Investigation of crystalline lens overshooting: ex vivo experiment and optomechanical simulation results.

Introduction: Crystalline lens overshooting refers to a situation in which the lens momentarily shifts too much from its typical location immediately after stopping the rotational movement of the eye globe. This movement can be observed using an optical technique called Purkinje imaging. Methods: In this work, an experimental setup was designed to reproduce this effect ex vivo using a fresh porcine eye. The sample was rotated 90° around its centroid using a high-velocity rotation stage, and the Purkinje image sequences were recorded, allowing us to quantify the overshooting effect. The numerical part of the study consisted of developing a computational model of the eye, based on the finite element method, that allowed us to understand the biomechanical behavior of the different tissues in this dynamic scenario. A 2D fluid-structure interaction model of the porcine eye globe, considering both the solid parts and humors, was created to reproduce the experimental outcomes. Results: Outputs of the simulation were analyzed using an optical simulation software package to assess whether the mechanical model behaves optically like the real ex vivo eye. The simulation predicted the experimental results by carefully adjusting the mechanical properties of the zonular fibers and the damping factor. Conclusion: This study effectively demonstrates the importance of characterizing the dynamic mechanical properties of the eye tissues to properly comprehend and predict the overshooting effect.

Complexity of crystalline lens wobbling investigated by means of combined mechanical and optical simulations.

Crystalline lens wobbling is a phenomenon when the lens oscillates briefly from its normal position immediately after stopping the rotational movement of the eye globe. It can be observed by means of Purkinje imaging. The aim of this research is to present the data and computation workflow that involve both biomechanical and optical simulations that can mimic this effect, aimed to better understanding of lens wobbling. The methodology described in the study allows to visualize both the dynamic changes of the lens conformation within the eye and its optical effect in terms of Purkinje performance.

Novel Method of Measuring Corneal Viscoelasticity Using the Corvis ST Tonometer.

BACKGROUND: The process of rapid propagation of the corneal deformation in air puff tonometer depends not only on intraocular pressure, but also on the biomechanical properties of the cornea and anterior eye. One of the biomechanical properties of the cornea is viscoelasticity, which is the most visible in its high-speed deformations. It seems reasonable to link the corneal viscoelasticity parameter to two moments of the highest speed of corneal deformations, when the cornea buckles. The aim of this work is to present a method of determining the time and place of occurrence of corneal buckling, examine spatial and temporal dependencies between two corneal applanations and bucklings in the Corvis ST tonometer, and correlate these dependencies with corneal viscoelastic properties. METHODS: Images of the horizontal cross section of the Corvis ST deformed cornea from the air puff tonometer Corvis ST were used. 14 volunteers participated in the study, each of them had one eye measured eight times. Mutual changes in the profile slopes of the deformed corneas were numerically determined. They describe pure corneal deformation, eliminating the influence of rotation, and displacement of the entire eyeball. For each point in the central area of the corneal profile, the maximum velocities of mutual slope changes accompanying the applanations were estimated. The times of their occurrence were adopted as buckling times. RESULTS: The propagation of buckling along the corneal profile is presented, as well as the repeatability and mutual correlations between the buckling parameters and intraocular pressure. Based on the relationship between them, a new parameter describing corneal hysteresis: Corvis Viscoelasticity (CVE) is introduced. It is characterized by high repeatability: ICC = 0.82 (0.69-0.93 CI) and low and insignificant correlation with intraocular pressure: r = 0.25 (p-value = 0.38). CONCLUSION: The results show for the first time how to measure the corneal buckling and viscoelastic effects with Corvis ST. CVE is a new proposed biomechanical parameter related to the viscoelastic properties of the cornea, which has high repeatability for the examined subject. The distribution of its values is planned to be tested on different groups of patients in order to investigate its clinical applicability.

Non-contact tonometry using Corvis ST: analysis of corneal vibrations and their relation with intraocular pressure.

The aim of this study was to determine characteristic frequencies of corneal vibrations occurring during air-puff intraocular pressure (IOP) measurement using the Corvis ST tonometer. Relations of frequency of the corneal vibrations with IOP were examined. Two selected vibration frequencies-frequency with maximum amplitude, and mass center of the frequency distribution area, for which the amplitude was higher than 50% (CM50)-present significant correlations with non-corrected IOP and biomechanical corrected IOP (bIOP). The highest correlation was found between the mean values of CM50 and bIOP (r=0.91). Based on the results obtained, it can be stated that the vibration frequencies of corneal peaks are closely related to the measured non-corrected and biomechanical corrected IOPs.

Eye retraction and rotation during Corvis ST 'air puff' intraocular pressure measurement and its quantitative analysis.

PURPOSE: The aim of this study was to analyse the indentation and deformation of the corneal surface, as well as eye retraction, which occur during air puff intraocular pressure (IOP) measurement. METHODS: A group of 10 subjects was examined using a non-contact Corvis ST tonometer, which records image sequences of corneas deformed by an air puff. Obtained images were processed numerically in order to extract information about corneal deformation, indentation and eyeball retraction. RESULTS: The time dependency of the apex deformation/eye retraction ratio and the curve of dependency between apex indentation and eye retraction take characteristic shapes for individual subjects. It was noticed that the eye globes tend to rotate towards the nose in response to the air blast during measurement. This means that the eye globe not only displaces but also rotates during retraction. Some new parameters describing the shape of this curve are introduced. Our data show that intraocular pressure and amplitude of corneal indentation are inversely related (r8  = -0.83, P = 0.0029), but the correlation between intraocular pressure and amplitude of eye retraction is low and not significant (r8  = -0.24, P = 0.51). CONCLUSIONS: The curves describing corneal behaviour during air puff tonometry were determined and show that the eye globe rotates towards the nose during measurement. In addition, eye retraction amplitudes may be related to elastic or viscoelastic properties of deeper structures in the eye or behind the eye and this should be further investigated. Many of the proposed new parameters present comparable or even higher repeatability than the standard parameters provided by the Corvis ST.

Numerical analysis of corneal curvature dynamics based on Corvis tonometer images.

The paper presents numerical analysis of corneal curvature distribution, based on Corvis ST images. It was shown that a new approach to analysis of corneal curvature from tonometer images enables a better description and understanding of processes during fast corneal deformation. Ten healthy volunteers participated in nine repeated measurements on one eye. 90 sequences of images were processed with software written in Matlab, with the use of the Image Processing Toolbox. Time-spatial distribution of the local curvature distribution of the corneal profile was obtained for each and every measurement. Some new curvature parameters were proposed and analyzed. A high repeatability for individual subjects was obtained for the proposed parameters. For four of these new parameters, the ICC coefficients were higher than 0.85. The ICC value for the calculated curvature of the cornea before deformation reaches 0.989. Such high repeatability of the proposed new parameters can be useful in examination and differentiation of corneas due to their geometrical and biomechanical properties.