Scleral biomechanics in the aging monkey eye

Relationship between Intraocular Pressure Fluctuation and Visual Field Progression Rates in the United Kingdom Glaucoma Treatment Study

PURPOSE

To investigate whether intraocular pressure (IOP) fluctuation is associated independently with the rate of visual field (VF) progression in the United Kingdom Glaucoma Treatment Study.

DESIGN

Randomized, double-masked, placebo-controlled multicenter trial.

PARTICIPANTS

Participants with ≥5 VFs (213 placebo, 217 treatment).

METHODS

Associations between IOP metrics and VF progression rates (mean deviation [MD] and five fastest locations) were assessed with linear mixed models. Fluctuation variables were mean Pascal ocular pulse amplitude (OPA), standard deviation (SD) of diurnal Goldmann IOP (diurnal fluctuation), and SD of Goldmann IOP at all visits (long-term fluctuation). Fluctuation values were normalized for mean IOP to make them independent from the mean IOP. Correlated nonfluctuation IOP metrics (baseline, peak, mean, supine, and peak phasing IOP) were combined with principal component analysis, and principal component 1 (PC1) was included as a covariate. Interactions between covariates and time from baseline modeled the effect of the variables on VF rates. Analyses were conducted separately in the two treatment arms.

MAIN OUTCOME MEASURES

Associations between IOP fluctuation metrics and rates of MD and the five fastest test locations.

RESULTS

In the placebo arm, only PC1 was associated significantly with the MD rate (estimate, -0.19 dB/year [standard error (SE), 0.04 dB/year]; P < 0.001), whereas normalized IOP fluctuation metrics were not. No variable was associated significantly with MD rates in the treatment arm. For the fastest five locations in the placebo group, PC1 (estimate, -0.58 dB/year [SE, 0.16 dB/year]; P < 0.001), central corneal thickness (estimate, 0.26 dB/year [SE, 0.10 dB/year] for 10 μm thicker; P = 0.01) and normalized OPA (estimate, -3.50 dB/year [SE, 1.04 dB/year]; P = 0.001) were associated with rates of progression; normalized diurnal and long-term IOP fluctuations were not. In the treatment group, only PC1 (estimate, -0.27 dB/year [SE, 0.12 dB/year]; P = 0.028) was associated with the rates of progression.

CONCLUSIONS

No evidence supports that either diurnal or long-term IOP fluctuation, as measured in clinical practice, are independent factors for glaucoma progression; other aspects of IOP, including mean IOP and peak IOP, may be more informative. Ocular pulse amplitude may be an independent factor for faster glaucoma progression.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Displacement of the Lamina Cribrosa With Acute Intraocular Pressure Increase in Brain-Dead Organ Donors

Purpose

To examine deformations of the optic nerve head (ONH) deep tissues in response to acute elevation of intraocular pressure (IOP).

Methods

Research-consented brain-dead organ donors underwent imaging by spectral domain optical coherence tomography (OCT). OCT imaging was repeated while the eye was sequentially maintained at manometric pressures of 10, 30, and 50 mm Hg. Radial scans of the ONH were automatically segmented by deep learning and quantified in three dimensions by a custom algorithm. Change in lamina cribrosa (LC) depth and choroidal thickness was correlated with IOP and age by linear mixed-effect models. LC depth was computed against commonly utilized reference planes.

Results

Twenty-six eyes from 20 brain-dead organ donors (age range, 22-62 years; median age, 43 years) were imaged and quantified. LC depth measured against a reference plane based on Bruch's membrane (BM), BM opening, and an anterior sclera canal opening plane showed both a reduction and an increase in LC depth with IOP elevation. LC depth universally increased in depth when measured against a sclera reference plane. Choroidal (-0.5222 µm/mm Hg, P < 0.001) and retinal nerve fiber layer thickness (-0.0717 µm/mm Hg, P < 0.001) significantly thinned with increasing IOP. The magnitude of LC depth change with IOP was significantly smaller with increasing age (P < 0.03 for all reference planes).

Conclusions

LC depth changes with IOP reduce with age and are significantly affected by the reference plane of choice, which highlights a need for standardizing LC metrics to properly follow progressive remodeling of the loadbearing tissues of the ONH by OCT imaging and for the definition of a reference database.

Intraocular Pressure and Myopia Progression, Axial Length Elongation in Rural Chinese Children

SIGNIFICANCE

This study reported the relationship between intraocular pressure (IOP) and myopia progression, which helps to understand more comprehensively whether IOP can be an important reference factor to intervene in the progression of myopia.

PURPOSE

This study aimed to investigate the association between IOP and myopia progression as well as axial length elongation in rural Chinese children.

METHODS

A total of 598 (598 of 878 [68.1%]) children (6 to 17 years) from the baseline Handan Offspring Myopia Study who completed a 3.5-year follow-up vision examination were included. Ocular examinations at both visits included cycloplegic autorefraction, IOP, and axial length measurements.

RESULTS

Children with myopia had the highest baseline IOP of the three refractive groups (14.13 ± 1.31, 13.78 ± 1.71, and 13.59 ± 1.64 mmHg in myopes, emmetropes, and hyperopes, respectively, P = .002). However, IOPs showed no significant difference between eyes with or without newly developed myopia (13.63 ± 1.68 vs. 13.89 ± 1.68, P = .16), with or without faster myopia progression (13.75 ± 1.61 vs. 13.86 ± 1.63, P = .46), or with axial length elongation (13.80 ± 1.61 vs. 13.76 ± 1.64, P = .80). The multivariate regression analysis demonstrated that neither baseline refractive error ( β = -0.082, P = .13) nor baseline axial length ( β = -0.156, P = .08) was associated with baseline IOP.

CONCLUSIONS

Myopic eyes have slightly higher IOP compared with emmetropic and hyperopic eyes, although it was not clinically significant. However, IOP was not found to be associated with either myopia progression or axial length elongation in this cohort sample of rural Chinese children.

Finite Deformation of Scleral Tissue under Electrical Stimulation: An Arbitrary Lagrangian-Eulerian Finite Element Method

The sclera is considered as the principal load-bearing tissue within the eye. The sclera is negatively charged; thus, it exhibits mechanical response to electrical stimulation. We recently demonstrated the electroactive behavior of sclera by performing experimental measurements that captured the deformation of the tip of scleral strips subjected to electric voltage. We also numerically analyzed the electromechanical response of the tissue using a chemo-electro-mechanical model. In the pre-sent study, we extended our previous work by experimentally characterizing the deformation profile of scleral strips along their length under electrical stimulation. In addition, we improved our previous mathematical model such that it could numerically capture the large deformation of samples. For this purpose, we considered the transient variability of the fixed charge density and the coupling between mechanical and chemo-electrical phenomena. These improvements in-creased the accuracy of the computational model, resulting in a better numerical representation of experimentally measured bending angles.

Assessment of age-related change of the ocular support system

To estimate the material stiffness of the orbital soft tissue in human orbits using an inverse numerical analysis approach, which could be used in future studies to understand the behaviour under dynamic, non-contact tonometry or simulate various ophthalmological conditions. Clinical data were obtained for the left eye of 185 Chinese participants subjected to a complete ophthalmic examination, including tests by the Corvis ST and Pentacam. 185 numerical models of the eye globes were built with idealised geometry of the sclera while considering the corneal tomography measured by the Pentacam. The models were extended to include representations of the orbital soft tissue (OST), which were given idealised geometry. The movement of the whole eye in response to an air-puff directed at the central cornea was examined and used in an inverse analysis process to estimate the biomechanical stiffness parameters of the OST. The results indicated a weak correlation of E _t with the progression of age, regardless of the stress at which E _t was calculated. However, there was evidence of significant differences in E _t between some of the age groups. There was statistical evidence of significant differences between E _t in the age range 20< years < 43 relative to E _t in OST with age ranges 43< years < 63 (p = 0.022) and 63< years < 91 (p = 0.011). In contrast, E _t in OST with age ranges 43< years < 63 and 63< years < 91 were not significantly different (p = 0.863). The optimised mechanical properties of the OST were found to be almost four times stiffer than properties of fatty tissue of previous experimental work. This study consolidated previous findings of the role of extraocular muscles on the ocular suppor system. In addition, the rotation of the globe during corvis loading is suggested to be of posterior components of the globe and shall be further investigated.

A direct fiber approach to model sclera collagen architecture and biomechanics

Sclera collagen fiber microstructure and mechanical behavior are central to eye physiology and pathology. They are also complex, and are therefore often studied using modeling. Most models of sclera, however, have been built within a conventional continuum framework. In this framework, collagen fibers are incorporated as statistical distributions of fiber characteristics such as the orientation of a family of fibers. The conventional continuum approach, while proven successful for describing the macroscale behavior of the sclera, does not account for the sclera fibers are long, interwoven and interact with one another. Hence, by not considering these potentially crucial characteristics, the conventional approach has only a limited ability to capture and describe sclera structure and mechanics at smaller, fiber-level, scales. Recent advances in the tools for characterizing sclera microarchitecture and mechanics bring to the forefront the need to develop more advanced modeling techniques that can incorporate and take advantage of the newly available highly detailed information. Our goal was to create a new computational modeling approach that can represent the sclera fibrous microstructure more accurately than with the conventional continuum approach, while still capturing its macroscale behavior. In this manuscript we introduce the new modeling approach, that we call direct fiber modeling, in which the collagen architecture is built explicitly by long, continuous, interwoven fibers. The fibers are embedded in a continuum matrix representing the non-fibrous tissue components. We demonstrate the approach by doing direct fiber modeling of a rectangular patch of posterior sclera. The model integrated fiber orientations obtained by polarized light microscopy from coronal and sagittal cryosections of pig and sheep. The fibers were modeled using a Mooney-Rivlin model, and the matrix using a Neo-Hookean model. The fiber parameters were determined by inversely matching experimental equi-biaxial tensile data from the literature. After reconstruction, the direct fiber model orientations agreed well with the microscopy data both in the coronal plane (adjusted R2 = 0.8234) and in the sagittal plane (adjusted R2 = 0.8495) of the sclera. With the estimated fiber properties (C10 = 5746.9 MPa; C01 = -5002.6 MPa, matrix shear modulus 200 kPa), the model's stress-strain curves simultaneously fit the experimental data in radial and circumferential directions (adjusted R2's 0.9971 and 0.9508, respectively). The estimated fiber elastic modulus at 2.16% strain was 5.45 GPa, in reasonable agreement with the literature. During stretch, the model exhibited stresses and strains at sub-fiber level, with interactions among individual fibers which are not accounted for by the conventional continuum methods. Our results demonstrate that direct fiber models can simultaneously describe the macroscale mechanics and microarchitecture of the sclera, and therefore that the approach can provide unique insight into tissue behavior questions inaccessible with continuum approaches.

High-frequency ultrasound point-of-care device to quantify myopia-induced microstructural changes in the anterior sclera

PURPOSE

To develop a point-of-care (POC) device using high-frequency ultrasound (US) for evaluating microstructural changes in the anterior sclera associated with myopia.

METHODS

The proposed POC device must satisfy four primary requirements for effective clinical use: the measurement component is handheld; the software must be simple and provide real-time feedback; patient safety and health data security requirements set forth by relevant governing bodies must be satisfied and the measurement data must have sufficient signal-to-noise ratio (SNR) and repeatability. Radiofrequency (RF) echo data acquired by the POC device will be processed using our quantitative US methods to characterise tissue microstructure and biomechanical properties.

RESULTS

All stated requirements have been met in the developed POC device. The high-frequency transducer is housed in a custom, 3D-printed, pen-like holder that allows for easy measurements of the anterior sclera. Custom software provides a simple interface for data acquisition, real-time data display and secure data storage. Exposimetry measurements of the US pressure field indicate device compliance with United States Food and Drug Administration limits for ophthalmic US. In vivo measurements on a volunteer suggest the RF data SNR and acquisition consistency are suitable for quantitative analysis.

CONCLUSIONS

A fully functioning POC device using high-frequency US has been created for evaluating the microstructure of the anterior sclera. Planned studies using the POC device to scan the eyes of myopia patients will help clarify how the anterior sclera microstructure may be affected by myopia. If effective, this portable, inexpensive and user-friendly system could be an important part of routine eye examinations.

Neuroretinal Rim Response to Transient Intraocular Pressure Challenge Predicts the Extent of Retinal Ganglion Cell Loss in Experimental Glaucoma

Purpose

To determine if the optic nerve head (ONH) response to transient elevated intraocular pressure (IOP) can predict the extent of neural loss in the nonhuman primate experimental glaucoma model.

Methods

The anterior chamber pressure of 21 healthy animals (5.4 ± 1.2 years, 8 female) was adjusted to 25 mm Hg for two hours followed by 10 mm Hg for an additional two hours. For the duration of IOP challenge the ONH was imaged using radial optical coherence tomography (OCT) scans at five-minute intervals. Afterward, a randomized sample of 14 of these subjects had unilateral experimental glaucoma induced and were monitored with OCT imaging, tonometry, and ocular biometry at two-week intervals.

Results

With pressure challenge, the maximum decrease in ONH minimum rim width (MRW) was 40 ± 10.5 µm at 25 mm Hg and was correlated with the precannulation MRW, Bruch's membrane opening (BMO) position, and the anterior lamina cribrosa surface position (P = 0.01). The maximum return of MRW at 10 mm Hg was 16.1 ± 5.0 µm and was not associated with any precannulation ONH feature (P = 0.24). However, healthy eyes with greater thickness return at 10 mm Hg had greater loss of MRW and retinal nerve fiber layer (RNFL) at a cumulative IOP of 1000 mm Hg · days after induction of experimental glaucoma. In addition, MRW and RNFL thinning was correlated with an increase in axial length (P < 0.01).

Conclusion

This study's findings suggest that the ONH's response to transient changes in IOP are associated with features of the ONH and surrounding tissues. The neural rim properties at baseline and the extent of axial elongation are associated with the severity of glaucomatous loss in the nonhuman primate model.

Stretch-Induced Uncrimping of Equatorial Sclera Collagen Bundles

Stretch-induced collagen uncrimping underlies the nonlinear mechanical behavior of the sclera according to what is often called the process of recruitment. We recently reported experimental measurements of sclera collagen crimp and pressure-induced uncrimping. Our studies, however, were cross-sectional, providing statistical descriptions of crimp with no information on the effects of stretch on specific collagen bundles. Data on bundle-specific uncrimping is necessary to better understand the effects of macroscale input on the collagen microscale and tissue failure. Our goal in this project was to measure bundle-specific stretch-induced collagen uncrimping of sclera. Three goat eyes were cryosectioned sagittally (30 μm). Samples of equatorial sclera were isolated, mounted to a custom uni-axial stretcher and imaged with polarized light microscopy at various levels of clamp-to-clamp stretch until failure. At each stretch level, local strain was measured using image tracking techniques. The level of collagen crimping was determined from the bundle waviness, defined as the circular standard deviation of fiber orientation along a bundle. Eye-specific recruitment curves were then computed using eye-specific waviness at maximum stretch before sample failure to define fibers as recruited. Nonlinear mixed effect models were used to determine the associations of waviness to local strain and recruitment to clamp-to-clamp stretch. Waviness decreased exponentially with local strain (p < 0.001), whereas bundle recruitment followed a sigmoidal curve with clamp-to-clamp stretch (p < 0.001). Individual bundle responses to stretch varied substantially, but recruitment curves were similar across sections and eyes. In conclusion, uni-axial stretch caused measurable bundle-specific uncrimping, with the sigmoidal recruitment pattern characteristic of fiber-reinforced soft tissues.

Corneal Hysteresis for the Diagnosis of Glaucoma and Assessment of Progression Risk: A Report by the American Academy of Ophthalmology

PURPOSE

To review the current published literature on the utility of corneal hysteresis (CH) to assist the clinician in the diagnosis of glaucoma or in the assessment of risk for disease progression in existing glaucoma patients.

METHODS

Searches of the peer-reviewed literature in the PubMed database were performed through July 2022. The abstracts of 423 identified articles were examined to exclude reviews and non-English articles. After inclusion and exclusion criteria were applied, 19 articles were selected, and the panel methodologist rated them for level of evidence. Eight articles were rated level I, and 5 articles were rated level II. The 6 articles rated level III were excluded.

RESULTS

Corneal hysteresis is lower in patients with primary open-angle glaucoma, primary angle-closure glaucoma, pseudoexfoliative glaucoma, and pseudoexfoliation syndrome compared with normal subjects. Interpretation of low CH in patients with high intraocular pressure (IOP) or on topical hypotensive medications is complicated by the influence of these parameters on CH measurements. However, CH is also lower in treatment-naïve, normal-tension glaucoma patients compared with normal subjects who have a similar IOP. In addition, lower CH is associated with an increased risk of progression of glaucoma based on visual fields or structural markers in open-angle glaucoma patients, including those with apparently well-controlled IOP.

CONCLUSIONS

Corneal hysteresis is lower in glaucoma patients compared with normal subjects, and lower CH is associated with an increased risk of disease progression. However, a causal relationship remains to be demonstrated. Nevertheless, measurement of CH complements current structural and functional assessments in determining disease risk in glaucoma suspects and patients.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

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.

Deep learning-based 3D OCT imaging for detection of lamina cribrosa defects in eyes with high myopia

The lamina cribrosa (LC) is a collagenous tissue located in the optic nerve head, and its dissection is observed in eyes with pathologic myopia as a LC defect (LCD). The diagnosis of LCD has been difficult because the LC is located deep beneath the retinal nerve fibers. The purpose of this study was to determine the prevalence and three-dimensional shape of LCDs in highly myopic eyes. Swept-source OCT scan images of a 3 × 3-mm cube centered on the optic disc were obtained from 119 eyes of 62 highly myopic patients. Each LC was manually labelled in cross-sectional OCT images along the axial, coronal, and sagittal planes. A deep convolutional neural network (DCNN) was trained with the manually labeled images, and the trained DCNN was applied to the detection of the LC in every image in each plane. Three-dimensional images of the LC were generated from the labeled image of each eye. The results showed that LCDs were detected in 12 of the 42 (29%) eyes in which an LC was visible. The LCDs ran vertically at the temporal edge of the optic disc. In conclusion, 3D OCT imaging with the application of DCNN is helpful in diagnosing LCDs.

Ocular Pulse Amplitude Correlates With Ocular Rigidity at Native IOP Despite the Variability in Intraocular Pulse Volume With Each Heartbeat

Purpose

The purpose of this study was to assess ocular coat mechanical behavior using controlled ocular microvolumetric injections (MVI) of 15 µL of balanced salt solution (BSS) infused over 1 second into the anterior chamber (AC) via a syringe pump.

Methods

Intraocular pressure (IOP) was continuously recorded at 200 Hz with a validated implantable IOP telemetry system in 7 eyes of 7 male rhesus macaques (nonhuman primates [NHPs]) during 5 MVIs in a series at native (3 trials), 15 and 20 mm Hg baseline IOPs, repeated in 2 to 5 sessions at least 2 weeks apart. Ocular rigidity coefficients (K) and ocular pulse volume (PV) were calculated for each trial. Data were averaged across sessions within eyes; PV was analyzed with a three-level nested ANOVA, and parameter relationships were analyzed with Pearson Correlation and linear regression.

Results

After MVI at native baseline IOP of 10.4 ± 1.6 mm Hg, IOP increased by 9.1 ± 2.8 mm Hg (∆IOP) at a 9.6 ± 2.7 mm Hg/s slope, ocular pulse amplitude (OPA) was 0.70 ± 0.13 mm Hg on average; the average K was 0.042 ± 0.010 µL-1 and average PV was 1.16 ± 0.43 µL. PV varied significantly between trials, days, and eyes (P ≤ 0.05). OPA was significantly correlated with K at native IOP: Pearson coefficients ranged from 0.71 to 0.83 (P ≤ 0.05) and R2 ranged from 0.50 to 0.69 (P ≤ 0.05) during the first trial.

Conclusions

The MVI-driven ∆IOP and slope can be used to assess ocular coat mechanical behavior and measure ocular rigidity.

Translational Relevance

Importantly, OPA at native IOP is correlated with ocular rigidity despite the significant variability in PV between heartbeats.

Modeling and experimental investigation of electromechanical properties of scleral tissue; a CEM model using an anisotropic hyperelastic constitutive relation

The sclera is a soft tissue primarily consisting of collagen fibers, elastin, and proteoglycans. The proteoglycans are composed of a core protein and negatively charged glycosaminoglycan side chains. The fixed electric charges inside the scleral extracellular matrix play a key role in its swelling and are expected to cause the tissue to deform in response to an electric field. However, the electroactive response of the sclera has not yet been investigated. The present work experimentally demonstrates that sclera behaves similar to an anionic electrosensitive hydrogel and develops a chemo-electro-mechanical (CEM) mathematical framework for its electromechanical response. In the numerical model, a hyperelastic constitutive law with distributed collagen fibers is used to capture the nonlinear mechanical properties of the sclera, and the coupled Poisson-Nernst-Planck equations represent the distribution of mobile ions throughout the domain. After calibrating the proposed numerical CEM model against the experimental measurements, we employ it to investigate the effects of different parameters on the scleral electromechanical response including the voltage and fixed charge density. The experimental and numerical findings of the present study confirm that sclera behaves as an electroactive hydrogel and provide new insight into the mechanical response of this ocular tissue.

Exploring the Vitreoretinal Interface: a Key Instigator of Unique Retinal Hemorrhage Patterns in Pediatric Head Trauma

Purpose

Various types of trauma can cause retinal hemorrhages in children, including accidental and nonaccidental head trauma. We used animal eyes and a finite element model of the eye to examine stress patterns produced during purely linear and angular accelerations, along with stresses attained during simulated repetitive shaking of an infant.

Methods

Using sheep and primate eyes, sclerotomy windows were created by removing the sclera, choroid, and retinal pigment epithelium to expose the retina. A nanofiber square was glued to a 5 mm² area of retina. The square was pulled and separated from vitreous while force was measured. A finite element model of the pediatric eye was used to computationally measure tension stresses during shaking.

Results

In both sheep and primate eyes, tension stress required for separation of retina from vitreous range from 1 to 5 kPa. Tension stress generated at the vitreoretinal interface predicted by the computer simulation ranged from 3 to 16 kPa during a cycle of shaking. Linear acceleration generated lower tension stress than angular acceleration. Angular acceleration generated maximal tension stress along the retinal vasculature. Linear acceleration produced more diffuse force distribution centered at the poster pole.

Conclusions

The finite element model predicted that tension stress attained at the retina during forcible shaking of an eye can exceed the minimum threshold needed to produce vitreoretinal separation as measured in animal eyes. Furthermore, the results show that movements that involve significant angular acceleration produce strong stresses localized along the vasculature, whereas linear acceleration produces weaker, more diffuse stress centered towards the posterior pole of the eye.

Scleral Cross-Linking in Form-Deprivation Myopic Guinea Pig Eyes Leads to Glaucomatous Changes

Purpose

To investigate the potential glaucomatous changes caused by scleral cross-linking (CXL) in a guinea pig form-deprivation (FD) myopia model.

Methods

Eighty 4-week-old tricolor guinea pigs were divided into four groups: FD only, genipin CXL only, FD plus CXL, and control. Refractive error, axial length (AL), intraocular pressure (IOP), and structural and vasculature optic disc changes in optical coherence tomography (OCT) and OCT angiography (OCTA) were measured at baseline and day 21. CXL efficacy was evaluated by scleral rigidity Young's modulus values. Histological and molecular changes in the anterior chamber angle, retina, and sclera were assessed.

Results

Baseline parameters were similar among groups (P > 0.05). The FD plus CXL group at day 21 had the least increase of AL (0.14 ± 0.08 mm) and highest IOP elevation (31.5 ± 3.6 mmHg) compared with the FD-only group (AL: 0.68 ± 0.17 mm; IOP: 22.2 ± 2.6 mmHg) and the control group (AL: 0.24 ± 0.09 mm; IOP: 17.4 ± 1.8 mmHg) (all P < 0.001). OCT and OCTA parameters of the optic disc in the FD plus CXL group at day 21 showed glaucomatous changes and decreased blood flow signals. Sclera rigidity increased in the CXL and FD plus CXL groups. Advanced glycation end products deposited extensively in the retina, choroid, and sclera of FD plus CXL eyes.

Conclusions

CXL causes increased IOP and subsequent optic disc, anterior segment, and scleral changes while inhibiting myopic progression and axial elongation in FD guinea pig eyes. Therefore, applying CXL to control myopia raises safety concerns.

Age and intraocular pressure in murine experimental glaucoma

Age and intraocular pressure (IOP) are the two most important risk factors for the development and progression of open-angle glaucoma. While IOP is commonly considered in models of experimental glaucoma (EG), most studies use juvenile or adult animals and seldom older animals which are representative of the human disease. This paper provides a concise review of how retinal ganglion cell (RGC) loss, the hallmark of glaucoma, can be evaluated in EG with a special emphasis on serial in vivo imaging, a parallel approach used in clinical practice. It appraises the suitability of EG models for the purpose of in vivo imaging and argues for the use of models that provide a sustained elevation of IOP, without compromise of the ocular media. In a study with parallel cohorts of adult (3-month-old, equivalent to 20 human years) and old (2-year-old, equivalent to 70 human years) mice, we compare the effects of elevated IOP on serial ganglion cell complex thickness and individual RGC dendritic morphology changes obtained in vivo. We also evaluate how age modulates the impact of elevated IOP on RGC somal and axonal density in histological analysis as well the density of melanopsin RGCs. We discuss the challenges of using old animals and emphasize the potential of single RGC imaging for understanding the pathobiology of RGC loss and evaluating new therapeutic avenues.

The effect of central corneal thickness on Goldmann tonometry: a retrospective study

PURPOSE

To determine the relation between central corneal thickness and intraocular pressure.

PATIENTS AND METHODS

This retrospective study investigated 112 eyes from 56 individuals who underwent photorefractive keratectomy in a single private medical center between May 2018 and September 2019. Intraocular pressure readings were obtained with Goldmann applanation tonometry, and central corneal thickness measurements were evaluated preoperatively. All the examinations were repeated at 3 and 6 months postoperative.

RESULTS

At 3 and 6 months postoperative, the mean intraocular pressure was only slightly reduced from baseline (mean reduction of 0.6 ± 2.0 mmHg, P < 0.001 and 0.73 ± 2.14 mmHg, P < 0.001, respectively). The change in intraocular pressure following photorefractive keratectomy was not clinically significant, and this change was not correlated with postoperative central corneal thickness at 3 months (p = 0.620, r = 0.047).

CONCLUSION

This study showed that the change in intraocular pressure following photorefractive keratectomy was not clinically significant, and ruled out a correlation in this context between the change in central corneal thickness and the delta intraocular pressure. Our results might question the axiom between central corneal thickness and intraocular pressure and may thus challenge the current clinical setting for evaluating glaucoma.

Biomechanics of the optic nerve head and sclera in canine glaucoma: A brief review

Glaucoma is a leading cause of irreversible blindness, a progressive optic neuropathy with retinal ganglion cell (RGC) death beginning in the optic nerve head (ONH). A primary risk factor for developing glaucoma is elevated intraocular pressure (IOP). Reducing IOP is the only treatment proven to be effective at delaying disease progression. Nevertheless, even when patients have their IOP reduced, the majority of them continue to lose vision. There are, in both humans and dogs, significant interindividual variabilities in susceptibilities to IOP-induced optic nerve damage. Vision loss progresses much more slowly in Beagles with open-angle glaucoma (OAG) caused by ADAMTS10 mutation. This can be attributed to the mutation-related altered ocular biomechanical properties. The principal site of optic nerve (ON) damage in glaucoma is the ONH. It is suggested that the biomechanical properties of the ONH and the surrounding peripapillary sclera (PPS) contribute to glaucoma development and progression. As far as the beneficial biomechanical properties of the ONH and PPS for a decreased susceptibility and slow progression of glaucoma, data are inconsistent and conflicting. Recent biomechanical studies on beagles with ADAMTS10 mutation demonstrated that the mutant dogs have mechanically weak posterior sclera. This weakness was associated with a reduced collagen density and a lower proportion of insoluble collagen. These changes, observed before glaucoma development, were considered intrinsic characteristics caused by the mutation rather than a secondary effect of IOP elevation. Further studies of ADAMTS10-OAG may elucidate the effects of altered biomechanical properties of ONH and PPS in determining the glaucoma progression.

Effect of different preconditioning protocols on the viscoelastic inflation response of the posterior sclera

Preconditioning by repeated cyclic loads is routinely used in ex vivo mechanical testing of soft biological tissues. The goal of preconditioning is to achieve a steady and repeatable mechanical response and to measure material properties that are representative of the in vivo condition. Preconditioning protocols vary across studies, and their effect on the viscoelastic response of tested soft tissue is typically not reported or analyzed. We propose a methodology to systematically analyze the preconditioning process with application to inflation testing. We investigated the effect of preconditioning on the viscoelastic inflation response of tree shrew posterior sclera using two preconditioning protocols: (i) continuous cyclic loading-unloading without rest and (ii) cyclic loading-unloading with 15-min rest between cycles. Posterior scleral surface strain was measured using three-dimensional Digital Image Correlation (3D-DIC). We used five variables of characterizing features of the stress-strain loop curve to compare the two preconditioning protocols. Our results showed protocol-dependent differences in the tissue response during preconditioning and at the preconditioned state. Incorporating a resting time between preconditioning cycles significantly decreased the number of cycles (10.5 ± 2.9 cycles vs. 3.1 ± 0.5 cycles, p < 0.001) but increased the total time (15.8 ± 4.4 min vs. 51.2 ± 8.3 min, p < 0.001) needed to reach preconditioned state. At the preconditioned state, 2 of 5 characteristic variables differed significantly between protocols: hysteresis loop area (difference=0.023 kJ/m³, p = 0.0020) and elastic modulus at high IOPs (difference=24.0 MPa, p = 0.0238). Our results suggest that the analysis of the preconditioning process is an essential part of inflation experiments and a prerequisite to properly characterize the tissue viscoelastic response. Furthermore, material properties obtained at the preconditioned state can be impacted by the resting time used during preconditioning and may not be directly compared across studies if the resting time varies by 15 min between studies. STATEMENT OF SIGNIFICANCE: Although applying a preconditioning protocol by repeated cyclic loads is common practice in ex vivo mechanical characterization of soft tissues, the tissue response is typically not reported or analyzed, and the protocol's potential effect on the response remains unclear. This is partially caused by lack of a standardized methodology to precondition soft tissues. We present the first systematic analysis of two representative preconditioning protocols used during inflation testing in ocular biomechanics. Our results show protocol-dependent differences in the viscoelastic response during the preconditioning process and at the preconditioned state. Consequently, the analysis of the preconditioning response represents an essential part of mechanical testing and a prerequisite to properly characterize the tissue viscoelastic response. The effect of preconditioning on the preconditioned state response must be considered when comparing results across studies with different preconditioning protocols.

Ultrasonic elastography to assess biomechanical properties of the optic nerve head and peripapillary sclera of the eye

PURPOSE

To quantitatively investigate both optic nerve head (ONH) and peripapillary sclera (PPS) biomechanical properties of porcine eyes through an ultrasonic elastography imaging system in response to both increasing and decreasing intraocular pressure (IOP).

METHODS

The Young's modulus of the ONH and PPS were assessed using our high resolution ultrasonic imaging system which utilized a mechanical shaker to induce shear waves and an off-axis aligned 40 MHz needle transducer to track micron-level displacement along the direction of wave propagation. In this study, imaging on a total of 8 ex vivo porcine eyes preloaded with IOPs from 6 mmHg to 30 mmHg was performed. To have a better understanding of the effect of varying IOP on biomechanics, both increasing and decreasing IOPs were investigated.

RESULTS

The increase of the Young's modulus of ONH (92.4 ± 13.9 kPa at 6 mmHg to 224.7 ± 71.1 kPa at 30 mmHg) and PPS (176.8 ± 14.3 kPa at 6 mmHg to 573.5 ± 64.4 kPa at 30 mmHg) following IOP elevation could be observed in the reconstructed Young's modulus of the shear wave elasticity (SWE) imaging while the B-mode structural images remained almost unchanged. In addition, for the same IOP level, both ONH and PPS have a tendency to be stiffer with decreasing IOP as compared to increasing IOP.

CONCLUSIONS

Our results demonstrate the feasibility of using our ultrasonic elastography system to investigate the stiffness mapping of posterior eye with high resolution in both increasing and decreasing IOPs, making this technique potentially useful for glaucoma.

The Shape of Posterior Sclera as a Biometric Signature in Open-angle Glaucoma: An Intereye Comparison Study

Supplemental Digital Content is available in the text.

Purpose:

To characterize intereye differences in posterior segment parameters and determine their significance in open-angle glaucoma patients with unilateral damage.

Methods:

Both eyes from 65 subjects without any nerve damage and 43 patients undergoing treatment for unilateral open-angle glaucoma were included in this study. A 12.0×9.0×2.6 mm volume of the posterior segment in each eye was scanned with swept-source optical coherence tomography. Coronally reconstructed optical coherence tomography images were analyzed to determine the deepest point of the eye (DPE), which we then calculated the distance (Disc-DPE distance), depth (Disc-DPE depth), angle (Disc-DPE angle) from the optic disc center to the DPE. Posterior pole shape was analyzed measuring the posterior pole-cross-sectional area, posterior pole-horizontal width (PP-HW), and posterior pole-vertical width) of the posterior pole. These measurements and their intereye absolute difference (IAD; absolute difference in measurements between the right and left eyes) values were compared between the healthy and unilateral glaucomatous patients.

Results:

The posterior sclera measurements, including the Disc-DPE distance, Disc-DPE depth, and posterior pole-cross-sectional area, were significantly different between the unilateral glaucoma eyes and contralateral healthy eyes (P=0.043, P=0.035, and P=0.049, respectively). By contrast, none of the intereye differences in optic nerve head parameters were significant in the unilateral glaucoma patients. In comparison with the IAD values, the baseline intraocular pressure and PP-HW of the posterior segment showed significant differences between the healthy and the unilateral glaucoma patients (P=0.019 and P=0.036, respectively). A multivariate analysis showed that a larger baseline intraocular pressure IAD [odds ratio (OR), 1.381; P=0.009)] and larger PP-HW IAD (OR, 1.324; P=0.032) were significantly associated with the presence of glaucoma.

Conclusions:

Compared with the fellow healthy eyes, glaucomatous eyes had larger and more steeply curved posterior poles, which represent a structural variation of the posterior sclera that might be associated with glaucomatous optic neuropathy.

Gaze-evoked deformations of the optic nerve head in thyroid eye disease

PURPOSE

To assess gaze evoked deformations of the optic nerve head (ONH) in thyroid eye disease (TED), using computational modelling and optical coherence tomography (OCT).

METHODS

Multiple finite element models were constructed: one model of a healthy eye, and two models mimicking effects of TED; one with proptosis and another with extraocular tissue stiffening. Two additional hypothetical models had extraocular tissue softening or no extraocular tissue at all. Horizontal eye movements were simulated in these models. OCT images of the ONH of 10 healthy volunteers and 1 patient with TED were taken in primary gaze. Additional images were recorded in the same subjects performing eye movements in adduction and abduction. The resulting ONH deformation in the models and human subjects was measured by recording the 'tilt angle' (relative antero-posterior deformation of the Bruch's membrane opening).

RESULTS

In our computational models the eyes with proptosis and stiffer extraocular tissue had greater gaze-evoked deformations than the healthy eye model, while the models with softer or no extraocular tissue had lesser deformations, in both adduction and abduction. In healthy subjects, the mean tilt angle was 1.46°±0.25 in adduction and -0.42°±0.12 in abduction. The tilt angle measured in the subject with TED was 5.37° in adduction and -2.21° in abduction.

CONCLUSION

Computational modelling and experimental observation suggest that TED can cause increased gaze-evoked deformations of the ONH.

Identification of key genes and pathways in scleral extracellular matrix remodeling in glaucoma: Potential therapeutic agents discovered using bioinformatics analysis

Background: Glaucoma is a leading cause of irreversible blindness. Remodeling of the scleral extracellular matrix (ECM) plays an important role in the development of glaucoma. The aim of this study was to identify the key genes and pathways for the ECM remodeling of sclera in glaucoma by bioinformatics analysis and to explore potential therapeutic agents for glaucoma management. Methods: Genes associated with glaucoma, sclera and ECM remodeling were detected using the text mining tool pubmed2ensembl, and assigned Gene Ontology (GO) biological process terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways using the GeneCodis program. A protein-protein interaction (PPI) network was constructed by STRING and visualized in Cytoscape, module analysis was performed using the Molecular Complex Detection (MCODE) plugin, and GO and KEGG analyses of the gene modules were performed using the Database of Annotation, Visualization and Integrated Discovery (DAVID) platform. The genes that clustered in the significant module were selected as core genes, and functions and pathways of the core genes were visualized using ClueGO and CluePedia. Lastly, the drug-gene interaction database was used to explore drug-gene interactions of the core genes to find drug candidates for glaucoma. Results: We identified 125 genes common to "Glaucoma", "Sclera", and "ECM remodeling" by text mining. Gene functional enrichment analysis yielded 30 enriched GO terms and 20 associated KEGG pathways. A PPI network that included 60 nodes with 249 edges was constructed, and three gene modules were obtained using the MCODE. We selected 13 genes that clustered in module 1 as core candidate genes that were associated mainly with ECM degradation and cell proliferation and division. The HIF-1 signaling pathway, FOXO signaling pathway, PI3K-Akt signaling pathway and TGFB signaling pathway were found to be enriched. We found that 11 of the 13 selected genes could be targeted by 26 existing drugs. Conclusions: The results showed that VEGFA, TGFB1, TGFB2, TGFB3, IGF2, IGF1, EGF, FN1, KNG1, TIMP1, SERPINE1, THBS1, and VWF were potential key genes involved to scleral ECM remodeling. Furthermore, 26 drugs were identified as potential therapeutic agents for glaucoma treatment and management.

Peripapillary sclera exhibits a v-shaped configuration that is more pronounced in glaucoma eyes

AIMS

To compare the shape of the anterior surface of the peripapillary sclera (PPS) between glaucoma and healthy subjects.

METHODS

88 primary open angle glaucoma (POAG), 98 primary angle closure glaucoma (PACG) and 372 age-matched and gender-matched healthy controls were recruited in this study. The optic nerve head of one randomly selected eye of each subject was imaged with spectral domain optical coherence tomography. The shape of the PPS was measured through an angle defined between a line parallel to the nasal anterior PPS boundary and one parallel to the temporal side. A negative value indicated that the PPS followed an inverted v-shaped configuration (peak pointing towards the vitreous), whereas a positive value indicated that it followed a v-shaped configuration.

RESULTS

The mean PPS angle in normal controls (4.56±5.99°) was significantly smaller than that in POAG (6.60±6.37°, p=0.011) and PACG (7.90±6.87°, p<0.001). The v-shaped PPS was significantly associated with older age (β=1.79, p<0.001), poorer best-corrected visual acuity (β=3.31, p=0.047), central corneal thickness (β=-0.28, p=0.001), peripapillary choroidal thickness (β=-0.21, p<0.001) and presence of POAG (β=1.94, p<0.009) and PACG (β=2.96, p<0.001). The v-shaped configuration of the PPS significantly increased by 1.46° (p=0.001) in healthy controls for every 10-year increase in age, but not in glaucoma groups.

CONCLUSIONS

The v-shaped configuration of the PPS was more pronounced in glaucoma eyes than in healthy eyes. This posterior bowing of the PPS may have an impact on the biomechanical environment of the optic nerve head.

Numerical Investigation on the Role of Mechanical Factors Contributing to Globe Flattening in States of Elevated Intracranial Pressure

Flattening of the posterior eye globe in the magnetic resonance (MR) images is a sign associated with elevated intracranial pressure (ICP), often seen in people with idiopathic intracranial hypertension (IIH). The exact underlying mechanisms of globe flattening (GF) are not fully known but mechanical factors are believed to play a role. In the present study, we investigated the effects of material properties and pressure loads on GF. For this purpose, we used a generic finite element model to investigate the deformation of the posterior eyeball. The degree of GF in numerical models and the significance of different mechanical factors on GF were characterized using an automated angle-slope technique and a statistical measure. From the numerical models, we found that ICP had the most important role in GF. We also showed that the angle-slope graphs pertaining to MR images from five people with high ICP can be represented numerically by manipulating the parameters of the finite element model. This numerical study suggests that GF observed in IIH patients can be accounted for by the forces caused by elevation of ICP from its normal level, while material properties of ocular tissues, such as sclera (SC), peripapillary sclera (PSC), and optic nerve (ON), would impact its severity.

Experimental evaluation of stiffening effect induced by UVA/Riboflavin corneal cross-linking using intact porcine eye globes

UVA/riboflavin corneal cross-linking (CXL) is a common used approach to treat progressive keratoconus. This study aims to investigate the alteration of corneal stiffness following CXL by mimicking the inflation of the eye under the in vivo loading conditions. Seven paired porcine eye globes were involved in the inflation test to examine the corneal behaviour. Cornea-only model was constructed using the finite element method, without considering the deformation contribution from sclera and limbus. Inverse analysis was conducted to calibrate the non-linear material behaviours in order to reproduce the inflation test. The corneal stress and strain values were then extracted from the finite element models and tangent modulus was calculated under stress level at 0.03 MPa. UVA/riboflavin cross-linked corneas displayed a significant increase in the material stiffness. At the IOP of 27.25 mmHg, the average displacements of corneal apex were 307 ± 65 μm and 437 ± 63 μm (p = 0.02) in CXL and PBS corneas, respectively. Comparisons performed on tangent modulus ratios at a stress of 0.03 MPa, the tangent modulus measured in the corneas treated with the CXL was 2.48 ± 0.69, with a 43±24% increase comparing to its PBS control. The data supported that corneal material properties can be well-described using this inflation methods following CXL. The inflation test is valuable for investigating the mechanical response of the intact human cornea within physiological IOP ranges, providing benchmarks against which the numerical developments can be translated to clinic.

IOP-induced regional displacements in the optic nerve head and correlation with peripapillary sclera thickness

Mechanical insult induced by intraocular pressure (IOP) is likely a driving force in the disease process of glaucoma. This study aimed to evaluate regional displacements in human optic nerve head (ONH) and peripapillary tissue (PPT) in response to acute IOP elevations, and their correlations with morphological characteristics of the posterior eye. Cross-sectional (2D) images of the ONH and PPT in 14 globes of 14 human donors were acquired with high-frequency ultrasound during whole globe inflation from 5 to 30 mm Hg. High-frequency ultrasound has a spatial resolution of tens of micrometers and is capable of imaging through the ONH and PPT thickness. Tissue displacements were calculated using a correlation-based speckle tracking algorithm for a dense matrix of kernels covering the 2D imaging plane. The ONH was manually segmented in the ultrasound B-mode images acquired at 5 mmHg based on echogenicity. The lamina cribrosa (LC) boundaries were visible in eight of the fourteen eyes and the LC region was segmented using a semi-automated superpixel-based method. The ONH had larger radial displacement than the PPT in all tested eyes and the difference increased with increasing IOP. A significant negative correlation was found between ONH-PPT displacement difference and PPT thickness (p < 0.05), while no significant correlations were found between ONH-PPT displacement difference and other morphological parameters including PPT radius of curvature, scleral canal size, LC thickness and anterior LC surface depth. Within the ONH, the radial displacement decreased in the region anterior to and across LC but not in the region posterior to LC. Finite element models using simplified geometry and material properties confirmed the role of LC in reducing the overall ONH radial displacements, but did not predict the displacement gradient change observed experimentally. These results suggested that a thinner PPT may be associated with a larger relative posterior motion of the ONH with respect to the surrounding PPT and the LC may play a major role in preventing excessive posterior displacement of ONH during acute IOP elevations.

Age and Menopause Effects on Ocular Compliance and Aqueous Outflow

Purpose

Glaucoma is the second leading cause of blindness worldwide. Recent work suggests that estrogen and the timing of menopause play a role in modulating the risk of developing glaucoma. Menopause is known to cause modest changes in intraocular pressure; yet, whether this change is mediated through the outflow pathway remains unknown. Menopause also affects tissue biomechanical properties throughout the body; however, the impact of menopause on ocular biomechanical properties is not well characterized.

Methods

Here, we simultaneously assessed the impact of menopause on aqueous outflow facility and ocular compliance, as a measure of corneoscleral shell biomechanics. We used young (3-4 months old) and middle-aged (9-10 months old) Brown Norway rats. Menopause was induced by ovariectomy (OVX), and control animals underwent sham surgery, resulting in the following groups: young sham (n = 5), young OVX (n = 6), middle-aged sham (n = 5), and middle-aged OVX (n = 5). Eight weeks postoperatively, we measured outflow facility and ocular compliance.

Results

Menopause resulted in a 34% decrease in outflow facility and a 19% increase in ocular compliance (P = 0.011) in OVX animals compared with sham controls (P = 0.019).

Conclusions

These observations reveal that menopause affects several key physiological factors known to be associated with glaucoma, suggesting that menopause may contribute to an increased risk of glaucoma in women.

Alterations in proteome of human sclera associated with primary open-angle glaucoma involve proteins participating in regulation of the extracellular matrix

Purpose

Primary open-angle glaucoma (POAG) is a common ocular disease, associated with abnormalities in aqueous humor circulation and an increase in intraocular pressure (IOP), leading to progressive optical neuropathy and loss of vision. POAG pathogenesis includes alterations of the structural properties of the sclera, especially in the optic nerve head area, contributing to the degeneration of the retinal ganglion cells. Abnormal sclera biomechanics hinder adequate compensation of IOP fluctuations, thus aggravating POAG progression. The proteomic basis of biomechanical disorders in glaucomatous sclera remains poorly understood. This study is aimed at revealing alterations in major scleral proteins, associated with POAG, at different stages of the disease and with different IOP conditions.

Methods

Samples of sclera were collected from 67 patients with POAG during non-penetrating deep sclerectomy and from nine individuals without POAG. Scleral proteins were extracted with a strong lysis buffer, containing a combination of an ionic detergent, a chaotropic agent, and a disulfide reducing agent, and were separated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The major scleral proteins were selected, subjected to in-gel digestion, and identified using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF)/TOF mass spectrometry (MS), coupled with tandem mass spectrometry (MS/MS). The specific POAG-associated alterations of the selected proteins were analyzed with SDS-PAGE and confirmed with western blotting of the scleral extracts, using the respective antibodies. The group of POAG-associated proteins was analyzed using Gene Ontology and genome-wide association study enrichment and protein-protein interaction network prediction.

Results

A total of 11 proteins were identified, among which six proteins, namely, vimentin, angiopoietin-related protein 7, annexin A2, serum amyloid P component, serum albumin, and thrombospondin-4, were found to be upregulated in the sclera of patients with advanced and terminal POAG. In the early stages of the disease, thrombospondin-4 level was, on the contrary, reduced when compared with the control, whereas the concentration of vimentin varied, depending on the IOP level. Moreover, angiopoietin-related protein 7 manifested as two forms, exhibiting opposite behavior: The common 45 kDa form grew with the progression of POAG, whereas the 35 kDa (apparently non-glycosylated) form was absent in the control samples, appeared in patients with early POAG, and decreased in concentration over the course of the disease. Functional bioinformatics analysis linked the POAG-associated proteins with IOP alterations and predicted their secretion into extracellular space and their association with extracellular vesicles and a collagen-containing extracellular matrix.

Conclusions

POAG is accompanied by alterations of the scleral proteome, which represent a novel hallmark of the disease and can reflect pathological changes in scleral biochemistry and biomechanics. The potential mechanisms underlying these changes relate mainly to the structure of the extracellular matrix, protein glycosylation, and calcium binding, and may involve fibroblast cytoskeleton regulation, as well as oxidative and inflammatory responses.

Collagen fiber interweaving is central to sclera stiffness

The mechanical properties of the microstructural components of sclera are central to eye physiology and pathology. Because these parameters are extremely difficult to measure directly, they are often estimated using inverse-modeling matching deformations of macroscopic samples measured experimentally. Although studies of sclera microstructure show collagen fiber interweaving, current models do not account for this interweaving or the resulting fiber-fiber interactions, which might affect parameter estimates. Our goal was to test the hypothesis that constitutive parameters estimated using inverse modeling differ if models account for fiber interweaving and interactions. We developed models with non-interweaving or interweaving fibers over a wide range of volume fractions (36-91%). For each model, we estimated fiber stiffness using inverse modeling matching biaxial experimental data of human sclera. We found that interweaving increased the estimated fiber stiffness. When the collagen volume fraction was 64% or less, the stiffness of interweaving fibers was about 1.25 times that of non-interweaving fibers. For higher volume fractions, the ratio increased substantially, reaching 1.88 for a collagen volume fraction of 91%. Simulating a model (interweaving/non-interweaving) using the fiber stiffness estimated from the other model produced substantially different behavior, far from that observed experimentally. These results show that estimating microstructural component mechanical properties is highly sensitive to the assumed interwoven/non-interwoven architecture. Moreover, the results suggest that interweaving plays an important role in determining the structural stiffness of sclera, and potentially of other soft tissues in which the collagen fibers interweave. STATEMENT OF SIGNIFICANCE: The collagen fibers of sclera are interwoven, but numerical models do not account for this interweaving or the resulting fiber-fiber interactions. To determine if interweaving matters, we examined the differences in the constitutive model parameters estimated using inverse modeling between models with interweaving and non-interweaving fibers. We found that the estimated stiffness of the interweaving fibers was up to 1.88 times that of non-interweaving fibers, and that the estimate increased with collagen volume fraction. Our results suggest that fiber interweaving is a fundamental characteristic of connective tissues, additional to anisotropy, density and orientation. Better characterization of interweaving, and of its mechanical effects is likely central to understanding microstructure and biomechanics of sclera and other soft tissues.

Effect of Changing Heart Rate on the Ocular Pulse and Dynamic Biomechanical Behavior of the Optic Nerve Head

Purpose

To study the effect of changing heart rate on the ocular pulse and the dynamic biomechanical behavior of the optic nerve head (ONH) using a comprehensive mathematical model.

Methods

In a finite element model of a healthy eye, a biphasic choroid consisted of a solid phase with connective tissues and a fluid phase with blood, and the lamina cribrosa (LC) was viscoelastic as characterized by a stress-relaxation test. We applied arterial pressures at 18 ocular entry sites (posterior ciliary arteries), and venous pressures at four exit sites (vortex veins). In the model, the heart rate was varied from 60 to 120 bpm (increment: 20 bpm). We assessed the ocular pulse amplitude (OPA), pulse volume, ONH deformations, and the dynamic modulus of the LC at different heart rates.

Results

With an increasing heart rate, the OPA decreased by 0.04 mm Hg for every 10 bpm increase in heart rate. The ocular pulse volume decreased linearly by 0.13 µL for every 10 bpm increase in heart rate. The storage modulus and the loss modulus of the LC increased by 0.014 and 0.04 MPa, respectively, for every 10 bpm increase in heart rate.

Conclusions

In our model, the OPA, pulse volume, and ONH deformations decreased with an increasing heart rate, whereas the LC became stiffer. The effects of blood pressure/heart rate changes on ONH stiffening may be of interest for glaucoma pathology.

Morphometric, Hemodynamic, and Biomechanical Factors Influencing Blood Flow and Oxygen Concentration in the Human Lamina Cribrosa

Purpose

We developed a combined biomechanical and hemodynamic model of the human eye to estimate blood flow and oxygen concentration within the lamina cribrosa (LC) and rank the factors that influence LC oxygen concentration.

Methods

We generated 5000 finite-element eye models with detailed microcapillary networks of the LC and computed the oxygen concentration of the lamina retinal ganglion cell axons. For each model, we varied the intraocular pressure (IOP) from 10 mm Hg to 55 mm Hg in 5-mm Hg increments, the cerebrospinal fluid pressure (13 ± 2 mm Hg), cup depth (0.2 ± 0.1 mm), scleral stiffness (±20% of the mean values), LC stiffness (0.41 ± 0.2 MPa), LC radius (1.2 ± 0.12 mm), average LC pore size (5400 ± 2400 µm²), the microcapillary arrangement (radial, isotropic, or circumferential), and perfusion pressure (50 ± 9 mm Hg). Blood flow was assumed to originate from the LC periphery and drain via the central retinal vein. Finally, we performed linear regressions to rank the influence of each factor on the LC tissue oxygen concentration.

Results

LC radius and perfusion pressure were the most important factors in influencing the oxygen concentration of the LC. IOP was another important parameter, and eyes with higher IOP had higher compressive strain and slightly lower oxygen concentration. In general, superior–inferior regions of the LC had significantly lower oxygen concentration than the nasal–temporal regions, resulting in an hourglass pattern of oxygen deficiency.

Conclusions

To the best of our knowledge, this study is the first to implement a comprehensive hemodynamical model of the eye that accounts for the biomechanical forces and morphological parameters of the LC. The results provide further insight into the possible relationship of biomechanical and vascular pathways leading to ischemia-induced optic neuropathy.

Role of radially aligned scleral collagen fibers in optic nerve head biomechanics

Collagen fibers organized circumferentially around the canal in the peripapillary sclera are thought to provide biomechanical support to the sensitive tissues within the optic nerve head (ONH). Recent studies have demonstrated the existence of a family of fibers in the innermost sclera organized radially from the scleral canal. Our goal was to determine the role of these radial fibers in the sensitivity of scleral canal biomechanics to acute increases in intraocular pressure (IOP). Following the same general approach of previous parametric sensitivity studies, we created nonlinear generic finite element models of a posterior pole with various combinations of radial and circumferential fibers at an IOP of 0 mmHg. We then simulated the effects of normal and elevated IOP levels (15 and 30 mmHg). We monitored four IOP-induced geometric changes: peripapillary sclera stretch, scleral canal displacement, lamina cribrosa displacement, and scleral canal expansion. In addition, we examined the radial (maximum tension) and through-thickness (maximum compression) strains within the ONH tissues. Our models predicted that: 1) radial fibers reduced the posterior displacement of the lamina, especially at elevated IOP; 2) radial fibers reduced IOP-induced radial strain within the peripapillary sclera and retinal tissue; and 3) a combination of radial and circumferential fibers maintained strains within the ONH at a level similar to those conferred by circumferential fibers alone. In conclusion, radial fibers provide support for the posterior globe, additional to that provided by circumferential fibers. Most importantly, a combination of both fiber families can better protect ONH tissues from excessive IOP-induced deformation than either alone.

The Dynamic Scleral Extracellular Matrix Alterations in Chronic Ocular Hypertension Model of Rats

Intraocular pressure (IOP) generates stress and strains in the laminar cribrosa and sclera, which may affect the development and progression of glaucoma. Scleral stiffness and material components have changed under elevated IOP. However, the detailed changes of the components of the hypertensive sclera are not well understood. In this study, we aimed to investigate the changes of the main components in the scleral extracellular matrix (ECM), and matrix metalloproteinase 2 (MMP2) and their relationship with time under chronic elevated IOP in Sprague–Dawley rats. An ocular hypertension model was established in the right eyes by anterior chamber injection with 0.3% carbomer solution. The left eye was used as the contralateral control. Immunofluorescent imaging of the tissue frozen sections, Western blot analysis, and quantitative PCR (qPCR) were performed to detect the expressions of type I collagen (COL1), elastin, and MMP2 in the sclera. The ocular hypertension model was successfully established. As compared to the left eyes, the immunofluorescence imaging, Western blot analysis, and qPCR showed that COL1, elastin, and MMP2 were significantly increased in the right eyes at 1 week (all P < 0.05). At 2 weeks, COL1 in the right eyes tended to be lower than that in the left eyes, while elastin and MMP2 were still higher (all P < 0.05) in the right eyes. When the IOP was elevated for 4 weeks, both COL1 and MMP2 were lower than those in the left eyes (all P < 0.05), while elastin between the two eyes was similar (P > 0.05). Under this 4-week hypertensive state, COL1 and elastin were initially elevated at 1 week, and then obviously reduced from 2 to 4 weeks. Consistently, MMP2 was gradually increased, with a peak at 2 weeks, and then decreased at 4 weeks. In conclusion, the chronic elevated IOP induced dynamic scleral ECM alterations in rats in a pressure- and time-dependent manner. MMP2 may play an important role in the balance between ECM synthesis and degradation and could potentially be a novel target for glaucoma intervention.

Biomechanical properties of the rat sclera obtained with inverse finite element modeling

It is widely accepted that biomechanics plays an important role in glaucoma pathophysiology, but the mechanisms involved are largely unknown. Rats are a common animal model of glaucoma, and finite element models are being developed to provide much-needed insight into the biomechanical environment of the posterior rat eye. However, material properties of rat ocular tissues, including the sclera, are currently unknown. Since the sclera plays a major role in posterior ocular biomechanics, our goal was to use inverse finite element modeling to extract rat scleral material properties. We first used digital image correlation to measure scleral surface displacement during whole-globe inflation testing. We modeled the sclera as a nonlinear material with embedded collagen fibers and then fit modeled displacements to experimental data using a differential evolution algorithm. Subject-specific models were constructed in which 3 parameters described the stiffness of the ground substance and collagen fibers in the posterior eye, and 16 parameters defined the primary orientation and alignment of fibers within eight scleral sub-regions. We successfully extracted scleral material properties for eight rat eyes. Model displacements recreated general patterns of the experimental displacements but did not always match local patterns. The fiber directions and fiber concentration parameters were highly variable, but on average, fibers were aligned circumferentially and were more aligned in the peripapillary sclera than in the peripheral sclera. The material properties determined here will be used to inform future finite element models of the rat posterior eye with the goal of elucidating the role of biomechanics in glaucoma pathophysiology.

Neuroretinal rim response to transient changes in intraocular pressure in healthy non-human primate eyes

Optic nerve head (ONH) neuroretinal rim thickness, quantified as minimum rim width (BMO-MRW), is a sensitive measure for assessing early glaucomatous disease. The BMO-MRW is sensitive to transient fluctuations in intraocular pressure (IOP), but the time course over which BMO-MRW decreases and recovers with changes in IOP remains unknown. The goal of this study was to investigate the dynamics of BMO-MRW changes over 2-h periods of mild or moderate IOP elevation, and subsequent recovery, in healthy non-human primate eyes. Eight non-human primates were included in the study. For each animal, in two different sessions separated by at least 2 weeks, the anterior chamber IOP of one eye was maintained at either 25 mmHg or 40 mmHg for 2 h and, subsequently, at 10 mmHg for 2 h. For the duration of anterior chamber cannulation, optical coherence tomography (OCT) radial scans centered on the ONH were acquired every 5 min and used to quantify BMO-MRW. An exponential decay or rise to maximum function was used to determine the extent and rate of structural change. Additionally, Bruch's membrane opening (BMO) area, BMO height/displacement, and BMO-referenced anterior lamina cribrosa surface depth (BMO-ALCSD) were computed from radial scans. A circular scan was used to quantify retinal nerve fiber layer thickness (RNFLT) and circumpapillary choroid thickness. The primary results demonstrated that the BMO-MRW changed over an extended duration, while BMO displacement was rapid and remained stable with sustained IOP. The mean maximum predicted BMO-MRW thinning following 2 h of IOP elevation was significantly related to pressure (34.2 ± 13.8 μm for an IOP of 25 mmHg vs 40.5 ± 12.6 μm for 40 mmHg, p = 0.03). The half-life for BMO-MRW thinning was 21.9 ± 9.2 min for 25 mmHg and 20.9 ± 4.2 min for 40 mmHg, not significantly different between IOP levels (p = 0.76). Subsequently, after 2 h of IOP at 10 mmHg, all animals exhibited partial recovery of BMO-MRW with similar degrees of persistent residual thinning for the two IOP levels (21.5 ± 13.7 vs 21.0 ± 12.3 μm, p = 0.88). Similar to BMO-MRW, choroid thickness exhibited gradual thinning with IOP elevation and residual thinning following IOP reduction. However, there was no significant change in BMO area or BMO-ALCSD in either experimental session. The RNFLT gradually decreased over the duration of IOP elevation, with continued decreases following IOP reduction for the 40 mmHg session, resulting in total changes from baseline of -2.24 ± 0.81 and -2.45 ± 1.21 μm for 25 and 40 mmHg, respectively (p < 0.001). The sum of the results demonstrate that the ONH neural tissue is sensitive to changes in IOP, the effects of which are gradual over an extended time course and different for increased vs. decreased pressure. Understanding the duration over which IOP influences BMO-MRW has important implications for studies investigating the effects of IOP on the ONH. Additionally, individual variability in ONH response to IOP may improve our understanding of the risk and progression of disease.

Transient Intraocular Pressure Fluctuations: Source, Magnitude, Frequency, and Associated Mechanical Energy

Purpose

To characterize intraocular pressure (IOP) dynamics by identifying the sources of transient IOP fluctuations and quantifying the frequency, magnitude, associated cumulative IOP-related mechanical energy, and temporal distribution.

Methods

IOP was monitored at 500 Hz for periods of 16 to 451 days in nine normal eyes of six conscious, unrestrained nonhuman primates using a validated, fully implanted wireless telemetry system. IOP transducers were calibrated every two weeks via anterior chamber cannulation manometry. Analysis of time-synchronized, high-definition video was used to identify the sources of transient IOP fluctuations.

Results

The distribution of IOP in individual eyes is broad, and changes at multiple timescales, from second-to-second to day-to-day. Transient IOP fluctuations arise from blinks, saccades, and ocular pulse amplitude and were as high as 14 mm Hg (>100%) above momentary baseline. Transient IOP fluctuations occur ∼10,000 times per waking hour, with ∼2000 to 5000 fluctuations per hour greater than 5 mm Hg (∼40%) above baseline. Transient IOP fluctuations account for up to 17% (mean of 12%) of the total cumulative IOP-related mechanical energy that the eye must withstand during waking hours.

Conclusions

Transient IOP fluctuations occur frequently and comprise a large and significant portion of the total IOP loading in the eye and should, therefore, be considered in future studies of cell mechanotransduction, ocular biomechanics, and/or clinical outcomes where transient IOP fluctuations may be important. If IOP dynamics are similar in humans, clinical snapshot IOP measurements are insufficient to capture true IOP.

Full-field displacement measurement of corneoscleral shells by combining multi-camera speckle interferometry with 3D shape reconstruction

Changes in the biomechanical properties of the connective tissue of the eye occur with age and underlie the development of several ocular diseases, such as glaucoma, myopia, and keratoconus. The biomechanical dynamics of ocular connective tissue are measured by ex vivo inflation testing, in which intraocular pressure (IOP) is varied and optical methods are used to produce maps of corneal and scleral displacement. Current optical methods are limited by acquisition rate, occlusions, poor spatial resolution, and insufficient 3D mapping. We developed an interferometric optical method integrates four-camera electronic speckle pattern interferometry (ESPI) and a novel three-dimensional (3D) shape reconstruction process to measure shape and full-field mechanical deformations of corneal and scleral shells during ex vivo inflation testing. Each camera provides accurate measurements of the laser beam phase related to deformations of the specimen surface; a multi-view stereovision method generates the shape of the specimen and a functional form that links every pixel of a given camera to 3D points on the specimen's visible surface. In this way, dynamic deformations of the specimen are localized, with quantification of the time-dependent 3D displacements of the specimen at nanometric accuracy. The ESPI-3D system is suitable for analyzing scleral deformation and morphological changes caused by time-varying IOP.

Risk Factors for Fast Visual Field Progression in Glaucoma

PURPOSE

To identify baseline and longitudinal risk factors for fast visual field (VF) decay in patients with open-angle glaucoma.

DESIGN

Retrospective cohort study.

METHODS

Patients with open-angle glaucoma with ≥6 VFs and ≥4 years of follow-up were included. VF decay rates were measured with the following methods: mean deviation (MD) rate, VF index (VFI) rate, and the Glaucoma Rate Index (GRI). The relationship between VF rates and clinical variables were investigated with linear mixed models. Logistic regression analysis was performed to determine which factors were associated with fast progression.

RESULTS

A total of 1317 eyes of 745 patients with a mean (± SD) age of 63.3 (±10.9) years and a median (interquartile range) MD -2.4 (-0.7 to -5.6) dB at baseline were analyzed. The median (interquartile range) number of VFs was 12 (9 to 16), and mean follow-up duration was 11.5 (±3.7) years. Older age (P < .001), higher peak intraocular pressure (IOP) (P < .001), and glaucoma surgery during the study period (P < .001) were associated with faster rates of progression regardless of the method used. Worse baseline MD was associated with MD rate (P = .02), but neither with VFI rate (P = .37) nor GRI (P = .31); whereas pseudoexfoliative glaucoma was associated with faster rates of progression with MD (P = .008) and VFI (P = .01) rates, but not with GRI. Higher peak IOPs (P = .005) was a significant predictor for fast progression.

CONCLUSION

In this cohort, older age, peak IOP, pseudoexfoliative glaucoma, and baseline MD were associated with the rate of glaucomatous VF worsening. Fast progressors had a higher peak IOP than non-fast progressors. The identification and appropriately aggressive treatment of fast progressors would reduce visual disability from glaucoma.

Cyclic Pattern of Intraocular Pressure (IOP) and Transient IOP Fluctuations in Nonhuman Primates Measured with Continuous Wireless Telemetry

Purpose: Most studies on intraocular pressure (IOP) to monitor IOP "fluctuations" in glaucoma patients have been performed with snapshot tonometry techniques that obtain IOP measurements at single time points weeks to months apart. However, IOP telemetry has shown that IOP varies from second-to-second due to blinks, saccades, and systolic vascular filling. The purpose of this study was to characterize the cyclic pattern of baseline IOP and transient IOP fluctuations in 3 nonhuman primates (NHPs).Methods: Bilateral IOP was measured using a proven implantable telemetry system and recorded 500 times per second, 24 hours a day, up to 451 continuous days in 3 male rhesus macaques aged 4 to 5 years old. The IOP transducers were calibrated every two weeks via anterior chamber cannulation manometry and all data were continuously corrected for signal drift via software, filtered for signal noise and dropout, and peaks and troughs were quantified and counted using a finite impulse response filter; waking hours were defined as 6:00-18:00 hours based on room light cycle.Results: Fourier transform analyses of baseline IOP and the hourly mean frequency of transient IOP fluctuations > 0.6 mmHg, 0.6-5 mmHg and > 5 mmHg above baseline during waking hours exhibited an approximate 16- to 91-day cyclic pattern in all NHPs. There were no measured environmental or experimental factors associated with this cyclical pattern.Conclusions: While the importance of the cyclic pattern identified in IOP and its fluctuations is unknown at this time, it is plausible that this pattern is relevant to both homeostasis and pathophysiology of the ONH, corneoscleral shell, and aqueous outflow pathways.

Inflation experiments and inverse finite element modelling of posterior human sclera

The complexity of inverse finite element modelling methods used in ocular biomechanics research has significantly increased in recent years in order to produce material parameters that capture microscale tissue behaviour. This study presents a more accessible method for researchers to optimise sclera material parameters for use in finite element studies where macroscale sclera displacements are required. Five human donor sclerae aged between 36 and 72 years were subjected to cycles of internal pressure up to 61 mmHg using a custom-built inflation rig. Displacements were measured using a laser beam and two cameras through a digital image correlation algorithm. Specimen-specific finite element models incorporating regional thickness variation and sclera surface topography were divided into six circumferential regions. An inverse finite element procedure was used to optimise Ogden material parameters for each region. The maximum root mean squared (RMS) error between the numerical and experimental displacements within individual specimens was 17.5 µm. The optimised material parameters indicate a gradual reduction in material stiffness (as measured by the tangent modulus) from the equator to the posterior region at low-stress levels up to 0.005 MPa. The variation in stiffness between adjacent regions became gradually less apparent and statistically insignificant at higher stresses. The study demonstrated how inflation testing combined with inverse modelling could be used to effectively characterise regional material properties capable of reproducing global sclera displacements. The material properties were found to vary between specimens, and it is expected that age could be a contributing factor behind this variation.

The Relationship Between Scleral Strain Change and Differential Cumulative Intraocular Pressure Exposure in the Nonhuman Primate Chronic Ocular Hypertension Model

Purpose

To determine the relationship between peripapillary scleral strain change and cumulative differential IOP exposure in nonhuman primates (NHPs) with unilateral chronic ocular hypertension.

Methods

Posterior scleral shells from 6 bilaterally normal and 10 unilateral chronic ocular hypertension NHPs were pressurized from 5 to 45 mm Hg, and the resulting full-field, three-dimensional, scleral surface deformations were acquired using laser speckle interferometry. Scleral tensile strain (local tissue deformation) was calculated by analytical differentiation of the displacement field; zero strain was assumed at 5 mm Hg. Maximum principal strain was used to represent the scleral strain, and strains were averaged over a 15°-wide (∼3.6-mm) circumpapillary region adjacent to the ONH. The relative difference in mean strain was calculated between fellow eyes and compared with the differential cumulative IOP exposure within NHPs during the study period. The relationship between the relative difference in scleral strain and the differential cumulative IOP exposure in fellow eyes was assessed using an F test and quadratic regression model.

Results

Relative differential scleral tensile strain was significantly associated with differential cumulative IOP exposure in contralateral eyes in the chronic ocular hypertension NHPs, with the bilaterally normal NHPs showing no significant strain difference between fellow eyes. The sclera in the chronic ocular hypertension eyes was more compliant than in their fellow eyes at low levels of differential cumulative IOP exposure, but stiffer at larger differential IOPs (P < 0.0001).

Conclusions

These cross-sectional findings suggest that longitudinal IOP-induced changes in scleral mechanical behavior are dependent on the magnitude of differential cumulative IOP exposure.

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.

A Protective Eye Shield Reduces Limbal Strain and Its Variability During Simulated Sleep in Adults With Glaucoma

PURPOSE

To determine the effect of wearing a protective eye shield (mask) on limbal strain magnitude and variability in glaucoma eyes when sleeping with 1 side of the face down (FD) against a pillow.

METHODS

A prospective, randomized, interventional trial was conducted at the Wilmer Eye Institute with 36 glaucoma patients. A contact lens sensor measured limbal strain (output in equivalent millivolts) during intervals of up to 60 minutes in lateral decubitus, FD, and supine positions. Eighteen subjects wore a mask during 1 of 2 FD intervals, with randomized assignment of the interval. Data from additional trials with no mask were included in some analyses. In addition, some facial-feature dimensions from 3D scanned images of 23 subjects were compared with limbal strain data.

RESULTS

Wearing a mask trends toward a reduced mean change in contact lens sensor output (limbal strain) on moving to a FD positions [+34.1 mVeq, P=0.01 reduced by -22.3 mVeq, P=0.09 (n=36)]. Mask wearing reduced variability in strain while FD [-22.8 mVeq, P=0.04 (n=18)]. In eyes with past progressive visual field loss, the effect of the mask reduced mean strain change when moving to FD [-44.8 mVeq, P=0.02 (n=31)]. Longer corneal apex to nose-tip and to temple lengths were associated with reduced variability while FD [P=0.02 and 0.04, respectively (n=23)]. Treating both lengths as confounding factors increased statistical significance, particularly for analysis of the no-mask change in strain data moving to and from the FD position [P=0.004 to 0.002 and P=0.03 to 0.01 (n=23)].

CONCLUSION AND RELEVANCE

Wearing a mask reduced limbal strain and variation in limbal strain during simulated FD sleep, particularly in eyes with past field worsening, as did some facial features.

Variation of Peripapillary Scleral Shape With Age

Purpose

To define the shape of the anterior surface of the peripapillary sclera (PPS) and evaluate its relationship with age and ocular determinants in a population-based Chinese cohort.

Methods

The optic nerve heads of 619 healthy Chinese subjects were imaged with spectral-domain optical coherence tomography. To assess the shape of the PPS/Bruch's membrane (BM), we measured the angle between a line parallel to the nasal anterior PPS/BM boundary and one parallel to the temporal side. A negative value indicated that the PPS/BM followed an inverted v-shaped configuration (peak pointing toward the vitreous), whereas a positive value indicated that it followed a v-shaped configuration (peak pointing toward the orbital tissues). A linear regression model was used to evaluate the relationship between the PPS angle and other ocular parameters.

Results

The mean PPS angle was 3.68° ± 6.73° and the BM angle was 9.69° ± 5.05°. The PPS angle increased on average by 0.233 deg/y. A v-shaped PPS was significantly associated with age (β = 0.087, P = 0.004), peripapillary choroidal thickness (β = -0.479, P < 0.001), lamina cribrosa depth (β = 0.307, P < 0.001), and BM angle (β = 0.487, P < 0.001) after adjusting for best corrected visual acuity, central corneal thickness, and axial length.

Conclusions

The anterior surface of PPS of an elderly adult population had a v-shaped configuration and was more pronounced with increasing age, thin peripapillary choroid, and a deep cup. Such a change in shape with age could have an impact on the biomechanical environment of the optic nerve head.

Determination of Corneal Biomechanical Behavior in-vivo for Healthy Eyes Using CorVis ST Tonometry: Stress-Strain Index

Purpose: This study aims to introduce and clinically validate a new algorithm that can determine the biomechanical properties of the human cornea in vivo.

Methods: A parametric study was conducted involving representative finite element models of human ocular globes with wide ranges of geometries and material biomechanical behavior. The models were subjected to different levels of intraocular pressure (IOP) and the action of external air puff produced by a non-contact tonometer. Predictions of dynamic corneal response under air pressure were analyzed to develop an algorithm that can predict the cornea's material behavior. The algorithm was assessed using clinical data obtained from 480 healthy participants where its predictions of material behavior were tested against variations in central corneal thickness (CCT), IOP and age, and compared against those obtained in earlier studies on ex-vivo human ocular tissue.

Results: The algorithm produced a material stiffness parameter (Stress-Strain Index or SSI) that showed no significant correlation with both CCT (p > 0.05) and IOP (p > 0.05), but was significantly correlated with age (p < 0.01). The stiffness estimates and their variation with age were also significantly correlated (p < 0.01) with stiffness estimates obtained earlier in studies on ex-vivo human tissue.

Conclusions: The study introduced and validated a new method for estimating the in vivo biomechanical behavior of healthy corneal tissue. The method can aid optimization of procedures that interfere mechanically with the cornea such as refractive surgeries and introduction of corneal implants.

Age-Related Changes in Quantitative Strain of Mouse Astrocytic Lamina Cribrosa and Peripapillary Sclera Using Confocal Microscopy in an Explant Model

Purpose

The purpose of this study was to measure the full-field deformation response to IOP change in the peripapillary sclera (PPS) and astrocytic lamina cribrosa (ALC) of young and old mouse eyes ex vivo.

Methods

Thirty-eight transgenic reporter mice with green fluorescent protein–expressing astrocytes were studied at 2 to 4 months and 13 to 15 months old. The ALC and PPS of the explant eyes were imaged using laser scanning microscopy under controlled inflation from 10 to 30 mm Hg. Strains were estimated for the ALC and PPS from imaged volumes using digital volume correlation.

Results

ALC strains were significantly greater than zero nasal–temporally for both age groups (mean = 4.3% and 4.0%; each P ≤ 0.004) and significantly greater than zero in the inferior–superior direction for younger mice (P = 0.0004). Younger mice had larger ALC inferior–superior strains than older mice (P = 0.002). The ALC area and perimeter enlarged with inflation in both age groups, with a greater increase in younger than in older mice (all P ≤ 0.004). The ALC nasal–temporal diameter change was greater than inferior–superiorly, and younger mice had greater enlargement nasal–temporally than older. PPS maximum shear strain was greater in the older mice (P = 0.002). The axial lengths of older mice were 14% longer and the PPS was 16% thinner than younger mice (both P = 0.0003).

Conclusions

The behavior of the ALC in younger mice with inflation exhibited greater strains and enlargement of ALC area than older mice. Some strain measures in the PPS were greater in older mice, likely related to their longer axial length and thinner PPS.