Research On The Nondestructive Inspection Technique For Measuring The Biomechanical Properties Of Ocular Cornea

A precise measurement of the biomechanical parameters, such as the stress, strain, stress relaxation, elasticity modulus, hysteresis and ocular rigidity of the ocular cornea, is very important in the guiding of the laser corneal refractive surgery for myopia and in the diagnosis and treatment about the glaucoma and corneal diseases. Having destroyed the physiological environment and the intrinsic structure of the cornea, these inspection technologies of the biomechanical parameter measurements in vitro,such as the corneal extension testing, the corneal inflation testing, the optical interferometry, and the whole ocular eyeball perfusion testing, cannot effectively achieve the biomechanical parameter measurements in vivo. Nowadays, Reichert ocular response analyzer (ORA), a tonometer device, can successfully measure both the corneal hysteresis (CH) and the corneal resistance factor (CRF) in vivo. In the clinical pratice, however, ORA isn’t be extensively employed by the ophthalmologists, because it doesn’t quantitatively relate CH or CRF to a classic mechanical parameter,such as the modulus of elasticity, stress, and strain of the corneal tissue.Therefore, it is necessary to develop the nondestructive inspection technology for the measurement of the corneal biomechanical parameters in vivo.The current work is supported by the Natural Science Foundation of Anhui Province (11040606M128) and the Natural Science Foundation of Anhui Provincial Education Department (KJ2011z060).In this dissertation, several inspection devices and their key techniques, related to the nondestructive measurement of the biomechanical parameters of the ocular cornea, are investigated in detailed.(1)Tonometry, based on the measurement of the corneal deformation, is the widely accepted and generally preferred method of evaluating intraocular pressure in clinical practice today. The work principle of applanation tonometry is introduced in detailed. The various inspection technologies of the corneal deformation measurement in tonometry, from the conventional tonometers (applanation or indention) to the high-technology pressure tonometers that were recently presented for clinical use, are presented. In tonometry, the effect of the corneal configuration, material behavior and biomechanical property on the measurements of intraocular pressure is also analyzed. Compared with the indention tonometry, applanation tonometry has more advantages for the measurement of the deformed cornea, such as non-invasion, fast speed, high precision, and so on. Here, a new method, of simultaneously measuring the applanation cornea area, displacement and the force exerted to the cornea, is presented for applanation tonometry. The measurement principle is described in detail, and a new kind of applanation tonometry device is constructed based on this principle. Experimental results on a simulated eyeball show the present device reading has good agreement with that of the Goldmann applanation tonometer.Therefore, the new applanation tonometry device can effectively achieve the successive measurement of the applanation cornea area, displacement and the force exerted to the cornea and the intraocular pressure in vivo.(2) Based on a corneal shell model, the quantitative elasticity modulus of the ocular comea, related to the applanation deformed cornea area and the measured intraocular pressure and the true intraocular pressure, is derived from the analyses of the relation between the corneal deformation and the load of the cornea during applanation tonometry.A new applanation method for measuring the elasticity modulus of the ocular cornea in vivo is introduced. Experimental results show that the elasticity modulus and creep behavior of the ocular cornea can be nondestructively measured by utilizing the presented applanation tonometry device.(3) A new inspection technology of the ocular rigidity measurement in vivo is presented. The success of the ocular rigidity measuring technique is also based on the development of the presented applanation tonometry device. In the current experiment, the applanation displacement versus applanation force curve is recorded. The test results on simulated eyeballs with different radii of curvature of the ocular cornea show that there are different curves for the applanation displacement and applanation force under the same applanation deformed cornea area. Therefore, the presented applanation tonometry device can achieve the nondestructive testing of the ocular rigidity in vivo.(4) Digital image correlation (DIC) is a typical non-contact, full-field experimental technique that allows rapid and highly accurate measurement of deformation and strain distributions with high resolution, simply optical-configuration and wider measurement range. Applications of the3-D digital image correlation method (3-D DIC) for the determination of the biomechanical behavior of the ocular cornea under the inflation are investigated. Experiments are performed on a simulated corneal specimen using special loading procedures. The full-field displacements on the surface of the corneal sample are calculated using3-D DIC. Experimental results show that3D-DIC method can achieve nondestructive testing of the biomechanical behavior of the ocular cornea, such as creep, hysteresis, stress relaxation, and so on.(5) Considering the potential clinical importance, the surface stress of the ocular cornea under the action of normal physiological intraocular pressure is estimated, and a novel3-D DIC technique and a simple mechanical model for determining the stress are also presented. The inspection device embodying mainly two CCD cameras and a manometer is developed primarily to measure the surface point displacement and to control the intraocular pressure of the eyeball. A simple theoretical model is used to characterize the membrane stresses of the ocular cornea under the action of the intraocular pressure. Laboratory experiments are carried out on a simulated cornea specimen. Experimental results show that the present technique is applicable to nondestructively estimate the membrane stresses. In the normal physiological intraocular pressure range, both meridian and circumference tensions of the ocular cornea along the radial coordinate distribute are not uniform.