Measuring Human Corneal Stromal Biomechanical Properties Using Tensile Testing Combined With Optical Coherence Tomography

Analysis of the sclera elasticity properties and comparison with corneal biomechanics properties in vivo and in vitro

PURPOSE

To explore scleral elastic modulus and its correlation with corneal biomechanics in vivo and in vitro, and to introduce a predictive model for non-destructive characterization of scleral and corneal elastic modulus.

METHODS

In vivo biomechanical parameters of 21 rabbit eyes were measured using the Corvis ST. Uniaxial tensile tests on corneal and scleral (anterior, equatorial, and posterior sections) strips provided low-strain and high-strain tangent elastic modulus (LSTM and HSTM). Pearson's and Spearman's correlation analyses were used to examine the relationship. A multivariate linear regression model was used to characterize the elastic modulus in vivo. Transmission electron microscopy was used to elucidate the arrangement of collagen fibers in the anterior, equatorial, and posterior sclera.

RESULTS

Twelve in vivo parameters significantly correlate with the corneal LSTM (P < 0.05), whereas no parameters are correlated with the corneal HSTM. SP-HC (Rho = 0.442) and HC Deflection Amp (HC DA, r = -0.605) correlated with anterior sclera LSTM. DA ratio 1 mm (r = 0.446) was correlated with the equatorial sclera LSTM. Integrated Radius (Rho = 0.483) and Radius (r = -0.473) were correlated with the equatorial sclera HSTM. The representative multivariate linear regression model indicated the follows: Corneal LSTM = 0.046 + 0.01 ∗ SP-A1 + 0.066 ∗ SSI. Anterior sclera LSTM = 0.294-0.120 ∗ HC DA. Equatorial sclera LSTM = 0.22 + 0.114 ∗ DA Ratio 1 mm - 0.049 ∗ HC DA.

CONCLUSIONS

Biomechanical parameters, such as SP-A1 and SSI, correlate with the low-strain elasticity of the cornea, which primarily reflects the properties of the corneal ground matrix. Corvis ST may not be able to detect biomechanics changes in the cornea collagen structure. The scleral elastic modulus prediction model offers new insights into the non-destructive characterization of scleral biomechanics.

Biomechanical properties of lenticules in the ReLEx® SMILE® minimally invasive surgery: Do the age and degree of myopia matter?

BACKGROUND

Myopia is currently the most common refractive disorder of the eye. The important role of the cornea's biomechanical parameters in relation to myopia has long been acknowledged, and ReLEx® SMILE minimally invasive refractive surgery offers new possibilities in the corneal biomechanical studies. The reported data on the role of the age and myopia degree in the corneal biomechanics are inconsistent.

METHODS

We have examined a considerable number (122 in total) of SMILE-derived lenticules from patients of different ages and with different severity of myopia, using atomic force microscopy, microtester and tensile testing.

FINDINGS

The mechanical properties of lenticules varied in a wide range and differed for the different techniques used. No differences between the Young's moduli of the anterior and posterior sides of lenticules were observed. The age-related stiffening of the cornea found in some studies was not registered in our study, with >150 samples of patients aged from 17 to 47 years. Significantly lower tensile moduli of lenticules from moderate and high myopic eyes were measured by tensile testing in high-strain conditions, while no correlation was found using other techniques. The structural atomic force microscopy imaging studies demonstrated a mostly chaotic 3D network of collagen fibrils, while the roughness was independent of the myopia severity.

INTERPRETATION

No fundamental effect of the age and myopia degree on the corneal mechanical properties and morphology has been shown. The inconsistency of findings reported in the literature are due to the high dispersion of experimental data and specifics of the measurement techniques.

Biomechanical Properties Measured With Dynamic Scheimpflug Analyzer in Myopic Maculopathy

PURPOSE

Corneal biomechanical properties are associated with axial elongation. We aimed to characterize corneal biomechanical properties in highly myopic eyes with myopic maculopathy (MM).

DESIGN

Retrospective cross-sectional study.

METHODS

We included patients examined between June 2022 and August 2023 who underwent corneal visualization Scheimpflug technology (Corvis ST) measurements. MM in highly myopic eyes (axial length >26.0 mm) was evaluated using META-PM (meta-analyses of pathologic myopia) study classification based on fundus photographs with subfoveal choroidal thickness measured via spectral domain optical coherence tomography. A linear mixed model was used to analyze the association of MM features with axial length (AL) and corneal biomechanical parameters, followed by model selection using the second-order-corrected Akaike information criterion.

RESULTS

We included 189 eyes from 109 participants. A significant correlation was observed between AL and biomechanical parameters that characterize maximal corneal deformation (maximal deflection amplitude and peak distance) and stiffness parameter (stress-strain index) (P < .05). Model selection revealed that both AL and maximal deflection amplitude were independently associated with MM with category ≥2 severe, as well as with subfoveal choroidal thickness in highly myopic eyes (AL >26.0 mm).

CONCLUSIONS

In highly myopic eyes, a greater maximal deflection amplitude was identified as a risk factor for MM. Corneal biomechanical properties may serve as biomarkers for predicting the development and progression of MM.

Corneal Stress Distribution Using the Procedure of Goldmann Applanation Tonometry, as Tested on a Human Cornea Model

OBJECTIVE

To assess the effect of corneal thinning and changes in intraocular pressure (IOP) on the distribution of corneal stress induced by Goldmann applanation tonometry (GAT).

METHODS

A 2D model of a human cornea was created using a computer-aided design and finite element analysis software, employing previously reported corneal biomechanical properties. The GAT procedure was simulated, and the magnitude and distribution of stress in the corneal stroma were obtained for several corneal thicknesses, stiffnesses, and IOP.

RESULTS

A significant increase in stress was found in the outer and inner layers of the central cornea and in the inner layers of the surrounding central region. The maximal stress value was observed in the central outer layers when the stiffness was doubled, as in our theoretical baseline cornea (125.16 kPa). Minimal stress was observed in the central inner layers for a central corneal thickness of 300 µm (28.17 kPa). The thickness and stiffness of the cornea significantly influenced the magnitude of the stress, whereas the stress distribution in the cornea did not show significant changes. The change in IOP did not induce significant changes in either stress magnitude or stress distribution.

CONCLUSIONS

The changes and distribution of corneal stress when a GAT procedure is performed support the idea that variations in corneal thickness and stiffness induce changes in corneal biomechanics that may be relevant for IOP readings. These findings are relevant for assessing IOP in corneas that have undergone surgical procedures or have diseases that alter their layers.

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.

Research progress on measurement methods and clinical applications of corneal elastic modulus

Various corneal diseases are strongly associated with corneal biomechanical characteristics, and early measurement of patients' corneal biomechanics can be utilized in their diagnosis and treatment. Measurement methods for corneal biomechanical characteristics are classified into ex vivo and in vivo. Some of these methods can directly measure certain corneal biomechanical parameters, while others require indirect calculation through alternative methods. However, due to diversities in measurement techniques and environmental conditions, significant differences may exist in the corneal mechanical properties measured by these two methods. Therefore, comprehensive research on current measurement methods and the exploration of novel measurement techniques may have great clinical significance. The corneal elastic modulus, a critical indicator in corneal biomechanics, reflects the cornea's ability to return to its initial shape after undergoing stress. This review aims to provide a comprehensive summary of the corneal elastic modulus, which is a critical biomechanical parameter, and discuss its direct, indirect, and potential measurement methods and clinical applications.

A sustainable approach to derive sheep corneal scaffolds from stored slaughterhouse waste

Aim: The escalating demand for corneal transplants significantly surpasses the available supply. To bridge this gap, we concentrated on ethical and sustainable corneal grafting sources. Our objective was to create viable corneal scaffolds from preserved slaughterhouse waste.Materials & methods: Corneas were extracted and decellularized from eyeballs that had been refrigerated for several days. These scaffolds underwent evaluation through DNA quantification, histological analysis, surface tension measurement, light propagation testing, and tensile strength assessment.Results: Both the native and acellular corneas (with ~90% DNA removed using a cost-effective and environmentally friendly surfactant) maintained essential optical and biomechanical properties for potential clinical use.Conclusion: Our method of repurposing slaughterhouse waste, stored at 4°C for several days, to develop corneal scaffolds offers a sustainable and economical alternative xenograft model.

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.

The alteration of the structure and macroscopic mechanical response of porcine patellar tendon by elastase digestion

Background: The treatment of patellar tendon injury has always been an unsolved problem, and mechanical characterization is very important for its repair and reconstruction. Elastin is a contributor to mechanics, but it is not clear how it affects the elasticity, viscoelastic properties, and structure of patellar tendon. Methods: The patellar tendons from six fresh adult experimental pigs were used in this study and they were made into 77 samples. The patellar tendon was specifically degraded by elastase, and the regional mechanical response and structural changes were investigated by: (1) Based on the previous study of elastase treatment conditions, the biochemical quantification of collagen, glycosaminoglycan and total protein was carried out; (2) The patellar tendon was divided into the proximal, central, and distal regions, and then the axial tensile test and stress relaxation test were performed before and after phosphate-buffered saline (PBS) or elastase treatment; (3) The dynamic constitutive model was established by the obtained mechanical data; (4) The structural relationship between elastin and collagen fibers was analyzed by two-photon microscopy and histology. Results: There was no statistical difference in mechanics between patellar tendon regions. Compared with those before elastase treatment, the low tensile modulus decreased by 75%-80%, the high tensile modulus decreased by 38%-47%, and the transition strain was prolonged after treatment. For viscoelastic behavior, the stress relaxation increased, the initial slope increased by 55%, the saturation slope increased by 44%, and the transition time increased by 25% after enzyme treatment. Elastin degradation made the collagen fibers of patellar tendon become disordered and looser, and the fiber wavelength increased significantly. Conclusion: The results of this study show that elastin plays an important role in the mechanical properties and fiber structure stability of patellar tendon, which supplements the structure-function relationship information of patellar tendon. The established constitutive model is of great significance to the prediction, repair and replacement of patellar tendon injury. In addition, human patellar tendon has a higher elastin content, so the results of this study can provide supporting information on the natural properties of tendon elastin degradation and guide the development of artificial patellar tendon biomaterials.

Patient-specific finite element analysis of human corneal lenticules: An experimental and numerical study

The number of elective refractive surgeries is constantly increasing due to the drastic increase in myopia prevalence. Since corneal biomechanics are critical to human vision, accurate modeling is essential to improve surgical planning and optimize the results of laser vision correction. In this study, we present a numerical model of the anterior cornea of young patients who are candidates for laser vision correction. Model parameters were determined from uniaxial tests performed on lenticules of patients undergoing refractive surgery by means of lenticule extraction, using patient-specific models of the lenticules. The models also took into account the known orientation of collagen fibers in the tissue, which have an isotropic distribution in the corneal plane, while they are aligned along the corneal curvature and have a low dispersion outside the corneal plane. The model was able to reproduce the experimental data well with only three parameters. These parameters, determined using a realistic fiber distribution, yielded lower values than those reported in the literature. Accurate characterization and modeling of the cornea of young patients is essential to study better refractive surgery for the population undergoing these treatments, to develop in silico models that take corneal biomechanics into account when planning refractive surgery, and to provide a basis for improving visual outcomes in the rapidly growing population undergoing these treatments.

Effect of corneal stiffness decrease on axial length elongation in myopia determined based on a mathematical estimation model

Purpose: To investigate the relationship between the corneal material stiffness parameter stress-strain index (SSI) and axial length (AL) elongation with varying severities of myopia, based on a mathematical estimation model. Methods: This single-center, cross-sectional study included data from healthy subjects and patients preparing for refractive surgery in the Qingdao Eye Hospital of Shandong First Medical University. Data were collected from July 2021 to April 2022. First, we performed and tested an estimated AL model ( A L M o r g a n ) based on the mathematical equation proposed by Morgan. Second, we proposed an axial increment model ( Δ A L ) corresponding to spherical equivalent error (SER) based on A L e m m e t r o p i a ( A L M o r g a n at SER = 0) and subject's real AL. Finally, we evaluated the variations of Δ A L with SSI changes based on the mathematical estimation model. Results: We found that AL was closely associated with A L M o r g a n (r = 0.91, t = 33.8, p < 0.001) with good consistency and SER was negatively associated with Δ A L (r = -0.89, t = -30.7, p < 0.001). The association of SSI with AL, A L e m m e t r o p i a , and Δ A L can be summarized using the following equations: A L = 27.7 - 2.04 × S S I , A L e m m e t r o p i a = 23.2 + 0.561 × S S I , and Δ A L = 4.52 - 2.6 × S S I . In adjusted models, SSI was negatively associated with AL (Model 1: β = -2.01, p < 0.001) and Δ A L (Model 3: β = -2.49, p < 0.001) but positively associated with A L e m m e t r o p i a (Model 2: β = 0.48, p < 0.05). In addition, SSI was negatively associated with Δ A L among subjects with AL ≥ 26 mm (β = -1.36, p = 0.02). Conclusion: AL increased with decreasing SSI in myopia.