Age-related variations in the biomechanical properties of human sclera

Scleral ultrastructure in young rhesus monkeys: A stereological approach

The sclera provides mechanical support to the globe and plays a key role in maintenance of ocular structural integrity. Evidence shows that scleral remodeling contributes to biomechanical weakening, increasing the risk of pathologies, such as glaucoma and posterior staphyloma in high myopia. A precise characterization of normal regional ultrastructure is needed for a better understanding of scleral remodeling. The purpose of this study was to develop and implement a stereological analysis method to characterize scleral ultrastructure in healthy young rhesus monkey eyes. Monkeys (N = 4 eyes) first underwent normal emmetropization in a primate nursery. At 150 days of age, scleral tissue was collected in 1 mm2 sections from three regions, equatorial nasal, posterior, and equatorial temporal, and processed for transmission electron microscopy (TEM). Images were captured at three depths: outer, middle, and inner, for all regions. Scleral thickness, fibroblast volume fraction, volume fraction of fibrillin microfibrils, and collagen fibril diameters were quantified using stereology and manual segmentation. The sclera was thickest in the posterior region with a mean thickness of 217.1 ± 10.1 μm. A total of 103 micrographs were analyzed to compute the volume fraction of fibroblasts and fibrillin microfibrils. Median fibroblast volume fraction was 7.4 %, and fibrillin microfibril volume fraction was 1.8 %. A total of 3564 collagen fibrils, encompassing 99 fibrils per region and depth per eye, were analyzed. Median collagen fibril diameter was 79.8 nm and ranged from 23 nm to 205 nm. This stereological approach provides a foundation for future studies investigating myopic scleral remodeling, where axial elongation and scleral thinning are known to show regional differences.

Longitudinal corneal hysteresis changes predict structural progression in medically controlled, early-to-moderate, open-angle glaucoma with a history of refractive surgery

BACKGROUND

The present study aims to identify the relationship between longitudinal changes in corneal hysteresis (CH) and progressive retinal nerve fibre layer (RNFL) thinning in a cohort of medically controlled, early-to-moderate open-angle glaucoma (OAG) patients with a history of laser refractive surgery (LRS).

METHODS

A total of 123 consecutive eyes with a diagnosis of medically controlled (peak intraocular pressure (IOP)<18 mm Hg), early-to-moderate OAG with a history of LRS underwent measurements of CH, corneal-compensated intraocular pressure (IOPcc) and RNFL thicknesses every 6 months. Linear models were used to investigate the relationship between CH change and RNFL thickness change over time.

RESULTS

Of 123 eyes, 30 eyes (24.4%, 42.9±9.3 years, 36.7% males) demonstrated RNFL loss (93 eyes no progression, 44.4±9.6 years, 30.1% males). No statistically significant difference was found in IOP, but significantly greater decrease in CH was noted in the progression group (-2.525% baseline (95% CI -4.974 to -0.076) vs 1.068% baseline (95% CI, -0.322 to 2.458); p=0.013). Relative CH change was greater for more advanced stage of OAG among the progression group. Patients with the greatest relative CH decrease over time was 1.7 times more likely to present RNFL loss (HR 1.705, 95% CI 1.113 to 2.611, p=0.014).

CONCLUSIONS

Longitudinal decrease in CH over time was greater in those showing structural progression than those without progression among medically controlled, early-to-moderate OAG with a history of LRS. Decrease in CH was significantly associated with faster RNFL loss. Larger CH changes indicate a higher risk of OAG progression in those with a history of LRS.

Finite element model and experimental validation of a solitary wave-based tonometer

Glaucoma is an age-related incurable disease and the second cause of blindness in the world. The risk of developing glaucoma increases in the presence of elevated intraocular pressure (IOP), a risk factor that can be modified through surgery. In clinical practice, IOP remains the cornerstone of the diagnosis and management of glaucoma, and Goldmann applanation tonometry (GAT) is considered the gold standard in IOP measurements. However, GAT, as any other commercial system, provides a single value that is unable to frame the circadian rhythm and sporadic surges of IOP. This capability is only possible with a portable device that patients can eventually self-administer anytime anywhere. To serve the purpose, the device must be easy-to-use and not requiring sterilization or topical anesthesia. Additional desirable features are low price and adaptability to patient's eyeball geometry. To address these needs, our group has conceptualized, assembled, and tested a new tonometer based on highly nonlinear solitary waves propagating along a chain of particles, the last of which is in dry contact with the eye to be evaluated. The hypothesis is that the travel time of the waves propagating within the chain is monotonically associated with the IOP. In this study, the effect of central corneal thickness (CCT) and IOP of artificial corneas made of polydimethylsiloxane was quantified experimentally, and modeled numerically using static and dynamic finite element analyses. The numerical and experimental results agreed in identifying a correlation between ToF and IOP and CCT. However, some quantitative discrepancies between numerical and experimental results warrant the improvement of the model.

Asymmetric stress distribution on lamina cribrosa in glaucoma patients with high myopia

PURPOSE

To investigate the impact of biomechanical changes in the optic nerve head on glaucoma progression during ocular rotation in highly myopic eyes with primary open-angle glaucoma.

METHODS

This study retrospectively enrolled 147 patients with primary open-angle glaucoma, including 57 with high myopia and 90 with non-high myopia. Personalized optic nerve head models were designed, and simulations were performed using finite element analysis to evaluate the effect of ocular rotation on the optic nerve head. Biomechanical changes in the optic nerve head were analyzed and compared between the groups.

RESULTS

The mean strain on the lamina cribrosa was significantly higher in the high myopia group compared to the non-high myopia group during ocular rotation. The stress ratio, indicating asymmetric stress distribution, was higher in the high myopia group than in the non-high myopia group. The stress ratio increased significantly as axial length increased. Both stress ratio and mean lamina cribrosa strain were correlated with rate of retinal nerve fiber layer thickness reduction (stress ratio: partial coefficient = -0.171, P = 0.040; mean lamina cribrosa strain: partial coefficient = -0.196, P = 0.018).

CONCLUSION

High myopia leads to increased lamina cribrosa strain and asymmetric stress distribution during ocular rotation. These findings underscore the importance of considering optic nerve head biomechanics in patients with high myopia. Additionally, the stress ratio may serve as a novel indicator for predicting glaucoma progression.

KEY MESSAGES

What is known Biomechanical changes in the optic nerve head differ between patients with high myopia and those with non-high myopia. What is new Asymmetric stress distribution on the lamina cribrosa during ocular movement was higher in the high myopia group and was associated with structural progression of glaucoma. A longer axial length increased the asymmetric stress on the lamina cribrosa during ocular movement.

Effect of Hydration on Viscoelastic Tensile Properties of Sclera

The present work characterized the effects of hydration on the viscoelastic tensile properties of the sclera. Scleral strips were dissected from the posterior region near the optic nerve head of porcine eyes in the superior-inferior direction. The samples were divided into four hydration groups and their mechanical response was characterized by conducting uniaxial tensile stress-relaxation experiments. An exponential relation and logarithmic expression were used to numerically represent the experimental measurements during the ramp and relaxation periods, respectively. A nonlinear increase in the tensile stress during the ramp period was observed for all strips. Furthermore, it was found that dehydrated specimens had stiffer tensile properties. In particular, it was observed that the maximum and equilibrium stresses increased significantly with decreasing hydration. Furthermore, it was found that the viscoelastic tensile response of porcine scleral strips at different hydration levels could be collapsed onto a single normalized curve. The findings of the present work showed that hydration had significant effects on the viscoelastic tensile properties of sclera.

Shear viscoelastic properties of human orbital fat

The shear viscoelastic behavior of eye's supporting orbital fat is unstudied in humans, yet is important during and after rapid movement. This investigation quantified viscoelastic characteristics of human orbital fat in constitutive form suitable for numerical simulation. Fresh human orbital fat was harvested postmortem from 6 male and 7 female donors of average age 78 ± 13 years. Fat samples were trimmed to disks of 20 ± 3.0 (standard deviation) mm average diameter and 2.1 ± 0.2 mm thickness. In 8 samples each, the following four testing protocols were performed: strain sweep from 0.0015 to 50 % at 1 Hz; viscometry at 0.1 s-1 shear rate; stress relaxation at physiological temperature; and frequency sweep from 0.159 to 15.9 Hz at 0.5 % strain to validate the Prony series parameters fitting stress relaxation behavior. Orbital fat exhibited viscoelastic behavior under dynamic shear with a 0.5 % linear viscoelastic strain limit. Storage modulus G' averaged 737 ± 310 Pa, and loss modulus G″ averaged 197 ± 76 Pa. Values were similar for strain and frequency sweep testing. At rupture, shear stress averaged 617 ± 366 Pa and rupture strain averaged 200 ± 70 %. The long-term relaxation modulus averaged 646 ± 264 Pa at 100 s. Frequency sweep testing validated the parameters of the Prony series fitted to the experimental stress relaxation data. Human orbital fat is linearly viscoelastic within a range typical of biological materials, and exhibits similar viscoelastic behavior for strain and frequency sweep testing. Stress relaxation data for human orbital fat has been parameterized for constitutive models that can be implemented in finite element analysis.

Tensile properties of glaucomatous human sclera, optic nerve, and optic nerve sheath

We characterized the tensile behavior of sclera, optic nerve (ON), and ON sheath in eyes from donors with glaucoma, for comparison with published data without glaucoma. Twelve freshly harvested eyes were obtained from donors with history of glaucoma, of average age 86 ± 7 (standard deviation) years. Rectangular samples were taken from anterior, equatorial, posterior, and peripapillary sclera, and ON sheath, while ON was in native form and measured using calipers. Under physiological temperature and humidity, tissues were preconditioned at 5% strain before loading at 0.1 mm/s. Force-displacement data were converted into engineering stress-strain curves fit by reduced polynomial hyperelastic models and analyzed by tangent moduli at 3% and 7% strain. Data were compared with an age-matched sample of 7 published control eyes. Optic atrophy was supported by significant reduction in ON cross section to 73% of normal in glaucomatous eyes. Glaucomatous was significantly stiffer than control in equatorial and peripapillary regions (P < 0.001). However, glaucomatous ON and sheath were significantly less stiff than control, particularly at low strain (P < 0.001). Hyperelastic models were well fit to stress-strain data (R2 > 0.997). Tangent moduli had variability similar to control in most regions, but was abnormally large in peripapillary sclera. Tensile properties were varied independently among various regions of the same eyes. Glaucomatous sclera is abnormally stiff, but the ON and sheath are abnormally compliant. These abnormalities correspond to properties predicted by finite element analysis to transfer potentially pathologic stress to the vulnerable disk and lamina cribrosa region during adduction eye movement.

Comparing IOP-Induced Scleral Deformations in the Myopic and Myopic Glaucoma Spectrums

Purpose

To compare changes in macular curvature following acute IOP elevation across a range of myopic conditions.

Methods

We studied 328 eyes from 184 subjects, comprising 32 emmetropic controls (between +2.75 and -2.75 diopters), 50 eyes with high myopia (<-5 diopters; HM), 108 highly myopic with glaucoma (HMG) and 105 pathologic myopia (PM) eyes, and 33 PM with staphyloma (PM+S) eyes. For each eye, we imaged the macula using optical coherence tomography (OCT) under the baseline condition and under acute IOP elevation (to ∼40 mm Hg) achieved through ophthalmodynamometry. We manually aligned the scans (baseline and IOP elevation) using three vascular landmarks in the macula tissue. We then automatically segmented the sclera and the choroid tissues using a deep learning algorithm and extracted the sclera-choroid interface. We calculated the macula curvatures, determined by the radius of curvature of the sclera-choroid interface in the nasal-temporal and superior-inferior direction. Differences in macula curvatures between baseline and elevated IOP scans were calculated at corresponding locations, and the mean curvature difference was reported for each eye.

Results

IOP elevation resulted in a significantly higher macula curvature change along the nasal-temporal direction in the PM+S (13.5 ± 8.2 × 10-5 µm-1), PM (9.0 ± 7.9 × 10-5 µm-1), and HMG (5.2 ± 5.1 × 10-5 µm-1) eyes as compared to HM (3.1 ± 2.7 × 10-5 µm-1) eyes (all P < 0.05). Interestingly, HM and HMG eyes had the same curvature change in the nasal-temporal direction as emmetropic control eyes (4.2 ± 4.3 × 10-5 µm-1).

Conclusions

Our findings indicate that the macula in HMG, PM, and PM+S eyes showed greater curvature changes under IOP elevation compared to HM and emmetropic eyes. These preliminary results suggest that HM eyes with conditions such as glaucoma or staphyloma are more sensitive to acute IOP elevation.

In-vivo high-frequency quantitative ultrasound-derived parameters of the anterior sclera correlated with level of myopia and presence of staphyloma

BACKGROUND

A high-frequency point-of-care (POC) ultrasound instrument was used to evaluate the microstructural and biomechanical properties of the anterior sclera in vivo using parameters computed from quantitative ultrasound (QUS) methods.

METHODS

In this cross-sectional study, both eyes of 85 enrolled patients were scanned with the POC instrument and ultrasound data were processed to obtain QUS parameters. Pearson correlation and multi-linear regression were used to identify relationships between QUS parameters and refractive error (RE) or axial length. After categorising eyes based on RE, binary support vector machine (SVM) classifiers were trained using the QUS or ophthalmic parameters (anterior chamber depth, central corneal thickness, corneal power, and intraocular pressure) to classify each eye. Classifier performance was evaluated by computing the area under the receiver-operating characteristic curve (AUC).

RESULTS

Individual QUS parameters correlated with RE and axial length (p < 0.05). Multi-linear regression revealed significant correlation between the set of QUS parameters and both RE (R = 0.49, p < 0.001) and axial length (R = 0.46, p = 0.001). Classifiers trained with QUS parameters achieved higher AUC (𝑝 = 0.06) for identifying myopic eyes (AUC = 0.71) compared to classifiers trained with ophthalmic parameters (AUC = 0.63). QUS-based classifiers attained the highest AUC when identifying highly myopic eyes (AUC = 0.77).

CONCLUSIONS

QUS parameters correlate with progressing myopia and may be indicative of myopia-induced microstructural and biomechanical changes in the anterior sclera. These methods may provide critical clinical information complementary to standard ophthalmic measurements for predicting myopia progression and risk assessment for posterior staphyloma formation.

Age-related alterations in vortex veins on indocyanine green angiography

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

Reversing Aging and Improving Health Span in Glaucoma Patients: The Next Frontier?

How to cite this article: Dada T, Mahalingam K, Bhartiya S. Reversing Aging and Improving Health Span in Glaucoma Patients: The Next Frontier? J Curr Glaucoma Pract 2024;18(3):87-93.

The anterior scleral thickness in primary open-angle glaucoma with high myopia

Purpose

This study aimed to investigate and compare the anterior scleral thickness (AST) among high myopia (HM), primary open-angle glaucoma (POAG), and POAG with HM (HMPOAG) groups.

Methods

Thirty-two HM eyes, 30 POAG eyes, and 31 HMPOAG eyes were included. The Schlemm's canal (SC) area, trabecular meshwork (TM) thickness, scleral spur (SS) length, and AST were measured using swept-source optical coherence tomography. AST was measured at 0 mm (AST0), 1 mm (AST1), 2 mm (AST2), and 3 mm (AST3) from SS.

Results

The HMPOAG group had significantly thinner AST, SS length, and TM thickness than the HM and POAG groups (all p < 0.05). In addition, the SC area of the HMPOAG group was also significantly smaller than that of the HM group (p < 0.001).

Conclusion

The HMPOAG group had the thinnest AST, shortest SS, thinnest TM, and smallest SC. The thinnest AST might contribute to the shortest SS, and further to the thinnest TM and smallest SC in the HMPOAG group. AST might be a novel clinical indicator in the prediction and evaluation of POAG.

Postmortem Digital Image Correlation and Finite Element Modeling Demonstrate Posterior Scleral Deformations during Optic Nerve Adduction Tethering

Postmortem human eyes were subjected to optic nerve (ON) traction in adduction and elevated intraocular pressure (IOP) to investigate scleral surface deformations. We incrementally adducted 11 eyes (age 74.1 ± 9.3 years, standard deviation) from 26° to 32° under normal IOP, during imaging of the posterior globe, for analysis by three-dimensional digital image correlation (3D-DIC). In the same eyes, we performed uniaxial tensile testing in multiple regions of the sclera, ON, and ON sheath. Based on individual measurements, we analyzed eye-specific finite element models (FEMs) simulating adduction and IOP loading. Analysis of 3D-DIC showed that the nasal sclera up to 1 mm from the sheath border was significantly compressed during adduction. IOP elevation from 15 to 30 mmHg induced strains less than did adduction. Tensile testing demonstrated ON sheath stiffening above 3.4% strain, which was incorporated in FEMs of adduction tethering that was quantitatively consistent with changes in scleral deformation from 3D-DIC. Simulated IOP elevation to 30 mmHg did not induce scleral surface strains outside the ON sheath. ON tethering in incremental adduction from 26° to 32° compressed the nasal and stretched the temporal sclera adjacent to the ON sheath, more so than IOP elevation. The effect of ON tethering is influenced by strain stiffening of the ON sheath.

The Temporal Relation Between Rates of Retinal Nerve Fiber Layer and Minimum Rim Width Changes in Glaucoma

Purpose

This study aims to determine whether OCT-derived rates of change in minimum rim width (MRW) are associated with and can potentially predict corresponding alterations in retinal nerve fiber layer thickness (RNFLT) in people with glaucoma.

Methods

The rates of change between six-monthly visits were taken from 568 eyes of 278 participants in the P3 Study. Structural equation models (SEM) assessed whether one parameter was predicted by the concurrent or previous rate of the other parameter, after adjusting for its own rate in the previous time interval. Root mean square error of approximation (RMSEA, with 90% confidence intervals [CI]), Tucker Lewis index (TLI) and the comparative fit index (CFI) assessed goodness of fit.

Results

Models without a time lag provided a better fit for the data (RMSEA = 0.101 [CI, 0.089, 0.113]), compared to a model featuring a time lag in RNFLT (RMSEA = 0.114 [CI, 0.102, 0.126]) or MRW (RMSEA = 0.114 [CI, 0.102, 0.127]). The SEMs indicated that rates for both MRW and RNFLT were predicted by their own rate in the previous time interval and by the other measure's change in the concurrent time interval (P > 0.001 for all). No evidence of a clinically significant time lag for either parameter was determined.

Conclusions

MRW and RNFLT exhibit concurrent changes over time in patients with glaucoma, with no clinically significant time lag determined.

Translational Relevance

RNFLT may be more useful than MRW in early glaucoma assessment because of its previously reported lower variability and reduced sensitivity to intraocular pressure changes.

Scanning Laser Ophthalmoscopy Demonstrates Pediatric Optic Disc and Peripapillary Strain During Horizontal Eye Rotation

Purpose: We employed automated analysis of scanning laser ophthalmoscopy (SLO) to determine if mechanical strains imposed on disc, and retinal and choroidal vessels during horizontal duction in children differ from those of adults.Methods: Thirty-one children aged 11.3 ± 2.7 (standard deviation) years underwent SLO in central gaze, and 35° ab- and adduction. Automated registration with deep learning-based optical flow analysis quantified vessel deformations as horizontal, vertical, shear, and equivalent strains. Choroidal vessel displacements in lightly pigmented fundi, and central disc vessel displacements, were also observed.Results: As in adults, strain in vessels during horizontal duction was greatest at the disc and decreased with distance from it. Strain in the pediatric disc was similar to published values in young adults,1 but in the peripapillary region was greater and propagated significantly more peripherally to at least three disc radii from it. During adduction in children, the nasal disc was compressed and disc vessels distorted, but the temporal half experienced tensile strain, while peripapillary tissues were compressed. The pattern was similar but strains were less in abduction (p < .001). Choroidal vessels were visualized in 24 of the 62 eyes and shifted directionally opposite overlying retinal vessels.Conclusions: Horizontal duction deforms the normal pediatric optic disc, central retinal vessels, peripapillary retina, and choroid, shearing the inner retina over the choroid. These mechanical effects occur at the sites of remodeling of the disc, sclera, and choroid associated with typical adult features that later emerge later, including optic cup enlargement, temporal disc tilting, and peripapillary atrophy.

Three-dimensional choroidal contour mapping in healthy population

Purpose was to study 3-dimensional choroidal contour at choroidal inner boundary (CIB) and choroidal outer boundary (COB) in healthy eyes. Healthy eyes imaged on wide field swept-source optical coherence tomography were included. Delineation of CIB and COB was done based on our previously reported methods. Quantitative analysis of the surfaces of CIB and COB was based on analyzing best fit spherical radius (R) (overall and sectoral). One hundred and seven eyes of 74 subjects with a mean age of 46.4 ± 19.3 years were evaluated. Overall, R COB (mean ± SD: 22.5 ± 4.8 mm) < R CIB (32.4 ± 9.4 mm). Central sector had the least R at COB (7.2 ± 5.9 mm) as well as CIB (25.1 ± 14.3 mm) across all age groups. Regression analysis between R (CIB) and age (r = -0.31, r2 = 0.09) showed negative correlation (P < 0.001) and that between R (COB) and age was positive (r = 0.26, r2 = 0.07) (P = 0.01). To conclude, central sector is the steepest sector in comparison to all the other sectors. This is indicative of a prolate shape of choroidal contour at CIB and COB. Outer boundary of choroid is steeper than inner boundary across all age groups. However, with ageing, outer boundary becomes flatter and inner boundary becomes steeper.

Effect of digital ocular massage on intraocular pressure and Schlemm's canal dimensions

Digital ocular massage has been reported to temporarily lower intraocular pressure (IOP). This could be related to an enhanced aqueous humor outflow; however, the mechanism is not clearly understood. Using anterior segment optical coherence tomography, the Schlemm's canal (SC) and trabecular meshwork (TM) can be imaged and measured. Here, 66 healthy adults underwent digital ocular massage for 10 min in their right eyes. The IOP and dimensions of the SC and TM were measured before and after ocular massage. All subjects demonstrated IOP reduction from 15.7 ± 2.5 mmHg at baseline to 9.6 ± 2.2 mmHg immediately after, and median of 11.6 mmHg 5-min after ocular massage (Friedman's test, p < 0.001). There was significant change in SC area (median 10,063.5 μm2 at baseline to median 10,151.0 μm2 after ocular massage, Wilcoxon test, p = 0.02), and TM thickness (median 149.8 μm at baseline to 144.6 ± 25.3 μm after ocular massage, Wilcoxon test, p = 0.036). One-third of the subjects demonstrated collapse of the SC area (-2 to -52%), while two-thirds showed expansion of the SC area (2 to 168%). There were no significant changes in SC diameter (270.4 ± 84.1 μm vs. 276.5 ± 68.7 μm, paired t-test, p = 0.499), and TM width (733.3 ± 110.1 μm vs. 733.5 ± 111.6 μm, paired t-test, p = 0.988). Eyes with a higher baseline IOP demonstrated a greater IOP reduction (Pearson correlation coefficient r = -0.521, p < 0.001). Eyes with smaller SC area at baseline showed greater SC area expansion (Pearson correlation coefficient = -0.389, p < 0.001). Greater IOP reduction appeared in eyes with greater SC area expansion (Pearson correlation coefficient r = -0.306, p = 0.01). Association between change in IOP and change in TM thickness was not significant (Spearman's ρ = 0.015, p = 0.902). Simple digital ocular massage is an effective method to lower IOP values, and change in the SC area was significantly associated with IOP changes.

A new look at an old problem: 3D modeling of accommodation reveals how age-related biomechanical changes contribute to dysfunction in presbyopia

Presbyopia is an age-related ocular disorder where accommodative ability declines so that an individual's focusing range is insufficient to provide visual clarity for near and distance vision tasks without corrective measures. With age, the eye exhibits changes in biomechanical properties of many components involved in accommodation, including the lens, sclera, and ciliary muscle. Changes occur at different rates, affecting accommodative biomechanics differently, but individual contributions to presbyopia are unknown. We used a finite element model (FEM) of the accommodative mechanism to simulate age-related changes in lens stiffness, scleral stiffness, and ciliary contraction to predict differences in accommodative function. The FEM predicts how ciliary muscle action leads to lens displacement by initializing a tensioned unaccommodated lens (Phase 0) then simulating ciliary muscle contraction in accommodation (Phase 1). Model inputs were calibrated to replicate experimentally measured lens and ciliary muscle in 30-year-old eyes. Predictions of accommodative lens deformation were verified with additional imaging studies. Model variations were created with altered lens component stiffnesses, scleral stiffness, or ciliary muscle section activations, representing fifteen-year incremental age-related changes. Model variations predict significant changes in accommodative function with age-related biomechanical property changes. Lens changes only significantly altered lens thickening with advanced age (46% decrease at 75 years old) while sclera changes produced progressive dysfunction with increasing age (23%, 36%, 49% decrease at 45, 60, and 75 years old). Ciliary muscle changes effected lens position modulation. Model predictions identified potential mechanisms of presbyopia that likely work in combination to reduce accommodative function and could indicate effectiveness of treatment strategies and their dependency on patient age or relative ocular mechanical properties.

The Effect of Scleral Buckle Surgery on Tonographic Outflow Facility, Positional Intraocular Pressure, and Ocular Biomechanics

PURPOSE

To investigate the in vivo effect of scleral buckle surgery on ocular biomechanics and aqueous humor dynamics.

DESIGN

Prospective observational cross-sectional study.

PARTICIPANTS

Nine patients with unilateral 360 degree encircling scleral buckles without vitrectomy for rhegmatogenous retinal detachments, between 3 and 39 months postoperative.

METHODS

All measurements were performed in both eyes of all participants. Intraocular pressure (IOP) was measured in the seated and supine positions using pneumatonometry. Outflow facility was measured using 2-minute weighted pneumatonography. Ocular rigidity coefficient was determined from the Friedenwald equations based on the difference in IOP with and without a weighted tonometer tip. The percentage change in IOP upon transitioning from seated to supine was calculated. Measurements for buckled and nonbuckled eyes were compared using paired Student t test of means.

MAIN OUTCOME MEASURES

Sitting and supine IOP and percentage difference between the 2 positions; outflow facility; ocular rigidity coefficient.

RESULTS

Seated IOP was similar between buckled and nonbuckled eyes (16.1 ± 2.5 vs. 16.7 ± 2.7 mmHg; P = 0.5) whereas supine IOP was lower in buckled eyes compared with nonbuckled eyes (18.7 ± 2.6 vs. 21.3 ± 2.5 mmHg; P = 0.008). The percentage increase in IOP upon change in body position from seated to supine was greater in nonbuckled eyes (17.4 ± 9.4% vs. 27.6 ± 9.5%; P = 0.005). Ocular rigidity coefficient was lower in buckled (9.9 × 10-3 ± 1.4 × 10-3 μL-1) vs. nonbuckled eyes (14.4 × 10-3 ± 3.1 × 10-3 μL-1; P = 0.006). Outflow facility was not significantly different in buckled and nonbuckled eyes.

CONCLUSIONS

Scleral buckling decreases ocular rigidity but does not affect outflow facility. This change in ocular biomechanics likely results in the attenuated IOP change from seated to supine position. Decreased ocular rigidity may also reduce IOP fluctuations and potentially reduce the risk for glaucoma progression.

FINANCIAL DISCLOSURE(S)

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

Direct measurements of collagen fiber recruitment in the posterior pole of the eye

Collagen is the main load-bearing component of the peripapillary sclera (PPS) and lamina cribrosa (LC) in the eye. Whilst it has been shown that uncrimping and recruitment of the PPS and LC collagen fibers underlies the macro-scale nonlinear stiffening of both tissues with increased intraocular pressure (IOP), the uncrimping and recruitment as a function of local stretch have not been directly measured. This knowledge is crucial to understanding their functions in bearing loads and maintaining tissue integrity. In this project we measured local stretch-induced collagen fiber bundle uncrimping and recruitment curves of the PPS and LC. Thin coronal samples of PPS and LC of sheep eyes were mounted and stretched biaxially quasi-statically using a custom system. At each step, we imaged the PPS and LC with instant polarized light microscopy and quantified pixel-level (1.5 μm/pixel) collagen fiber orientations. We used digital image correlation to measure the local stretch and quantified collagen crimp by the circular standard deviation of fiber orientations, or waviness. Local stretch-recruitment curves of PPS and LC approximated sigmoid functions. PPS recruited more fibers than the LC at the low levels of stretch. At 10% stretch the curves crossed with 75% bundles recruited. The PPS had higher uncrimping rate and waviness remaining after recruitment than the LC: 0.9º vs. 0.6º and 3.1º vs. 2.7º. Altogether our findings support describing fiber recruitment of both PPS and LC with sigmoid curves, with the PPS recruiting faster and at lower stretch than the LC, consistent with a stiffer tissue. STATEMENT OF SIGNIFICANCE: Peripapillary sclera (PPS) and lamina cribrosa (LC) collagen recruitment behaviors are central to the nonlinear mechanical behavior of the posterior pole of the eye. How PPS and LC collagen fibers recruit under stretch is crucial to develop constitutive models of the tissues but remains unclear. We used image-based stretch testing to characterize PPS and LC collagen fiber bundle recruitment under local stretch. We found that fiber-level stretch-recruitment curves of PPS and LC approximated sigmoid functions. PPS recruited more fibers at a low stretch, but at 10% bundle stretch the two curves crossed with 75% bundles recruited. We also found that PPS and LC fibers had different uncrimping rates and non-zero waviness's when recruited.

Scleral remodeling in early adulthood: the role of FGF-2

Emmetropization, a natural process of ocular elongation, is closely associated with scleral remodeling. The Fibroblast growth factor-2 (FGF-2) was reported involved in scleral remodeling in myopia models. Herein, we aimed to investigate the role of scleral fibroblast-to-myofibroblast differentiation and FGF-2 in scleral remodeling during maturation. Our findings revealed that the posterior scleral fibroblasts (SFs) from mature guinea pigs exhibit increased stiffness compared to those from young guinea pigs. Moreover, mature SFs displayed decreased cell proliferation but increased levels of α-SMA, matrix metalloproteinase 2 (MMP2), and collagen 1, when compared to young SFs. Additionally, the mRNA expression of scleral Fgf-2, Fgf receptor 1 (Fgfr1), Fgfr2, Fgfr3, and Fgfr4 was increased in mature SFs. Notably, exogenous FGF-2 showed increased cell proliferation and led to decreased expression of α-SMA, MMP2, and collagen 1 in mature SFs. Overall, our findings highlight the influence of maturation on SFs from posterior scleral shells, resulting in increased stiffness and the manifestation of fibroblast-to-myofibroblast differentiation during development. Exogenous FGF-2 increased cell proliferation and reversed the age-related fibroblast-to-myofibroblast differentiation, suggesting a potential role of FGF-2 in regulating scleral remodeling.

Scanning Laser Ophthalmoscopy Demonstrates Disc and Peripapillary Strain During Horizontal Eye Rotation in Adults

PURPOSE

We used automated image analysis of scanning laser ophthalmoscopy (SLO) to investigate mechanical strains imposed on disc, and retinal and choroidal vessels during horizontal duction in adults.

DESIGN

Deep learning analysis of optical images.

METHODS

The peripapillary region was imaged by SLO in central gaze, and 35° abduction and adduction, in younger and older healthy adults. Automated image registration was followed by deep learning-based optical flow analysis to track determine local tissue deformations quantified as horizontal, vertical, and shear strain maps relative to central gaze. Choroidal vessel displacements were observed when fundus pigment was light.

RESULTS

Strains in the retina and disc could be quantified in 22 younger (mean ± SEM, 26 ± 5 years) and 19 older (64 ± 10 years) healthy volunteers. Strains were predominantly horizontal and greater for adduction than for abduction. During adduction, maximum horizontal strain was tensile in the nasal hemi-disc, and declined progressively with distance from it. Strain in the temporal hemi-retina during adduction was minimal, except for compressive strain on the disc of older subjects. In abduction, horizontal strains were less and largely confined to the disc, greater in older subjects, and generally tensile. Vertical and shear strains were small. Nasal to the disc, choroidal vessels shifted nasally relative to overlying peripapillary retinal vessels.

CONCLUSIONS

Strain analysis during horizontal duction suggests that the optic nerve displaces the optic canal, choroid, and peripapillary sclera relative to the overlying disc and retina. This peripapillary shearing of the optic nerve relative to the choroid and sclera may be a driver of disc tilting and peripapillary atrophy.

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.

Direct measurements of collagen fiber recruitment in the posterior pole of the eye

Collagen is the main load-bearing component of the peripapillary sclera (PPS) and lamina cribrosa (LC) in the eye. Whilst it has been shown that uncrimping and recruitment of the PPS and LC collagen fibers underlies the macro-scale nonlinear stiffening of both tissues with increased intraocular pressure (IOP), the uncrimping and recruitment as a function of local stretch have not been directly measured. This knowledge is crucial for the development of constitutive models associating micro and macro scales. In this project we measured local stretch-induced collagen fiber bundle uncrimping and recruitment curves of the PPS and LC. Thin coronal samples of PPS and LC of sheep eyes were mounted and stretched biaxially quasi-statically using a custom system. At each step, we imaged the PPS and LC with instant polarized light microscopy and quantified pixel-level (1.5 μm/pixel) collagen fiber orientations. We used digital image correlation to measure the local stretch and quantified collagen crimp by the circular standard deviation of fiber orientations, or waviness. Local stretch-recruitment curves of PPS and LC approximated sigmoid functions. PPS recruited more fibers than the LC at the low levels of stretch. At 10% stretch the curves crossed with 75% bundles recruited. The PPS had higher uncrimping rate and waviness remaining after recruitment than the LC: 0.9° vs. 0.6° and 3.1° vs. 2.7°. Altogether our findings support describing fiber recruitment of both PPS and LC with sigmoid curves, with the PPS recruiting faster and at lower stretch than the LC, consistent with a stiffer tissue.

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.

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.

Typical localised element-specific finite element anterior eye model

Purpose

The study presents an averaged anterior eye geometry model combined with a localised material model that is straightforward, appropriate and amenable for implementation in finite element (FE) modelling.

Methods

Both right and left eye profile data of 118 subjects (63 females and 55 males) aged 22-67 years (38.5 ± 7.6) were used to build an averaged geometry model. Parametric representation of the averaged geometry model was achieved through two polynomials dividing the eye into three smoothly connected volumes. This study utilised the collagen microstructure x-ray data of 6 ex-vivo healthy human eyes, 3 right eyes and 3 left eyes in pairs from 3 donors, 1 male and 2 females aged between 60 and 80 years, to build a localised element-specific material model for the eye.

Results

Fitting the cornea and the posterior sclera sections to a 5th-order Zernike polynomial resulted in 21 coefficients. The averaged anterior eye geometry model recorded a limbus tangent angle of 37° at a radius of 6.6 mm from the corneal apex. In terms of material models, the difference between the stresses generated in the inflation simulation up to 15 mmHg in the ring-segmented material model and localised element-specific material model were significantly different (p < 0.001) with the ring-segmented material model recording average Von-Mises stress 0.0168 ± 0.0046 MPa and the localised element-specific material model recording average Von-Mises stress 0.0144 ± 0.0025 MPa.

Conclusions

The study illustrates an averaged geometry model of the anterior human eye that is easy to generate through two parametric equations. This model is combined with a localised material model that can be used either parametrically through a Zernike fitted polynomial or non-parametrically as a function of the azimuth angle and the elevation angle of the eye globe. Both averaged geometry and localised material models were built in a way that makes them easy to implement in FE analysis without additional computation cost compared to the limbal discontinuity so-called idealised eye geometry model or ring-segmented material model.

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.

Application of Decellularized Porcine Sclera in Repairing Corneal Perforations and Lamellar Injuries

Scleras are mainly used for the treatment of glaucoma, eyelid damage, and scleral ulcers. Given that the sclera and cornea collectively constitute the complete external structure of the eyeball and both have the same tissue and cell origin, we attempted to identify scleral materials to treat lamellar and penetrating corneal injuries. Based on research in our center, antigenic components in decellularized porcine sclera (DPS) were removed using a simplified decellularization method, leaving the collagen structure and active components undamaged. DPS preserved the mechanical properties and did not significantly inhibit the proliferation and replication of human corneal epithelial cells. In vivo, the graft epithelium healed well after lamellar and penetrating scleral grafting, and the graft thickness did not change evidently. DPS can resist suture traction during scleral transplantation and maintain anterior chamber stability until day 28 post-operatively, especially in penetrating repairs. No obvious immune rejection of lamellar or penetrating scleral grafts was found 28 days after DPS transplantation. This study shows that DPS could be used as an alternative material for the emergency repair of corneal perforations and lamellar injuries, representing another application of sclera.

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.

The Anterior Segment Biometrics in High Myopia Eyes

INTRODUCTION

The aim of this study was to investigate and compare the anterior segment biometrics in high myopia and control groups.

METHODS

Thirty-four eyes of 34 high myopia patients and 42 eyes of 42 control subjects were included. Schlemm's canal (SC) area, trabecular meshwork (TM) thickness and length, scleral spur (SS) length, and anterior scleral thickness (AST) were measured using swept-source optical coherence tomography. Associations between SC area, TM thickness, TM length, SS length, and AST were also estimated.

RESULTS

SC area, TM thickness, and SS length were significantly associated with AST0 (AST at 0 mm from SS) in both high myopia and control groups. AST0 (702.61 ± 78.05 vs. 729.12 ± 95.87 μm, p = 0.085) and SS length (206.25 ± 52.25 vs. 212.09 ± 51.86 μm, p = 0.556) were not significantly different between high myopia and control groups, whereas SC area (6,622.68 ± 1,130.06 vs. 6,105.85 ± 1,297.84 μm2, p = 0.015) was significantly greater and TM thickness (96.15 ± 34.40 vs. 107.93 ± 29.97 μm, p = 0.048) was significantly thinner in high myopia group than in control group.

CONCLUSION

SC area and TM thickness were significantly associated with AST0, while AST0 and SS length were not significantly different between high myopia and control groups. The changes in SC and TM dimensions in high myopia eyes might be caused by factors other than AST0 and SS length.

The anterior scleral thickness in eyes with primary open-angle glaucoma

Purpose

To investigate the anterior scleral thickness (AST) and its associations with Schlemm’s canal (SC) area, trabecular meshwork (TM) thickness and length, and scleral spur (SS) length in healthy and primary open-angle glaucoma (POAG) groups.

Methods

Thirty-five eyes of 35 healthy subjects and 23 eyes of 23 patients with POAG were included. The AST, SC area, TM thickness and length, and SS length were measured using swept-source optical coherence tomography. AST was measured at 0 mm (AST0), 1 mm (AST1), 2 mm (AST2), and 3 mm (AST3) from SS. Associations between AST and SC area, TM thickness and length, and SS length were also estimated.

Results

AST0 (728.84 ± 99.33 vs. 657.39 ± 67.02 μm, p < 0.001), AST1 (537.79 ± 79.55 vs. 506.83 ± 57.37 μm, p = 0.038), AST3 (571.09 ± 79.15 vs. 532.13 ± 59.84 μm, p = 0.009), SC area (6304.26 ± 1238.72 vs. 4755.64 ± 1122.71 μm2, p < 0.001), TM thickness (107.21 ± 31.26 vs. 94.51 ± 24.18 μm, p = 0.035), TM length (736.20 ± 141.85 vs. 656.43 ± 127.03 μm, p = 0.004), and SS length (219.89 ± 50.29 vs. 174.54 ± 35.58 μm, p < 0.001) were significantly greater in healthy group than in POAG group. In addition, SC area, TM thickness, and SS length were significantly and positively associated with AST0 in the healthy group, whereas no similar associations were observed in the POAG group.

Conclusions

Compared with the healthy group, AST was significantly thinner in the POAG group, which also had smaller SC and TM dimensions. Moreover, the SC area, TM thickness, and SS length were significantly and positively associated with AST in the healthy group. Thus, AST might play an important role in maintaining TM and SC morphology and further in the pathogenesis of POAG.

The Upper Paralimbal Zone is Especially Susceptible to Ruptures Due to Blunt Trauma

BACKGROUND

Globe ruptures are ophthalmological emergencies where the primary diagnosis using a slit lamp is often a challenge.

OBJECTIVES

The retrospective case study aimed to characterise the predilection sites, the demographic profile and the causes of bursts due to blunt globe trauma.

MATERIALS AND METHODS

The surgical plan of the Eye Centre of the Medical Centre (University of Freiburg) was electronically searched using the keyword "globe rupture" over a period of 18 years (2000 - 2018). This led to 350 hits, which were scrutinised by hand. 134 globe ruptures due to blunt globe trauma were then identified.

RESULTS

The most common predilection sites were the upper nasal and the upper temporal paralimbal zones with 37 and 32% of the examined globe ruptures, respectively. The average age at the time of rupture was 60 (from 2 to 97) years. The median age was 62.5 years. 37% (49/134) of patients were female. The most frequent causes of accidents were falls (43%), "flying" wooden and metal objects (18%) and acts of violence (10%).

CONCLUSIONS

Special attention should be paid to the upper paralimbal zone in the exploratory primary care of globe ruptures. If the rupture occurred due to "flying" wooden and metal objects, the posterior sclera burst most frequently. The epidemiological data suggest that targeted prevention against falls for individuals over 60 years and consistent wearing of protective eyewear when working with wood or metal could reduce the incidence of bulbar bursting. It was also noticeable that acts of violence were the third most frequent cause of rupture, although woman did not report that they were affected by this. Given the high occurrence of violence against women, shame and stigma may lead to an incorrect inquiry about the cause of the accident. This could be mitigated by training employees specifically to take a sensitive history.

Comparison of Corneal Wave Speed and Ocular Rigidity in Normal and Glaucomatous Eyes

PRECIS

Ocular biomechanics were compared between treated glaucoma patients and healthy subjects matched for age, intraocular pressure (IOP), and axial length. There was no difference in corneal wave propagation speed, but ocular rigidity was lower in glaucomatous eyes.

PURPOSE

Ocular biomechanical properties are important in understanding glaucoma pathogenesis but the affected tissues are unclear. In this study, we compared corneal wave speed (a measure of corneal elasticity) and ocular rigidity coefficient between glaucomatous and normal eyes.

MATERIALS AND METHODS

Twenty glaucomatous eyes from 10 patients and 20 normal eyes from 13 controls, matched for age, IOP, and axial length were included. Ocular rigidity was calculated based on the difference in supine IOP by pneumatonometry with and without a 10-g weight. Corneal wave speed was determined by ultrasound surface wave elastography. A small, 0.1-second harmonic vibration at 100 Hz was generated through the closed eyelids. Wave propagation was captured by an ultrasound transducer, and wave speed was determined from the phase change with distance. Comparisons were performed using generalized estimating equation models.

RESULTS

There were no significant differences in corneal wave speed between glaucomatous and normal eyes (2.16±0.25 vs. 2.07±0.16 m/s, P=0.17). However, ocular rigidity was significantly lower in glaucomatous eyes (0.0218±0.0033 vs. 0.0252±0.0050/μL, P=0.01). Corneal wave speed was not correlated with age and IOP in either group (P≥0.23) but was correlated with ocular rigidity (R=0.48, P=0.02) and inversely correlated with axial length (R=-0.53, P=0.01) in glaucomatous eyes.

CONCLUSION

Glaucomatous eyes tend to have lower ocular rigidity than healthy eyes with similar age, IOP, and axial length. However, the lack of a difference in corneal wave speed suggests that corneal tissue may not be significantly affected, and scleral changes likely play a more important role in glaucoma.

In vivo measurement of the attenuation coefficient of the sclera and ciliary muscle

We acquired 1325 nm OCT images of the sclera and ciliary muscle of human subjects. The attenuation coefficients of the sclera and ciliary muscle were determined from a curve fit of the average intensity profile of about 100 A-lines in a region of interest after correction for the effect of beam geometry, using a single scattering model. The average scleral attenuation coefficient was 4.13 ± 1.42 mm-1 with an age-related decrease that was near the threshold for statistical significance (p = 0.053). The average ciliary muscle attenuation coefficient was 1.72 ± 0.88 mm-1, but this value may be an underestimation due to contributions from multiple scattering. Overall, the results suggest that inter-individual variations in scleral attenuation contribute to variability in the quality of transscleral OCT images of the ciliary muscle and the outcome of transscleral laser therapies.

Tensile Viscoelastic Properties of the Sclera after Glycosaminoglycan Depletion

PURPOSE

Fibrillar collagen network and glycosaminoglycans (GAGs) are the primary components of extracellular matrix (ECM) of the sclera. The main goal of this study was to investigate the possible structural roles of GAGs in the scleral tensile properties as a function of preconditioning and displacement rate.

METHODS

Four-step uniaxial stress-relaxation tests were used for characterizing the viscoelastic tensile response of the posterior porcine sclera with and without enzymatic GAG removal. The scleral strips were divided into different groups based on the displacement rate and the presence or absence of a preconditioning step in the loading protocol. The groups were (1) displacement rate of 0.2 mm/min without preconditioning, (2) displacement rate of 1 mm/min without preconditioning, (3) displacement rate of 0.2 mm/min with preconditioning, and (4) displacement rate of 1 mm/min with preconditioning. The peak stress, equilibrium stress, and the equilibrium elastic modulus were calculated for all specimens and compared against each other.

RESULTS

Increasing the displacement rate from 0.2 mm/min to 1.0 mm/min was found to cause an insignificant change in the equilibrium stress and equilibrium elastic modulus of porcine scleral strips. Removal of GAGs resulted in an overall stiffer tensile behavior independent of the displacement rate in samples that were not preconditioned (P < .05). The behavior of preconditioned samples with and without GAG removal was not significantly different from each other.

CONCLUSIONS

The experimental measurements of the present study showed that GAGs play an important role in the mechanical properties of the posterior porcine sclera. Furthermore, using a preconditioning step in the uniaxial testing protocol resulted in not being able to identify any significant difference in the tensile behavior of GAG depleted and normal scleral strips.

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.

Future Trends in Presbyopia Correction

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

Material properties and effect of preconditioning of human sclera, optic nerve, and optic nerve sheath

The optic nerve (ON) is a recently recognized tractional load on the eye during larger horizontal eye rotations. In order to understand the mechanical behavior of the eye during adduction, it is necessary to characterize material properties of the sclera, ON, and in particular its sheath. We performed tensile loading of specimens taken from fresh postmortem human eyes to characterize the range of variation in their biomechanical properties and determine the effect of preconditioning. We fitted reduced polynomial hyperelastic models to represent the nonlinear tensile behavior of the anterior, equatorial, posterior, and peripapillary sclera, as well as the ON and its sheath. For comparison, we analyzed tangent moduli in low and high strain regions to represent stiffness. Scleral stiffness generally decreased from anterior to posterior ocular regions. The ON had the lowest tangent modulus, but was surrounded by a much stiffer sheath. The low-strain hyperelastic behaviors of adjacent anatomical regions of the ON, ON sheath, and posterior sclera were similar as appropriate to avoid discontinuities at their boundaries. Regional stiffnesses within individual eyes were moderately correlated, implying that mechanical properties in one region of an eye do not reliably reflect properties of another region of that eye, and that potentially pathological combinations could occur in an eye if regional properties are discrepant. Preconditioning modestly stiffened ocular tissues, except peripapillary sclera that softened. The nonlinear mechanical behavior of posterior ocular tissues permits their stresses to match closely at low strains, although progressively increasing strain causes particularly great stress in the peripapillary region.

Scleral necrosis after brachytherapy for uveal melanoma: Analysis of risk factors

BACKGROUND

Radiation-induced scleral necrosis (RISN) is a rare, but a serious complication of brachytherapy for uveal melanoma. We aimed at analysing the incidence, timing and risk factors associated with development of RISN in a large institutional series.

METHODS

All consecutive cases with brachytherapy for uveal melanoma treated by the Departments of Ophthalmology and Radiotherapy at University Hospital Essen between 1999 and 2016 were eligible. Development of RISN during the post-treatment follow-up was recorded. A 1:2 propensity score matched case-control study was performed for the evaluation of the prognostic value of different tumour- and treatment-associated parameters.

RESULTS

RISN was documented in 115 (2.9%) of 3960 patients with uveal melanoma included in the final analysis, and occurred at the mean 30.3 months (range: 1.26-226 months) after brachytherapy. In the whole cohort, younger age (p = 0.042), plaque type (p = 0.001) and ciliary body involvement (p < 0.0001) were independently associated with the RISN occurrence. In the case-control study, multivariable weighted proportional hazard analysis discovered the association of the following additional tumour- and treatment-associated characteristics with RISN: posterior tumour margin anterior to equatorial region (p = 0.0003), extraocular tumour extension (p = <0.0001), scleral contact dose (p = <0.0001), conjunctival dehiscence after therapy (p = 0.0001), disinsertion of the superior rectus muscle (p = 0.001) and the glaucoma medication (p = 0.014).

CONCLUSIONS

Our study confirms RISN as a rare complication, which might occur even years later after the brachytherapy for uveal melanoma. Alongside with scleral dose five other tumour and therapy related factors predict the risk of RISN after brachytherapy for uveal melanoma were established.

Effect of biomechanical properties on myopia: a study of new corneal biomechanical parameters

Background

To assess the corneal stress-strain index (SSI), which is a marker for material stiffness and corneal biomechanical parameters, in myopic eyes.

Methods

A total of 1054 myopic patients were included in this study. Corneal visualisation Scheimpflug technology was used to measure the SSI. Corneal biomechanics were assessed using the first and second applanation times (A1-and A2-times); maximum deflection amplitude (DefAmax); deflection area (HCDefArea); the highest concavity peak distance (HC-PD), time (HC-time), and deflection amplitude (HC-DefA); integrated radius (IR); whole eye movement (WEM); stiffness parameter (SP-A1;, biomechanically corrected intraocular pressure (BIOP); and Corvis biomechanical index (CBI). Scheimpflug tomography was used to obtain the mean keratometery (Km) and central corneal thickness (CCT). According to the spherical equivalent (SE) (low myopia: SE ≥ − 3.00D and high myopia: SE ≤ − 6.00D.), the suitable patients were divided into two groups.

Results

The mean SSI value was 0.854 ± 0.004. The SSI had a positive correlation with A1-time ((r = 0.272), HC-time (r = 0.218), WEM (r = 0.288), SP-A1 (r = 0.316), CBI (r = 0.199), CCT (r = 0.125), bIOP (r = 0.230), and SE (r = 0.313) (all p-values<0.01). The SSI had a negative correlation with HCDefA (r = − 0.721), HCDefArea (r = − 0.665), HC-PD(r = − 0.597), IR (r = − 0.555), DefAmax (r = − 0.564), and Km (r = − 0.103) (all p-values<0.01). There were significant differences in SSI (t = 8.960, p<0.01) and IR (t = − 3.509, p<0.01) between the low and high myopia groups.

Conclusions

In different grades of myopia, the SSI values were lower in eyes with higher SEs. It indicates that the mechanical strength of the cornea may be compromised in high myopia. The SSI was positively correlated with the spherical equivalent, and it may provide a new way to study the mechanism of myopia.

Limbus misrepresentation in parametric eye models

Purpose

To assess the axial, radial and tangential limbus position misrepresentation when parametric models are used to represent the cornea and the sclera.

Methods

This retrospective study included 135 subjects aged 22 to 65 years (36.5 mean ±9.8 STD), 71 females and 64 males. Topography measurements were taken using an Eye Surface Profiler topographer and processed by a custom-built MATLAB code. Eye surfaces were freed from edge-effect artefacts and fitted to spherical, conic and biconic models.

Results

When comparing the radial position of the limbus, average errors of -0.83±0.19mm, -0.76±0.20mm and -0.69±0.20mm were observed within the right eye population for the spherical, conic and biconic models fitted up to 5mm. For the same fitting radius, the average fitting errors were -0.86±0.23mm, -0.78±0.23mm and -0.73±0.23mm for the spherical, conic and biconic models respectively within the left eye population. For the whole cornea fit, the average errors were -0.27±0.12mm and -0.28±0.13mm for the spherical models, -0.02±0.29mm and -0.05±0.27mm for the conic models, and -0.22±0.16mm and 0.24±0.17mm for the biconic models in the right and left eye populations respectively.

Conclusions

Through the use of spherical, conic and biconic parametric modelling methods, the eye’s limbus is being mislocated. Additionally, it is evident that the magnitude of fitting error associated with the sclera may be propagating through the other components of the eye. This suggests that a corneal nonparametric model may be necessary to improve the representation of the limbus.

A full-field 3D digital image correlation and modelling technique to characterise anterior cruciate ligament mechanics ex vivo

It is limiting to use conventional methods when characterising material properties of complex biological tissues with inhomogeneous and anisotropic structure, such as the anterior cruciate ligament (ACL) in the knee joint. This study aims to develop and utilise a three-dimensional digital image correlation method (3D DIC) for the purpose of determining material properties of femur-ACL-tibia complex across the surface without any contact between the tissue and the loading equipment. A full-field (360° view) 3D DIC test setup consisting of six digital single-lens reflex cameras was developed and ACL specimens from skeletally mature dog knee joints were tested. The six cameras were arranged into three pairs and the cameras within each pair were positioned with 25° in between to obtain the desired stereovision output. The test setup was calibrated twice: first to obtain the intrinsic and extrinsic parameters within camera pairs, and second to align the 3D surfaces from each camera pair in order to generate the full view of the ACLs. Using the undeformed 3D surfaces of the ligaments, ACL-specific finite element models were generated. Longitudinal deformation of ligaments under tensile loads obtained from the 3D DIC, and this was analysed to serve as input for the inverse finite element analysis. As a result, hyperelastic coefficients from the first-order Ogden model that characterise ACL behaviour were determined with a marginal error of <1.5%. This test setup and methodology provides a means to accurately determine inhomogeneous and anisotropic material properties of ACL. The methodology described in this study could be adopted to investigate other biological and cultured tissues with complex structure. STATEMENT OF SIGNIFICANCE: Determining the material properties of soft tissues with complex anatomical structure, such as the anterior cruciate ligament (ACL), is important to better understand their contribution to musculoskeletal biomechanics. Current conventional methods for characterising material properties of the ACL are often limited to a contact measurement approach, however an improved understanding of the mechanics of this complex tissue is vital in terms of preventing injury and developing novel therapies. This article reports the development and utilisation of non-contact optical methodology involving full-field three-dimensional digital image correlation and finite element analysis to accurately investigate material properties of the ACL, in a controlled environment. This technique reduces inaccuracies due to specimen clamping and more importantly considers the inhomogeneous nature of the examined tissue.

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.

Bilaminar Mechanics of the Human Optic Nerve Sheath

PURPOSE/AIM

The adult human optic nerve (ON) sheath has recently been recognized to be bilaminar, consisting of inner layer (IL) and outer layer (OL). Since the ON and sheath exert tension on the globe in large angle adduction as these structures transmit reaction force of the medial rectus muscle to the globe, this study investigated the laminar biomechanics of the human ON sheath.

MATERIALS AND METHODS

Biomechanical characterization was performed in ON sheath specimens from 12 pairs of fresh, post-mortem adult eyes. Some ON sheath specimens were tested completely, while others were separated into IL and OL. Uniaxial tensile loading under physiological temperature and humidity was used to characterize a linear approximation as Young's modulus, and hyperelastic non-linear behavior using the formulation of Ogden. Micro-indentation was performed by imposing small compressive deformations with small, hard spheres. Specimens of the same sheaths were paraffin embedded, sectioned at 10 micron thickness, and stained with van Gieson's stain for anatomical correlation.

RESULTS

Mean (± standard error of the mean, SEM) tensile Young's modulus of the inner sheath at 19.8 ± 1.6 MPa significantly exceeded that for OL at 9.7 ± 1.2 MPa; the whole sheath showed intermediate modulus of 15.4 ± 1.1 MPa. Under compression, the inner sheath was stiffer (7.9 ± 0.5 vs 5.2 ± 0.5 kPa) and more viscous (150.8 ± 10.6 vs 75.6 ± 6 kPa s) than outer sheath. The inner sheath had denser elastin fibers than outer sheath, correlating with greater stiffness.

CONCLUSIONS

We conclude that maximum tensile stiffness occurs in the elastin-rich ON sheath IL that inserts near the lamina cribrosa where tension in the sheath exerted during adduction tethering may be concentrated adjacent the ON head.

In vivo measures of anterior scleral resistance in humans with rebound tonometry

PURPOSE

To measure regional variations in anterior scleral resistance (ASR) using a ballistic rebound tonometer (RBT) and examine whether the variations are significantly affected by ethnicity and refractive error (RE).

METHODS

ASR was measured using a RBT (iCare TA01) following calibration against the biomechanical properties of agarose biogels. Eight scleral regions (nasal, temporal, superior, inferior, inferior-nasal, inferior-temporal, superior-nasal and superior-temporal) were measured at locations 4mm from the limbus. Subjects were 130 young adults comprising three ethnic groups whose RE distributions [MSE (D) ± S.D.] incorporated individuals categorised as without-myopia (NM; MSE ≥ -0.50) and with-myopia (WM; MSE < -0.50); British-White (BW): 26 NM + 0.52 ± 1.15D; 22 WM -3.83 ± 2.89D]; British-South-Asian (BSA): [9 NM + 0.49 ± 1.06D; 11 WM -5.07 ± 3.76D; Hong-Kong-Chinese (HKC): [11 NM + 0.39 ± 0.66D; 49 WM -4.46 ± 2.70D]. Biometric data were compiled using cycloplegic open-field autorefraction and the Zeiss IOLMaster. Two- and three-way repeated measures analysis of variances (anovas) tested regional differences for RBT values across both refractive status and ethnicity whilst stepwise forward multiple linear regression was used as an exploratory test.

RESULTS

Significant regional variations in ASR were identified for the BW, BSA and HKC (p < 0.001) individuals; superior-temporal region showed the lowest levels of resistance whilst the inferior-nasal region the highest. Compared to the BW and BSA groups, the HKC subjects displayed a significant increase in mean resistance for each respective region (p < 0.001). With the exception of the inferior region, ethnicity was found to be the chief predictor for variation in the scleral RBT values for all other regions. Mean RE group differences were insignificant.

CONCLUSIONS

The novel application of RBT to the anterior sclera confirm regional variation in ASR. Greater ASR amongst the HKC group than the BW and BSA individuals suggests that ethnic differences in anterior scleral biomechanics may exist.

Microstructure-based numerical simulation of the mechanical behaviour of ocular tissue

This paper aims to present a novel full-eye biomechanical material model that incorporates the characteristics of ocular tissues at microstructural level, and use the model to analyse the age-related stiffening in tissue behaviour. The collagen content in ocular tissues, as obtained using X-ray scattering measurements, was represented by sets of Zernike polynomials that covered both the cornea and sclera, then used to reconstruct maps of collagen fibril magnitude and orientation on the three-dimensional geometry of the eye globe. Fine-mesh finite-element (FE) models with eye-specific geometry were built and supported by a user-defined material model (UMAT), which considered the regional variation of fibril density and orientation. The models were then used in an iterative inverse modelling study to derive the material parameters that represent the experimental behaviour of ocular tissues from donors aged between 50 and 90 years obtained in earlier ex vivo studies. Sensitivity analysis showed that reducing the number of directions that represented the anisotropy of collagen fibril orientation at each X-ray scattering measurement point from 180 to 16 would have limited and insignificant effect on the FE solution (0.08%). Inverse analysis resulted in material parameters that provided a close match with experimental intraocular pressure-deformation behaviour with a root mean square of error between 3.6% and 4.3%. The results also demonstrated a steady increase in mechanical stiffness in all ocular regions with age. A constitutive material model based on distributions of collagen fibril density and orientation has been developed to enable the accurate representation of the biomechanical behaviour of ocular tissues. The model offers a high level of control of stiffness and anisotropy across ocular globe, and therefore has the potential for use in planning surgical and medical procedures.