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.

Ex vivo, in vivo and in silico studies of corneal biomechanics: a systematic review

Healthy cornea guarantees the refractive power of the eye and the protection of the inner components, but injury, trauma or pathology may impair the tissue shape and/or structural organization and therefore its material properties, compromising its functionality in the ocular visual process. It turns out that biomechanical research assumes an essential role in analysing the morphology and biomechanical response of the cornea, preventing pathology occurrence, and improving/optimising treatments. In this review, ex vivo, in vivo and in silico methods for the corneal mechanical characterization are reported. Experimental techniques are distinct in testing mode (e.g., tensile, inflation tests), samples' species (human or animal), shape and condition (e.g., healthy, treated), preservation methods, setup and test protocol (e.g., preconditioning, strain rate). The meaningful results reported in the pertinent literature are discussed, analysing differences, key features and weaknesses of the methodologies adopted. In addition, numerical techniques based on the finite element method are reported, incorporating the essential steps for the development of corneal models, such as geometry, material characterization and boundary conditions, and their application in the research field to extend the experimental results by including further relevant aspects and in the clinical field for diagnostic procedure, treatment and planning surgery. This review aims to analyse the state-of-art of the bioengineering techniques developed over the years to study the corneal biomechanics, highlighting their potentiality to improve diagnosis, treatment and healing process of the corneal tissue, and, at the same, pointing out the current limits in the experimental equipment and numerical tools that are not able to fully characterize in vivo corneal tissues non-invasively and discourage the use of finite element models in daily clinical practice for surgical planning.

How does atmospheric pressure cold helium plasma affect the biomechanical behaviour on alkali-lesioned corneas?

BACKGROUND

Melting corneal ulcers are a serious condition that affects a great number of animals and people around the world and it is characterised by a progressive weakening of the tissue leading to possible severe ophthalmic complications, such as visual impairment or blindness. This disease is routinely treated with medical therapy and keratoplasty, and recently also with alternative regenerative therapies, such as cross-linking, amniotic membrane transplant, and laser. Plasma medicine is another recent example of regenerative treatment that showed promising results in reducing the microbial load of corneal tissue together with maintaining its cellular vitality. Since the effect of helium plasma application on corneal mechanical viscoelasticity has not yet been investigated, the aim of this study is first to evaluate it on ex vivo porcine corneas for different exposition times and then to compare the results with previous data on cross-linking treatment.

RESULTS

94 ex vivo porcine corneas divided into 16 populations (healthy or injured, fresh or cultured and treated or not with plasma or cross-linking) were analysed. For each population, a biomechanical analysis was performed by uniaxial stress-relaxation tests, and a statistical analysis was carried out considering the characteristic mechanical parameters. In terms of equilibrium normalised stress, no statistically significant difference resulted when the healthy corneas were compared with lesioned plasma-treated ones, independently of treatment time, contrary to what was obtained about the cross-linking treated corneas which exhibited more intense relaxation phenomena.

CONCLUSIONS

In this study, the influence of the Helium plasma treatment was observed on the viscoelasticity of porcine corneas ex vivo, by restoring in lesioned tissue a degree of relaxation similar to the one of the native tissue, even after only 2 min of application. Therefore, the obtained results suggest that plasma treatment is a promising new regenerative ophthalmic therapy for melting corneal ulcers, laying the groundwork for further studies to correlate the mechanical findings with corneal histology and ultrastructural anatomy after plasma treatment.

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

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

A composite hydrogel membrane with shape and water retention for corneal tissue engineering

Tissue engineering (TE) cornea is one of the most potential alternatives to the shortage of corneal donors in cornea transplantation. Sodium alginate (SA) hydrogel is commonly used as scaffold in TE. Herein, we present an approach to construct a composite hydrogel, which with SA fiber skeleton structure for shape retention and gelatin surface modification for water retention. The light transmittance, water retention rate, and swelling rate of hydrogels were characterized, and the tensile mechanical properties were also investigated. Keratinocytes were treated with material extract liquor and the results showed that the gelatin modified SA hydrogel has good cytocompatibility. Furthermore, human corneal stromal fibroblasts (HCSFs) from the lenticules were implanted on the surface of gels, and the SA-gelatin hydrogel significantly improved the adhesion and spreading of HCSFs. Finally, we discussed the improvement and application prospect of the composite hydrogel as cornea equivalents.

Noncontact Acoustic Micro-Tapping Optical Coherence Elastography for Quantification of Corneal Anisotropic Elasticity: In Vivo Rabbit Study

Purpose

The purpose of this study was to demonstrate accurate measurement of corneal elastic moduli in vivo with noncontact and noninvasive optical coherence elastography.

Methods

Elastic properties (in-plane Young's modulus, E, and both in-plane, μ, and out-of-plane, G, shear moduli) of rabbit cornea were quantified in vivo using noncontact dynamic acoustic micro-tapping optical coherence elastography (AµT-OCE). The intraocular pressure (IOP)-dependence of measured mechanical properties was explored in extracted whole globes following in vivo measurement. A nearly incompressible transverse isotropic (NITI) model was used to reconstruct moduli from AµT-OCE data. Independently, cornea elastic moduli were also measured ex vivo with traditional, destructive mechanical tests (tensile extensometry and shear rheometry).

Results

Our study demonstrates strong anisotropy of corneal elasticity in rabbits. The in-plane Young's modulus, computed as E = 3μ, was in the range of 20 MPa to 44 MPa, whereas the out-of-plane shear modulus was in the range of 34 kPa to 261 kPa. Both pressure-dependent ex vivo OCE and destructive mechanical tests performed on the same samples within an hour of euthanasia strongly support the results of AµT-OCE measurements.

Conclusions

Noncontact AµT-OCE can noninvasively quantify cornea anisotropic elastic properties in vivo.

Translational Relevance

As optical coherence tomography (OCT) is broadly accepted in ophthalmology, these results suggest the potential for rapid translation of AµT-OCE into clinical practice. In addition, AµT-OCE can likely improve diagnostic criteria of ectatic corneal diseases, leading to early diagnosis, reduced complications, customized surgical treatment, and personalized biomechanical models of the eye.

Acoustic Micro-Tapping Optical Coherence Elastography to Quantify Corneal Collagen Cross-Linking: An Ex Vivo Human Study

Purpose

To evaluate changes in the anisotropic elastic properties of ex vivo human cornea treated with ultraviolet cross-linking (CXL) using noncontact acoustic micro-tapping optical coherence elastography (AμT-OCE).

Design

Acoustic micro-tapping OCE was performed on normal and CXL human donor cornea in an ex vivo laboratory study.

Subjects

Normal human donor cornea (n = 22) divided into 4 subgroups. All samples were stored in optisol.

Methods

Elastic properties (in-plane Young's, E, and out-of-plane, G, shear modulus) of normal and ultraviolet CXL-treated human corneas were quantified using noncontact AμT-OCE. A nearly incompressible transverse isotropic model was used to reconstruct moduli from AμT-OCE data. Independently, cornea elastic moduli were also measured with destructive mechanical tests (tensile extensometry and shear rheometry).

Main Outcome Measures

Corneal elastic moduli (in-plane Young's modulus, E, in-plane, μ, and out-of-plane, G, shear moduli) can be evaluated in both normal and CXL treated tissues, as well as monitored during the CXL procedure using noncontact AμT-OCE.

Results

Cross-linking induced a significant increase in both in-plane and out-of-plane elastic moduli in human cornea. The statistical mean in the paired study (presurgery and postsurgery, n = 7) of the in-plane Young's modulus, E = 3 μ , increased from 19 MPa to 43 MPa, while the out-of-plane shear modulus, G, increased from 188 kPa to 673 kPa. Mechanical tests in a separate subgroup support CXL-induced cornea moduli changes and generally agree with noncontact AμT-OCE measurements.

Conclusions

The human cornea is a highly anisotropic material where in-plane mechanical properties are very different from those out-of-plane. Noncontact AμT-OCE can measure changes in the anisotropic elastic properties in human cornea as a result of ultraviolet CXL.

Orientation and depth dependent mechanical properties of the porcine cornea: Experiments and parameter identification

The porcine cornea is a standard animal model in ophthalmic research, making its biomechanical characterization and modeling important to develop novel treatments such as crosslinking and refractive surgeries. In this study, we present a numerical model of the porcine cornea based on experimental measurements that captures both the depth dependence and orientation dependence of the mechanical response. The mechanical parameters of the established anisotropic hyperelastic material models of Gasser, Holzapfel and Ogden (HGO) and Markert were determined using tensile tests. Corneas were cut with a femtosecond laser in the anterior (100 μm), central (350 μm), and posterior (600 μm) regions into nasal-temporal, superior-inferior, and diagonal strips of 150 μm thickness. These uniformly thick strips were tested at a low speed using a single-axis testing machine. The results showed that the corneal mechanical properties remained constant in the anterior half of the cornea regardless of orientation, but that the material softened in the posterior layer. These results are consistent with the circular orientation of collagen observed in porcine corneas using X-ray scattering. In addition, the parameters obtained for the HGO model were able to reproduce the published inflation tests, indicating that it is suitable for simulating the mechanical response of the entire cornea. Such a model constitutes the basis for in silico platforms to develop new ophthalmic treatments. In this way, researchers can match their experimental surrogate porcine model with a numerical counterpart and validate the prediction of their algorithms in a complete and accessible environment.

Promoting keratocyte stem like cell proliferation and differentiation by aligned polycaprolactone-silk fibroin fibers containing Aloe vera

There is a long history behind applying biological macromolecules like Aloe vera (AV) in regenerative medicine; endowed with anti-inflammatory and antimicrobial activities besides improving immune activity, AV has always been of particular interest to regenerate/reconstruct injuries and burns. In the present study, aligned electrospun polycaprolactone (PCL)-silk fibroin (SF) fibers containing different percentages of AV (0, 2.5, 5, and 7.5%wt) were fabricated for stromal regeneration. The results illustrated that a uniform bead-free structure was obtained, and the AV incorporation decreased the mean fiber diameter from 552 down to 182 nm and led to more alignment in the fibers. The Young's modulus raised from 4.96 to 5.26 MPa by higher amount of AV up to 5%wt. It is noteworthy that both the fiber alignment and AV affected the scaffolds' transparency and water uptake to increase. The human stromal keratocyte cells (hSKC)s culture revealed that the addition of AV and morphological properties of scaffolds encouraged cell adhesion and proliferation. The mRNA expression level for keratocan and ALDH3A1 and immunocytochemistry F-actin revealed the positive effect of AV on hSKCs differentiation. Our study indicated the promising potential of AV as a biological macromolecule for stromal tissue regeneration.

Eye orbit effects on eyeball resonant frequencies and acoustic tonometer measurements

The eye orbit has mechanical and acoustic characteristics that determine resonant frequencies and amplify acoustic signals in certain frequency ranges. These characteristics also interfere with the acoustic amplitudes and frequencies of eyeball when measured with an acoustic tonometer. A model in which a porcine eyeball was embedded in ultrasonic conductive gel in the orbit of a model skull was used to simulate an in vivo environment, and the acoustic responses of eyeballs were detected. The triggering source was a low-power acoustic speaker contacting the occipital bone, and the detector was a high-resolution microphone with a dish detecting the acoustic signals without contacting the cornea. Dozens of ex vivo porcine eyeballs were tested at various intraocular pressure levels to detect their resonant frequencies and acoustic amplitudes in their power spectra. We confirmed that the eyeballs' resonant frequencies were proportional to intraocular pressure, but interference from orbit effects decreased the amplitudes in these resonant frequency ranges. However, we observed that the frequency amplitudes of eyeballs were correlated with intraocular pressure in other frequency ranges. We investigated eye orbit effects and demonstrated how they interfere with the eyeball's resonant frequencies and frequency amplitudes. These results are useful for developing advanced acoustic tonometer.

Mechanical behaviour of healthy versus alkali-lesioned corneas by a porcine organ culture model

BACKGROUND

Cornea is a composite tissue exhibiting nonlinear and time-dependent mechanical properties. Corneal ulcers are one of the main pathologies that affect this tissue, disrupting its structural integrity and leading to impaired functions. In this study, uniaxial tensile and stress-relaxation tests are developed to evaluate stress-strain and time-dependent mechanical behaviour of porcine corneas.

RESULTS

The samples are split in two groups: some corneas are analysed in an unaltered state (healthy samples), while others are injured with alkaline solution to create an experimental ulcer (lesioned samples). Furthermore, within each group, corneas are examined in two conditions: few hours after the enucleation (fresh samples) or after 7 days in a specific culture medium for the tissue (cultured samples). Finally, another condition is added: corneas from all the groups undergo or not a cross-linking treatment. In both stress-strain and stress-relaxation tests, a weakening of the tissue is observed due to the imposed conditions (lesion, culture and treatment), represented by a lower stiffness and increased stress-relaxation.

CONCLUSIONS

Alkali-induced corneal stromal melting determines changes in the mechanical response that can be related to a damage at microstructural level. The results of the present study represent the basis for the investigation of traditional and innovative corneal therapies.

Estimation of scleral mechanical properties from air-puff optical coherence tomography

We introduce a method to estimate the biomechanical properties of the porcine sclera in intact eye globes ex vivo, using optical coherence tomography that is coupled with an air-puff excitation source, and inverse optimization techniques based on finite element modeling. Air-puff induced tissue deformation was determined at seven different locations on the ocular globe, and the maximum apex deformation, the deformation velocity, and the arc-length during deformation were quantified. In the sclera, the experimental maximum deformation amplitude and the corresponding arc length were dependent on the location of air-puff excitation. The normalized temporal deformation profile of the sclera was distinct from that in the cornea, but similar in all tested scleral locations, suggesting that this profile is independent of variations in scleral thickness. Inverse optimization techniques showed that the estimated scleral elastic modulus ranged from 1.84 ± 0.30 MPa (equatorial inferior) to 6.04 ± 2.11 MPa (equatorial temporal). The use of scleral air-puff imaging holds promise for non-invasively investigating the structural changes in the sclera associated with myopia and glaucoma, and for monitoring potential modulation of scleral stiffness in disease or treatment.

Delineating Corneal Elastic Anisotropy in a Porcine Model Using Noncontact OCT Elastography and Ex Vivo Mechanical Tests

Purpose

To compare noncontact acoustic microtapping (AμT) OCT elastography (OCE) with destructive mechanical tests to confirm corneal elastic anisotropy.

Design

Ex vivo laboratory study with noncontact AμT-OCE followed by mechanical rheometry and extensometry.

Participants

Inflated cornea of whole-globe porcine eyes (n = 9).

Methods

A noncontact AμT transducer was used to launch propagating mechanical waves in the cornea that were imaged with phase-sensitive OCT at physiologically relevant controlled pressures. Reconstruction of both Young's modulus (E) and out-of-plane shear modulus (G) in the cornea from experimental data was performed using a nearly incompressible transversely isotropic (NITI) medium material model assuming spatial isotropy of corneal tensile properties. Corneal samples were excised and parallel plate rheometry was performed to measure shear modulus, G. Corneal samples were then subjected to strip extensometry to measure the Young's modulus, E.

Main Outcome Measures

Strong corneal anisotropy was confirmed with both AμT-OCE and mechanical tests, with the Young's (E) and shear (G) moduli differing by more than an order of magnitude. These results show that AμT-OCE can quantify both moduli simultaneously with a noncontact, noninvasive, clinically translatable technique.

Results

Mean of the OCE measured moduli were E = 12 ± 5 MPa and G = 31 ± 11 kPa at 5 mmHg and E = 20 ± 9 MPa and G = 61 ± 29 kPa at 20 mmHg. Tensile testing yielded a mean Young's modulus of 1 MPa - 20 MPa over a strain range of 1% to 7%. Shear storage and loss modulus (G'/G'') measured with rheometry was approximately 82/13 ± 12/4 kPa at 0.2 Hz and 133/29 ± 16/3 kPa at 16 Hz (0.1% strain).

Conclusions

The cornea is confirmed to be a strongly anisotropic elastic material that cannot be characterized with a single elastic modulus. The NITI model is the simplest one that accounts for the cornea's incompressibility and in-plane distribution of lamellae. AμT-OCE has been shown to be the only reported noncontact, noninvasive method to measure both elastic moduli. Submillimeter spatial resolution and near real-time operation can be achieved. Quantifying corneal elasticity in vivo will enable significant innovation in ophthalmology, helping to develop personalized biomechanical models of the eye that can predict response to ophthalmic interventions.

Goldmann Tonometry and Corneal Biomechanics

Glaucoma is the second cause of irreversible blindness in the world. Intraocular pressure (IOP) is a recognized major risk factor for the development and progression of glaucomatous damage. Goldmann applanation tonometry (GAT) is internationally accepted as the gold standard for the measurement of IOP. The purpose of this study was to search for correlations between Goldmann tonometry and corneal mechanical properties and thickness by means of in vitro tests. IOP was measured by the Goldmann applanation tonometer (GIOP), and by a pressure transducer inserted in the anterior chamber of the eye (TIOP), at increasing pressure levels by addition of saline solution in the anterior chamber of enucleated pig eyes (n = 49). Mechanical properties were also determined by inflation tests. The GAT underestimated the real measurements made by the pressure transducer, with most common differences in the range 15–28 mmHg. The difference between the two instruments, highlighted by the Bland–Altman test, was confirmed by ANOVA, normality tests, and Mann–Whitney’s tests, both on the data arranged for infusions and for the data organized by pressure ranges. Pearson correlation tests revealed a negative correlation between (TIOP-GIOP) and both corneal stiffness and corneal thickness. In conclusion, data obtained showed a discrepancy between GIOP and TIOP more evident for softer and thinner corneas, that is very important for glaucoma detection.

Experimental in-vitro investigation on Epi-Off-Crosslinking on porcine corneas

Aim

To evaluate quantitatively the effects of the Epi-Off-CXL irradiance dose on the stromal stiffening of pig corneas.

Setting

Laboratory of Biological structures (LaBS), Politecnico di Milano, Milano, Italy.

Methods

Inflation tests have been carried on 90 excised and de-epithelized pig corneas, monitoring the change of configuration of the corneal dome at specific pressures. Test have been carried out twice on each cornea, once before and once after Epi-Off-CXL performed at a constant irradiance of 9 mW/cm² and variable UV-A exposure times. Corneas were grouped according to the exposure time (2.5, 5, 10, 15 and 20 min), proportional to the irradiation dose (1.35, 2.7, 5.4, 8.1, and 10.8 J/cm²). A theoretical model based on linearized shell theory has been used to estimate the increment of the corneal stiffness.

Results

The linearized shell theory allowed to establish a quantitative relation between the increment of the stiffness parameters and the irradiation dose. Relative to the pre-treatment values, in all experiments the post-treatment corneal stiffness revealed a pronounced increase. In general, the stiffness gain increased with the exposure time. No significant differences in stiffening was observed between tests conducted at 2.5, 5, and 10 min exposure.

Conclusions

Qualitatively, the effectiveness of accelerated CXL treatments observed in pig corneas complies very well with in-vivo clinical results in humans, suggesting that experimental data in pigs can be very useful for the design of the procedure in humans. A larger irradiation dose provides a larger increment of the corneal stiffness. Due to the biological variability of the tissues, however, it is difficult to distinguish quantitatively the level of the reinforcement induced by accelerated protocols (low doses with < = 10 min exposure), less prone to induce damage in the corneal tissue. Therefore, the definition of personalized treatments must be related to the actual biomechanics of the cornea.

The effect of intraocular pressure elevation and related ocular biometry changes on corneal OCT speckle distribution in porcine eyes

The aim of this study was to evaluate the influence of increase in intraocular pressure (IOP) and cooccurring changes in ocular biometry parameters on the corneal optical coherence tomography (OCT) speckle distribution in ex-vivo experiments on porcine intact eyes. Twenty-three eyeballs were used in the inflation test where IOP in the anterior chamber was precisely set from 10 mmHg to 40 mmHg in steps of 5 mmHg and where eye biometry was utilized (IOL Master 700). To assess the influence of the duration of the experiment on the OCT speckle statistics, the second experiment was performed with 10 eyeballs at the constant IOP of 15 mmHg. Based on the OCT scans of central cornea (Copernicus REVO), spatial maps of the scale parameter (a) and the shape parameter (v) of the gamma distribution speckle model were estimated. The means of both parameters for each spatial map were computed within the 2 mm of the central stroma. Both distributional parameters statistically significantly varied with IOP and time (one way repeated measures ANOVA, all p-values < 0.001). The a parameter revealed a faster statistically significant increase in IOP up to 25 mmHg, regardless of time. Central corneal thickness (CCT), the anterior chamber depth, and the mean equivalent spherical power varied significantly with IOP, whereas CCT and axial length changed statistically significantly with time. Statistically significant correlation was found between CCT and the a parameter, after removing IOP as a confounding factor (r = -0.576, p < 0.001). The parameters of the gamma distribution can be used not only for identifying IOP induced changes in the optical scattering within the corneal stroma, but also in corneal geometry. The approach of corneal speckle analysis could be potentially utilized for an indirect and noninvasive assessment of some properties of corneal stroma.

Nearly-incompressible transverse isotropy (NITI) of cornea elasticity: model and experiments with acoustic micro-tapping OCE

The cornea provides the largest refractive power for the human visual system. Its stiffness, along with intraocular pressure (IOP), are linked to several pathologies, including keratoconus and glaucoma. Although mechanical tests can quantify corneal elasticity ex vivo, they cannot be used clinically. Dynamic optical coherence elastography (OCE), which launches and tracks shear waves to estimate stiffness, provides an attractive non-contact probe of corneal elasticity. To date, however, OCE studies report corneal moduli around tens of kPa, orders-of-magnitude less than those (few MPa) obtained by tensile/inflation testing. This large discrepancy impedes OCE's clinical adoption. Based on corneal microstructure, we introduce and fully characterize a nearly-incompressible transversely isotropic (NITI) model depicting corneal biomechanics. We show that the cornea must be described by at least two shear moduli, contrary to current single-modulus models, decoupling tensile and shear responses. We measure both as a function of IOP in ex vivo porcine cornea, obtaining values consistent with both tensile and shear tests. At pressures above 30 mmHg, the model begins to fail, consistent with non-linear changes in cornea at high IOP.

Fiber reinforced GelMA hydrogel to induce the regeneration of corneal stroma

Regeneration of corneal stroma has always been a challenge due to its sophisticated structure and keratocyte-fibroblast transformation. In this study, we fabricate grid poly (ε-caprolactone)-poly (ethylene glycol) microfibrous scaffold and infuse the scaffold with gelatin methacrylate (GelMA) hydrogel to obtain a 3 D fiber hydrogel construct; the fiber spacing is adjusted to fabricate optimal construct that simulates the stromal structure with properties most similar to the native cornea. The topological structure (3 D fiber hydrogel, 3 D GelMA hydrogel, and 2 D culture dish) and chemical factors (serum, ascorbic acid, insulin, and β-FGF) are examined to study their effects on the differentiation of limbal stromal stem cells to keratocytes or fibroblasts and the phenotype maintenance, in vitro and in vivo tissue regeneration. The results demonstrate that fiber hydrogel and serum-free media synergize to provide an optimal environment for the maintenance of keratocyte phenotype and the regeneration of damaged corneal stroma.

Regeneration of corneal stroma has been a challenge due to its sophisticated structure and the easy transformation of the keratocyte. Here, the authors use a hydrogel reinforced with orthogonally aligned fibres and serum free medium to maintain keratocyte phenotype for the in vivo stromal regeneration.

Cornea modelling

Background

Biomechanics introduces numerous technologies to support clinical practice in ophthalmology, with the goal of improving surgical outcomes and to develop new advanced technologies with minimum impact on clinical training. Unfortunately, a few misconceptions on the way that computational methods should be applied to living tissues contributes to a lack of confidence towards computer-based approaches.

Methods

Corneal biomechanics relies on sound theories of mechanics, including concepts of equilibrium, geometrical measurements, and complex material behaviors. The peculiarities of biological tissues require the consideration of multi-physics, typical of the eye environment, and to adopt customized geometrical models constructed on the basis of advanced optical imaging and in-vivo testing.

Results

Patient-specific models are able to predict the outcomes of refractive surgery and to exploit the results of in-vivo test to characterize the material properties of the corneal tissue.

Conclusions

Corneal biomechanics can become an important support to clinical practice, provided that methods are based on the actual multi-physics and use customized geometrical and mechanical models.

Influence of needling conditions on the corneal insertion force

Needle insertion plays an important part in the process of corneal graft surgery. In this paper, a three-dimensional symmetry model of the human cornea is constructed using the finite element method. Simplification of specific optic physiology is defined for the model: The cornea constrained by the sclera is presented as two layers consisting of epithelium and stroma. A failure criterion based on the distortion energy theory has been proposed to predict the insertion process of the needle. The simulation results show a good agreement with the experimental data reported in the literature. The influence of needling conditions (e.g. insertion velocity, rotation parameters and vibration parameters) on the insertion force are then discussed. In addition, a multi-objective optimization based on particle swarm optimization (PSO) is applied to reduce the insertion force. The numerical results provide guidelines for selecting the motion parameters of the needle and a potential basis for further developments in robot-assisted surgery.

Line-Field Optical Coherence Tomography as a tool for In vitro characterization of corneal biomechanics under physiological pressures

There has been a lot of interest in accurately characterising corneal biomechanical properties under intraocular pressure (IOP) to help better understand ocular pathologies that are associated with elevated IOP. This study investigates the novel use of Line-Field Optical Coherence Tomography (LF-OCT) as an elastographic tool for accurately measuring mechanical properties of porcine corneas based on volumetric deformation following varying IOPs. A custom-built LF-OCT was used to measure geometrical and corneal surface displacement changes in porcine corneas under a range of IOPs, from 0-60 mmHg. Corneal thickness, elastic properties and hysteresis were calculated as a function of pressure. In addition, the effects of hydration were explored. We found that the elastic modulus increased in a linear fashion with IOP. Corneal thickness was found to reduce with IOP, decreasing 14% from 0 to 60 mmHg. Prolonged hydration in phosphate buffered saline (PBS) was found to significantly increase the elastic modulus and corneal hysteresis. Our study demonstrates that LF-OCT can be used to accurately measure the elastic properties based on volumetric deformation following physiological pressures. Furthermore, we show that prolonged hydration in PBS has a significant effect on the measured corneal properties.

A 3D fluid-solid interaction model of the air puff test in the human cornea

We present a numerical model of a contactless test commonly used to assess the biomechanics of the human cornea. The test, consisting in a rapid air jet applied to the anterior surface of the cornea, is controversial. Although the numerous studies documented in the literature have not been able yet to clarify its relevance as a diagnostic tool, the test has the potential to be combined with inverse analysis procedures to characterize the parameters of numerical models of the cornea. With the final goal of employing the air puff test in advanced material identification algorithms, here we propose to model the cornea with standard finite elements and the fluids filling the anterior chamber of the eye with a meshfree discretization. The interaction between moving fluids and deforming cornea is accounted for by modifying the interface boundary conditions of both fluid and solid. The proposed model represents the first fully 3D example of an aqueous-cornea fluid-solid interaction analysis which uses a robust meshfree approach for the fluid. Although we restrict our scope to isotropic nonlinear materials, numerical results confirm the undeniable importance of including internal fluids in the simulation of the air puff test. Thus the proposed approach stands as a procedural paradigm for the identification of the mechanical parameters of the human cornea.

Assessment of the influence of viscoelasticity of cornea in animal ex vivo model using air-puff optical coherence tomography and corneal hysteresis

Application of the air-puff swept source optical coherence tomography (SS-OCT) instrument to determine the influence of viscoelasticity on the relation between overall the air-puff force and corneal apex displacement of porcine corneas ex vivo is demonstrated. Simultaneous recording of time-evolution of the tissue displacement and air pulse stimulus allows obtaining valuable information related in part to the mechanical properties of the cornea. A novel approach based on quantitative analysis of the corneal hysteresis of OCT data is presented. The corneal response to the air pulse is assessed for different well-controlled intraocular pressure (IOP) levels and for the progression of cross-linking-induced stiffness of the cornea. Micrometer resolution, fast acquisition and noncontact character of the air-puff SS-OCT measurements have potential to improve the in vivo assessment of mechanical properties of the human corneas.

STUDY OF THE TRANSVERSAL DEFORMATION OF CORNEAL STRIP UNDER UNIAXIAL LOADING

Uniaxial test is easy to access and to obtain accuracy data, but it is difficult to acquire two-dimensional deformation information. We investigated the relationship between the two strain components of corneal strip in uniaxial tests, which is the basis for determining of anisotropic strain energy function of cornea via uniaxial tests. Nine rabbits were taken. The left and right corneas were cut along superior-inferior (SI) and nasal-temporal (NT) direction, respectively. For each strip the uniaxial test was carried out, and the tensile displacements, strip images and loads were recorded. Then the stretching strain, the transversal strains and stress were obtained. Optimization based inverse analysis was utilized to find the best among six fitting models that characterizes the relationship between two strain components in uniaxial tests. All models fitted well the experimental data gathered for corneal strips ([Formula: see text]). According to the model selection index, the power model achieved the best performance index: 0.1268 for SI strips and 0.1063 for NT strips versus 0.151 (SI strips) and 0.107 (NT strips) found at most by other models. Thus, it is the most suitable one for describing the relationship between the two strain components of corneal strip during uniaxial stretching.

Natural Biomaterials for Corneal Tissue Engineering, Repair, and Regeneration

Corneal blindness is a major cause of vision loss, estimated to affect over 10 million people worldwide. Once impaired through clouding or shape change, the best treatment option for restoring vision is corneal transplantation using full or partial thickness cadaveric grafts. However, donor corneas are globally limited and face rejection and graft failure, similar to other transplanted organs. Thus, there is a need for viable alternatives to donor corneas in order to increase supply, reduce rejection, and to minimize variability in tissue quality. To address this, researchers have developed new materials and strategies to tissue engineer full or partial thickness cornea grafts in order to repair, regenerate, or replace the diseased cornea. This progress report first reviews the anatomy and physiology of the cornea to frame the biological requirements and discuss the injuries and diseases that necessitate the need fortransplantation, as well as the requirements for a suitable donor tissue alternative. This is followed by recent progress using naturally derived biomaterials including silk, collagen, amniotic membranes, and decellularized corneas. Finally, remaining challenges in the field as they relate to the biomaterials discussed are identified, and the future research directions that should result in further advances in restoring corneal vision are highlighted.

Perilimbal sclera mechanical properties: Impact on intraocular pressure in porcine eyes

There is extensive knowledge on the relationship of posterior scleral biomechanics and intraocular pressure (IOP) load on glaucomatous optic neuropathy; however, the role for biomechanical influence of the perilimbal scleral tissue on the aqueous humor drainage pathway, including the distal venous outflow system, and IOP regulation is not fully understood. The purpose of this work is to study the outflow characteristics of perfused porcine eyes relative to the biomechanical properties of the perilimbal sclera, the posterior sclera and the cornea. Enucleated porcine eyes from eleven different animals were perfused with surrogate aqueous at two fixed flow rates while monitoring their IOP. After perfusion, mechanical stress-strain and relaxation tests were conducted on specimens of perilimbal sclera, posterior sclera, and cornea from the same perfused eyes. Statistical analysis of the data demonstrated a strong correlation between increased tangent modulus of the perilimbal sclera tissues and increased perfusion IOP (R² = 0.74, p = 0.0006 at lower flow rate and R² = 0.71, p = 0.0011 at higher flow rate). In contrast, there were no significant correlations between IOP and the tangent modulus of the other tissues (Posterior sclera: R² = 0.17 at lower flow rate and R² = 0.30 at higher flow rate; cornea: R² = 0.02 at lower flow rate and R²<0.01 at higher flow rate) nor the viscoelastic properties of any tissue (R² ≤ 0.08 in all cases). Additionally, the correlation occurred for IOP and not net outflow facility (R² ≤ 0.12 in all cases). These results provide new evidence that IOP in perfused porcine eyes is strongly influenced by the tangent modulus, sometimes called the tissue stiffness, of the most anterior portion of the sclera, i.e. the limbus.

Corneal Biomechanical Properties after FS-LASIK with Residual Bed Thickness Less Than 50% of the Original Corneal Thickness

Background

The changes in corneal biomechanical properties after LASIK remain an unknown but important topic for surgical design and prognostic evaluation. This study aims to observe the postoperative corneal biomechanical properties one month after LASIK with amount of corneal cutting (ACC) greater than 50% of the central corneal thickness (CCT).

Methods

FS-LASIK was performed in 10 left rabbit eyes with ACC being 60% (L60) and 65% (L65) of the CCT, while the right eyes (R) were the control. After 4 weeks, rabbits were executed and corneal strip samples were prepared for uniaxial tensile tests.

Results

At the same strain, the stresses of L65 and L60 were larger than those of R. The elastic moduli of L60 and L65 were larger than those of R when the stress was 0.02 MPa, while they began to be less than those of R when stress exceeds the low-stress region. After 10 s relaxation, the stress of specimens L65, L60, and R increased in turn.

Conclusion

The elastic moduli of the cornea after FS-LASIK with ACC greater than 50% of the CCT do not become less under normal rabbit IOP. The limit stress grows with the rise of ACC when relaxation becomes stable.

Engineering of Corneal Tissue through an Aligned PVA/Collagen Composite Nanofibrous Electrospun Scaffold

Corneal diseases are the main reason of vision loss globally. Constructing a corneal equivalent which has a similar strength and transparency with the native cornea, seems to be a feasible way to solve the shortage of donated cornea. Electrospun collagen scaffolds are often fabricated and used as a tissue-engineered cornea, but the main drawback of poor mechanical properties make it unable to meet the requirement for surgery suture, which limits its clinical applications to a large extent. Aligned polyvinyl acetate (PVA)/collagen (PVA-COL) scaffolds were electrospun by mixing collagen and PVA to reinforce the mechanical strength of the collagen electrospun scaffold. Human keratocytes (HKs) and human corneal epithelial cells (HCECs) inoculated on aligned and random PVA-COL electrospun scaffolds adhered and proliferated well, and the aligned nanofibers induced orderly HK growth, indicating that the designed PVA-COL composite nanofibrous electrospun scaffold is suitable for application in tissue-engineered cornea.

The mechanics of corneal deformation and rupture for penetrating injury in the human eye

Penetrating eye injuries are surgical emergencies with guarded visual prognosis. The purpose of the current study was to determine the force required to rupture the cornea with a penetrating object, and to study how this force is affected by the object geometry. Thirty-six human cadaveric eyes from donors of various ages were characterized for diameter, axial length, and pre-test intraocular pressure. In order to investigate the effects of specimen storage time on the tissue response, half of the specimens were tested within two weeks of donor expiration, and half of the specimens were stored at -4°C for 12-18 months. Indenters of three different diameters (1.0, 1.5, and 2.0mm) were lowered into the apex of the cornea until rupture. Resistance to displacement (stiffness), displacement at failure, and the force at failure were determined. Multi-variable regression analysis was used to determine associations of the input variables (indenter size, test speed, and tissue postmortem time) on the mechanics of the tissue response. Twenty-nine of the 36 specimens failed at the indenter location in the cornea, four failed at the limbus, and three failed in the sclera near sites of muscle attachment. The average force at failure caused by the 1.0mm, 1.5mm, and 2.0mm indenters increased from 30.5±5.5N to 40.5±8.3N to 58.2±14.5N, respectively (p<0.002). The force at failure was associated with the donor age (p<0.001), and globe diameter (p<0.041), but was not associated with pre-test intraocular pressure, tissue postmortem time, axial length, or speed of the indenter. This study has quantified the force-displacement and failure response of a large series of human cadaveric eyes subjected to penetrating indentation loads on the cornea. The results provide useful data for characterizing the relationship between corneal rupture and the geometry of a penetrating object.

Assessment of Corneal Biomechanical Properties with Inflation Test Using Optical Coherence Tomography

Biomechanical properties are important for the cornea to maintain its normal shape and function. There is still a need to develop better methods for accurate measurement of corneal mechanical properties. In this study, we propose to introduce the optical coherence tomography (OCT) in inflation test for the imaging of corneal deformation in order to measure its biomechanical properties. Ten cornea-mimicking silicone phantoms with different stiffness and five fresh porcine corneas were tested using the proposed method. Intra-ocular pressure was changed from 10 to 90 mmHg using two different loading rates to observe the pressure-apex displacement relationship and calculate the apparent stiffness of the corneas. Stiffness of the corneal phantoms obtained by the inflation test ranged from 0.2 to 1 MPa, which was highly consistent with the results from the mechanical tensile test (y = 0.70x, p < 0.001). The porcine corneas showed highly viscoelastic behavior with obvious hysteresis in inflation. The apparent stiffness of the porcine corneas was 0.63 ± 0.07 and 1.05 ± 0.08 MPa with loading rates of 3.3 and 33 mmHg/min, respectively. Mapping of corneal surface displacement was also generated for both the phantom and porcine corneas. This study showed that it is feasible to incorporate the high resolution OCT imaging in inflation test to measure the biomechanical properties of the cornea.

UNDERSTANDING THE VISCOELASTIC PROPERTIES OF RABBIT CORNEA BASED ON STRESS RELAXATION TESTS AND CYCLIC UNIAXIAL TESTS

Background: Determining the viscoelastic properties of cornea is important in the fields of understanding of the tissue’s response to mechanical actions and the accurate numerical simulation of corneal biomechanical behavior under the effects of keratoconus and refractive surgery. To address this need, we present an approach to model the viscoelastic response of rabbit cornea from uniaxial test data. Methods: The corneal strip samples from six rabbits were obtained to perform cyclic uniaxial tension tests and stress relaxation tests. We investigated the suitability of six constitutive models, including empirical models and hyperelastic models, by a quasi-linear viscoelastic law. Applying non-linear optimization techniques, we found material parameters for each different strip sample. Results and conclusions: The model gave a better fit to loading data with [Formula: see text], and predicted the unloading data in the cyclic uniaxial tests with errors-of-fit ranging from 0.03 to 0.06. The results indicate that the best model is the power of the first invariant of strain with Prony form relaxation model, and that the method to identify the material parameters are valid for modeling the visoelastic response of cornea from uniaxial test data.

Aging and ocular tissue stiffness in glaucoma

Glaucoma is a progressive and chronic neurodegenerative disorder characterized by damage to the inner layers of the retina and deformation of the optic nerve head. The degeneration of retinal ganglion cells and their axons results in an irreversible loss of vision and is correlated with increasing age. Extracellular matrix changes related to natural aging generate a stiffer extracellular environment throughout the body. Altered age-associated ocular tissue stiffening plays a major role in a significant number of ophthalmic pathologies. In glaucoma, both the trabecular meshwork and the optic nerve head undergo extensive extracellular matrix remodeling, characterized by fibrotic changes associated with cellular and molecular events (including myofibroblast activation) that drive further tissue fibrosis and stiffening. Here, we review the literature concerning the role of age-related ocular stiffening in the trabecular meshwork, lamina cribrosa, sclera, cornea, retina, and Bruch membrane/choroid and discuss their potential role in glaucoma progression. Because both trabecular meshwork and lamina cribrosa cells are mechanosensitive, we then describe molecular mechanisms underlying tissue stiffening and cell mechanotransduction and how these cellular activities can drive further fibrotic changes within ocular tissues. An improved understanding of the interplay between age-related tissue stiffening and biological responses in the trabecular meshwork and optic nerve head could potentially lead to novel therapeutic strategies for glaucoma treatment.

Plastic modification of the cornea by pneumatic force corrects myopia: Pneumatic keratology

PurposeThe eyeball and, in particular, the cornea deform in vitro by the application of a distending force like other visco-elastic tissue. If the force is large enough, the cornea strains beyond the elastic range, and a permanent deformation occurs. Such permanent strain is referred to as 'plastic' strain. The phenomenon, however, has never been observed or produced on living tissue. This report seeks to demonstrate that the central radius of a patient's cornea can be altered in a controlled manner designed to correct refractive errors.MethodsTo plastically deform the living cornea, we applied a vacuum to the cornea of eight rabbits and five human eyes with a novel device. This device consists of a chamber of 11 mm in diameter. The chamber is radially divided into four interconnected sub-chambers.ResultsHere we show that a strain can be achieved in vivo with a force produced by the application of the specially designed chamber where air is evacuated. An anatomical modification of the cornea of humans and rabbits was achieved. The deformation of the cornea was plastic, and therefore permanent.ConclusionsThe method described here-Pneumatic Keratology- can be used to alter the cornea by non-invasive means. A vacuum chamber with radial openings alters the collagen fibers in the stroma and flattens the cornea. A flatter cornea corrects or reduces myopia.

Tissue-engineered cornea constructed with compressed collagen and laser-perforated electrospun mat

While Plastic Compressed (PC) collagen technique is often used to fabricate bioengineered constructs, PC collagen gels are too weak to be sutured or conveniently handled for clinical applications. To overcome this limitation, electrospun poly (lactic-co-glycolide) (PLGA) mats, which have excellent biocompatibility and mechanical properties, were combined with PC collagen to fabricate sandwich-like hybrid constructs. By laser-perforating holes with different sizes and spacings in the electrospun mats to regulate the mechanical properties and light transmittance of the hybrid constructs, we produced hybrid constructs with properties very suitable to apply in corneal tissue engineering. The maximum tensile stress of the optimal hybrid construct was 3.42 ± 0.22 MPa. The light transmittance of the hybrid construct after perforation was approximately 15-fold higher than before, and light transmittance increased gradually with increasing time. After immersing into PBS for 7 days, the transmittance of the optimal construct changed from 63 ± 2.17% to 72 ± 1.8% under 500 nm wavelength. The live/dead staining, cell proliferation assay and immunohistochemistry study of human corneal epithelial cells (HCECs) and human keratocytes (HKs) cultured on the optimal hybrid construct both demonstrated that the cells adhered, proliferated, and maintained their phenotype well on the material. In addition, after culturing for 2 weeks, the HCECs could form stratified layers. Thus, our designed construct is suitable for the construction of engineered corneal tissue.

Finite element analysis and experimental evaluation of penetrating injury through the cornea

Penetration injuries of the eye are among the most frequent causes of permanent visual impairment resulting from trauma. The purpose of this study was to determine the peak strain at which rupture occurs in the cornea due to a penetrating object. Probes of varying diameters (1.0, 1.5, and 2.0mm) were pressed into the apex of the cornea of 36 human cadaveric eye specimens until perforation or rupture of the specimen at the cornea, limbus, or sclera occurred. An axisymmetric finite element model of the human globe was created to replicate the experimental set-up. The models were used to map the force-displacement response of the experiments and quantitatively determine a peak strain at which the eye ruptures. For the experiments, the average force at failure increased from the smallest to largest probe (p<0.002). The average forces at failure are as follows: 30.5±5.5N (1.0mm probe); 40.5±8.3N (1.5mm probe); 58.2±14.5N (2.0mm probe). The force-displacement responses of the finite element models of all three probe sizes bounded and tracked the experimental data. In all cases, the peak strain at failure in the cornea was located on the posterior surface of the cornea, directly adjacent to the corneal apex. This strain was in the range of 29% to 33% for all models analyzed. In addition to determining an objective failure strain of corneal tissue, the model developed in this study can provide quantitative information for understanding the risk of penetrating eye injuries.

Material Properties from Air Puff Corneal Deformation by Numerical Simulations on Model Corneas

OBJECTIVE

To validate a new method for reconstructing corneal biomechanical properties from air puff corneal deformation images using hydrogel polymer model corneas and porcine corneas.

METHODS

Air puff deformation imaging was performed on model eyes with artificial corneas made out of three different hydrogel materials with three different thicknesses and on porcine eyes, at constant intraocular pressure of 15 mmHg. The cornea air puff deformation was modeled using finite elements, and hyperelastic material parameters were determined through inverse modeling, minimizing the difference between the simulated and the measured central deformation amplitude and central-peripheral deformation ratio parameters. Uniaxial tensile tests were performed on the model cornea materials as well as on corneal strips, and the results were compared to stress-strain simulations assuming the reconstructed material parameters.

RESULTS

The measured and simulated spatial and temporal profiles of the air puff deformation tests were in good agreement (< 7% average discrepancy). The simulated stress-strain curves of the studied hydrogel corneal materials fitted well the experimental stress-strain curves from uniaxial extensiometry, particularly in the 0-0.4 range. Equivalent Young´s moduli of the reconstructed material properties from air-puff were 0.31, 0.58 and 0.48 MPa for the three polymer materials respectively which differed < 1% from those obtained from extensiometry. The simulations of the same material but different thickness resulted in similar reconstructed material properties. The air-puff reconstructed average equivalent Young´s modulus of the porcine corneas was 1.3 MPa, within 18% of that obtained from extensiometry.

CONCLUSIONS

Air puff corneal deformation imaging with inverse finite element modeling can retrieve material properties of model hydrogel polymer corneas and real corneas, which are in good correspondence with those obtained from uniaxial extensiometry, suggesting that this is a promising technique to retrieve quantitative corneal biomechanical properties.

Interrelation of Hydration, Collagen Cross-Linking Treatment, and Biomechanical Properties of the Cornea

PURPOSE

The present study was designed to investigate the effects of hydration and collagen cross-linking treatment on biomechanical properties of the cornea.

METHODS

The original corneal collagen cross-linking protocol was used to induce cross-links in bovine corneas. The thickness of samples was used as a measure of their hydration and five different thickness groups (n = 5 each) were considered. The cross-linked corneal strips were allowed to hydrate/dehydrate until their thickness reached 500, 700, 900, 1100, and 1500 μm. The tensile behavior of specimens in each thickness group was characterized by conducting uniaxial tensile experiments. The experiments were done in mineral oil in order to keep the thickness of samples constant and minimize hydration changes.

RESULTS

It was observed that collagen cross-linking treatment significantly increased both the maximum tensile stress and the equilibrium (relaxed) stress of the bovine cornea (p < 0.01). Furthermore, with increasing the thickness (hydration) of the collagen cross-linked samples, their tensile stiffness significantly decreased (p < 0.01). An exponential relation and a logarithmic expression successfully represented experimentally measured stress-strain behavior and relaxation response of all groups (r(2 )> 0.99), respectively.

CONCLUSION

Hydration and collagen cross-linking treatment concomitantly affect biomechanical properties of the cornea. Therefore, an accurate estimate of stiffening effects of collagen cross-linking treatment option using uniaxial tensile experiments is only possible if the hydration of specimens is fully controlled.

Customized Finite Element Modelling of the Human Cornea

Aim

To construct patient-specific solid models of human cornea from ocular topographer data, to increase the accuracy of the biomechanical and optical estimate of the changes in refractive power and stress caused by photorefractive keratectomy (PRK).

Method

Corneal elevation maps of five human eyes were taken with a rotating Scheimpflug camera combined with a Placido disk before and after refractive surgery. Patient-specific solid models were created and discretized in finite elements to estimate the corneal strain and stress fields in preoperative and postoperative configurations and derive the refractive parameters of the cornea.

Results

Patient-specific geometrical models of the cornea allow for the creation of personalized refractive maps at different levels of IOP. Thinned postoperative corneas show a higher stress gradient across the thickness and higher sensitivity of all geometrical and refractive parameters to the fluctuation of the IOP.

Conclusion

Patient-specific numerical models of the cornea can provide accurate quantitative information on the refractive properties of the cornea under different levels of IOP and describe the change of the stress state of the cornea due to refractive surgery (PRK). Patient-specific models can be used as indicators of feasibility before performing the surgery.

Analysis of the viscoelastic properties of the human cornea using Scheimpflug imaging in inflation experiment of eye globes

PURPOSE

To demonstrate a Scheimpflug-based imaging procedure for investigating the depth- and time-dependent strain response of the human cornea to inflation testing of whole eye globes.

METHODS

Six specimens, three of which with intact corneal epithelium, were mounted in a customized apparatus within a humidity and temperature-monitored wet chamber. Each specimen was subjected to two mechanical tests in order to measure corneal strain resulting from application of cyclic (cyclic regimen) and constant (creep regimen) stress by changing the intra-ocular pressure (IOP) within physiological ranges (18-42 mmHg). Corneal shape changes were analyzed as a function of IOP and both corneal stress-strain curves and creep curves were generated.

RESULTS

The procedure was highly accurate and repeatable. Upon cyclic stress application, a biomechanical corneal elasticity gradient was found in the front-back direction. The average Young's modulus of the anterior cornea ranged between 2.28±0.87 MPa and 3.30±0.90 MPa in specimens with and without intact epithelium (P = 0.05) respectively. The Young's modulus of the posterior cornea was on average 0.21±0.09 MPa and 0.17±0.06 MPa (P>0.05) respectively. The time-dependent strain response of the cornea to creep testing was quantified by fitting data to a modified Zener model for extracting both the relaxation time and compliance function.

CONCLUSION

Cyclic and creep mechanical tests are valuable for investigating the strain response of the intact human cornea within physiological IOP ranges, providing meaningful results that can be translated to clinic. The presence of epithelium influences the results of anterior corneal shape changes when monitoring deformation via Scheimpflug imaging in inflation experiments of whole eye globes.

Hydration dependent viscoelastic tensile behavior of cornea

The cornea is a protective transparent connective tissue covering the front of the eye. The standard uniaxial tensile experiments are among the most popular techniques for investigating biomechanical properties of the cornea. This experimental method characterizes the stress-strain response of corneal strips immersed in a bathing solution. In the present study, the important roles of corneal hydration on tensile viscoelastic properties were investigated. The thickness was used as a surrogate for hydration and uniaxial tensile experiments were performed on bovine corneal samples with four different average thickness (hydration), i.e., 1100 μm (4.87 mg water/mg dry tissue), 900 μm (4.13 mg water/mg dry tissue), 700 μm (3.20 mg water/mg dry tissue), and 500 μm (1.95 mg water/mg dry tissue). The samples were immersed in mineral oil in order to prevent their swelling during the experiments. A quasilinear viscoelastic (QLV) model was used to analyze the experimental measurements and determine viscoelastic material constants. It was observed that both maximum and equilibrium (relaxed) stresses were exponentially increased with decreasing tissue thickness (hydration). Furthermore, the QLV model successfully captured the corneal viscoelastic response with an average R (2) value greater than 0.99. Additional experiments were conducted in OBSS in order to confirm that these significant changes in viscoelastic properties were because of corneal hydration and not the bathing solution. The findings of this study suggest that extra care must be taken in interpreting the results of earlier uniaxial tensile testings and their correspondence to the corneal biomechanical properties.

Swelling pressure and hydration behavior of porcine corneal stroma

PURPOSE

The aim of this study was to characterize swelling pressure-thickness, swelling pressure-hydration and hydration-thickness relations of porcine cornea.

METHODS

Mechanical compression tests and free swelling experiments were performed on porcine cornea. A rheometer (DHR-2, TA Instruments) with a thermally controlled fluid chamber filled with 0.9% NaCl solution was used to measure the equilibrium swelling pressure of (n = 17) corneal stromal specimens. The samples were compressed incrementally and their swelling pressure-thickness relations were obtained. In parallel to this investigation, a transient digital imaging microscope (H800-CL, American Scope Inc.), a USB autofocus camera (UM05, ViTiny), and a precision weighing scale (AGZN100, Torbal) were simultaneously used to measure the weight-thickness relation of (n = 8) corneal specimens. This experimental study gave the thickness-hydration relationship required for expressing swelling pressure measurements as a function of hydration.

RESULTS

At the in vivo 666 ± 68 µm central corneal thickness, an average swelling pressure of 52 ± 13 mmHg and hydration of 3.36 ± 0.25 mg H2O/mg dry tissue were found. The swelling pressure was reported as functions of both tissue thickness and hydration. The average fixed charge density of ρF/F ~ 42.8 mM and dry density of 1.47±0.15 g/cm3 were found. The thickness-hydration relationship was only linear when the tissue thickness was within the range of physiological thickness.

CONCLUSION

Overall, the physiological hydration and swelling pressure of the porcine cornea were within the same range of those reported previously for other mammalian corneas such as steers, rabbits and humans. Nevertheless, the thickness-hydration behavior of the porcine cornea was only similar to that of the human cornea.