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

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.

Revealing regional variations in scleral shear modulus in a rabbit eye model using multi-directional ultrasound optical coherence elastography

The mechanical properties of the sclera play a critical role in supporting the ocular structure and maintaining its shape. However, non-invasive measurements to quantify scleral biomechanics remain challenging. Recently introduced multi-directional optical coherence elastography (OCE) combined with an air-coupled ultrasound transducer for excitation of elastic surface waves was used to estimate phase speed and shear modulus in ex vivo rabbit globes (n = 7). The scleral phase speed (12.1 ± 3.2 m/s) was directional-dependent and higher than for corneal tissue (5.9 ± 1.4 m/s). In the tested locations, the sclera proved to be more anisotropic than the cornea by a factor of 11 in the maximum of modified planar anisotropy coefficient. The scleral shear moduli, estimated using a modified Rayleigh-Lamb wave model, showed significantly higher values in the circumferential direction (65.4 ± 31.9 kPa) than in meridional (22.5 ± 7.2 kPa); and in the anterior zone (27.3 ± 9.3 kPa) than in the posterior zone (17.8 ± 7.4 kPa). The multi-directional scanning approach allowed both quantification and radial mapping of estimated parameters within a single measurement. The results indicate that multi-directional OCE provides a valuable non-invasive assessment of scleral tissue properties that may be useful in the development of improved ocular models, the evaluation of potential myopia treatment strategies, and disease characterization and monitoring.

Planar biaxial testing of CXL strengthening effects

The stiffening effect of corneal crosslinking (CXL) treatment, a therapeutic approach for managing the progression of keratoconus, has been primarily investigated using uniaxial tensile experiments. However, this testing technique has several drawbacks and is unable to measure the mechanical response of cornea under a multiaxial loading state. In this work, we used biaxial mechanical testing method to characterize biomechanical properties of porcine cornea before and after CXL treatment. We also investigated the influence of preconditioning on measured properties and used TEM images to determine microstructural characteristics of the extracellular matrix. The conventional method of CXL treatment was used for crosslinking the porcine cornea. The biaxial experiments were done by an ElectroForce TestBench system at a stretch ratio of 1:1 and a displacement rate of 2 mm/min with and without preconditioning. The experimental measurements showed no significant difference in the mechanical properties of porcine cornea along the nasal temporal (NT) and superior inferior (SI) direction. Furthermore, the CXL therapy significantly enhanced the mechanical properties along both directions without creating anisotropic response. The samples tested with preconditioning showed significantly stiffer response than those tested without preconditioning. The TEM images showed that the CXL therapy did not increase the diameter of collagen fibers but significantly decreased their interfibrillar spacing, consistent with the mechanical property improvement of CXL treated samples.

Biomechanics Explains Variability of Response of Small Hypertropia to Graded Vertical Rectus Tenotomy

PURPOSE

Small angle hypertropia in sagging eye syndrome is conveniently treated by graded vertical rectus tenotomy, yet an adjustable technique under topical anesthesia has been recommended because of variability of effect. We performed graded tenotomy in an experimental model to elucidate the reason for variability of response to this surgical procedure.

DESIGN

Experimental study.

METHODS

Thirty-two fresh bovine rectus musculotendon specimens were prepared including continuity with insertional sclera, and extending for a total 40 mm length to the proximal muscle bellies, and trimmed to 16 mm width. Specimens were anchored by the clamps at the scleral insertion and muscle belly ends within a physiological chamber. After preconditioning and elongation to 10% strain was imposed by a linear motor, tensile force was allowed to stabilize at a plateau state. Then 25%, 50%, 75%, 90%, and 100% marginal tenotomies were performed progressively as remnant forces were measured.

RESULTS

Tendon thickness averaged 0.29 ± 0.05 mm and width 19.71 ± 2.25 mm. On average, remnant force decreased linearly (R2 = 0.985) from 4.23 ± 1.34, 2.76 ± 0.88, 1.70 ± 0.73, 1.01 ± 0.49, 0.39 ± 0.10, and 0 N, at 0%, 25%, 50%, 75%, 90%, and 100% tenotomy. However, there was marked individual variability in effect among specimens, with coefficients of variation of 32%, 32%, 43%, 49%, and 27%, respectively.

CONCLUSION

On average, there is a linear relationship between graded rectus tenotomy and percentage force reduction, but the effect among individual tendons is large, paralleling the reported variation in surgical effect. This explains and implies continued advisability of adjustable technique in this procedure.

Effects of Different Scleral Photo-Crosslinking Modalities on Scleral Stiffness and Hydration

Purpose

The purpose of this study was to evaluate the biomechanical and hydration differences in scleral tissue after two modalities of collagen cross-linking.

Methods

Scleral tissue from 40 adult white rabbit eyes was crosslinked by application of 0.1% Rose Bengal solution followed by 80 J/cm2 green light irradiation (RGX) or by application of 0.1% riboflavin solution followed by 5.4 J/cm2 ultraviolet A irradiation (UVX). Posterior scleral strips were excised from treated and untreated sclera for tensile and hydration-tensile tests. For tensile tests, the strips were subjected to uniaxial extension after excision. For hydration-tensile tests, the strips were dehydrated, rehydrated, and then tested. Young's modulus at 8% strain and swelling rate were estimated. ANOVAs were used to test treated-induced differences in scleral biomechanical and hydration properties.

Results

Photo-crosslinked sclera tissue was stiffer (Young's modulus at 8% strain: 10.7 ± 4.5 MPa, on average across treatments) than untreated scleral tissue (7.1 ± 4.0 MPa). Scleral stiffness increased 132% after RGX and 90% after UVX compared to untreated sclera. Scleral swelling rate was reduced by 11% after RGX and by 13% after UVX. The stiffness of the treated sclera was also associated with the tissue hydration level. The lower the swelling, the higher the Young's modulus of RGX (-3.8% swelling/MPa) and UVX (-3.5% swelling/MPa) treated sclera.

Conclusions

Cross-linking with RGX and UVX impacted the stiffness and hydration of rabbit posterior sclera. The Rose Bengal with green light irradiation may be an alternative method to determine the efficacy and suitability of inducing scleral tissue stiffening in the treatment of myopia.

Effect of twisting of intravitreal injections on ocular bio-mechanics: a novel insight to ocular surgery

Although intravitreal (IVT) injections provide several advantages in treating posterior segment eye diseases, several associated challenges remain. The current study uses the finite element method (FEM) to highlight the effect of IVT needle rotation along the insertion axis on the reaction forces and deformation inside the eye. A comparison of the reaction forces at the eye's key locations has been made with and without rotation. In addition, a sensitivity analysis of various parameters, such as the needle's angular speed, insertion location, angle, gauge, shape, and intraocular pressure (IOP), has been carried out to delineate the individual parameter's effect on reaction forces during rotation. Results demonstrate that twisting the needle significantly reduces the reaction forces at the penetration location and throughout the needle travel length, resulting in quicker penetration. Moreover, ocular biomechanics are influenced by needle insertion location, angle, shape, size, and IOP. The reaction forces incurred by the patient may be reduced by using a bevel needle of the higher gauge when inserted close to the normal of the local scleral surface toward the orra serrata within the Pars Plana region. Results obtained from the current study can deepen the understanding of the twisting needle's interaction with the ocular tissue.

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.

Finite element model of ocular adduction with unconstrained globe translation

Details of the anatomy and behavior of the structures responsible for human eye movements have been extensively elaborated since the first modern biomechanical models were introduced. Based on these findings, a finite element model of human ocular adduction is developed based on connective anatomy and measured optic nerve (ON) properties, as well as active contractility of bilaminar extraocular muscles (EOMs), but incorporating the novel feature that globe translation is not otherwise constrained so that realistic kinematics can be simulated. Anatomy of the hemisymmetric model is defined by magnetic resonance imaging. The globe is modeled as suspended by anatomically realistic connective tissues, orbital fat, and contiguous ON. The model incorporates a material subroutine that implements active EOM contraction based on fiber twitch characteristics. Starting from the initial condition of 26° adduction, the medial rectus (MR) muscle was commanded to contract as the lateral rectus (LR) relaxed. We alternatively modeled absence or presence of orbital fat. During pursuit-like adduction from 26 to 32°, the globe translated 0.52 mm posteriorly and 0.1 mm medially with orbital fat present, but 1.2 mm posteriorly and 0.1 mm medially without fat. Maximum principal strains in the optic disk and peripapillary reached 0.05-0.06, and von-Mises stress 96 kPa. Tension in the MR orbital layer was ~ 24 g-force after 6° adduction, but only ~ 3 gm-f in the whole LR. This physiologically plausible simulation of EOM activation in an anatomically realistic globe suspensory system demonstrates that orbital connective tissues and fat are integral to the biomechanics of adduction, including loading by the ON.

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.

Interaction between eye movements and adhesion of extraocular muscles

The contact and pull-off tests and finite element simulations were used to study the extraocular muscle-sclera adhesion and its variation with eye movement in this research. The effect of the adhesion on the eye movements was also determined using equilibrium equations of eye motion. The contact and pull-off tests were performed using quasi-static and non-quasi-static unloading velocities. Finite element models were developed to simulate these tests in cases with high unloading velocity which could not be achieved experimentally. These velocities range from the eye's fixation to saccade movement. The tests confirmed that the pull-off force is related to the unloading velocity. As the unloading velocity increases, the pull-off force increases, with an insignificant increase at the high ocular saccade velocities. The adhesion moment between the extraocular muscles and the sclera exhibited the same trend, increasing with higher eye movement velocities and higher separation angles between the two interfaces. The adhesion moment ratio to the total moment was calculated by the traditional model and the active pulley model of eye movements to assess the effect of adhesion behavior on eye movements. At the high ocular saccade velocities (about 461 deg/s), the adhesion moment was found to be 0.53% and 0.50% of the total moment based on the traditional and active pulley models, respectively. The results suggest that the adhesion behavior between the extraocular muscles and the sclera has a negligible effect on eye movements. At the same time, this adhesion behavior can be ignored in eye modeling, which simplifies the model reasonably well. STATEMENT OF SIGNIFICANCE: 1. Adhesion behavior between the extraocular muscles and the sclera at different indenter unloading velocities determined by contact and pull-off tests. 2. A finite element model was developed to simulate the adhesive contact between the extraocular muscles and the sclera at different indenter unloading velocities. The bilinear cohesive zone model was used for adhesive interactions. 3. The elastic modulus and viscoelastic parameters of the extraocular muscle along the thickness direction were obtained by using compressive stress-relaxation tests. 4. The influence of the adhesion moment between the extraocular muscles and the sclera on eye movement was obtained according to the equation of oculomotor balance. The adhesion moment between the extraocular muscles and the sclera was found to increase with increased eye movement velocity and increased separation angle between the two interfaces.

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.

Investigation of microstructural failure in the human cornea through fracture tests

Fracture toughness of the human cornea is one of the critical parameters in suture-involved corneal surgeries and the development of bioengineered mimetics of the human cornea. The present article systematically studied the fracture characteristics of the human cornea to evaluate its resistance to tear in the opening (Mode-I) and trouser tear mode (Mode-III). Tear experiments reveal the dependency of the fracture behavior on the notch size and its location created in the corneal specimens. The findings indicate lamellar tear and collagen fiber pull-out as a failure mechanism in trouser tear and opening mode tests, respectively. Experimental results have shown a localized variation of tear behavior in trouser tear mode and indicated an increasing resistance to tear from the corneal center to the periphery. This article demonstrated the complications of evaluating fracture toughness in opening mode and showed that the limbus was weaker than the cornea and sclera against tearing. The overall outcomes of the present study help in designing experiments to understand the toughness of the diseased tissues, understanding the effect of the suturing location and donor placement, and creating numerical models to study parameters affecting corneal replacement surgery.

Finite element modeling of effects of tissue property variation on human optic nerve tethering during adduction

Tractional tethering by the optic nerve (ON) on the eye as it rotates towards the midline in adduction is a significant ocular mechanical load and has been suggested as a cause of ON damage induced by repetitive eye movements. We designed an ocular finite element model (FEM) simulating 6° incremental adduction beyond the initial configuration of 26° adduction that is the observed threshold for ON tethering. This FEM permitted sensitivity analysis of ON tethering using observed material property variations in measured hyperelasticity of the anterior, equatorial, posterior, and peripapillary sclera; and the ON and its sheath. The FEM predicted that adduction beyond the initiation of ON tethering concentrates stress and strain on the temporal side of the optic disc and peripapillary sclera, the ON sheath junction with the sclera, and retrolaminar ON neural tissue. However, some unfavorable combinations of tissue properties within the published ranges imposed higher stresses in these regions. With the least favorable combinations of tissue properties, adduction tethering was predicted to stress the ON junction and peripapillary sclera more than extreme conditions of intraocular and intracranial pressure. These simulations support the concept that ON tethering in adduction could induce mechanical stresses that might contribute to ON damage.

Optic nerve-associated connective tissue structures revisited: a histological study using human fetuses and adult cadavers

Unlike the usual peripheral nerve, the optic nerve accompanies a thick "dural sheath," a thin "sheath of pia mater" (SPM), and multiple "septa," which divides the nerve fibers into fascicles. We collected specimens from 25 adult cadavers and 15 fetuses and revisited the histological architecture of the optic and oculomotor nerves. In the optic chiasma, the meningeal layer of the dura joins the pia to form a thick SPM, and the periosteum of the sphenoid is continuous with the dural sheath at the orbital exit of the bony optic canal. The septa appeared as a cluster of irregularly arrayed fibrous plates in the intracranial course near the chiasma. Thus, the septa were not derived from either the SPM or the dural sheath. In the orbit, the central artery of the retina accompanies collagenous fibers from the dural sheath and the SPM to provide the vascular sheath in the optic nerve. These connective tissue configurations were the same between adult and fetal specimens. At the optic disk, the dural sheath and SPM merged with the sclera, whereas the septa appeared to end at the lamina cribrosa. However, in fetuses without lamina cribrosa, the septa extend into the nerve fiber layer of the retina. The SPM and septa showed strong elastin immunoreactivity, in contrast to the absence of reactivity in the sheaths of the oculomotor nerve. Each S100 protein-positive Schwann sheath of the oculomotor nerve was surrounded by collagenous endoneurium. Glial fibrillary acidic protein-positive astrocytes showed a linear arrangement along the septa. This article is protected by copyright. All rights reserved.

Glaucoma and biomechanics

PURPOSE OF REVIEW

Biomechanics is an important aspect of the complex family of diseases known as the glaucomas. Here, we review recent studies of biomechanics in glaucoma.

RECENT FINDINGS

Several tissues have direct and/or indirect biomechanical roles in various forms of glaucoma, including the trabecular meshwork, cornea, peripapillary sclera, optic nerve head/sheath, and iris. Multiple mechanosensory mechanisms and signaling pathways continue to be identified in both the trabecular meshwork and optic nerve head. Further, the recent literature describes a variety of approaches for investigating the role of tissue biomechanics as a risk factor for glaucoma, including pathological stiffening of the trabecular meshwork, peripapillary scleral structural changes, and remodeling of the optic nerve head. Finally, there have been advances in incorporating biomechanical information in glaucoma prognoses, including corneal biomechanical parameters and iridial mechanical properties in angle-closure glaucoma.

SUMMARY

Biomechanics remains an active aspect of glaucoma research, with activity in both basic science and clinical translation. However, the role of biomechanics in glaucoma remains incompletely understood. Therefore, further studies are indicated to identify novel therapeutic approaches that leverage biomechanics. Importantly, clinical translation of appropriate assays of tissue biomechanical properties in glaucoma is also needed.

Trabecular Meshwork Motion Profile from Pulsatile Pressure Transients: A New Platform to Simulate Transitory Responses in Humans and Nonhuman Primates

Trabecular meshwork (TM) motion abnormality is the leading cause of glaucoma. With technique limitations, how TM moves is still an enigma. This study describes a new laboratory platform to investigate TM motion responses to ocular transients in ex vivo eyes. The anterior segments of human cadaver and primate eyes were mounted in a perfusion system fitting. Perfusion needles were placed to establish mean baseline pressure. A perfusion pump was connected to the posterior chamber and generated an immediate transient pressure elevation. A phase-sensitive optical coherent tomography system imaged and quantified the TM motion. The peak-to-peak TM displacements (ppTMD) were determined, a tissue relaxation curve derived, and a time constant obtained. This study showed that the ppTMD increased with a rise in the pulse amplitude. The ppTMD was highest for the lowest mean pressure of 16 mmHg and decreased with mean pressure increase. The pulse frequency did not significantly change ppTMD. With a fixed pulse amplitude, an increase in mean pressure significantly reduced the time constant of recoil from maximum distension. Our research platform permitted quantitation of TM motion responses to designed pulse transients. Our findings may improve the interpretation of new TM motion measurements in clinic, aiding in understanding mechanisms and management.