Ageing of the human corneal stroma: structural and biochemical changes

Factors Associated With Corneal Hysteresis in an Elderly White Population: The Thessaloniki Eye Study

PRCIS

Corneal hysteresis (CH) was positively associated with female sex and central corneal thickness, and negatively with age and axial length. Diabetes showed no significant association with CH in the adjusted models.

PURPOSE

To provide values of corneal hysteresis (CH) in an elderly, healthy Greek population and to investigate its association with demographic, ocular, and systemic factors.

METHODS

Cross-sectional population-based study. Out of 1092 participants in the incidence cohort of the Thessaloniki Eye Study (TES) who were re-examined between 2013 and 2015, there were 801 eligible subjects in this study according to prespecified criteria. All subjects underwent an interview and a comprehensive clinical examination. Demographic data and medical and ophthalmic, family, and smoking histories were recorded. CH was evaluated with an Ocular Response Analyzer (ORA).

RESULTS

The mean age of study subjects was 79.7±3.9 years and 345 out of 801 participants (43.1%) were female. The mean CH among all subjects was 10.1±1.5 mm Hg. After adjusting for age, sex, central corneal thickness (CCT), axial length (AL), and diabetes, CH was positively associated with female sex (β=0.446; P <0.0001) and CCT (β=0.017; P <0.0001), and negatively associated with age (β=-0.024; P =0.038) and AL (β=-0.181; P <0.0001). There was no association between CH and diabetes (β=0.186; P =0.10). In a supplementary multivariable analysis of a smaller sample size, where we included corneal curvature (CC) as an additional covariate, no association was found between CH and CC (β=-0.235; P =0.26) or age (β=-0.023; P =0.09).

CONCLUSION

This study outlined variations in CH among a Greek population. CH decreased with advancing age while women had greater CH compared with men. In addition, eyes with thinner corneas and longer AL had lower CH values. No significant associations were found between CH and CC or the presence of diabetes.

Comparison of Keratoconus Progression Rate between Adolescents Aged 19-24 Years and Young Adults: Impact on Indication for Crosslinking

PURPOSE

To investigate the keratoconus characteristics and progression rate in a patient group of adolescents aged 19-24 years and to compare the results with young adults aged 25-30 years.

METHODS

A total of 158 keratoconic eyes (82 and 76 eyes in the adolescent and young adult groups) of the Homburg Keratoconus Center, which were examined by Scheimpflug tomography at least two times, were included in this retrospective study. The visual characteristics, corneal tomography, and biomechanical measurements were noted at the initial visit, 6 months, 12 months, and 24 months after the initial visit. Progression rates and the amounts of change were calculated and compared between groups.

RESULTS

Baseline visual, tomographical, and biomechanical characteristics along with keratoconus staging were similar between groups at the initial visit (p > .05 for all values). Progression rate was 76.8% vs 14.9% with respect to Kmax change, 72.3% vs 12.0% with respect to ABC parameters, and 26.4% vs 16.4% with respect to E staging in the adolescent vs young adult groups, respectively (p < .001, p < .001, and p = .25).

CONCLUSIONS

Keratoconus may worsen more rapidly in adolescent patients aged 19-24 years compared to young adult patients aged 25-30 years. This study emphasizes the high progression rate in adolescent patients and the consideration of early cross-linking and/or at least closer follow-ups in this age group.

Effect of axial elongation on anterior scleral thickness in myopia

OBJECTIVE

This study aimed to investigate the effect of axial elongation on anterior scleral thickness (AST) in myopia.

METHODS

The convenience sampling method was used to select 122 patients without ocular or systemic diseases affecting ametropia in Ya'an People's Hospital between March 2020 and January 2022. According to the diopter and axial length (AL), the patients were divided into an emmetropia group (32 cases), a low-to-moderate myopia group (40 cases), and a high myopia group (40 cases). Relevant data were collected and analyzed.

RESULTS

The AST of the four diameter lines was highest at the scleral process and measured. The results, in descending order, were as follows: temporal side (716.45 ± 65.55 μm), lower side (692.32 ± 74.76 μm), nasal side (671.54 ± 61.76 μm), and superior side (550.76 ± 51.34 μm). There was a correlation between the AL and the average AST of the four diameter lines. The correlations, from highest to lowest, were as follows: the mean AST of the nasal side (r = -0.455, P < 0.001), the mean AST of the upper side (r = -0.431, P < 0.001), the mean AST of the lower side (r = -0.402, P < 0.001) and the mean AST of the temporal side (r = -0.363, P < 0.001). Age was associated with the average AST of the upper, nasal, and temporal sides. The correlations, from highest to lowest, were as follows: the average AST of the upper side (r = 0.545, P < 0.001), the average AST of the temporal side (r = 0.422, P < 0.001), and the average AST of the nasal side (r = 0.411, P < 0.001). There were significant differences in AST among the three groups at 0, 1, 2, 3, 4, 5, and 6 mm from the scleral bulge (P < 0.05). AST was positively correlated with age and negatively correlated with AL (R2= 0.432, F = 231.73, P < 0.05).

CONCLUSION

The thickness of the sclera changed with the AL and measurement position. AST was negatively correlated with the AL and positively correlated with age. Compared with emmetropia and low-to-moderate myopia, the anterior sclera in cases of high myopia was thinner. Myopia affects not only the thickness of the posterior sclera but also the AST.

The Impact of Standard Cross-Linking on the Corneal Optical Density-Age Relationship in Keratoconus After Mechanical Stripping of the Epithelium

Background: To determine if cross-linking (CXL) treatment modifies any pre-existing association between corneal optical density (COD) and age in keratoconus free of corneal scarring. Methods: COD was monitored in two groups (i) before and after standard CXL treatment for keratoconus (de-epithelization with a crescent blade, n = 69 eyes) and (ii) age/gender-matched cases without any signs of keratoconus (n = 24 eyes). Seven different markers of COD were quantified using a 0-100 grey scale, supplied with Pentacam™ software. Results: Mean age (±sd, range) in Group I (19 females and 50 males) was 24.2 years (±7.2, 11-44) and in Group II (9 females and 15 males), it was 24.7 years (±7.6, 17-45). COD over the apex and along the depth of the cornea (y, arbitrary scale units) was associated with age (x, in years) in Group I at preop (y = 0.08x + 13.12, r s  = 0.350 and p=0.003), at 12 months postop (y = 0.08x + 15.15, r s  = 0.295 and p=0.014) and in Group II, at the start (y = 0.16x + 11.28, r s  = 0.474 and p=0.019) and 12 months later (y = 0.24x + 8.63, r s  = 0.600 and p=0.002). The change in COD following CXL was significantly associated with the preop value. Conclusion: CXL initially breaks down the pre-existing relationship between COD and age. This is re-established by 12 months postop. The CXL induced change in COD depends on the preop value but not on the patient's age.

Age-related analysis of corneal biomechanical parameters in healthy Chinese individuals

To report the correlation between corneal biomechanical parameters and age in healthy Chinese individuals. This cross-sectional study was conducted on 864 eyes of 543 healthy participants. A comprehensive ophthalmic examination and corneal biomechanics examination using Corneal Visualization Scheimpflug Technology (Corvis ST) were conducted. Based on age, all participants were further divided into five age groups (n) as follows: group A, 11-20 years (105); group B, 21-30 years (112); group C, 31-40 years (113); group D, 41-50 years (100); and group E, > 50 years (113). Using Corvis ST, we examined 35 corneal biomechanical parameters and compared them across the different age groups. Spearman's correlation coefficients and stepwise multivariate linear regression models were used to investigate whether the corneal biomechanical parameters were related to demographic and ocular characteristics. A correlation analysis between the left and right eyes revealed that 13 parameters were significantly associated with eye differences. Among the 35 corneal biomechanical parameters, 28 exhibited significant differences across the age groups, with stiffness parameter at applanation 1(SPA1) showing an upward trend after the age of 30 and stress-strain index (SSI) demonstrates a statistically significant upward trend when comparing the five age groups in the study. Additionally, Spearman's correlation analysis and stepwise multivariate linear regression analysis revealed that 11 corneal biomechanical parameters were positively correlated with age and 10 were negatively correlated with age. Corvis biomechanical index (CBI) was significantly negatively correlated with intraocular pressure (IOP) and central corneal thickness (CCT), SSI was significantly positively correlated with age and IOP, and SPA1 were positively correlated with IOP and CCT. In conclusion, most corneal biomechanical parameters showed a significant correlation with age, with corneal stiffness progressively increasing alongside advancing age, IOP, or CCT.

Crosslinking with UV-A and riboflavin in progressive keratoconus: From laboratory to clinical practice - Developments over 25 years

Changes in the biomechanical and biochemical properties of the human cornea play an important role in the pathogenesis of ectatic diseases. A number of conditions in primarily acquired (keratoconus or pellucid marginal degeneration) or secondarily induced (iatrogenic keratectasia after refractive laser surgeries) ectatic disorders lead to decreased biomechanical stability. Corneal collagen cross-linking (CXL) represents a technique to slow or even halt the progression of ectatic pathologies. In this procedure, riboflavin is applied in combination with ultraviolet A radiation. This interaction induces the production of reactive oxygen species, which leads to the formation of additional covalent bonds between collagen molecules and subsequent biomechanical corneal strengthening. This procedure is so far the only method that partially interferes etiopathogenetically in the treatment of ectatic diseases that slows or stops the process of corneal destabilization, otherwise leading to the need for corneal transplantation. Besides, CXL process increases markedly resistance of collagenous matrix against digesting enzymes supporting its use in the treatment of corneal ulcers. Since the discovery of this therapeutic procedure and the first laboratory experiments, which confirmed the validity of this method, and the first clinical studies that proved the effectiveness and safety of the technique, it has been spread and adopted worldwide, even with further modifications. Making use of the Bunsen-Roscoe photochemical law it was possible to shorten the duration of this procedure in accelerated CXL and thus improve the clinical workflow and patient compliance while maintaining the efficacy and safety of the procedure. The indication spectrum of CXL can be further expanded by combining it with other vision-enhancing procedures such as individualized topographically-guided excimer ablation. Complementing both techniques will allow a patient with a biomechanically stable cornea to regularize it and improve visual acuity without the need for tissue transplantation, leading to a long-term improvement in quality of life.

Age-related alterations in vortex veins on indocyanine green angiography

To determine age-related alterations in vortex veins in healthy subjects. A total of 228 healthy subjects (aged 4 to 86 years) were recruited and divided into four groups (G1, <21 years; G2, 21-40 years; G3, 41-60 years; and G4, 61-86 years). The clinical characteristics of the participants were recorded, and parameters including the number of vortex vein roots (NVVR), the central vortex vein diameter (CVVD), the mean root area of the vortex vein (MRAVV), and the weighted mean of the thickest branch diameter (WMTBD) were obtained by marking the vortex veins on indocyanine green angiography (ICGA). The NVVR in the age group over 60 years old was significantly lower than that in other age groups (P < 0.05). The CVVD, MRAVV, and WMTBD of all age groups increased with increasing age (P < 0.05). The NVVR was unevenly distributed among the quadrants (P < 0.001). The proportions of type four vortex veins (complete systems including ampulla) and anastomotic branches of the vortex veins were significantly increased in elderly participants over 50 years of age (P < 0.05). Subfoveal choroidal thickness was significantly correlated with age, NVVR, CVVD and MRAVV (P < 0.05). This is the first study to reveal age-related alterations in vortex veins on ICGA in a healthy population. Aging may lead to partial vortex occlusion and residual vortex dilation. As age increases, anastomotic branches increasingly appear between the originally independent vortex veins. Translational relevance: Aging may lead to partial vortex occlusion and residual vortex dilation.

Changes in Stress-Strain Index in School-Aged Children: A 3-Year Longitudinal Study

Purpose

To determine three-year change of the corneal biomechanical parameter stress-strain index (SSI) in schoolchildren aged 7- 9 years and their correlation with refractive error and axial length (AL).

Methods

This is a prospective cohort study. Data of the AL, refractive error, and corneal biomechanical parameter SSI were collected at baseline and a 3-year follow-up for 217 schoolchildren. SSI, AL, and refractive error were measured via corneal visualization Scheimpflug technology (Corvis ST), IOLMaster biometry, and cycloplegic refraction. Three years of changes in SSI and its association with refractive error and AL were analyzed. Participants were divided into persistent nonmyopia (PNM), newly developed myopia (NDM), and persistent myopia (PM). The three-year difference in SSI among the three groups was analyzed.

Results

After three years of follow-up, the corneal biomechanical parameter SSI decreased in all participants (P < 0.01). There was a negative correlation between the change in SSI and the change in AL (r = -0.205, P=0.002) and a positive correlation between the change in refractive error (r = 0.183, P=0.007). After three years of follow-up, there was a decrease in the SSI for the NDM, PM, and PNM participants, with a median change of -0.05 for PNM and -0.13 and -0.09 for the NDM and PM, respectively. There was a significant decrease in corneal biomechanical properties for NDM patients compared with PNM patients (P < 0.01).

Conclusion

In 7- to 9-year-old schoolchildren, SSI decreased after three years of the longitudinal study, and the change in SSI was correlated with the change in AL and refractive error. There was a rapid decrease in corneal biomechanical properties among newly developed myopic patients.

Twelve-Month Clinical and Histopathological Performance of a Novel Synthetic Cornea Device in Rabbit Model

Purpose

To report the biological stability and postoperative outcomes of a second-generation, single-piece, flexible synthetic cornea in a rabbit model.

Methods

Device materials and design were amended to enhance biointegration. Optic skirt design devices were made from compact perfluoroalkoxy alkane with porous expanded polytetrafluoroethylene ingrowth surface overlying the skirt and optic wall. Sixteen devices were implanted into intrastromal pocket in rabbit eyes. Rabbits were randomly assigned to 6- and 12-month follow-up cohorts (n = 8 in each) postoperatively. Monthly examinations and optical coherence tomography assessed cornea-device integration, iridocorneal angle, optic nerve, and retina.

Results

There were no intraoperative complications. All devices were in situ at exit, with clear optics. No retroprosthetic membrane, glaucoma, cataract formation, or retinal detachment was observed. Two rabbits in the 6-month group had mild, focal anterior lamella thinning without retraction adjacent to the optic near tight sutures. Three postoperative complications occurred in the 12-month group. One rabbit diagnosed with endophthalmitis was euthanized on day 228. Mild sterile focal retraction of anterior lamella occurred in two rabbits, which were terminated on days 225 and 315. Light microscopic examination of enucleated globes demonstrated fibroplasia with new collagen deposition into the porous scaffold without significant inflammation, encapsulation, or granuloma formation.

Conclusions

Clinical evaluations, imaging, and histopathological findings indicate favorable outcomes of this synthetic corneal device in a rabbit model. Early feasibility studies in humans are being planned.

Translational Relevance

Favorable 12-month results of the device in rabbits demonstrate vision-restoring potential in corneally blind individuals at high risk of failure with donor keratoplasty.

Analysis of potential impact factors of corneal biomechanics in myopia

PURPOSE

To investigate potential impact factors associated with corneal biomechanical properties in Chinese myopia and further to investigate quantifying corneal biomechanics in clinical work.

METHODS

Three hundred fifty-five eyes from 181 healthy myopic subjects with a mean age of 25.1 ± 9.4 were recruited in this study. Each patient carried out a comprehensive ophthalmic examination, including corneal hysteresis(CH), corneal resistance factor(CRF), central corneal thickness(CCT), axial length(AL), intraocular pressure(IOP), spherical equivalence(SE) and corneal curvature (K). CH and CRF were measured with the ocular response analyzer(ORA). To analyze the corneal biomechanical characteristics of myopia patients and their association with other parameters.

RESULT

The multiple linear regression analysis showed that CH was positively associated with CCT, and corneal curvature (all with P < 0.05) and negatively associated with SE and AL)(all with P < 0.05); CRF was positively correlated with CCT, corneal curvature and IOP(all with P < 0.05), but negatively correlated with SE and AL(all with P < 0.05). The CH and CRF values in children group were both higher than adults group (≥ 18 years old) (P < 0.05), but it attributed to that the CCT of children was thicker than adults. Excluding factor of CCT, there was no significant difference in CH and CRF between children group and adult group. Excluding factor of CCT, there was no significant difference in CH and CRF among different stage of age (age 18-48).

CONCLUSION

The CCT played the most important role of affecting the CH and CRF. The SE, corneal curvature, AL and IOP had a certain influence on corneal biomechanics. Whether the CH and CRF values of individual patient are normal in clinical work, it should refer to the CH and CRF values corresponding CCT sectional range and SE.

Colombian Ocular Diseases Epidemiology Study (CODES): incidence and sociodemographic characterisation of keratoconus between 2015 and 2020

OBJECTIVE

To estimate the incidence and describe the demographic characteristics of keratoconus (KC) in Colombia using national health registry data between January 1st 2015 and December 31th 2020.

METHODS AND ANALYSIS

We conducted a nationwide, population-based study using the Integrated Social Protection Information System from the Colombian Ministry of Health, the unique official database in the country. We used the International Classification of Diseases code for KC (H186) to identify the number of new cases of KC and estimate the incidence rates both overall and according to age and sex. We made a standard morbidity ratio map to graph Colombia's morbidity risk of KC onset.

RESULTS

Of 50 372 424 subjects, 21 710 had KC between 2015 and 2020. However, due to the COVID-19 pandemic, all the incidence rates of this study were based on the 18 419 reported until 2019. The incidence rate in the general population was 10.36 (95% CI 10.08 to 10.64) per 100 000 inhabitants. The incidence peak among males was in their early 20s and females in their late 20s. The overall male to female incidence rate ratio was 1.60. Regarding the distribution of the disease, most cases were reported in Bogotá (48.64%), Antioquia (14.04%) and Cundinamarca (10.38%).

CONCLUSION

We performed the first nationwide, population-based study of KC in Latin America, finding distribution patterns similar to those reported in the literature. This study provides valuable information on the epidemiology of KC in Colombia, which is helpful in the development of policies for the diagnosis, prevention and treatment of the disease.

Comparison of anterior sclera thickness in emmetropes and myopes

BACKGROUND

This study aimed to compare anterior scleral thicknesses (ASTs) in people with emmetropia and myopia to explore the effect of myopia on AST.

METHODS

In this cross-sectional study, 93 participants (i.e., 93 eyes) with emmetropia and myopia underwent ocular imaging via anterior segment optical coherence tomography. We acquired raw B-scan OCT images along each of the four meridians (superior, inferior, nasal, and temporal), The AST was estimated from the limbus to a distance of 6 mm. The participants were aged between 20 and 50 years (mean age: 30.2 ± 8.8 years). The axial length (AL) was 22.50 ~ 33.04 mm (mean AL: 26.51 ± 2.65 mm), and the spherical equivalent (SE) was + 0.50 ~ 27.5 D (mean SE: -7.20 ± 6.5 D). The selected sample comprised 37 males and 56 females who were categorized as emmetropes, mild-moderate myopes, or high myopes. The four meridians of AST, AL, and refractive error were observed.

RESULTS

The AL was significantly negatively correlated with the four meridians of AST (the r value ranged between - 0.511 and - 0.228, P < 0.05). There was no significant correlation between age and inferior diameter (r = 0.113, P = 0.314), but age was positively correlated with the average AST of the superior, temporal, and nasal diameters (the r value ranged between 0.452 and 0.552, P < 0.05). There was no significant correlation between sex and AST (the T value ranged between - 1.816 and - 0.130, P > 0.05). Except for the inferior diameters of 1 mm, 5 mm, and 6 mm and the temporal diameter of 1 mm, the four diameters in the emmetropia group and the high myopia group were statistically significant at a distance of 0 ~ 6 mm from the limbus (P < 0.05).

CONCLUSION

The AST is negatively correlated with AL and positively correlated with age. Compared with emmetropic eyes, the AST is thinner in highly myopic eyes. Myopia affects AST, which may be useful for monitoring progression in cases of myopia.

Aging Effects on Optic Nerve Neurodegeneration

Common risk factors for many ocular pathologies involve non-pathologic, age-related damage to the optic nerve. Understanding the mechanisms of age-related changes can facilitate targeted treatments for ocular pathologies that arise at any point in life. In this review, we examine these age-related, neurodegenerative changes in the optic nerve, contextualize these changes from the anatomic to the molecular level, and appreciate their relationship with ocular pathophysiology. From simple structural and mechanical changes at the optic nerve head (ONH), to epigenetic and biochemical alterations of tissue and the environment, multiple age-dependent mechanisms drive extracellular matrix (ECM) remodeling, retinal ganglion cell (RGC) loss, and lowered regenerative ability of respective axons. In conjunction, aging decreases the ability of myelin to preserve maximal conductivity, even with "successfully" regenerated axons. Glial cells, however, regeneratively overcompensate and result in a microenvironment that promotes RGC axonal death. Better elucidating optic nerve neurodegeneration remains of interest, specifically investigating human ECM, RGCs, axons, oligodendrocytes, and astrocytes; clarifying the exact processes of aged ocular connective tissue alterations and their ultrastructural impacts; and developing novel technologies and pharmacotherapies that target known genetic, biochemical, matrisome, and neuroinflammatory markers. Management models should account for age-related changes when addressing glaucoma, diabetic retinopathy, and other blinding diseases.

Crosslinking vs. Observation in Fellow Eyes of Keratoconus Patients

Purpose

To evaluate whether unilateral crosslinking (CXL) and conservative follow-up of the fellow eye is an acceptable management strategy in patients with keratoconus (KC).

Methods

Seventy-nine fellow eyes of KC subjects that initially underwent unilateral CXL were included. Thirty fellow eyes ultimately received CXL (group 1) whereas 49 fellow eyes were followed (group 2). Best spectacle corrected visual acuity (BSCVA) and corneal tomographic parameters were collected in all eyes preoperatively and at the last follow-up.

Results

Subjects who received CXL in the fellow eye (group 1) were younger than subjects who did not (group 2, p=0.026). Group 1 eyes had higher baseline K1 (p=0.026), K2 (p=0.006), Km (p=0.01), and Kmax (p=0.002) compared to group 2 eyes. Amongst the 49 naïve fellow eyes (group 2), 19 eyes showed evidence of progression. Progressing naïve eyes had higher baseline K1, K2, Km, and Kmax (p < 0.01); progressors also had thinner pachymetry at the pupil, apex, and thinnest point (p < 0.01). Baseline values of K1 ≥ 43.5 Diopter (D), K2 > 45.1D, Km > 44.3D, K_max > 47.9D, astigmatism > 1.4D, pachymetry at the pupil <475 μm, and thinnest pachymetry <478 μm were tentative predictors of progression in the naïve fellow eye.

Conclusions

Unilateral CXL with vigilant follow up of the fellow eye may be an acceptable management strategy in a subset of KC eyes.

Materials for Implantable Surface Electrode Arrays: Current Status and Future Directions

Surface electrode arrays are mainly fabricated from rigid or elastic materials, and precisely manipulated ductile metal films, which offer limited stretchability. However, the living tissues to which they are applied are nonlinear viscoelastic materials, which can undergo significant mechanical deformation in dynamic biological environments. Further, the same arrays and compositions are often repurposed for vastly different tissues rather than optimizing the materials and mechanical properties of the implant for the target application. By first characterizing the desired biological environment, and then designing a technology for a particular organ, surface electrode arrays may be more conformable, and offer better interfaces to tissues while causing less damage. Here, the various materials used in each component of a surface electrode array are first reviewed, and then electrically active implants in three specific biological systems, the nervous system, the muscular system, and skin, are described. Finally, the fabrication of next-generation surface arrays that overcome current limitations is discussed.

Ten-year longitudinal investigation of astigmatism: The Yamagata Study (Funagata)

Despite numerous investigations into ocular or corneal astigmatism, the dynamic nature of astigmatism remains poorly understood. To reveal potential associations between age and astigmatism, 264 Japanese participants who underwent systemic and ophthalmological examinations in Funagata Town (Yamagata Prefecture, Japan) were evaluated over a 10-year period. Astigmatism was evaluated with regard to the cylinder power, cylinder axis, and vector analyses. Whereas the refractive cylinders showed age-related increases in patients in their 40s to 60s, the corneal cylinders did not change over 10 years. Nevertheless, cylindrical axis of the cornea demonstrated a continuous shift toward against-the-rule (ATR) astigmatism. Vector analyses revealed that the astigmatic shift toward ATR progressed continually after patients reached their 40s, although the shift did not accelerate with age. These novel insights may pave the way for the development of potential strategies for vision correction, including refractive surgeries, and vision-quality maintenance in the elderly.

Distribution of pediatric keratoconus by different age and gender groups

PURPOSE

To evaluate the distribution of pediatric keratoconus patients and the disease severity based on different age and gender groups in China.

MATERIALS AND METHODS

A total of 446 keratoconus eyes in 266 pediatric patients from January 2019 to January 2022 were included in the cross-sectional study. The clinical findings and severity of keratoconus were recorded and Kruskal-Wallis test, chi-squared test, and Cochran-Mantel-Haenszel (CMH) test were used to compare the parameters between different gender and age groups.

RESULTS

The male/female ratio was 353/93 (3.8:1), and the median age was 16 years (range: 6-17 years). Male patients were statistically younger than female patients (P = 0.041). The male/female ratio decreased with age (P for trend = 0.011). The distribution of the topographic keratoconus classification (TKC) stage was significantly different between gender and age groups (all P < 0.05). Male patients had a higher ratio of advanced keratoconus eyes (TKC ≥ 3) than female patients (P < 0.001), and CMH analysis indicated that being a male was a risk factor for advanced keratoconus after controlling for age (odds ratio: 2.581, P < 0.001).

CONCLUSION

Male keratoconus patients were younger, with a higher ratio of advanced keratoconus than female patients in the Chinese pediatric patients evaluated. Multicenter studies with larger sample sizes are necessary in the future.

Comparing the Differences in Slowing Myopia Progression by Riboflavin/Ultraviolet A Scleral Cross-linking before and after Lens-induced Myopia in Guinea Pigs

PURPOSE

To compare the effectiveness and differences in slowing myopia progression in Guinea pigs by riboflavin/ultraviolet A (UVA) scleral cross-linking (sCXL) before and after lens-induced myopia (LIM).

METHODS

Forty 4-week-old Guinea pigs were randomly divided into four groups (n = 10 per group): CXL-A, CXL-B, LIM, and Control groups. The right eyes in CXL-A, CXL-B, LIM groups were treated with -10.00 D lenses from 4 to 10-week old and the left eyes were untreated. In CXL-A and CXL-B groups, riboflavin/UVA sCXL was performed on the right eyes at 4 weeks and 8 weeks of age, respectively. Both eyes were untreated in Control group. The intraocular pressure (IOP), the axial length (AXL), and the refraction were measured in vivo at 4, 8, and 10 weeks of age. At 10 weeks of age, the right eyes were enucleated for the tensile test and transmission electron microscopy observations.

RESULTS

The myopia has been successfully induced in LIM and CXL-B groups during 4-8 weeks. In CXL-A group, the growth rate of AXL and myopic refraction was markedly inhibited during 4-8 weeks and the inhibitory effects diminished during 8-10 weeks. During 8-10 weeks, the growth rate of AXL and myopic refraction in CXL-B were marked suppressed. At 10 weeks of age, the myopia refraction was lower and the AXL was shorter in CXL-A group in comparison to CXL-B group. The IOP was not significantly different among the 4 groups of eyes at 4, 8, and 10 weeks of age. The scleral stiffness, the fibril diameters, and the fibril density of the sclera were significantly increased in CXL-A and CXL-B groups compared to LIM group.

CONCLUSION

Riboflavin/UVA sCXL administrated before and after the myopia modeling could both slow the myopia progression in Guinea pigs. The before-myopia preventative sCXL showed lower myopic refraction in the same age comparison between the cross-linked groups. The effect of riboflavin/UVA sCXL might reduce over time and the long-term effect should be further investigated. This sCXL intervention might control the ultrastructure alterations of the sclera during the myopia remodeling.

Changes in Corneal Morphology with Age in Asian Population: A Multicenter Study of 30,618 Cases

Introduction

To evaluate normal reference ranges for corneal morphological parameters and investigate age-related changes in these parameters in Asian subjects with healthy eyes in order to provide reference data for preoperative evaluation of corneal refractive surgery and the early differential diagnosis of subclinical and asymptomatic keratoconus.

Methods

This cross-sectional, multicenter, observational study was conducted in five provinces of China, from January 2014 through October 2019. It is a retrospective analysis. Examiner-blinded clinical measurements were performed after stratification of the subjects into the following age groups: < 18, 18–30, 31–40, 41–50. We evaluated 30,618 healthy eyes of Chinese subjects who exhibited a normal corneal morphology, had no history of eye surgery or trauma, stopped wearing soft contact lenses for at least 2 weeks (rigid contact lenses for at least 4 weeks), and underwent topographic studies for both eyes on the same day.

Results

While the anterior and posterior corneal curvatures (K1 and K2) increased with age, corneal astigmatism of the anterior and posterior surfaces (ΔK) and central, minimum, and overall corneal thicknesses decreased with age. Age-related decrease of the overall corneal thickness was more obvious toward the periphery. The anterior and posterior corneal surface heights exhibited a decrease and an increase, respectively. Both index of height asymmetry (IHA) and index of vertical asymmetry (IVA) exhibited an increase with age.

Conclusions

The cornea exhibits overall thinning with age and gradually changes from a flat ellipse to an elongated ellipse in Asian individuals with healthy eyes. However, the anterior and posterior surfaces become smoother with age. Owing to the very large number of cases, these small differences are statistically significant. The results obtained are consistent with the hypothesis that a normal cornea seems to withstand quite well the effect of IOP, external pressures, and the natural cross-linking.

Corneal biomechanical properties following corneal cross-linking: Does age have an effect?

PURPOSE

To explore the effect of age on corneal biomechanical properties following corneal cross-linking (CXL).

METHODS

A total of 12 pairs of human eye-banked corneas (24 corneas, from 14 females and 10 males) were used in the study. The mean donor age was 48.5 years (ranging from 26 to 71 years). Corneas were divided into three age groups: A (26-41 years), B (42-57 years) and C (58-71 years), with four pairs in each group. For each pair, the right corneas were cross-linked using accelerated CXL with UVA (10 mW/cm²) and riboflavin, while the left corneas served as controls and were not exposed to either UVA irradiation or riboflavin. The corneal elastic modulus of the anterior, mid and posterior corneal stroma was measured using nanoindentation.

RESULTS

The difference in the corneal elastic modulus following CXL was significant in the anterior (p = 0.00002) and mid stroma (p = 0.001); however, the difference was not significant in the posterior stroma (p = 0.27) when compared to control corneas. The corneal elastic modulus of the anterior stroma increased by 178.44% in Group A, 119.7% in Group B and 50.73% in Group C compared to control corneas. For the mid stroma, the elastic modulus increased by 47.35% in Group A, 25% in Group B and 24.56% in Group C. No differences were observed in the posterior stroma between age groups.

CONCLUSIONS

Corneal elasticity showed a greater response to CXL in the younger group compared to older groups. CXL treatment showed effectiveness in enhancing stromal strength, and the effect was concentrated in the anterior and mid stroma with minimal impact on the posterior stroma in all age groups.

Update and Review of Diagnosis and Management of Post-Refractive Surgery Ectasia

ABSTRACT

Post-Refractive Surgery Ectasia is a serious, sight-threatening, and highly - avoided complication seen after the following procedures: Laser in situ Keratomileusis, Photorefractive Keratectomy, Small Incision Lenticule Extraction, Radial and/or Arcuate Keratotomy. Specific risk factors may include age, corneal thickness, degree of refractive error, corneal topographic changes including irregular astigmatism, percent tissue ablation, and residual stromal bed. Biomarkers may be a new option to help indicate who is at greatest risk for ectasia. Visual aids including glasses or contacts lenses are often required to achieve optimal vision. Collagen crosslinking is the only treatment thought to stop progression of ectasia and prevent keratoplasty. Other surgical options may include topography-guided phototherapeutic keratectomy and intrastromal corneal ring segments. Ultimately an "ounce of prevention is a pound of cure" so careful preoperative screening and ultimately offering the safest and most effective treatments for patients is arguably the most important job of the refractive surgeon.

Prospective Clinical Study of Keratoconus Progression in Patients Awaiting Corneal Cross-linking

PURPOSE

Keratoconus progression should be treated with corneal cross-linking (CXL) in a timely manner. This study aimed to investigate patient factors associated with keratoconus progression between time of listing and at time of CXL.

METHODS

Prospective observational study at a tertiary center. Ninety-six eyes of 96 patients with keratoconus. Demographic, clinical, and tomographic parameters were analyzed to determine the risk factors for keratoconus progression. Analyzed tomographic indices included steepest keratometry, average keratometry, cornea thinnest point, index of surface variance, index of vertical asymmetry, keratoconus index, center keratoconus index, index of height asymmetry, and index of height decentration.

RESULTS

A total of 38 eyes (39.6%) were found to have keratoconus progression during an average waiting time of 153 ± 101 days. There were significant differences in preoperative tomographic parameters such as index of surface variance (111.3 ± 36.6 vs. 88.3 ± 31.8; P = 0.002), index of vertical asymmetry (1.1 ± 0.4 vs. 0.9 ± 0.4; P = 0.005), keratoconus index (1.31 ± 0.12 vs. 1.22 ± 0.11; P < 0.001), and index of height decentration (0.16 ± 0.07 vs. 0.11 ± 0.06; P = 0.015) between eyes that progressed and those that remained stable. There were no significant differences in steepest keratometry, average keratometry, cornea thinnest point, and center keratoconus index. Multivariate analysis did not reveal age, presence of atopy/atopic keratoconjunctivitis, eye rubbing, or waiting time to be a significant risk factor for progression; however, Maori ethnicity was a risk factor (odds ratio = 3.89; P = 0.02).

CONCLUSIONS

A significant proportion of eyes were found to be progressing while waiting for CXL. A risk stratification score for patients awaiting CXL may reduce the risk of progression.

Corneal Optical Coherence Tomography Speckle in Crosslinked and Untreated Rabbit Eyes in Response to Elevated Intraocular Pressure

Purpose

To ascertain the influence of intraocular pressure (IOP) on corneal optical coherence tomography (OCT) speckle in untreated and ultraviolet A–riboflavin induced corneal collagen crosslinked rabbit eyes.

Methods

Left corneas of eight rabbits were de-epithelialized and crosslinked by applying riboflavin and 30-minute ultraviolet A light exposure. After enucleation (6 months after treatment), each eyeball (treated and untreated) was mounted in a measurement setup, in which IOP was increased from 15 to 45 mm Hg in steps of 5 mm Hg. At each IOP value, single B-scans of the central cornea were acquired three times with the spectral-domain OCT Copernicus-HR. Then, three regions of interest, including the anterior, posterior, and entire corneal stroma, were automatically extracted. Five different probability distributions were used as a model for the corneal speckle and the one with the best goodness of fit was chosen for further analysis.

Results

The generalized gamma distribution achieved the best goodness of fit and its scale (a) and shape (v) parameters statistically significantly changed with increasing IOP in the three regions of analysis (two-way repeated measures analysis of variance, all P < 0.05). The statistically significant difference between untreated and crosslinked eyes was observed for the shape parameters of the posterior and entire corneal stroma.

Conclusions

Corneal OCT speckle is influenced by IOP and shows to be significantly different in untreated and crosslinked eyes. Corneal OCT speckle analysis has the potential to be indirectly used for assessing changes in corneal stroma in ex vivo and in vivo studies.

Translational Relevance

Investigation of corneal OCT speckle statistics can offer additional diagnostic biomarkers related to changes in the corneal stroma after ocular surgeries.

Future Trends in Presbyopia Correction

Presbyopia affects 1.8 billion people worldwide. This reduction in distance corrected near visual acuity impacts quality of life, which prompts patients to seek treatment. Presbyopia is an early manifestation of the "crystalline lens optical dysfunction through aging," or dysfunctional lens syndrome, and appropriate management of presbyopia is dependent on coexisting factors such as increased higher order aberrations, reduced contrast sensitivity, light scatter, and lenticular opacification. This review of published literature (PubMed and MEDLINE) is presented in narrative format and discusses medical and surgical treatments available to patients who experience presbyopia, while highlighting future therapies. Numerous strategies exist for the management of presbyopia. These strategies include pharmacological therapy, glasses and contact lenses, corneal, scleral, and lenticular procedures. This article discusses the role of several new and existing presbyopia treatments, as well as which patients are candidates for these novel therapies. Although no single treatment is ideal for all patients with presbyopia, new medical and surgical strategies increase the number of options available when addressing different stages of presbyopia and dysfunctional lens syndrome. [J Refract Surg. 2021;37(6 Suppl):S28-S34.].

Seven years follow-up of corneal cross-linking (CXL) in pediatric patients: Evaluation of treated and untreated eye

BACKGROUND

Retrospective cohort study evaluating long term keratoconus progression amongst cross-linking (CXL) treated pediatric patients in the treated and the fellow untreated eyes.

METHODS

Data on 60 eyes of 30 patients, 18 years old or younger, who underwent CXL in at least one eye was collected and analyzed. Follow-up measurements taken from the treated and untreated eye up to 7 years after CXL treatment, were compared to baseline measurements. Parameters included uncorrected distance visual acuity (UCDVA), best-corrected spectacle visual acuity (BCSVA), manifest refraction, pachymetry, corneal tomography, and topography.

RESULTS

Mean age of patients was 16 ± 2.1 years. For the treated eyes, during follow-up period mean UCDVA had improved (from 0.78 ± 0.22 at baseline to 0.58 ± 0.26 logMAR at 7 years; p = 0.13), as well as mean BCSVA (from 0.23 ± 0.107 at baseline to 0.172 ± 0.05 logMAR at 7 years; p = 0.37). The mean average keratometry showed a significant flattening (from 49.95 ± 4.04 to 47.94 ± 3.3 diopters (D); p < 0.001), However there was no change in the mean maximal keratometry. The mean minimal corneal thickness (MCT) showed a significant mild reduction of 26 µm (p = 0.006). Although statistically insignificant, the mean manifest cylinder was also reduced to 2D (p = 0.15). During the follow-up period, eight untreated eyes (26.6%) deteriorated and underwent CXL, while only one treated eye (3.33%) required an additional CXL.

CONCLUSION

CXL is a safe and efficient procedure in halting keratoconus progression in the pediatric population, the fellow eye needs to be carefully monitored but only a 25% of the patients will require CXL in that eye during a period of 7 years.

Comparative analysis of the corneal birefringence pattern in healthy children and adults

PURPOSE

To undertake a comparative analysis of the corneal shape, thickness and isochromatics in the eyes of children and adults in order to determine the extent of similarities and differences between the cohorts.

METHODS

The study involved 24 children (aged 8 years) and 37 young White adults (aged between 22-24 years) with no apparent or known health or ocular conditions. Measurements were made of corneal radius of curvature, both central (CCT) and paracentral (PCT) corneal thickness and intraocular pressure (IOP). Images of the isochromatics were captured using a slit lamp and a circular polarizer. The geometry of fringe I and II of the isochromatics was analysed.

RESULTS

Statistically significant differences were found between CCT and PCT in nasal and temporal regions for both the children and adult cohorts. The same trends were observed in the radii of the cornea. Statistically significant differences between side lengths and angles of isochromatic fringes were found. No differences in asymmetry of shape for fringe I between adults and children were detected; greater symmetry was seen in fringe II in children than for adults.

CONCLUSIONS

The asymmetry in corneal shape and curvature contributes to the shape of the isochromatic fringes. This is likely linked to the orientation and parameters of the collagen fibres and to the muscles' forces, and be relevant for surgical procedures such as corneal transplantation.

Corneal biomechanical properties after soft contact lens wear measured on a dynamic Scheimpflug analyzer: A before and after study

OBJECTIVES

To evaluate the corneal biomechanics before and after daily use of contact lenses (CLs), measured by Scheimpflug-based devices.

METHODS

This prospective clinical study includes participants who were scheduled to use CLs daily for refractive error. The biomechanical parameters were measured by the Corneal Visualization Scheimpflug Technology (Corvis ST) before and one month after using the soft CLs.

RESULTS

Twenty-three subjects (46 eyes), including 16 female (76.2%) with a mean age of 28±7.29 years, were enrolled. There was no significant difference among biomechanical factors measured before and after contact lens wear (P>0.05). Using regression analysis of the biomechanical markers, we found a statistically significant association between second applanation length (A2 length) (P=0.001), highest concavity radius (HCR) (P=0.05), deflection amplitude ratio (DA_ratio) (P=0.05) and integrated radius (P<0.001) with age. Regarding spherical equivalent, we found a statistically significant association between central corneal thickness (CCT) (P=0.05), A2 length (P=0.03) and stiffness parameter at first applanation (SPA₁) (P=0.02).

CONCLUSIONS

We did not find a significant difference in terms of corneal biomechanical parameters between baseline and month 1; but regression analyses showed a statistically significant association between A2 length, HCR, DA_ratio, integrated radius, CCT and SPA₁ and certain subject characteristics.

Advanced Research in Scleral Cross-Linking to Prevent From Progressive Myopia

ABSTRACT

Riboflavin-ultraviolet A (UVA) collagen cross-linking (CXL) has been applied in clinical settings to prevent the progression of keratoconus and corneal dilatation caused by other reasons in past decades. As CXL with riboflavin-UVA can enhance the stiffness of collagen-rich tissues, this technique has been further used on sclera to investigate as a safe and effective myopia prevention treatment. Despite the riboflavin-UVA scleral CXL is still in the animal and in vitro experimental phases and the mechanism is not very clear, it is promising to control myopia development clinically. In this article, researches on the laboratory experiments of riboflavin-UVA scleral CXL on scheme exploration and mechanism were reviewed in order to provide more laboratory evidence for scleral CXL in clinical myopia prevention and control in the future.

Keratoconus: cross-linking the window of the eye

Keratoconus is a condition in which the cornea progressively thins and weakens, leading to severe, irregular astigmatism and a significant reduction in quality of life. Although the precise cause of keratoconus is still not known, biochemical and structural studies indicate that overactive enzymes within the cornea break down the constituent proteins (collagen and proteoglycans) and cause the tissue to weaken. As the disease develops, collagen fibres slip past each other and are redistributed across the cornea, causing it to change shape. In recent years, it was discovered that the photochemical induction of cross-links within the corneal extracellular matrix, through the use of riboflavin and ultraviolet (UVA) light, could increase the strength and enzymatic resistance of the tissue and thereby halt keratoconus progression. Worldwide acceptance and use of riboflavin/UVA corneal cross-linking therapy for halting keratoconus progression has increased rapidly, in accordance with the growing body of evidence supporting its long-term effectiveness. This review focusses on the inception of riboflavin/UVA corneal cross-linking therapy for keratoconus, its clinical effectiveness and the latest scientific advances aimed at reducing patient treatment time, improving patient comfort and increasing patient eligibility for treatment.

Plain language summary

Review of current treatments using cross-linking to halt the progress of keratoconus Keratoconus is a disease in which the curved cornea, the transparent window at the front of the eye, weakens, bulges forward into a cone-shape and becomes thinner. This change of curvature means that light is not focussed onto the retina correctly and vision is progressively impaired. Traditionally, the effects of early keratoconus were alleviated by using glasses, specialist contact lenses, rings inserted into the cornea and in severe cases, by performing a corneal transplant. However, it was discovered that by inducing chemical bonds called cross-links within the cornea, the tissue could be strengthened and further thinning and shape changes prevented. The standard cross-linking procedure takes over an hour to perform and involves the removal of the cells at the front of the cornea, followed by the application of Vitamin B2 eye drops and low energy ultraviolet light (UVA) to create new cross-links within the tissue. Clinical trials have shown this standard procedure to be safe and effective at halting keratoconus progression. However, there are many treatment modifications currently under investigation that aim to reduce patient treatment time and increase comfort, such as accelerated cross-linking procedures and protocols that do not require removal of the surface cells. This review describes the different techniques being developed to carry out corneal cross-linking efficiently and painlessly, to halt keratoconus progression and avoid the need for expensive surgery.

Age distribution and associated factors of cornea biomechanical parameter stress-strain index in Chinese healthy population

Background

To investigate the new cornea biomechanical parameter stress-strain index (SSI) in Chinese healthy people and the factors associated with SSI.

Methods

A total of 175 eyes from 175 participants were included in this study. Axial length was measured with the Lenstar LS-900. Pentacam measured curvature of the cornea and anterior chamber volume (ACV). Cornea biomechanical properties assessments were performed by corneal visualization Scheimpflug technology (Corvis ST). Student’s t-test, one-way ANOVA, partial least square linear regression (PLSLR) and linear mixed effects (LME) model were used in the statistical analysis.

Results

The mean (±SD) SSI was 1.14 ± 0.22 (range, 0.66–1.78) in all subjects and affected by age significantly after age of 35 (P < 0.05). In LME models, SSI was significantly associated with age (β = 0.526, P < 0.001), axial length (AL) (β = − 0.541, P < 0.001), intraocular pressure (IOP) (β = 0.326, P < 0.001) and steepest radius of anterior corneal curvature (RsF) (β = 0.229, P < 0.001) but not with ACV, biomechanical corrected intraocular pressure (bIOP), flattest radius of anterior corneal curvature (RfF) or central corneal thickness (CCT) (P > 0.05 for each).

Conclusions

SSI increased with age after the age of 35. In addition to age, SSI was positively correlated with RsF and IOP, while negatively correlated with AL.

Age-Related Changes in Astigmatism and Potential Causes

Astigmatism causes deterioration of the retinal image and affects vision quality. Maintenance and improvement of visual function requires an understanding of the prevalence, age-related changes, and mechanisms of astigmatism. In this article, we discuss the findings of studies that investigated astigmatism. Some of these studies showed that the prevalence of high degrees of astigmatism in childhood typically decreases with emmetropization. With-the-rule astigmatism occurs most commonly in young adults. With age, the prevalence of astigmatism increases, and the axis shifts from a predominance of with-the-rule astigmatism to a predominance of against-the-rule astigmatism. This age-related change is caused by alterations in corneal curvature. Although the cause of this change is not fully understood, alterations in the position and tension of the eyelid, corneal stromal collagen fibrils, Descemet membrane, and extraocular muscles may influence the shape of the cornea. Furthermore, genetic factors may contribute to the development of astigmatism. Technological advances in ophthalmology are expected to improve our understanding of the etiology of astigmatism and enable the maintenance of quality of vision.

Regulation of Keratocyte Phenotype and Cell Behavior by Substrate Stiffness

Corneal tissue engineering is an alternative way to solve the problem of lack of corneal donor tissue in corneal transplantation. Keratocytes with a normal phenotype and function in tissue-engineered cornea would be critical for corneal regeneration. Although the role of extracellular/substrate material stiffness is well-known for the regulation of the cell phenotype and cell behavior in many different cell types, its effects in keratocyte culture have not yet been thoroughly studied. This project studied the effect of substrate stiffness on the keratocyte phenotype marker expression and typical cell behavior (cell adhesion, proliferation, and migration), and the possible mechanisms involved. Human primary keratocytes were cultured on tissue culture plastic (TCP, ∼10⁶ kPa) or on plates with the stiffness equivalent of physiological human corneal stroma (25 kPa) or vitreous body (1 kPa). The expression of keratocyte phenotype markers, cell adhesion, proliferation, and migration were compared. The results showed that the stiffness of the substrate material regulates the phenotype marker expression and cell behavior of cultured keratocytes. Physiological corneal stiffness (25 kPa) superiorly preserved the cell phenotype when compared to the TCP and 1 kPa group. Keratocytes had a larger cell area when cultured on 25 kPa plates as compared to on TCP. Treatment of cells with NSC 23766 (Rac1 inhibitor) mimicked the response in the cell phenotype and behavior seen in the transition from soft materials to stiff materials, including the cytoskeletal structure, expression of keratocyte phenotype markers, and cell behavior. In conclusion, this study shows that substrate stiffness regulates the cell phenotype marker expression and cell behavior of keratocytes by Rac1-mediated cytoskeletal reorganization. This knowledge contributes to the development of corneal tissue engineering.

Effects of Topical Ozone Application on Outcomes after Accelerated Corneal Collagen Cross-linking: An Experimental Study

Purpose

Ozone is a trioxygen molecule that spontaneously degrades into oxygen and oxygen free radicals. This study was designed to assess the effects of topical ozone application on outcomes after corneal collagen cross-linking (CXL).

Methods

Enucleated fresh cadaver yearling sheep eyes (n = 28) were divided into five groups: control (C, n = 6), sham (S, n = 6), ozone only (Z, n = 6), CXL only (X, n = 5), and Ozone + CXL (ZX, n = 5). In all groups, except C, the epithelial layer was removed. In group Z, 20 μg/mL liquid ozone was topically applied. In group X, CXL was performed in the accelerated pulse mode. In group ZX, both CXL and ozone were applied. Post-interventional oxygen levels were determined and corneal confocal microscopy and optical coherence tomography were performed. Corneas were evaluated using light and electron microscopy.

Results

Pre-interventional central corneal thickness (CCT) was highest in the control group and considerably similar in the remaining groups (P = 0.006). Pre- and post-interventional CCT were significantly different in the ozonated groups (Z and ZX) (P = 0.028; P = 0.043). Demarcation line depths were similar in groups Z, X, and ZX (P = 0.343). Increased stromal tissue reflectivity was observed in groups Z, X, and ZX. Oxygen levels were higher in the ozonated groups (Z and ZX) (P = 0.006), and caspase activity was higher in the CXL groups (X and ZX) (P = 0.028) as compared to the other groups. Group ZX showed tighter, more regular, and parallel fibrils.

Conclusion

Ozone increases corneal stromal oxygenation which can probably augment the effect of CXL. Future studies should investigate the safety and feasibility of ozone application during CXL.

Corneal Stiffness and Collagen Cross-Linking Proteins in Glaucoma: Potential for Novel Therapeutic Strategy

Biomechanical properties of the cornea have recently emerged as clinically useful in risk assessment of diagnosing glaucoma and predicting disease progression. Corneal hysteresis (CH) is a dynamic tool, which measures viscoelasticity of the cornea. It represents the overall deformability of the cornea, and reduces significantly with age. Low CH has also been associated with optic nerve damage and progression of visual field loss in glaucoma. The extracellular matrix (ECM) constituents of the cornea, trabecular meshwork (TM), sclera, and lamina cribrosa (LC) are similar, as they are predominantly made of fibrillar collagen. This suggests that biomechanical changes in the cornea may also reflect optic nerve compliance in glaucomatous optic neuropathy, and in the known increase of TM tissue stiffness in glaucoma. Increased collagen cross-linking contributes to tissue stiffening throughout the body, which is observed in normal aging and occurs at an accelerated rate in systemic conditions such as fibrotic and cardiovascular diseases, cancer, and glaucoma. We reviewed 3 ECM cross-linking proteins that may have a potential role in the disease process of increased tissue stiffness in glaucoma, including lysyl oxidase (LOX)/lysyl oxidase-like 1 (LOXL1), tissue transglutaminase (TG2), and advanced glycation end products. We also report elevated messenger RNA (mRNA) levels of LOX and TG2 in glaucoma LC cells to support our proposed theory that increased levels of cross-linking proteins in glaucoma play a role in LC tissue stiffness. We highlight areas of research that are needed to better understand the role of cross-linking in glaucoma pathogenesis, leading potentially to a novel therapeutic strategy.

The relationship between patient age and residual refractive error after uneventful laser in situ keratomileusis for moderate-to-high hyperopia

PURPOSE

To determine if the manifest sphero-cylindrical residual refractive error, at various time points over a 12-month postop period after laser in situ keratomileusis (LASIK) was associated with patient age at time of surgery.

METHODS

Patients with moderate to high hyperopia (3.00-7.00 DS) and astigmatism ⩽2 DC underwent LASIK using Wavelight Allegretto Eye Q (400 Hz). Treatments were centered on corneal vertex, flaps were made with Moria M2 mechanical microkeratome. Pre-and postoperative uncorrected and corrected distant visual acuity, best corrected spherical equivalent (SEQ) were measured. Measurements were taken at 1 week, 1, 3, 6, and 12 months after the surgery. Target refraction was emmetropia. Total of 161 patients were treated. In binocular cases, data from the right eyes were included for analysis. In this article, we report on refraction data only. Raw data were subjected to several permutations to elicit any links between refractive outcomes and patient age.

RESULTS

The key findings were as follows y = postop SEQ (diopters), x = patient age (years), ln(x) = natural logarithm of patient age: At 1 month, y = x[0.049 -0.011.ln(x)] (R = -0.205, p = 0.001, n = 161). At 3 months, y = x[0.077 -0.017.ln(x)] (R = -0.355, p < 0.001, n = 161). At 6 months, y = x[0.088 -0.020.ln(x)] (R = -0.382, p < 0.001, n = 161). At 12 months, y = x[0.093 -0.021.ln(x)] (R = -0.409, p < 0.001, n = 161). There was no significant association between x and y at 1 week (p > 0.05).

CONCLUSION

Residual postop refractive error after LASIK for hyperopia has a logarithmic association with patient age at time of surgery. In younger patients there is tendency toward undercorrection, the opposite occurs in older patients and this persists 1 year after treatment.

Comparison of Biomechanically Corrected Intraocular Pressure Obtained by Corvis ST and Goldmann Applanation Tonometry in Patients With Open-angle Glaucoma and Ocular Hypertension

PRECIS

Biomechanically corrected intraocular pressure (bIOP) measurements provided by the new Corvis ST (CST) were higher than measurements obtained with Goldman applanation tonometry (GAT) in eyes with ocular hypertension (OHT) or open-angle glaucoma (OAG).

PURPOSE

To compare bIOP obtained with a new version of CST with GAT measurements in patients with OAG and OHT, and to identify factors affecting IOP measurement differences between these methods.

METHODS

A total of 122 eyes with OAG or OHT were enrolled. Sixty eyes were treated with prostaglandin analogs (PGA) and 62 eyes with selective laser trabeculoplasty (SLT). IOP was measured with CST, followed by GAT. Central corneal thickness (CCT) was measured with ultrasound pachymetry. Measurements obtained with these 2 approaches were compared.

RESULTS

The overall mean IOP was 15.2±3.0 mm Hg and 14.1±3.2 mm Hg with the CST and GAT, respectively (r=0.74; P<0.0001). There was a 1.17 mm Hg bias between tonometers, with 95% limits of agreement of -2.66 to 5.01 mm Hg. According to multivariate regression analysis, differences between bIOP and GAT-IOP were associated with CCT (P=0.001) and age (P=0.007) in the PGA group, but only with CCT in the SLT group (P=0.002). bIOP and GAT-IOP values were influenced by age (PGA: P=0.014; SLT: P=0.006) and CCT (PGA: P=0.007; SLT: P=0.032), respectively.

CONCLUSIONS

BIOP values were higher and less affected by CCT than GAT-IOP values in eyes with OAG and OHT. However, these measurements may not be interchangeable in the clinic.

Endothelial Safety and Efficacy of Ex Vivo Collagen Cross-linking of Human Corneal Transplants

PURPOSE

To assess endothelial safety and efficacy of ex vivo corneal collagen cross-linking (CXL) in human corneal transplants stored in 2 different culture media.

DESIGN

Fellow-eye controlled laboratory study of ex vivo human donor corneas.

METHODS

Three sets of paired human donor corneas, 5 pairs each, were stored in organ culture medium before deswelling either at 31 C or at room temperature. One eye of each pair was cross-linked by 0.1% riboflavin in hydroxylpropyl methylcellulose (HPMC) instillation for 10 minutes followed by 10 minutes of ultraviolet-A (9 mW/cm²) irradiation while contralateral eyes served as controls. In Set 1, endothelial cell densities were determined. In Set 2, paired samples were assigned to the 2 deswelling media and CXL efficacy was assessed comparing to untreated controls using collagenase-A-assisted enzymatic digestion. In Set 3, biomechanical testing was performed in the eye pairs (treated vs control) by stress/strain measurements.

RESULTS

There was no difference in endothelial cell counts between CXL samples and controls (P = .21). No statistically significant difference in digestion dynamics was found between tissues stored in the 2 different culture media. Complete enzymatic digestion was slowed down by 3 hours in the cross-linked samples (P = .036). Stress needed for a 12% strain was increased by 34% in the treatment group compared to control (P = .04).

CONCLUSIONS

Ex vivo CXL of human donor tissue is an effective and safe procedure with no difference regarding efficacy between 2 commercially available deswelling media. Biochemical and biomechanical resistance were significantly increased after CXL. Patients requiring keratoplasty owing to corneal melting might benefit from the strengthening effect of preoperative CXL of donor tissue.

Age-dependent Deformation of the Optic Nerve Head and Peripapillary Retina by Horizontal Duction

PURPOSE

To study effects of age and horizontal duction on deformation of the optic nerve head (ONH) and peripapillary retina (PPR), as reflected by displacement of vascular landmarks, to explore the influence of adduction tethering.

DESIGN

Cross-sectional study.

METHODS

Setting: University.

STUDY POPULATION

Single eyes of 20 healthy young adults (average age 23.9 ± 3.9 [SD] years) were compared to 20 older subjects (average age 61.4 ± 9.3 years). Observational Procedure: The disc and PPR were imaged by scanning laser ophthalmoscopy in central gaze and at 35 degrees abduction and adduction.

MAIN OUTCOME MEASURE

Deformations of the disc and adjacent PPR were measured by comparing positions of epipapillary and epiretinal blood vessels.

RESULTS

Vessels within the ONH of younger subjects shifted temporally during adduction and nasally during abduction. Displacement of the nasal hemi-disc in adduction was greater at 38.5 ± 1.7 μm (standard error of mean) than the temporal half at 4.1 ± 2.1 μm (P < .001). PPR within 1 radius of the disc margin underwent 7.6 ± 1.6 μm average temporal displacement in adduction in young subjects. In abduction, the young temporal hemi-disc shifted 4.4 ± 0.6 μm nasally without significant displacement in the nasal half. Older subjects' ONH showed less temporal shift and less displacement in the PPR within 1 disc radius (P < .0001) in adduction; the nasal hemi-disc shifted 24.5 ± 1.3 μm compared with 4.4 ± 2.1 μm in the temporal half. There were no significant deformations of the disc during abduction by older subjects.

CONCLUSION

Large horizontal duction, particularly adduction, deforms the disc and peripapillary vasculature. This deformation, which is larger in younger than older subjects, may be due to optic nerve tethering in adduction.

[Refractive shift after glaucoma surgery]

Filtering glaucoma surgery can affect certain biometric features of the eye: it can change the axial length and anterior chamber depth, while redistribution of mechanical tensions in the fibrous tunic can alter the shape of the cornea. Among these changes that affect refraction, reshaping of corneal curvature is the principle one. Contrary to the expectations, and in contrast to changes associated with cataract surgery, glaucoma surgery led to decrease in vertical corneal radius (steepening) and development of the with-the-rule astigmatism. A number of studies helped reveal the features of corneal astigmatism that appears after glaucoma surgery: its power, duration, possibility of horizontal meridian flattening, influence of topical cytostatic drugs and drainage devices, etc. Potential reasons of astigmatism development were suggested: conjunctival incisions and sutures in the limbal area, quantity and strength of flap sutures, intraocular pressure level, shift of fistula edges, tissue contraction after cauterization, etc. The key role in pathogenesis of the refractive shift due to change of corneal curvature may belong to particularities of wound healing in the cornea and sclera: healing in sclera is similar to one of connective tissue - its ultrastructure undergoes aging process followed by change of rigidity. At the same time, the main reason for the development of corneal astigmatism after glaucoma surgery remains unclear.

Scleral structure and biomechanics

As the eye's main load-bearing connective tissue, the sclera is centrally important to vision. In addition to cooperatively maintaining refractive status with the cornea, the sclera must also provide stable mechanical support to vulnerable internal ocular structures such as the retina and optic nerve head. Moreover, it must achieve this under complex, dynamic loading conditions imposed by eye movements and fluid pressures. Recent years have seen significant advances in our knowledge of scleral biomechanics, its modulation with ageing and disease, and their relationship to the hierarchical structure of the collagen-rich scleral extracellular matrix (ECM) and its resident cells. This review focuses on notable recent structural and biomechanical studies, setting their findings in the context of the wider scleral literature. It reviews recent progress in the development of scattering and bioimaging methods to resolve scleral ECM structure at multiple scales. In vivo and ex vivo experimental methods to characterise scleral biomechanics are explored, along with computational techniques that combine structural and biomechanical data to simulate ocular behaviour and extract tissue material properties. Studies into alterations of scleral structure and biomechanics in myopia and glaucoma are presented, and their results reconciled with associated findings on changes in the ageing eye. Finally, new developments in scleral surgery and emerging minimally invasive therapies are highlighted that could offer new hope in the fight against escalating scleral-related vision disorder worldwide.

Brillouin Spectroscopy of Normal and Keratoconus Corneas

PURPOSE

To investigate the age dependence of Brillouin spectroscopy of the cornea and to compare normal and keratoconus corneas.

DESIGN

Retrospective case-control study.

METHODS

Study Population: Healthy patients and patients suffering from keratoconus seen at the Institut für Refraktive und Ophthalmo-Chirurgie (IROC) between December 2016 and March 2017. Brillouin frequency shifts of patients of 2 different groups were examined with Brillouin spectroscopy perpendicular to the corneal surface. Group 1 consisted of 47 healthy eyes, whereas Group 2 included 36 eyes with keratoconus of unclear progression. Besides Brillouin examinations, corneal tomographies were acquired so that correlations and comparisons between geometric parameters and Brillouin frequency shifts could be evaluated.

MAIN OUTCOME MEASURES

Corneal Brillouin frequency shifts averaged over full corneal thickness.

RESULTS

A significant correlation between age and central Brillouin frequency shift was identified (P = .011) with an increase in Brillouin frequency shift of 4 MHz per decade in normal corneas. Keratoconus corneas have a significantly reduced Brillouin frequency shift at the thinnest point compared to normal corneas (5.7072 ± 0.0214 vs 5.7236 ± 0.0146 GHz, P < .001). The Brillouin frequency shift at the point of maximum posterior elevation showed best correlation with geometry-derived keratoconus indices. The receiver operating characteristic curve analysis of Brillouin frequency shift showed substantially worse sensitivity and specificity compared to K_max and thinnest pachymetry for keratoconus detection.

CONCLUSION

Noninvasive Brillouin spectroscopy adds clinical information about the biomechanical state of the cornea perpendicular to the surface. An age-dependent stiffening of the cornea has been found and keratoconus corneas are statistically significantly different from normal corneas, but for precise differentiating of keratoconus stages (including normal corneas) the method is currently neither specific nor sensitive enough. Further development including standardized mapping and establishment of new indices may increase the potential of Brillouin spectroscopy substantially.

Spatially-resolved Brillouin spectroscopy reveals biomechanical abnormalities in mild to advanced keratoconus in vivo

Mounting evidence connects the biomechanical properties of tissues to the development of eye diseases such as keratoconus, a disease in which the cornea thins and bulges into a conical shape. However, measuring biomechanical changes in vivo with sufficient sensitivity for disease detection has proven challenging. Here, we demonstrate the diagnostic potential of Brillouin light-scattering microscopy, a modality that measures longitudinal mechanical modulus in tissues with high measurement sensitivity and spatial resolution. We have performed a study of 85 human subjects (93 eyes), consisting of 47 healthy volunteers and 38 keratoconus patients at differing stages of disease, ranging from stage I to stage IV. The Brillouin data in vivo reveal increasing biomechanical inhomogeneity in the cornea with keratoconus progression and biomechanical asymmetry between the left and right eyes at the onset of keratoconus. The receiver operating characteristic analysis of the stage-I patient data indicates that mean Brillouin shift of the cone performs better than corneal thickness and maximum curvature respectively. In conjunction with morphological patterns, Brillouin microscopy may add value for diagnosis of keratoconus and potentially for screening subjects at risk of complications prior to laser eye surgeries.

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

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

Effect of age on visual and refractive results after LASIK: mechanical microkeratome versus femtosecond laser

AIM

To evaluate the effect of age on visual and refractive results after laser in situ keratomileusis (LASIK) obtained with a mechanical microkeratome or a femtosecond laser.

METHODS

Retrospective, nonrandomized, cohort study. A total of 3826 eyes were included in the study (1725 eyes treated with mechanical LASIK and 2101 eyes treated with femtosecond LASIK). The relationship between patient age and the 3-month postoperative visual and refractive results of both procedures were analyzed by linear regression analysis.

RESULTS

Three months postoperatively, we found a significant correlation between age and the postoperative spherical equivalent (SE; r ²=0.004, P=0.006), efficacy (r ²=0.006, P=0.001), and safety indexes (r ²=0.05, P=0.0001) in the mechanical LASIK group. On the other hand, we found a significant correlation between age and the postoperative SE (r ²=0.02, P=0.0001) and the efficacy index (r ²=0.01, P=0.0001) but not the safety index in the femtosecond laser group. Mechanical LASIK provided slightly but significantly better efficacy and predictability in patients 18 to 40 years of age and femtosecond LASIK did so in patients older than 40 years of age. The femtosecond laser provided better safety results than the mechanical microkeratome in both age groups.

CONCLUSION

A tendency toward undercorrection and less predictability is found with aging after myopic LASIK regardless of whether the flap was created with a mechanical microkeratome or a femtosecond laser. However, femtosecond laser provides significantly better outcomes in terms of efficacy, safety and predictability compared to mechanical microkeratome for the correction of myopia in patients over 40y.

Comparison of Central Corneal Thickness Measurements Using Optical and Ultrasound Pachymetry in Glaucoma Patients and Elderly and Young Controls

PRéCIS:: Optical and ultrasound pachymetry cannot be used interchangeably in young, healthy adults. In elderly, healthy and glaucoma patients the speed of ultrasound in the cornea increases because of changing corneal biomechanical characteristics in age and disease and the different devices could be used interchangeably.

PURPOSE

The purpose of this study was to compare central corneal thickness (CCT) using optical and ultrasound pachymetry in patients with open-angle glaucoma and young as well as elderly, healthy controls. Further to investigate whether the devices could be used interchangeably.

METHODS

In total, 69 eyes of 41 glaucoma patients, 51 eyes of 32 elderly and 50 eyes of 25 young controls were consecutively included in this cross-sectional observational study. Optical CCT measurements were obtained using the noncontact Specular Microscope CEM-530 (NCSM). Ultrasound pachymetry (USP) was measured using the Pachy Meter SP 3000.

RESULTS

In young subjects (27.2±4.8 y), the mean CCT taken with NCSM and USP was 562.1±33.6 μm and 565.8±35.8 μm, respectively. This was significantly different (USP>NCSM, P=0.019). In elderly subjects (70.6±10.7 y) CCT measured with NCSM (562.5±27.8 μm) compared to USP (564.9±27.1 μm) was not statistically significantly different (P= 0.121). In glaucoma patients (65.0±11.1 y), USP measured thinner CCT values compared to NCSM, without significant differences between the devices (NCSM 525.3±32.3 μm; USP 522.9±33.15 μm; P=0.067). CCT was significantly thinner in comparison to both healthy groups (P< 0.001).

CONCLUSIONS

In young subjects, ultrasound pachymetry measurements are higher than they are with optical pachymetry. This difference is no longer observed in elderly subjects and is even reversed in glaucoma patients. A higher speed of ultrasound in the cornea due to changing corneal biomechanical properties in the elderly and glaucoma patients could explain this. The devices could be used interchangeably in older and glaucoma patients, but not in young individuals.

Comparison of corneal biomechanical changes after refractive surgery by noncontact tonometry: small-incision lenticule extraction versus flap-based refractive surgery - a systematic review

Corneal refractive surgery disrupts corneal integrity and reduces biomechanical stability in consequence of the beneficial refractive alteration. The minimal invasive cap-based refractive procedure, small-incision lenticule extraction (SMILE), has been proposed to affect corneal integrity less than flap-based procedures, due to the fibre-sparing incision of the strong anterior corneal lamellae. Flap-based procedures include laser-assisted in situ keratomileusis (LASIK), femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and femtosecond lenticule extraction (FLEx). The purpose of this systematic review was to evaluate corneal biomechanical changes by noncontact air pulse tonometry after treatment of myopia/myopic astigmatism with SMILE compared to flap-based refractive surgery. A total of 220 publications were identified through a systematic search in PubMed and Embase. Two levels of screening identified nine studies (three randomised controlled trials (RCT) and six nonrandomised clinical trials) eligible for the review. All the nonrandomised clinical trials were graded to have an overall serious risk of bias. Measurements with the Corvis ST were not included in any of the eligible studies. The RCTs found no statistical significant differences between SMILE or flap-based procedures concerning corneal hysteresis (CH) or corneal resistance factor (CRF), as measured with the Ocular Response Analyzer. However, a greater reduction in CRF and CH was found in the flap-based group in five and two of the nonrandomised studies, respectively. The findings in this review illustrate that the presumed biomechanical advantages of a cap-based small incision could not be demonstrated in existing studies and by commercially available technology. However, studies with lower levels of evidence suggest less affection of corneal viscoelastic properties after SMILE when evaluating corneal stability by noncontact tonometry.