatism, out of which four {anterior minimum sagittal curvature (ant. Rmin), posterior minimum sagittal curvature (post. Rmin), maximum Ambrosió relational thickness (ART max) and total deviation value (D)} indicated excellent (>90%) sensitivity and specificity in addition to the excellent AUROC values. Topographic and Belin-Ambrosió enhanced ectasia display (BAD) maps of a Scheimpflug principle-based Pentacam tomographer are useful in enhancing the diagnosis of keratoconus and may also provide valuable information in effectively screening for keratoconus cases among refractive surgery candidates with high myopic astigmatism.

Nonlinear optical properties of human cornea measured by spectral domain Z-scan method

In the myopia correction surgery by femtosecond laser, such as Laser in Situ Keratomileusis (LASIK) and Small Incision Lenticule Extraction (SMILE), the nonlinear refractive index of the cornea may cause the deviation of cutting depth. In order to improve the cornea cutting's accuracy and reduce the possibility of undercorrection, the nonlinear refractive index coefficient n₂ of the human cornea must be measured with high accuracy. The spectral domain Z-scan technique can measure n₂ of the highly scattering biological tissues with much better signal to noise ratio and thus better accuracy than the conventional methods. In this paper, the n₂ coefficient of one ex-vivo human corneal sample was measured by the spectral domain Z-scan technique. Experimental results show that as this corneal sample gradually dehydrates, its n₂ coefficients are 1.1 ± 0.1×10^-19 m²/W, 1.4 ± 0.2×10^-19 m²/W and 1.6 ± 0.2 ×10^-19 m²/W respectively for the corneal sample with water contents of 89%, 82%, and 78%. The increase of the water content reduces the value of n₂, which is reasonable since the nonlinear refractive index coefficient of water is one order of magnitude smaller than that of the cornea.

Semper's cells in the insect compound eye: Insights into ocular form and function

The arthropod compound eye represents one of two major eye types in the animal kingdom and has served as an essential experimental paradigm for defining fundamental mechanisms underlying sensory organ formation, function, and maintenance. One of the most distinguishing features of the compound eye is the highly regular array of lens facets that define individual eye (ommatidial) units. These lens facets are produced by a deeply conserved quartet of cuticle-secreting cells, called Semper cells (SCs). Also widely known as cone cells, SCs were originally identified for their secretion of the dioptric system, i.e. the corneal lens and underlying crystalline cones. Additionally, SCs are now known to execute a diversity of patterning and glial functions in compound eye development and maintenance. Here, we present an integrated account of our current knowledge of SC multifunctionality in the Drosophila compound eye, highlighting emerging gene regulatory modules that may drive the diverse roles for these cells. Drawing comparisons with other deeply conserved retinal glia in the vertebrate single lens eye, this discussion speaks to glial cell origins and opens new avenues for understanding sensory system support programs.

Ocular Delivery of Predatory Bacteria with Cryomicroneedles Against Eye Infection

The development of potent antibiotic alternatives with rapid bactericidal properties is of great importance in addressing the current antibiotic crisis. One representative example is the topical delivery of predatory bacteria to treat ocular bacterial infections. However, there is a lack of suitable methods for the delivery of predatory bacteria into ocular tissue. This work introduces cryomicroneedles (cryoMN) for the ocular delivery of predatory Bdellovibrio bacteriovorus (B. bacteriovorus) bacteria. The cryoMN patches are prepared by freezing B. bacteriovorus containing a cryoprotectant medium in a microneedle template. The viability of B. bacteriovorus in cryoMNs remains above 80% as found in long-term storage studies, and they successfully impede the growth of gram-negative bacteria in vitro or in a rodent eye infection model. The infection is significantly relieved by nearly six times through 2.5 days of treatment without substantial effects on the cornea thickness and morphology. This approach represents the safe and efficient delivery of new class of antimicrobial armamentarium to otherwise impermeable ocular surface and opens up new avenues for the treatment of ocular surface disorders.

Neuroprotective Effects and Therapeutic Potential of Transcorneal Electrical Stimulation for Depression

Transcorneal electrical stimulation (TES) has emerged as a non-invasive neuromodulation approach that exerts neuroprotection via diverse mechanisms, including neurotrophic, neuroplastic, anti-inflammatory, anti-apoptotic, anti-glutamatergic, and vasodilation mechanisms. Although current studies of TES have mainly focused on its applications in ophthalmology, several lines of evidence point towards its putative use in treating depression. Apart from stimulating visual-related structures and promoting visual restoration, TES has also been shown to activate brain regions that are involved in mood alterations and can induce antidepressant-like behaviour in animals. The beneficial effects of TES in depression were further supported by its shared mechanisms with FDA-approved antidepressant treatments, including its neuroprotective properties against apoptosis and inflammation, and its ability to enhance the neurotrophic expression. This article critically reviews the current findings on the neuroprotective effects of TES and provides evidence to support our hypothesis that TES possesses antidepressant effects.

Pivotal Role of Corneal Fibroblasts in Progression to Corneal Ulcer in Bacterial Keratitis

The shape and transparency of the cornea are essential for clear vision. However, its location at the ocular surface renders the cornea vulnerable to pathogenic microorganisms in the external environment. Pseudomonas aeruginosa and Staphylococcus aureus are two such microorganisms and are responsible for most cases of bacterial keratitis. The development of antimicrobial agents has allowed the successful treatment of bacterial keratitis if the infection is diagnosed promptly. However, no effective medical treatment is available after progression to corneal ulcer, which is characterized by excessive degradation of collagen in the corneal stroma and can lead to corneal perforation and corneal blindness. This collagen degradation is mediated by both infecting bacteria and corneal fibroblasts themselves, with a urokinase-type plasminogen activator (uPA)-plasmin-matrix metalloproteinase (MMP) cascade playing a central role in collagen destruction by the host cells. Bacterial factors stimulate the production by corneal fibroblasts of both uPA and pro-MMPs, released uPA mediates the conversion of plasminogen in the extracellular environment to plasmin, and plasmin mediates the conversion of secreted pro-MMPs to the active form of these enzymes, which then degrade stromal collagen. Bacterial factors also stimulate expression by corneal fibroblasts of the chemokine interleukin-8 and the adhesion molecule ICAM-1, both of which contribute to recruitment and activation of polymorphonuclear neutrophils, and these cells then further stimulate corneal fibroblasts via the secretion of interleukin-1. At this stage of the disease, bacteria are no longer necessary for collagen degradation. In this review, we discuss the pivotal role of corneal fibroblasts in corneal ulcer associated with infection by P. aeruginosa or S. aureus as well as the development of potential new modes of treatment for this condition.

Comparison of Corneal Biomechanical Properties and Corneal Tomography Between Customized and Accelerated Corneal Crosslinking in Eyes with Keratoconus

PURPOSE

To compare the changes in corneal biomechanical properties and corneal tomography between transepithelial customized corneal crosslinking (C-CXL) and epithelium-off accelerated corneal crosslinking (A-CXL) in eyes with keratoconus.

METHODS

Twenty eyes in 20 consecutive patients who underwent C-CXL (C-CXL group) and 20 eyes in 20 patients who underwent A-CXL (A-CXL group) were included in this retrospective comparative study. The corneal biomechanical properties were analyzed using a Scheimpflug-based tonometer, and all corneas were examined by anterior segment optical coherence tomography (AS-OCT) before and 3 months after surgery. The corneal biomechanical parameters analyzed were the maximum inverse radius, deformation amplitude (DA) ratio max (2 mm), stiffness parameter at applanation 1, and integrated radius. The AS-OCT parameters analyzed included average keratometry, corneal astigmatism, maximum keratometry reading (Kmax), higher-order irregularity, and asymmetry.

RESULTS

In the C-CXL group, there were significant improvements in biomechanical parameters, including the maximum inverse radius, the DA ratio max (2 mm), and the integrated radius after surgery (P = 0.037, P = 0.002, and P = 0.003, respectively). In the C-CXL group, there was a significant decrease in the Kmax, higher-order irregularity, and asymmetry components (P = 0.014, P = 0.008, and P = 0.016, respectively). The biomechanical properties and AS-OCT parameters did not change significantly in the A-CXL group after surgery. According to multiple regression analyses, C-CXL had a greater effect than A-CXL in improving the maximum inverse radius, DA ratio max (2 mm), integrated radius, Kmax, asymmetry component, and higher-order irregularity component.

CONCLUSIONS

C-CXL might improve the biomechanical properties and irregular shape of the cornea from the early postoperative period to a greater extent than A-CXL.

Modelling the visual world of a velvet worm

In many animal phyla, eyes are small and provide only low-resolution vision for general orientation in the environment. Because these primitive eyes rarely have a defined image plane, traditional visual-optics principles cannot be applied. To assess the functional capacity of such eyes we have developed modelling principles based on ray tracing in 3D reconstructions of eye morphology, where refraction on the way to the photoreceptors and absorption in the photopigment are calculated incrementally for ray bundles from all angles within the visual field. From the ray tracing, we calculate the complete angular acceptance function of each photoreceptor in the eye, revealing the visual acuity for all parts of the visual field. We then use this information to generate visual filters that can be applied to high resolution images or videos to convert them to accurate representations of the spatial information seen by the animal. The method is here applied to the 0.1 mm eyes of the velvet worm Euperipatoides rowelli (Onychophora). These eyes of these terrestrial invertebrates consist of a curved cornea covering an irregular but optically homogeneous lens directly joining a retina packed with photoreceptive rhabdoms. 3D reconstruction from histological sections revealed an asymmetric eye, where the retina is deeper in the forward-pointing direction. The calculated visual acuity also reveals performance differences across the visual field, with a maximum acuity of about 0.11 cycles/deg in the forward direction despite laterally pointing eyes. The results agree with previous behavioural measurements of visual acuity, and suggest that velvet worm vision is adequate for orientation and positioning within the habitat.

The relationship of pre-corneal to pre-contact lens non-invasive tear breakup time

Purpose

To examine the relationship between pre-corneal and pre-contact lens tear film stability (TFS), and to determine whether pre-corneal TFS is a reliable predictor of subsequent pre-lens TFS after a contact lens is placed on the eye.

Methods

667 records met inclusion criteria and were extracted from a soft contact lens multi-study database. Multivariable linear mixed effects models were fit to examine the association between pre-corneal and pre-lens TFS, adjusting for potential confounders and accounting for repeated measures. Receiver Operating Characteristic (ROC) analysis was employed to assess the predictive performance of pre-corneal TFS for subsequent pre-lens TFS. TFS was quantified for this analysis as the non-invasive tear breakup time (NITBUT).

Results

Pre-corneal NITBUT was significantly related to the pre-lens NITBUT at both 10 min (p<0.001) and 2–6 hrs (p<0.001) post-lens insertion. However, the sensitivities of pre-corneal NITBUT for predicting symptom-associated thresholds of pre-lens NITBUT ranged from 50–65%, and specificities ranged from 57–72%, suggesting poor-to-moderate diagnostic performance.

Conclusions

Despite the association of pre-corneal and pre-lens TFS, the inherent lability and sensitivity to environmental exposures of the tear film introduce significant variability into NITBUT measurements. Using pre-corneal NITBUT to identify likely successful contact lens candidates prior to fitting is thus not sufficiently accurate to be relied upon in the clinical setting.

Light Polarization by Biological Nanocoatings

Light plays paramount functions for living beings in nature. In addition to color, the polarization of light is used by many animals for navigation and communication. In this study, we describe the light polarizing role of special nanostructures coating cuticular surfaces of diverse arthropods. These structures are built as parallel nanoscale ridges covering the eyes of the sunlight-navigating spider Drassodes lapidosus and of the water pond-swarming black fly Simulium vittatum, as well as the light-emitting abdominal lantern of the firefly Aquatica lateralis. Exact topography and dimensions of the parallel nanoridges provide different light polarizing efficiencies and wavelength sensitivity. Optical modeling confirms that the nanoscale ridges are responsible for the spectral polarization dependency. Co-opting from our recent work on the self-assembly of Drosophila corneal nanostructures, we engineer arthropod-like parallel nanoridges on artificial surfaces, which recapitulate the light polarization effects. Our work highlights the fundamental importance of nanocoatings in arthropods for the light polarization management and provides a new biomimetic approach to produce ordered nanostructures under mild conditions.

In vivo measurements of corneal birefringence properties using the one-way reflective Mueller polarimetry

This work presents the results of in vivo measurements of human corneal birefringence properties using the one-way reflective polariscope, in which the same module is used to generate and analyze the light polarization state. Data analysis was performed with the use of Mueller polarimetry techniques. The distribution of the azimuth angle and the phase retardation was determined on the paracentral and limbal areas. The results indicate growth of the phase retardation magnitude and the radial orientation of the azimuth angle in the cornea's peripheral region, which confirms our assumptions and the results presented by other researchers.

The Role of Eph Receptors and Ephrins in Corneal Physiology and Diseases

The cornea, while appearing to be simple tissue, is actually an extremely complex structure. In order for it to retain its biomechanical and optical properties, perfect organization of its cells is essential. Proper regeneration is especially important after injuries and in the course of various diseases. Eph receptors and ephrin are mainly responsible for the proper organization of tissues as well as cell migration and communication. In this review, we present the current state of knowledge on the role of Eph and ephrins in corneal physiology and diseases, in particular, we focused on the functions of the epithelium and endothelium. Since the role of Eph and ephrins in the angiogenesis process has been well established, we also analyzed their influence on conditions with corneal neovascularization.

Applications of high‐speed videokeratoscopy

High-speed videokeratoscopy is an emerging technology that has the potential to provide new information on dynamic changes of corneal topography and tear film behaviour. We have developed a high-speed videokeratoscope that has the ability to acquire data at the rate of 50 Hz. Two major applications of the technology are considered in this paper. First, the analysis of tear film stability in the inter-blink interval is evaluated and techniques for estimating the tear film build-up and break-up times are considered. The second application involves the study of the dynamic response of the corneal anterior surface to mechanical forces exerted by the eyelids during horizontal eye movements in downward gaze. The limitations and potential opportunities for the use of this new technology are discussed.

Human Cadaveric Donor Cornea Derived Extra Cellular Matrix Microparticles for Minimally Invasive Healing/Regeneration of Corneal Wounds

Biological materials derived from extracellular matrix (ECM) proteins have garnered interest as their composition is very similar to that of native tissue. Herein, we report the use of human cornea derived decellularized ECM (dECM) microparticles dispersed in human fibrin sealant as an accessible therapeutic alternative for corneal anterior stromal reconstruction. dECM microparticles had good particle size distribution (≤10 µm) and retained the majority of corneal ECM components found in native tissue. Fibrin-dECM hydrogels exhibited compressive modulus of 70.83 ± 9.17 kPa matching that of native tissue, maximum burst pressure of 34.3 ± 3.7 kPa, and demonstrated a short crosslinking time of ~17 min. The fibrin-dECM hydrogels were found to be biodegradable, cytocompatible, non-mutagenic, non-sensitive, non-irritant, and supported the growth and maintained the phenotype of encapsulated human corneal stem cells (hCSCs) in vitro. In a rabbit model of anterior lamellar keratectomy, fibrin-dECM bio-adhesives promoted corneal re-epithelialization within 14 days, induced stromal tissue repair, and displayed integration with corneal tissues in vivo. Overall, our results suggest that the incorporation of cornea tissue-derived ECM microparticles in fibrin hydrogels is non-toxic, safe, and shows tremendous promise as a minimally invasive therapeutic approach for the treatment of superficial corneal epithelial wounds and anterior stromal injuries.

All-printed stretchable corneal sensor on soft contact lenses for noninvasive and painless ocular electrodiagnosis

Electroretinogram examinations serve as routine clinical procedures in ophthalmology for the diagnosis and management of many ocular diseases. However, the rigid form factor of current corneal sensors produces a mismatch with the soft, curvilinear, and exceptionally sensitive human cornea, which typically requires the use of topical anesthesia and a speculum for pain management and safety. Here we report a design of an all-printed stretchable corneal sensor built on commercially-available disposable soft contact lenses that can intimately and non-invasively interface with the corneal surface of human eyes. The corneal sensor is integrated with soft contact lenses via an electrochemical anchoring mechanism in a seamless manner that ensures its mechanical and chemical reliability. Thus, the resulting device enables the high-fidelity recording of full-field electroretinogram signals in human eyes without the need of topical anesthesia or a speculum. The device, superior to clinical standards in terms of signal quality and comfortability, is expected to address unmet clinical needs in the field of ocular electrodiagnosis.

Comparison of porcine corneal decellularization methods and importance of preserving corneal limbus through decellularization

Background

The aim of this study is to compare the three previously applied, conventional porcine corneal decellularization methods and to demonstrate the importance of preserving the corneal limbus through decellularization.

Methods

Fresh, wild-type (with or without) limbus porcine corneas were decellularized using three different methods, including (i) sodium dodecyl sulfate (SDS), (ii) hypertonic saline (HS), and (iii) N₂ gas (NG). Post-treatment evaluation was carried out using histological, residual nuclear material, and ultrastructural analyses. Glycerol was used to help reduce the adverse effects of decellularization. The corneas were preserved for two weeks in cornea storage medium.

Results

All three decellularization methods reduced the number of keratocytes at different rates in the stromal tissue. However, all methods, except SDS, resulted in the retention of large numbers of cells and cell fragments. The SDS method (0.1% SDS, 48h) resulted in almost 100% decellularization in corneas without limbus. Low decellularization capacity of the NG method (<50%) could make it unfavorable. Although HS method had a more balanced damage-decellularization ratio, its decellularization capacity was lower than SDS method. Preservation of the corneoscleral limbus could partially prevent structural damage and edema, but it would reduce the decellularization capacity.

Conclusion

Our results suggest that SDS is a very powerful decellularization method, but it damages the cornea irreversibly. Preserving the corneoscleral limbus reduces the efficiency of decellularization, but also reduces the damage.

Determining the willingness of Australians to export their corneas on death

Background

12.7 million people await a corneal transplant, but 53% are without access to corneal tissue. Sharing corneal tissue across nations can provide some access, however the willingness of export populations, like Australians, to export their donation on death, has never been evaluated. Our research samples the Australian population, determining their willingness to export.

Materials and method

We conducted e-surveys. N = 1044 Australians participated. The sample represented the Australian population, based on population demographics. Chi-Square and bivariate correlation coefficients examined associations between categorical variables, with a sample size of N = 1044, power of 0.80, and alpha of p = 0.05. Outcome measures were based on population sampling, by exploring willingness export, through the e-survey method.

Results

38% (n = 397) of respondents said yes to exportation, 23.8% (n = 248) said no, and 38.2% (n = 399) were undecided. We found no relationship between willingness to export and general demographics, though those registered on the Donatelife Register (p = < .001), and those already willing to donate their eyes (p = < .001) were significantly more willing to export.

Discussion

More Australians are willing to export their corneas than not, though a significant portion remain undecided. The Donatelife Register, and donation awareness, are key components of respondent decision making. Therefore, the provision of information about exportation prior to, and at the point-of-donation, is essential for assisting Australian’s to decide to export or not. Further examination and development of consent-for-export systems are necessary before routine exportation is undertaken.

Correlation between corneal thickness, keratometry, age, and differential pressure difference in healthy eyes

To determine the use of differential pressure difference (DPD), in air-puff differential tonometry, as a potential biomechanical measure of the cornea and elucidate its relationship with the intraocular pressure (IOP), central corneal thickness, corneal curvature, and age. This study comprised 396 eyes from 198 patients and was conducted at Acibadem University, School of Medicine, Department of Ophthalmology, Istanbul, Turkey. The central corneal curvature and refraction of the eyes were measured using an Auto Kerato-Refractometer (KR-1; Topcon Corporation, Tokyo, Japan). IOP and central corneal thickness were measured using a tono-pachymeter (CT-1P; Topcon Corporation, Tokyo, Japan), wherein two separate readings of IOP were obtained using two different modes: 1-30 and 1-60. The difference between these two readings was recorded as the DPD. The factors affecting the DPD were determined by stepwise multiple linear regression analysis. DPD varied over a dynamic range of - 3.0 to + 5.0 mmHg and was weakly correlated with the central corneal thickness (r = 0.115, p < 0.05). DPD showed no significant correlation with IOP 1-30 (p > 0.05). A weak but statistically significant (p < 0.05) positive correlation of DPD was observed with age (r = 0.123), Kavg (r = 0.102), and the CCT (r = 0.115). There was a significant correlation between DPD and Kavg, CCT, and age. There was no significant correlation between DPD and IOP 1-30. Age-related changes in the corneal ultrastructure may be a plausible explanation for the weak positive association between age and DPD. The proposed method may prove a valid non-invasive tool for the evaluation of corneal biomechanics and introduce DPD in the decision-making of routine clinical practice.