The aim of this study was to develop a comprehensive biomechanical model to predict biomechanical properties of all ocular tissues and to compare the simulations with air-puff swept-source OCT data.
We developed a novel rheological model of the whole eye. The cornea and the crystalline lens were modelled as a combination of spring and dashpot(s) to describe their viscoelastic properties. In addition, a mass element was included to model lenticular wobbling after air pulse. Finally, the eye retraction (depending on the fatty and muscle tissues behind the eye globe) was modeled by a parallel combination of a spring and dashpot elements, and a mass component described the weight of the eye.
We measured deformation profiles of ocular components of the human eyes in-vivo using long-range SS-OCT instrument integrated with air-puff stimulation, which enables to visualize the dynamics of eye through its entire length and to measure the intraocular distances (SS-OCT ocular biometer). The deformations calculated from the model were fitted to the measured profiles data using Levenberg-Marquardt method. The rheological model allowed for predicting the displacements of the cornea and the crystalline lens (1.25mm and 0.115mm, respectively), the eye retraction (0.28mm) and the axial wobbling of the lens within 40ms. The developed model outcomes match well to experimental data of corneal and lenticular hysteresis curves identifying viscoelastic properties of the ocular tissues
In conclusion, the proposed rheological model correctly predicts the effects observed with air-puff SS-OCT ocular biometer and it can be used in future modelling of the whole eye biomechanics.
The study presents measurement and numerical analysis of time variability of the eye pupil geometry and its
position, as well as their correlations with blood pulsation. The image of the eye pupil was recorded by use of the fast
CCD camera with 200 fps rates. Blood pulsation was synchronously recorded by use of pulse transducer with the
sampling frequency of 200 Hz. Each single image from a sequence was numerically processed. Contour of the eye pupil
was approximated, and its selected geometrical parameters as well as center positions were calculated. Spectral and
coherence analysis of time variability of calculated pupil parameters and blood pulsation were determined.
Paper presents method of numerical analysis of time variability the eye pupil geometry by use of the first and the second moments of irregular pupil form. Sequences of the pupil shape variations were recorded by use of high-speed CCD camera, with recording speed 250fps and next analyzed on a group of 11 patients. The shape of the pupil for each frame from the sequence was characterized numerically by use of second moments of inertia and orientation of its principal axes. Additionally the area and coordinates of center of gravity of the pupil were calculated. Kinetics of the pupil area, its moment of inertia and principal axes inclination angle were analyzed for each sequence. Results of calculations based on the real shape of the pupil were compared with the elliptic approximation of the pupil form. Calculations show high similarity of results for elliptic approximation of the pupil form and the real form approach. However, more exact analysis show a clear differences in the variability of pupil parameters.
A lateral shearing interferometer was used to examine the smoothness of the tear film. The information about the distribution and stability of the precorneal tear film is carried out by the wavefront reflected from the surface of tears and coded in interference fringes. Smooth and regular fringes indicate a smooth tear film surface. On corneae after laser in situ keratomileusis (LASIK) or radial keratotomy (RK) surgery, the interference fringes are seldom regular. The fringes are bent on bright lines, which are interpreted as tear film breakups. The high-intensity pattern seems to appear in similar location on the corneal surface after refractive surgery. Our purpose was to extract information about the pattern existing under the interference fringes and calculate its shape reproducibility over time and following eye blinks. A low-pass filter was applied and correlation coefficient was calculated to compare a selected fragment of the template image to each of the following frames in the recorded sequence. High values of the correlation coefficient suggest that irregularities of the corneal epithelium might influence tear film instability and that tear film breakup may be associated with local irregularities of the corneal topography created after the LASIK and RK surgeries.
A sequence of videokeratoscopic images was registered using commercially available instrument E300 at a rate of 50
fps. During the 20 seconds measurement, subject's head was fixed strongly. Acquired images were analyzed for
detecting fixational eye movements and corneal surface deformation. For this purpose two rings were extracted from
each frame and the ellipses were fitted to them, using least square method. The time series of the ellipses geometrical
parameters were considered: minor and major axes length as well as the ellipses center and the orientation. The
frequency spectra of mentioned parameters were obtained by application of the Fast Fourier Transform. The longitudinal
position of the corneal apex was controlled, thanks to the cone side viewer installed inside the videokeratoscope.
The average amplitude of the variation of the ellipse's axes length is around 20μm and of the orientation of the ellipse
around 0,1 rad. In the signals frequency characteristics, appear the peak corresponding to the heart rate. No clear
relationship was found between the variations of the fitted ellipse parameters and the longitudinal position of the corneal
apex.
The fixational eye movements were examined using two different methods. One of them consists of calculating the
correlation function between the first and successive frame of the sequence and searching its maximum. The other is
based on tracking the center of the ellipse fitted to particular ring of the videokeratoscopic image. The accuracy of the
second method found to be higher.
Simple methods proposed in this work can extend the application of videokeratoscopic measurements.
The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. [J. Opt. Soc. Am. A 24(10), 3274-3283 (2007)] showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight (~0.6 D) with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions (PSFs) at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.
Intraocular pressure (IOP) varies quasi-periodically due to blood pulsation in vessels inside the eye globe. This variations cause the eye deformations and displacements of the outer surface of the eye. The aim of this paper is to calculate the correlation between longitudinal corneal apex displacement and cardiovascular activity. Using ultrasound transducer at sampling frequency of 100Hz we have measured longitudinal corneal apex displacement (LCAD) of the left eye for 5 subjects. Synchronically we have registered ECG and blood pulsation signals at the same sampling frequency. Cross-correlation function was applied to investigate dependencies between these signals. To find time shift between LCAD and ECG or pulse, the time window of 3 seconds length have been chosen from all signals and had been shifting with the step of 0.01 seconds from 0 to 7s. For each shift the cross-correlation function and its extrema were calculated in the window area. We have obtained information about extrema position of cross-correlation function and its stability in time for particular subjects. The time shift between LCAD and ECG or pulse is individual feature of each subject. Such calculations may lead us to better understanding of pulse propagation in human eye and creation a non invasive method of eye hemodynamics and ocular diagnosis.
The lateral shearing interferometer was applied to in vivo investigate the stability of the tear film surface covering
the contact lenses. The 8 mW HeNe laser was used as the light source. The sequences of interferograms were recorded
by CCD camera in real time during the inter-blink intervals at 25 fps, stored in a computer memory and numerically
processed. Every frame illustrates the pattern of interference fringes that corresponds to temporal stage of prelens tear
film surface. Fast Fourier Transform was used to quantitative evaluate tear film surface irregularities and the numerical
measure M2 was used to obtain quantitative description of the tear film smoothness. The M2 index is the lowest for the
smooth and regular surface of the tear film and its values increase if the prelens tear film begins to be unstable.
The proposed way of analysis of each interferogram gives opportunity to calculate the credibility of given M2 index and
automatically reject a part of a frame, that is not covered by interference fringes with an appropriate contrast due to eye
movement. The tear film smoothness has been measured on different types of soft contact lenses of 4 companies. The
significance differences in tear film stability have been noticed between low and high water content materials of contact
lenses. The tear film was less stable on low water content materials.
The purpose of this study is to investigate the kinetics of precorneal tear film stabilization process after eye blink and the process of creating the break-up of the tear film layer. The tear film of patients were examined in vivo by used the lateral shearing interferometer. The information about the distribution and stability of the tear film over the cornea is carried by the wave front reflected from the surface of tears and coded in interference fringes. Smooth and regular fringes indicate the smooth surface of tears over the cornea. Immediately after eye blink the interference fringes are observed on background of bright and dark areas. The contrast of this structure fades with time slowly and after 1-3 sec the background of interference fringes becomes uniform. The vertical orientation and instability of this structure suggests connection with eyelid movement and the spread of tears. If the eye is kept open for a long time, bright lines appear in the background of fringes after a dozen seconds. The slowly appearing structure might signify the tear film break-up. In case of eyes after a LASIK surgery the shape of the background structure has different nature and might be stable in time suggesting the stability of the corneal surface irregularities.
Tear film stability plays an important role in the quality of vision. We present an interferometric method for assessing the stability of precorneal tear film in real time. A lateral shearing technique is applied as a noninvasive and sensitive method for investigating tear film stability and the smoothness of the tear film surface by quantitative evaluation of the interference fringe pattern. The evaporation of tears and the appearance of tear film break-up between blinks cause changes in the fringe geometry. For quantitative assessment of the fringe smoothness and consequently of the tear film structure, the fast Fourier transform (FFT) is applied. Four parameters are used to quantitatively estimate dynamic changes in the tear film stability of the cornea 20 s after the eye blink. Examples of examined interferograms recorded in patients with healthy eyes, patients suffering from dry-eye syndrome, and patients wearing contact lenses are given. Significant differences between the stability of the tear film in the healthy eye and that in the dry eye and the eye with a contact lens are observed. The favorable influence of artificial tears applied in patients with dry-eye syndrome or contact lenses is also discussed.
The pre-ocular tear film is the most anterior refractive surface of the eye. Its stability plays an important role in the condition of vision from the optical and physiological point of view. If the eye is opened for a significantly long time or suffers from an anormalities in tear production, there appear isolated dry islands - break-ups - with a random distribution in the continuous lacrimal film. We applied an interferometric method - Lateral Shearing Technique for investigating the tear film stability and the smoothness of the tear film surface. This method is non-invasive and it is characterised by the high accuracy and sensitivity. Interferometry allows dynamic measurements of the tear film stability in real time by observation of interference fringes. The evaporation of tears and appearance of the breakups causes changes in the fringe geometry. Fast Fourier Transform has been used for quantitative assessment of the fringe smoothness and as a consequence of the tear film surface geometry. This paper presents the method used for quantitative evaluation of the tear film distribution on the cornea. Examples of interferograms recorded on eyes of patients with healthy eyes, suffering from dry eye syndrome and wearing contact lenses are also given. With our technique we were able to observe distinct differences in stability of the tear film between healthy and dry eyes, and the tear film on contact lenses.
The human pupillomotor response to light (the pupil light reflex -- PLR) is an example of nonlinear biological systems. The time characteristic of the PLR response is determined by the successive activation of the parasympathetic and sympathetic innervation of the iris muscles. The shape of characteristics is typically evaluated from the changes in pupil size following a pulse stimulation. The paper presents the system for studies of the dynamic behavior of the pupillary system. Results of the experiments and analysis are presented.
The aim of our study was to define such shape of videokeratometric Placido disc based illuminator that would guarantee quasi flat image formation and due to that sharp and reliable information of corneal geometry receiving. The new shape of surface, which Placido rings would be painted on, is "cigar" like and was the result of our model study. Now analytical comparison with existing, commercially used videokeratometric illuminator was performed. Such cylindrical illuminator is used in the Tomey TMS-3 videotopographer. The study relied upon simple analytical path of rays calculations. The calculations were performed for 17 rays in inproportionate configuration at the entrance pupil plane. The results show the difference between image surfaces created by both illuminators on the average cornea. The image surface formed by introduced stimulator is much more flatter than the image of TMS-3 illuminator. This means that in order to receive sharp images providing reliable data, one can use the camera with smaller depth of sharpness.
The imaging polarimetry with carrier frequency uses periodically space variation of light polarization obtained with a birefringent wedge, inserted into an optical setup. Implementation of required phase shifting in measuring optical system is performed by use of Liquid Crystal Modulator (LCM). Application of LCM instead of rotational phase plates significantly accelerates measurements and enables recording fast, dynamical variations of both distributions of birefringence and azimuth angle of anisotropic objects. Accuracy of calculation of birefringence and azimuth angle distributions of measured object depends on orientation of LCM in relation to the other optical elements of the system and on accuracy of phase shift given by the LCM. This paper presents the model calculations, which enable to evaluate the influence of the adjustment parameters of LCM on reconstruction of the maps of birefringence and azimuth angle of the measured object.
The presented paper reports the continuation of research on the linear model of human eyeball. We use the physically linear eye model, also presented in the paper by Srodka et al.'), but we introduce a new element to this model- passive non-accommodating eye lens. We look for such conditions, that placed upon geometry and material parameters of the eyeball, would enable the model to show optical self-adjustment effect. The Finite Element Method was used to model the eyeball tissue by means of physically linear materials. The current paper also investigates limitations imposed on boundary conditions, as well as geometrical and material parameters of model, by postulated requirement of eyeball models the optical self-adjustment effect. Subsequently, the results of investigations carried out on the model with and without eye lens are compared.
This paper reports the method of recording the Pupil Light Reflex with using a CCD linear sensor as a detector. The system allows to obtain a linear resolution 0,005 mm and a temporary resolution 11 ins. The principle of measuring method and example results of PLR study are presented.
Paper presents results of in vivo measurements of dynamic variations of the corneal topography by use of the Twyman Green interferometer. Sequence of interferograms were recorded by the CCD camera and stored in the computer memory. Then the fringe tracking method was used separately to each interferogram giving the phase surface of the wave reflected from the cornea in the numerical form. Results from neighboring interferograms were subtracted giving new sequence of changes of the corneal topography within 40 ms. Obtained results show the complex space distribution of the corneal topography variations.
The method of the measurement of the corneal topography was worked out. This measurement system uses an interferometer based on radial shearing. This paper presents the preliminary results of the experiments. The results are compared with other methods.
The cornea does not play any significant role in accommodation and is not expected to alter in curvature during the process. Nevertheless, some residual shape change may occur. In this paper we present preliminary results of a study in which we looked at the shape of the cornea for two separate accommodative states. Corneal power was measured with a Javal keratometer in tow states of the eye on one eye each from 12 emmetropic subjects aged between 20 and 28 years. These results suggest that there may be slight changes in central corneal curvature with accommodation.
The cornea is the outermost segment of the eyeball. Due to it's lamellar structure the cornea indicates the optical anisotropy and effect of birefringence. We have applied phase stepping imaging polarimetry to measure birefringence of the human cornea in vivo and in vitro. Investigation of the corneal birefringence could be useful for examining the inner corneal structure, its lamellar arrangement and in medical diagnosing of corneal pathologies. There are also some potential applications of this method in transplantation of human corneas and refractive surgery procedures. The phase stepping imaging polarimetry technique used here, allows calculating azimuth angle, phase retardation and transmission coefficient of the sample. The method uses simple setup, sample at rest, and enables fast and accurate acquisition of data. The theory of the method is included. Experimental results of azimuth angle and phase retardation distribution for human cornea in vivo and in vitro are also presented.
The shearing interferometry is presented as in vivo method for testing artificial tears covering the cornea. The stability of two types of artificial tears (gel and liquid) on the surface of the cornea was estimated by the analysis of interferograms. The 3 mW He-Ne laser was used as the light source in the shearing interferometer and CCD camera for recording the interference images. The coherent light is reflected from the surface of the cornea covered with the artificial tears. The reflected wavefront is directed to the lateral shearing interferometer and forms the interference pattern. The sequence of the obtained interferograms were recorded on VHS video tape. Then selected frames were digitized and analyzed in computer by use of image processing software. The proposed method is noncontact and noninvasive.
The paper presents the complex structure of the human tear film. From the optics point of view the homogeneously distributed tear film on the corneal surface makes this diffused surface optically smooth. Every eye blink distributes a new portion of tears on the surface of the cornea or the contact lens. If the production of tears is not sufficient or their composition is inappropriate, the tear film is deteriorated and so called tear film break-up (TFBU) appears on the surface of the cornea. These local break-ups of the smooth tear film introduce disturbances in the phase of a quasiplane wavefront falling on the cornea from a far-located point source of light. Two types of the TFBUs are considered in this paper. Topography of a point one and a linear one form of the break ups are approximated by use of four different continuous functions. The modeled topography of the TFBU is used for calculation of the local variation of the wavefront phase. The Point Spread Function of the eye model is calculated by use of the Fast Fourier Transform without and with the different approximations of TFBUs. It is shown that different type of TFBU approximation influences significantly the form of the Point Spread Function of the eye.
In last years many algorithms and techniques based on whole field polarimetry has been presented. Most of them could be applied in photoelasticity. In this paper we propose a method supported with digital image processing that makes the evaluation of photoelastic patterns simpler. The phase retardation and the azimuth angle induced by photoelastic sample were measured using phase stepping imaging polarimetry technique. Some approaches taken from phase stepping technique were used to minimize number of data needed to calculate two maps of the phase retardation and the azimuth angle. In contrary to other methods this technique offers simple setup, few moving parts and fast acquisition of data. The theory and experimental results are included in the paper.
The paper presents an approach for measurements of the corneal topography by use of lateral-shearing interferometry. In shearing interferometry the wavefront reflected from the object is split in two wavefronts. The introduction of lateral shift and superposition of the wavefronts produces interference pattern. We used phase shifting algorithm to obtain the phase differences of the wavefront reflected from the cornea. The measurements of the phase differences are carried out in two orthogonal directions. The phase of the wavefront is reconstructed by use of algorithm for the large grid based on the discrete integration. The computer-generated interferograms are used to test the procedure and to estimate its accuracy.
Phase shifting imaging polarimetry was applied to measure birefringence of the human cornea in vitro and in vivo. Using computerized polarimeter, two maps representing distribution of the azimuth angle and the retardation are calculated and presented. Investigation of the corneal birefringence could be useful for examining the inner corneal structure and in diagnosis of some corneal pathologies.
The shearing interferometry is presented as an in vivo method for testing the stability of the tear film covering the contact lens. The material and quality of a contact lens and its correct fitting to the surface of cornea influence the stability of the tear film. By observation of the tear film distribution, the precise assessment of the quality of the contact lens surface can be performed. Moreover, the overused or damaged contact lens can be detected by the analysis of interferograms. In this study the following types of soft contact lenses were used: Dura Soft D3 (r equals 8,7 mm, r equals 8,4 mm), Medalist (r equals 8,7 mm) and Johnson and Johnson (r equals 8,8 mm). The contact lenses were placed on the patients' cornea. The 3 mW HeNe laser was used as the light source in shearing interferometer and the CCD camera for recording the interference images. The coherent light reflected from the surface of the contact lens covered with the tear film formed the interference pattern. The sequence of the obtained interferograms were recorded and then analyzed. The accuracy of the contact lens fitting can be estimated by the interferogram analysis. The examples of correct and incorrect fitting of the overused or damaged contact lenses are presented. The proposed method is noncontact, nondestructive and of the high accuracy.
The paper presents a modified method of the Shearing interferogram analysis by the use of the Fast Fourier Transform. The interference fringes are obtained in Shearing interferometer and are related to a wavefront reflected from the tear film over the cornea. The method is based on the measurement of interference fringes disturbances by calculation of the 2nd momentum of Fourier spectra of the interferogram. The less smooth tear film covering the cornea the more disturbed interference fringes and higher value of the 2nd momentum calculated from its Fourier spectra. The interferograms obtained from the proposed setup can differ also in the shape of fringes and in its density. This is caused by the differences in distance between the interferometer objective and the eye. Since the living eye is a continuously moving object, the fixation of this distance is difficult to achieve in practice. Obviously the fringes variations affect the Fourier spectra distribution and results of the numerical analysis based on FFT. The proposed method compensates the variations by applying the correcting factor determined from spectra distance from the origin of Fourier domain.
The first part of the paper presents the analytical curves used to approximate the corneal profile. Next, some definition of 3D surfaces curvature, like main normal sections, main radii of curvature and their orientations are given. The examples of four nonrotational 3D surfaces such as: ellipsoidal, surface based on hyperbolic cosine function, sphero-cylindrical and toroidal, approximating the corneal topography are proposed. The 3D surface and the contour plots of main radii of curvature and their orientation for four nonrotational approximation of the cornea are shown. Results of calculations are discussed from the point of view of videokeratometric images.
The paper presents preliminary results of applying two different interference techniques for evaluation of the breakup characteristics of the tear film. The Twyman-Green interferometer (TGI) and Lateral Shearing technique (LST) were applied in two separates set ups. To find irregularities in the tear film distribution, the sequence of interferograms of the cornea or contact lens were stored and processed in a computer by use of modular frame grabber. The interferograms of in vivo precorneal tear film breakup formation are presented for both techniques. The proposed methods have the advantage of being noncontact and applying the low energy laser beam.
We have applied imaging polarimetry to measure birefringence of the human cornea in vivo. A theoretical model of the corneal birefringence is considered. The polarized light transmission through the system is modelled theoretically by use of the Jones matrix formalism. The light passes some polarization components, the cornea, anterior chamber and is scattered at the iris. Using computerized polarimeter, two maps representing distribution of the azimuth angle and retardation are calculated and presented. Investigation of the corneal birefringence could be useful for examining the inner corneal structure, its lamellar arrangement and in medical diagnosing of keratoconic cornea.
The cornea is the outermost segment of the eyeball. Due to its lamellar structure the cornea indicates the optical anisotropy and effect of birefringence. In this study, the corneal birefringence properties have been investigated by numerical simulations and experimental measurements. A theoretical model of the corneal birefringence is considered. Ray tracing through such a model depending on different approximations of the corneal topography is calculated. In the paper photographs of the polarization patterns of the cornea in vivo with linearly polarized light are presented. Numerical and experimental results are compared and discussed.
The tear film fulfills several important functions in the eye. Apart of its physiologic functions like maintaining a moist environment for the epithelial cells of the cornea and conjunctiva, bacterial properties, transporting metabolic products etc., this film causes that the corneal surface has the optical quality. This smooth surface allows to apply interferometry for measurements. The paper presents tear's layer distribution on the soft contact lens and the cornea in comparison. Tv frame speed registration in the Twyman- Green interferometer was used to observe an unstable biomedical objects like the eye. The proposed method has the advantage of being noncontact and applies the low energy laser beam in interferometric set-up. This provides non- invasive testing of human cornea in vivo and enables observation the kinetics of its tear layer deterioration. The evaluation of non-invasive tear breakup time is possible by use of proposed setup. Further analysis of recorded interferograms helps to examine the matter of the breakup process and can be used for detection of the 'dry eye' symptoms.
An anisotropy of optical elements of the human eye is examined. A theoretical model of the corneal birefringence is considered. Ray tracing through such a model depending on different approximation of the corneal topography is calculated. In the paper photographs of the polarization patterns of the cornea in vivo with linearly polarized light are presented. Numerical and experimental results are compared and discussed.
Recent developments in optical, optoelectronic, and digital electronic imaging and metrology are creating opportunities for a new type of diagnostics methods and systems. Some of these techniques, established already in the field of technical and industrial non-destructive testing, have increasingly gained importance in biomedical research and may enter the clinical scene, as well. Even the laboratory investigations can have strong impact for further developments in this field. However, in experimental medicine the quantitative analysis of experimental data is sometimes required. When applying different interferometric methods, the obtained results are in the form of fringe pattern images. In this paper some of these methods, including holographic interferometry, laser interferometry and moire techniques are described and illustrated by experimental results. For acquisition and evaluation of the fringe pattern images, the Bioscan Optimas package from Bioscan, Incorporated of Edmonds, Wash., running under Microsoft Windows was used.
The cornea is the most refractive element in the eye. Its refractive power is about 70% of the power of the whole eye. The shape of the cornea is aspheric, and almost always has no rotational symmetry. Even small surface irregularities can cause a perceptible reduction in visual acuity. Standard methods for evaluation of the corneal topography used in clinical practice include keratometry, photokeratoscopy, and computer assisted videokeratography. All of these methods used the principles of geometrical optics, and their accuracy is about 0.25 D. An application of interference phenomenon's to examine the corneal contour map significantly increase the accuracy. Using the interferometric inspection of the corneal shape one can easily observe the fine corneal topography, the fast, dynamic changes of the corneal surface, and the topology of the tear film and its irregularities. The paper presents the Twyman Green interferometer, used in experiments, an example of sequence of interferograms and their 3D presentations.
The new interferometric method for investigation of the corneal surface is presented. Fundamentals of the proposed method, based on dual-wavefront illumination, are explained and the experimental set-up for practical realization is described. This technique can be suitable for measurement of the corneal surface in vivo.
The paper presents results of holographic experiments of expanding bovine cornea in vitro, after different refractive surgery procedure (keratoplasty). The corneas of fresh, enucleated bovine eyes were subjected to different refractive procedures, such as: radial and tangential incisions, mechanical ablation of the corneal layers as well as ablation by means of excimer laser. After keratoplasty, the eyeball was placed into the holographic set-up and the needle connected to a pressure meter was inserted into the anterior chamber of the eye. Double exposure, as well as real time holographic interferograms of expanding corneas were recorded by means of photothermoplastic camera, due to small intraocular pressure differences in order of a few Pa. The holographic interferograms were stored and processed in the computer memory by use of CCD camera and the image processing board. Quantitative results are presented in the form of a 3-D surface plot of the displacement vectors of the corneal surface and respective changes of the corneal curvature.
The application of speckle photography in biomechanical testing of bones and surgical fixing devices is presented. Double-exposure speckle photography is used for measuring the in-plane deformation of broken lower leg bones supported with different fixing devices under axial loading. An osteosynthesis plate, an external fixator, and an intramedullar nail mounted on the tibia shaft are tested. The results for different loading conditions are analyzed and compared with those obtained by holographic interferometry. Further, the human hyoid bone is investigated by this method. The load is applied to the anterior surface of the body of the bone. All tested specimen show an asymmetric displacement, the greatest in a plane vertical to the load. An evaluation of fracture behavior can be done from the displacement pattern.
The Young's Modulus for human tibia obtained from a non-destructive holographic interferometry method has been compared with the same parameter calculated from classical stress-strain test. The comparison has been performed for the same bone, first tested non- destructively and than destroyed during the stress-strain test. The results are in agreement with the results presented by other authors.
Double-exposure as well as real-time holographic interferometry are used to evaluate changes of the central curvature of the bovine cornea in vitro under small intraocular pressure differences. Double-exposure holographic interferograms of expanding corneas were used to get the positions of dark interference fringes on the surface of the cornea. Real-time holographic interferometry enables determining the absolute value of the displacement of the corneal center along the axis of the eye or the number of the fringe that is located closest to the corneal center. A mathematical model is given for the radial distensions and displacement of the center of curvature in two mutually perpendicular planes of a longitudinal section of the cornea along the axis of the eye. Examples of holographic interferograms of a bovine cornea recorded at an intraocular pressure of 1340 Pa and pressure differences of 10 and 20 Pa, respectively, as well as resulting changes of the corneal curvature are presented.
A simple method to determine the moduli of elasticity E and G and the Poisson coefficient v of a linear elastic isotropic structural material on the basis of a cylindrical beam, from a single double-exposure holographic interferogram, is described. The method is applicable to different types of materials. The measurement of a steel beam is presented as an example.
Double exposure as well as real time holographic interferometry were used to evaluate changes of the central curvature of the bovine cornea in vitro under small intraocular pressure (IOP) differences. Double exposure holographic interferograms of expanding corneas were used to get the positions of dark interference fringes on the surface of the cornea. Real time holographic interferometry allows one to determine the absolute value of the displacement of the corneal center along the axis of the eye or the number of the fringe, which is located closest to the corneal center. A mathematical model is given for the radial distentions and displacement of the center of curvature in two mutually perpendicular planes of a longitudinal- section of the cornea along the axis of the eye. Examples of holographic interferograms of a bovine cornea recorded at an intraocular pressure of 1340 Pa and pressure differences of 10 Pa and 20 Pa, respectively, as well as resulting changes of the corneal curvature are presented.
Concepts including the length of T-incisions are mainly based on empirical evaluation of the surgeons. We report on a holographic interferometric study of the effects of straight T- incisions of 5 mm, 10 mm, and 15 mm length using double-exposure holographic interferometry. A new classification of the central holographic interferometric fringe pattern including cross-like, parallel and convex fringes allows an objective qualitative analysis of 48 holographic interferograms of the corneas of six bovine eyes. The intraocular pressure is 1340 Pa with 10 Pa and 20 Pa pressure increase. T-incisions of 5 mm length of incision (incision depth 95%) change the holographic interferometric fringe pattern completely. An increase to 10 mm length of incision results in a change of holographic fringe pattern in 50% of the cases. With 15 mm length of incision only two types of holographic interferometric fringe pattern of the cornea can be seen: parallel fringes and irregular fringes indicating an instability of the cornea. This might be a reason for paradox flattening of the cornea in the unincised meridian.
In this preliminary study the effects of a simple anterior cut of a bovine cornea were investigated by means of double-exposure holographic interferometry. The resulting change of holographic fringe pattern was obvious. Further investigations under controlled conditions are envisaged.
The unilateral axially dynamic fixator (Orthofix) was mounted on a sheep tibial shaft. Three fixation modes: static, dynamic controlled, and dynamic free were examined by means of double exposure holographic interferometry. Simultaneously, the acceleration was measured by an accelerometer and displayed on the monitor together with loading characteristics. The first exposure was made before the acting force was applied to the tibia plateau. The second one after the moment when the acceleration wave started to propagate through the specimen. We stated that in the case of dynamization less torsion occurs at the fracture site. So far, we have not been able to determine any correlation between results of holographic and accelerometric measurements.
The expansion of the cornea of fresh enucleated bovine eyes was tested by means of double-exposure holographic interferometry. The tissue was subjected to small increases in intraocular pressure. Preliminary results indicate that each bovine cornea has its own special elastic expansion which appears in a different fringe pattern.
Double-exposure speckle photography was used for measuring the in-plane deformation of broken lower legs bones supported with different fixing devices under axial loading. An osteosynthesis plate, an external fixator, and an intramedullary nail mounted on the tibial shaft were tested. The results for different loading conditions were analyzed and compared with those obtained by holographic interferometry.
Of all the capabilities of holography for image processing and measuring purposes, holographic interferometry
has found the broadest application in biological and medical research. As a technique, which is non-destructive
and allows for high resolving deformation analysis, holographic interferometry has gained popularity over the recent
years in experimental biomechanics and orthopaedics. Some examples of holographic interferometry in experimental
orthopaedics are reviewed in this report. The possibility of using holographic interferometry to study the behaviour
of broken bones supported by fixing devices is pointed out in this review.
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