Review of ex-vivo characterisation of corneal biomechanics

Pilot study on the dynamic interactions between cardiac activity and corneal biomechanics during eye movements

Background and objective

The study examines the relationship between ocular rotations and cardiovascular functions through detailed biomechanical analysis. The study documents specific patterns of ocular movements and their synchronization with cardiovascular activity, highlighting significant correlations. These findings provide a basis for understanding the opto-biomechanical interplay between ocular and cardiovascular dynamics.

Methods

Authors employed a custom-designed prototype, integrating a camera and numerical pulse oximeter, to analyze the right eyeballs of participants. The corneal surface reflections were recorded, along with concurrent blood pulsation (BP) signal acquisition. Numerical analysis helped determine the reflection positions and orientations to study eye movements and corneal deformations. Time and frequency domain analyses, including coherence functions, were utilized.

Results

Significant correlations were found between variations in corneal curvature, selected fixational eye movements (FEM) parameters, and Pulsatile ocular blood flow (POBF), revealing synchronized expansions of the corneal surfaces with cardiovascular activity. Furthermore, FEM displayed spectral correlations with BP, indicating an interrelation between ocular rotations and cardiovascular functions.

Conclusion

These findings reveal the complex interactions between the cornea and Pulsatile Ocular Blood Flow (POBF), as well as between Fixational Eye Movements (FEM) and POBF. While the influence of POBF on both corneal dynamics and FEM is clear, further research is necessary to directly link corneal dynamics and FEM. These insights hold potential for non-invasive diagnostic applications and provide a deeper understanding of ocular biomechanics.

Effects of Genipin Crosslinking of Porcine Perilimbal Sclera on Mechanical Properties and Intraocular Pressure

The mechanical properties of sclera play an important role in ocular functions, protection, and disease. Modulating the sclera's properties by exogenous crosslinking offers a way to expand the tissue's range of properties for study of the possible influences on the eye's behavior and diseases such as glaucoma and myopia. The focus of this work was to evaluate the effects of genipin crosslinking targeting the porcine perilimbal sclera (PLS) since the stiffness of this tissue was previously found in a number of studies to influence the eye's intraocular pressure (IOP). The work includes experiments on tensile test specimens and whole globes. The specimen tests showed decreased strain-rate dependence and increased relaxation stress due to the cross-linker. Whole globe perfusion experiments demonstrated that eyes treated with genipin in the perilimbal region had increased IOPs compared to the control globes. Migration of the cross-linker from the target tissue to other tissues is a confounding factor in whole globe, biomechanical measurements, with crosslinking. A novel quantitative genipin assay of the trabecular meshwork (TM) was developed to exclude globes where the TM was inadvertently crosslinked. The perfusion study, therefore, suggests that elevated stiffness of the PLS can significantly increase IOP apart from effects of the TM in the porcine eye. These results demonstrate the importance of PLS biomechanics in aqueous outflow regulation and support additional investigations into the distal outflow pathways as a key source of outflow resistance.

A discrete-to-continuum model for the human cornea with application to keratoconus

We introduce a discrete mathematical model for the mechanical behaviour of a planar slice of human corneal tissue, in equilibrium under the action of physiological intraocular pressure (IOP). The model considers a regular (two-dimensional) network of structural elements mimicking a discrete number of parallel collagen lamellae connected by proteoglycan-based chemical bonds (crosslinks). Since the thickness of each collagen lamella is small compared to the overall corneal thickness, we upscale the discrete force balance into a continuum system of partial differential equations and deduce the corresponding macroscopic stress tensor and strain energy function for the micro-structured corneal tissue. We demonstrate that, for physiological values of the IOP, the predictions of the discrete model converge to those of the continuum model. We use the continuum model to simulate the progression of the degenerative disease known as keratoconus, characterized by a localized bulging of the corneal shell. We assign a spatial distribution of damage (i.e. reduction of the stiffness) to the mechanical properties of the structural elements and predict the resulting macroscopic shape of the cornea, showing that a large reduction in the element stiffness results in substantial corneal thinning and a significant increase in the curvature of both the anterior and posterior surfaces.

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.

New forays into measurement of ocular biomechanics

PURPOSE OF REVIEW

The field of corneal biomechanics has rapidly progressed in recent years, reflecting technological advances and an increased understanding of the clinical significance of measuring these properties. This review will evaluate in-vivo biomechanical properties obtained by current technologies and compare them regarding their relevance to established biomechanical properties obtained by gold-standard ex-vivo techniques normally conducted on elastic materials.

RECENT FINDINGS

Several new technologies have appeared in recent years, including vibrational optical coherence tomography (VOCT) and the corneal indentation device (CID). These techniques provide promising new opportunities for minimally invasive and accurate measurements of corneal viscoelastic properties.

SUMMARY

Alterations in corneal biomechanics are known to occur in several corneal degenerative diseases and after refractive surgical procedures. The measurement of corneal biomechanical properties has the capability to diagnose early disease and monitor corneal disease progression. Several new technologies have emerged in recent years, allowing for more accurate and less invasive measurements of corneal biomechanical properties, most notably the elastic modulus.

Accuracy of an Air-Puff Dynamic Tonometry Biomarker to Discriminate the Corneal Biomechanical Response in Patients With Keratoconus

PURPOSE

The aim of this study was to assess accuracy of the mean corneal stiffness ( kc , N/m) parameter to discriminate between patients with keratoconus and age-matched healthy subjects.

METHODS

Dynamic Scheimpflug imaging tonometry was performed with Corvis ST (Oculus Optikgeräte GmbH, Germany) in patients with keratoconus (n = 24; study group) and age-matched healthy subjects (n = 32; control). An image processing algorithm was developed to analyze the video sequence of the Corvis ST air-puff event and to determine the geometric and temporal parameters that correlated with the corneal tissue biomechanical properties. A modified 3-element viscoelastic model was used to derive the kc parameter, which represented the corneal tissue resistance to deformation under load. Receiver operating characteristic curves were used to assess the overall diagnostic performance for determining the area under the curve, sensitivity, and specificity of the kc in assessing the corneal tissue deformation to the Corvis ST air-puff event in keratoconus and control eyes. The Corvis Biomechanical Index ( CBI ) was analyzed for external validation.

RESULTS

The kc parameter was significantly different between keratoconus and controls ( P < 0.001), ranging from 24.9 ±3.0 to 34.2 ±3.5 N/m, respectively. It was highly correlated with CBI (r = -0.69; P < 0.001); however, the kc parameter had greater specificity (94%) than CBI (75%), whereas the 2 biomarkers had similar area under the curve (0.98 vs. 0.94) and sensitivity (96% vs. 92%) in predicting the occurrence of keratoconus.

CONCLUSIONS

The kc parameter extracted by video processing analysis of dynamic Scheimpflug tonometry data was highly accurate in discriminating patients with clinically manifest keratoconus compared with controls.

Torsional wave elastography to assess the mechanical properties of the cornea

Corneal mechanical changes are believed to occur before any visible structural alterations observed during routine clinical evaluation. This study proposed developing an elastography technique based on torsional waves (TWE) adapted to the specificities of the cornea. By measuring the displacements in the propagation plane perpendicular to the axis of the emitter, the effect of guided waves in plate-like media was proven negligible. Ex vivo experiments were carried out on porcine corneal samples considering a group of control and one group of alkali burn treatment ([Formula: see text]OH) that modified the mechanical properties. Phase speed was recovered as a function of intraocular pressure (IOP), and a Kelvin-Voigt rheological model was fitted to the dispersion curves to estimate viscoelastic parameters. A comparison with uniaxial tensile testing with thin-walled assumptions was also performed. Both shear elasticity and viscosity correlated positively with IOP, being the elasticity lower and the viscosity higher for the treated group. The viscoelastic parameters ranged from 21.33 to 63.17 kPa, and from 2.82 to 5.30 Pa s, for shear elasticity and viscosity, respectively. As far as the authors know, no other investigations have studied this mechanical plane under low strain ratios, typical of dynamic elastography in corneal tissue. TWE reflected mechanical properties changes after treatment, showing a high potential for clinical diagnosis due to its rapid performance time and paving the way for future in vivo studies.

A Review on Damage and Rupture Modelling for Soft Tissues

Computational modelling of damage and rupture of non-connective and connective soft tissues due to pathological and supra-physiological mechanisms is vital in the fundamental understanding of failures. Recent advancements in soft tissue damage models play an essential role in developing artificial tissues, medical devices/implants, and surgical intervention practices. The current article reviews the recently developed damage models and rupture models that considered the microstructure of the tissues. Earlier review works presented damage and rupture separately, wherein this work reviews both damage and rupture in soft tissues. Wherein the present article provides a detailed review of various models on the damage evolution and tear in soft tissues focusing on key conceptual ideas, advantages, limitations, and challenges. Some key challenges of damage and rupture models are outlined in the article, which helps extend the present damage and rupture models to various soft tissues.