Novel dynamic corneal response parameters in a practice use: a critical review

Background: Non-contact tonometers based on the method using air puff and Scheimpflug’s fast camera are one of the latest devices allowing the measurement of intraocular pressure and additional biomechanical parameters of the cornea. Biomechanical features significantly affect changes in intraocular pressure values, as well as their changes, may indicate the possibility of corneal ectasia. This work presents the latest and already known biomechanical parameters available in the new offered software. The authors focused on their practical application and the diagnostic credibility indicated in the literature. Discussion: An overview of available literature indicates the importance of new dynamic corneal parameters. The latest parameters developed on the basis of biomechanics analysis of corneal deformation process, available in non-contact tonometers using Scheimpflug’s fast camera, are used in the evaluation of laser refractive surgery procedures, e.g. LASIK procedure. In addition, the assessment of changes in biomechanically corrected intraocular pressure confirms its independence from changes in the corneal biomechanics which may allow an intraocular pressure real assessment. The newly developed Corvis Biomechanical Index combined with the corneal tomography and topography assessment is an important aid in the classification of patients with keratoconus. Conclusion: New parameters characterising corneal deformation, including Corvis Biomechanical Index and biomechanical compensated intraocular pressure, significantly extend the diagnostic capabilities of this device and may be helpful in assessing corneal diseases of the eye. Nevertheless, further research is needed to confirm their diagnostic pertinence

A new method for detecting the outer corneal contour in images from an ultra‑fast Scheimpflug camera

BACKGROUND: The Corvis® ST tonometer is an innovative device which, by combining a classic non-contact tonometer with an ultra-fast Scheimpflug camera, provides a number of parameters allowing for the assessment of corneal biomechanics. The acquired biomechanical parameters improve medical diagnosis of selected eye diseases. One of the key elements in biomechanical measurements is the correct corneal contour detection, which is the basis for further calculations. The presented study deals with the problem of outer corneal edge detection based on a series of images from the afore-mentioned device. Corneal contour detection is the first and extremely important stage in the acquisition and analysis of corneal dynamic parameters. RESULT: A total of 15,400 images from the Corvis® ST tonometer acquired from 110 patients undergoing routine ophthalmologic examinations were analysed. A method of outer corneal edge detection on the basis of a series of images from the Corvis® ST was proposed. The method was compared with known and commonly used edge detectors: Sobel, Roberts, and Canny operators, as well as others, known from the literature. The analysis was carried out in MATLAB® version 9.0.0.341360 (R2016a) with the Image Processing Toolbox (version 9.4) and the Neural Network Toolbox (version 9.0). The method presented in this paper provided the smallest values of the mean error (0.16%), stability (standard deviation 0.19%) and resistance to noise, characteristic for Corvis® ST tonometry tests, compared to the methods known from the literature. The errors were 5.78 ± 9.19%, 3.43 ± 6.21%, and 1.26 ± 3.11% for the Roberts, Sobel, and Canny methods, respectively. CONCLUSIONS: The proposed new method for detecting the outer corneal contour increases the accuracy of intraocular pressure measurements. It can be used to analyse dynamic parameters of the cornea

Quantitative assessment of the impact of blood pulsation on intraocular pressure measurement results in healthy subjects

Background. Blood pulsation affects the results obtained using various medical devices in many different ways. Method. The paper proves the effect of blood pulsation on intraocular pressure measurements. Six measurements for each of the 10 healthy subjects were performed in various phases of blood pulsation. A total of 8400 corneal deformation images were recorded. The results of intraocular pressure measurements were related to the results of heartbeat phases measured with a pulse oximeter placed on the index finger of the subject's left hand. Results. The correlation between the heartbeat phase measured with a pulse oximeter and intraocular pressure is 0.69±0.26 (p<0.05). The phase shift calculated for the maximum correlation is equal to 60±40° (p<0.05). When the moment of measuring intraocular pressure with an air-puff tonometer is not synchronized, the changes in IOP for the analysed group of subjects can vary in the range of ±2.31 mmHg (p<0.3). Conclusions. Blood pulsation has a statistically significant effect on the results of intraocular pressure measurement. For this reason, in modern ophthalmic devices, the measurement should be synchronized with the heartbeat phases

Dynamic OCT measurement of corneal deformation by an air puff in normal and cross-linked corneas

A new technique is presented for the non-invasive imaging of the dynamic response of the cornea to an air puff inducing a deformation. A spectral OCT instrument combined with an air tonometer in a non-collinear configuration was used to image the corneal deformation over full corneal cross-sections, as well as to obtain high speed measurements of the temporal evolution of the corneal apex. The entire deformation process can be dynamically visualized. A quantitative analysis allows direct extraction of several deformation parameters, such as amplitude, diameter and volume of the maximum deformation, as well as duration and speed of the increasing deformation period and the recovery period. The potential of the technique is demonstrated on porcine corneas in vitro under constant IOP for several conditions (untreated, after riboflavin instillation and under cross-linking with ultraviolet light), as well as on human corneas in vivo. The new technique has proved very sensitive to detect differences in the deformation parameters across conditions. We have confirmed non-invasively that Riboflavin and UV-cross-linking induce changes in the corneal biomechanical properties. Those differences appear to be the result of changes in constituent properties of the cornea, and not a consequence of changes in corneal thickness, geometry or IOP. These measurements are a first step for the estimation of the biomechanical properties of corneal tissue, at an individual level and in vivo, to improve diagnosis and prognosis of diseases and treatments involving changes in the biomechanical properties of the cornea

Automatic method of analysis and measurement of additional parameters of corneal deformation in the Corvis tonometer

Introduction: The method for measuring intraocular pressure using the Corvis tonometer provides a sequence of images of corneal deformation. Deformations of the cornea are recorded using the ultra-high-speed Scheimpflug camera. This paper presents a new and reproducible method of analysis of corneal deformation images that allows for automatic measurements of new features, namely new three parameters unavailable in the original software. Material and method: The images subjected to processing had a resolution of 200 × 576 × 140 pixels. They were acquired from the Corvis tonometer and simulation. In total 14000 2D images were analysed. The image analysis method proposed by the author automatically detects the edge of the cornea and sclera fragments. For this purpose, new methods of image analysis and processing proposed by the author as well as those well-known, such as Canny filter, binarization, median filtering etc., have been used. The presented algorithms were implemented in Matlab (version 7.11.0.584 - R2010b) with Image Processing toolbox (version 7.1 -R2010b) using both known algorithms for image analysis and processing and those proposed by the author. Results: Owing to the proposed algorithm it is possible to determine three parameters: (1) the degree of the corneal reaction relative to the static position; (2) the corneal length changes; (3) the ratio of amplitude changes to the corneal deformation length. The corneal reaction is smaller by about 30.40% compared to its static position. The change in the corneal length during deformation is very small, approximately 1% of its original length. Parameter (3) enables to determine the applanation points with a correlation of 92% compared to the conventional method for calculating corneal flattening areas. The proposed algorithm provides reproducible results fully automatically within a few seconds/per patient using Core i7 processor. Conclusions: Using the proposed algorithm, it is possible to measure new, additional parameters of corneal deformation, which are not available in the original software. The presented analysis method provides three new parameters of the corneal reaction. Detailed clinical studies based on this method will be presented in subsequent papers

Corneal biomechanical properties : Measurement, modification and simulation

Esta tesis aborda la medición de las propiedades biomecánicas de la córnea. Se desarrollaron técnicas para medir la rigidez de la córnea in vitro con el fin de estudiar el comportamiento de la córnea como una función de diferentes factores (tales como la hidratación, la geometría, la presión intraocular y la rigidez de la córnea). Los datos experimentales se utilizaron para construir modelos numéricos capaces de reproducir la respuesta biomecánica observada de la córnea. Se aplicaron modelos numéricos para recuperar los parámetros biomecánicos de mediciones de deformación in vivo y para estudiar el efecto de la implantación de segmentos de anillos intraestromales. En particular, se utilizaron el método de inflación en ojos enteros y botones córneales, la extensiometría bídimensional, un soplo de aire combinado con tomografía de coherencia óptica (OCT), microscopía de Brillouin y OCT-vibrografía para las mediciones experimentales. Para el análisis numérico, se construyeron modelos de elementos finitos para estudiar la inflación de ojos enteros y botones córneales, la respuesta de la córnea después de un soplo de aire, el comportamiento del ojo bajo vibración y los cambios refractivos después de la implantación de anillos intraestromales. This thesis addresses the measurement of the corneal biomechanical properties. Techniques were developed to measure the corneal stiffness in vitro in order to study the corneal behavior as a function of different factors (such as hydration, geometry, intraocular pressure, corneal stiffness). Experimental data were used to build numerical models, which were able to reproduce the observed biomechanical response of the cornea. Numerical models were applied to retrieve biomechanical parameters from in vivo deformation measurements and to study the outcome with implantation of intrastromal ring segments. In particular whole-eye / corneal inflation, 2D extensiometry, an air-puff technique combined with optical coherence tomography (OCT), Brillouin microscopy and OCT-vibrography were used for the experimental measurements. For the numerical analysis, finite element models were built for eye inflation, corneal response following an air-puff, ocular vibration behavior and refractive changes after ICRS implantation

In vivo biomechanical response of the human cornea to acoustic waves

The cornea is the optical window to the brain. Its optical and structural properties are responsible for optical transparency and vision. The shape, elasticity, rigidity, or stiffness are due to its biomechanical properties, whose stability results in ocular integrity and intraocular pressure dynamics. Here, we report in vivo observations of shape changes and biomechanical alterations in the human cornea induced by acoustic wave pressure within the frequency range of 50–350 Hz and the sound pressure level of 90 dB. The central corneal thickness (CCT) and eccentricity (e2) were measured using Scheimpflug imaging and biomechanical properties [corneal hysteresis (CH) and intraocular pressure (IOP)] were assessed with air-puff tonometry in six young, healthy volunteers. At the specific 150 Hz acoustic frequency, the variations in e2 and CCT were 0.058 and 7.33 µm, respectively. Biomechanical alterations were also observed in both the IOP (a decrease of 3.60 mmHg) and CH (an increase of 0.40 mmHg)

Noncontact elastic wave imaging optical coherence elastography for evaluating changes in corneal elasticity due to crosslinking

The mechanical properties of tissues can provide valuable information about tissue integrity and health and can assist in detecting and monitoring the progression of diseases such as keratoconus. Optical coherence elastography (OCE) is a rapidly emerging technique, which can assess localized mechanical contrast in tissues with micrometer spatial resolution. In this work we present a noncontact method of optical coherence elastography to evaluate the changes in the mechanical properties of the cornea after UV-induced collagen cross-linking. A focused air-pulse induced a low amplitude (μm scale) elastic wave, which then propagated radially and was imaged in three dimensions by a phase-stabilized swept source optical coherence tomography (PhSSSOCT) system. The elastic wave velocity was translated to Young’s modulus in agar phantoms of various concentrations. Additionally, the speed of the elastic wave significantly changed in porcine cornea before and after UV-induced corneal collagen cross-linking (CXL). Moreover, different layers of the cornea, such as the anterior stroma, posterior stroma, and inner region, could be discerned from the phase velocities of the elastic wave. Therefore, because of noncontact excitation and imaging, this method may be useful for in vivo detection of ocular diseases such as keratoconus and evaluation of therapeutic interventions such as CXL

In vivo Measurement of Corneal Stiffness and Intraocular Pressure to Enable Personalised Disease Management and Treatment

In ophthalmology, accurate measurement of intraocular pressure (IOP) and in vivo measurement of corneal material stiffness have been long-standing problems. Access to this information would transform the diagnosis and therapy of diseases and conditions such as glaucoma, refractive errors and keratoconus that are currently affecting over 50% of the world population. The aim of this study is to develop new methods for the accurate measurement of IOP and corneal material stiffness in vivo. To achieve this goal, a mathematical method was developed to analyse tomography data of keratoconic corneas, to estimate the area, height and location of the keratoconic cone. This information was utilised in the development of representative numerical models of the ocular globe. A large parametric study was then conducted, and with the aid of custom-built programming tools, high-performance computing and optimisation techniques, new methods were developed. These methods enabled the use of information obtained from a non-contact tonometry device to estimate biomechanically corrected IOP and corneal material stiffness. Methods developed in this study were validated on data collected from experimental tests as well as a large clinical database obtained from four continents. The results showed that the newly developed methods for measuring IOP are more accurate than those currently available in the market. IOP measurements were stable when compared in pre and post-surgical procedures such as refractive correction or corneal crosslinking. IOP values showed a weak/no correlation with geometrical or biomechanical parameters. Further methods for measuring corneal biomechanics in-vivo showed notable advancements compared to the existing method. Biomechanical values were weakly/not correlated with IOP and geometrical features while strongly correlated with age as an indication of changes in material stiffness. The experimental validation showed excellent agreement between the in-vivo measurements in comparison to ex-vivo findings. The outcome of this research will have an impact on the better diagnosis of glaucoma by eliminating misdiagnosis due to IOP measurement inaccuracies. Further, it enables personalised disease management and treatment through in-vivo measurement of corneal biomechanics that leads to optimisation of surgical procedures, most notably corneal crosslinking and refractive surgeries