Biomechanics of eye tissues is an important parameter of the state of the ocular system and its study is of undoubted interest since there are several clinical situations in which an in vivo assessment of mechanical properties can help both in diagnosis and in treatment. The risk of developing glaucoma and keratoconus of the eye is associated with pathological changes in the biomechanical properties of such eye tissues as the cornea and sclera. Thus, the problem arises of studying tissue biomechanics and the possibility of influencing it. For this purpose, experiments were carried out to determine the dynamics of elastic properties of intact and modified tissue of the sclera and cornea of the pig’s eye by OCT elastography and speckle interferometry. Internal stresses found by numerical simulation from a comparison of subsequent OCT frames demonstrate a dependence on the biomechanics of the tissue sample. It was also shown that the time and temperature dependences of the contrast and correlation functions in speckle interferometry make it possible to track the onset of structural changes in the tissues during repetitively pulsed laser heating. A comparison was made in the behaviour of these curves for the central and peripheral regions of the cornea. The results obtained and their repeatability allow us to conclude that the speckle interferometry can be used as the basis for a system for monitoring structural changes in the cornea associated with the thermal effect of laser radiation. OCT elastography is a sensitive method for studying the biomechanical properties of eye tissues (cornea and sclera) under laser exposure, reflecting the dependence on the intraocular pressure of the eye. This method can be used as the basis for a control system in the development of medical technology for influencing the sclera and cornea of the eye.
We use novel possibilities opened by compressional Optical Coherence Elastography (OCE) to characterize both natural interstitial gaps and laser-irradiation-produced porosity in collagenous tissues (corne and cartilage). Under increasing moderate compression up to strains ~several percent, the current Young modulus of cornea gradually increases from initial values below 100 kPa to values >MPa that are closer to Young's modulus of cartilages in which collagen fibers are much denser packed. The lower stiffness of cornea can reasonably be attributed to the initially looser packed collagenous fiber layers, so that initial high compressibility of cornea is dominated by interlayer voids (gaps). By analogy with geophysics, we apply a model describing the reduction in the tissue elastic modulus due to the presence of a system of nearly parallel, thin "crack-like" voids/pores between the collagenous fiber layers. Initially they are highly compressible, but with increasing compression are getting closed, so that the material gets stiffer. Characterization of such porous component in water-saturated packing of collagenous layers in the natural state is inaccessible to AFM and conventional microscopy, whereas OCE enables earlier unavailable possibility to non-invasively characterize such pores/gaps (their total volume and distribution over the aspect ratio) by analogy with crack characterization in geophysics. Also we apply OCE to characterize spatially-inhomogenous modification of pore characteristics by moderate IR-laser-irradiation in regimes typical of collagenous-tissue reshaping. The obtained results are important for obtaining better insight in the structural modificatons in collagenous packings in the context of the development of novel methods of laser-assisted non-surgical methods of cornea-refraction correction and biologically non-destructive reshaping of cartilaginous samples for fabrication of implants in otolaryngology and maxillofacial surgery.
We consider application of optical coherence elastography (OCE) to problems of laser-assisted structural modification/reshaping of avascular collagenous tissues used for fabrication of cartilaginous implants and corneal tissue reshaping for perspective technologies of vision-correction. The developed OCE technique allows one to quantitatively visualize aperiodic strains during the IR-laser irradiation of the tissue samples and evaluate cumulative strains produced by the laser irradiations. OCE can assess stability of laser-modeled implants via monitoring of post-irradiation slow strains and to study the interplay of temperature and thermal stresses to optimize tissue reshaping. Irradiation-induced micro-porosity in the tissue can be assessed via combination of strain mapping with compressional optical elastography.
We present a realization of real-time OCT-based strain mapping by estimating interframe phase-variation gradient using the developed "vector" method. This technique allows for mapping both fairly fast and large, as well as rather small strains, slowly-varying on intervals ~tens of minutes. Optimization of interframe interval for improving signal-to-noise ratio is discussed and experimentally demonstrated. Ultimate stability of strain estimation with the designed OCT setup is experimentally estimated using stable phantoms. Examples of spatially resolved maps of slowly-varying strains are demonstrated. The developed methods can be used in emerging techniques of laser-assisted modification of collagenous tissues (e.g., for such perspective application as fabrication of cartilaginous implants).
A new laser method for increasing uveoscleral outflow path for normalization of intraocular pressure in glaucomatous eyes is presented.
Nonuniform laser heating affects the porous system of biological tissues. Formation of new pores in the paralimbal region of the eye can accelerate the flow of the intraocular fluid through the eye sclera and, thus, facilitate normalization of the intraocular pressure. A positive effect of laser impact is achieved, as a rule, in a narrow range of laser radiation parameters, which makes it difficult to choose the intensity and time parameters of laser irradiation due to such factors as nonstationary temperature fields, thermotensions and pressure that can give rise to undesirable effects and complications.
The comparison between reflected and transmitted laser light through the eye tissue has allowed to establish the main requirements for laser settings parameters responsible for efficacy and safety of the laser irradiation. The positive effect is achieved only by using relatively small intensity of the laser radiation. At high intensity, the hydraulic permeability decreases due to denaturation and tissue hardening.
Atomic Force Microscopy (AFM) measurements with nanoindentation and optical coherence tomography (OCT) based compressional phase-sensitive optical coherence elastography (OCE). OCE measurements demonstrated laser-induced dilatation areas attributed to formation of ensembles of micro-and nano-pores in sclera providing increase in its hydraulic permeability. Much higher resolution AFM examinations directly demonstrated such individual irradiation-produced pores. At the same time, the collagen structure of the sclera is not destroyed, and tissue mechanical properties do not degrade under laser radiation. The process of pore formation is in good agreement with computer simulations of the dynamics of thermal stress fields induced by laser irradiation.
A new method for non-ablative correction of cornea shape is based on thermo-mechanical effect of laser radiation with ring-shaped laser beam. The results obtained demonstrated that the new method for correction of eye refraction yields a significant alteration in the eye refraction and the ring-shaped laser beam with various ring diameters for correction of the eye refraction allows obtaining controllable alterations of the eye refraction with axial symmetry without any pathological changes in central part of cornea.
Optical properties of cornea and sclera of the eye and their alterations under the effect of 1.56-μm laser radiation are studied. The laser settings corresponded to the laser treatment regimens used (1) to correct the shape of the cornea and change the refraction of the eye and (2) to improve the hydraulic permeability of the sclera in glaucoma cases. A fiber-optical system to investigate the dynamics of the reflected and transmitted scattered laser radiation and a setup with a double integrating sphere to determine the optical properties of the ocular tissues on the basis of the Monte-Carlo simulation of the propagation of light was used. When the radiation characteristics corresponded to the treatment regimens for correcting the shape of the cornea, no noticeable changes were detected in its optical properties. When irradiating the sclera in conditions corresponding to the treatment regimens for improving its hydraulic permeability, the optical characteristics of the tissue showed definite changes. The results obtained as to the dynamics of the optical signals during the course of laser irradiation of the cornea and sclera create prerequisites for designing test systems to be used with novel medical laser techniques for correcting visual abnormalities.
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