ABSTRACT
Objective To study the age-related changes in biomechanical properties of rabbit cornea by using the data from uniaxial extension testing on rabbit cornea strips. Methods To obtain the experiment data, the uniaxial extension test on rabbit cornea strips of both 3-month old and 7 to 8 month-old were performed. The exponential function model and power function model were used to fit the strain-stress curves, and two kinds of viscoelastic mechanical models were used to analyze the stress relaxation curves. Results The mechanical behavior of the rabbit cornea strip presented a nonlinear and viscoelastic property. Within the range of error permitting, the age was not an important factor to affect the stress-strain, as no significant difference was found in nonlinear stress-strain curve of rabbit cornea strips between different ages. The cornea strips of 7 to 8 month-old rabbit had a slightly bigger elastic modulus and a faster stress relaxation than those of 3 month-old one. Different stretching rates had no obvious influence on the nonlinear stress-strain of 3-month old rabbit cornea strips, but the cornea strips at high stretching rate could have a faster stress relaxation. Conclusions The tangent modulus of rabbit cornea increases slightly with age, but its relaxation properties would change greatly with age.
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Objective To establish a three-dimensional (3D) visualization model for the vessel system of rabbit eyes using X-ray phase contrast imaging（XPCI）technique, and observe the morphological characteristics of iris vessels of the rabbit eyes. Methods Angiography on vessels of the New Zealand rabbit eyes was conducted using Barium sulfate as the contrast medium. The projected images of in vitro rabbit eye samples with high precision were obtained by XPCI technique, and then converted to tomography images by filter back projection. The 3D reconstruction of the rabbit eyes was completed by commercial visualization software Amira 5.2.2. Results The main blood vessels of the rabbit eyes were clear and coherent in the projection images, and the distribution and trend of some small vessels could be observed, with the smallest distinguishable blood vessel diameter being about 10 μm. The 3D model for vessel network of the rabbit eyes was built after 3D reconstruction of CT scan images. The major arterial circle of the iris could be observed at level 4 branch structure of vessels in the fundus, and the minimum diameter of vessels that could be identified was 40 μm. Conclusions The vessels of the rabbit eyes can be clearly observed and 3D visualization of vessel network can be constructed by using XPCI technique, which would provide basis for the analysis on hemodynamics of blood vessels in the eye and reference for the clinical study of glaucoma.
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Objective To measure the rabbit retinal nerve fiber layer(RNFL) thickness in vivo under different intraocular pressures (IOP) and at different time intervals after acute high IOP, and to obtain the regularity on change of RNFL thickness with acute high IOP. Methods Four groups of acute high IOP model were formed by perfusing the saline water into the anterior chamber of rabbit eyes, then RNFL thickness under different IOPs was measured by optical coherence tomography with radial scanning mode whose center was optical papilla and diameter was 6 mm. Results The absolute change values of RNFL thickness were(-27.16±14.24), (-33.33±6.74), (-48.75±5.24), (-67.29±3.89) μm under different IOPs of (4.50±0.35), (6.07±0.31), (7.74±0.26), (10.71±0.07) kPa, respectively. The linear relationship was found between the relative change of RNFL thickness and IOP. IOP could return to the normal level within two days after acute high IOP, while the RNFL thickness could be restored at the second week, and it was easier to restore if the IOP was lower than 6.65 kPa. Conclusions The acute high IOP could cause RNFL thickness to decrease significantly with the increased acute high IOP, but it will be restored after some time.
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Objective To simulate the supination-external rotation ankle injury and establish a 3D finite element model of the ankle. Methods Based on CT images of the normal human ankle joint, the 3D model of the ankle with ligaments was established. The supination-external rotation ankle injuries with four different degrees of Lauge-Hanson were analyzed by finite element method. Distributions of the ankle joint stress and tibial articular surface pressure were obtained. Results The maximum stress was at the anterior tibiofibular ligament attachment point of the tibial under supination-external rotation loading. When the anterior tibiofibular ligament was ruptured, the maximum stress was at the interosseous membrane. After the interosseous membrane was ruptured, the high stress was at the posterior ligament of the ankle. When the posterior tibiofibular ligament was ruptured, the high stress was at the deltoid ligament. The high pressure was at the distal fibula or the rear of tibial articular surface. Conclusions The established ankle-foot 3D numerical model can be used for the mechanical analysis of supination-externalrotation ankle injury. The calculated distributions of the ankle stress and the tibial articular surface pressure were in agreement with the description of Lauge-Hanson classification.
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Objective To propose an inverse method for determining nonlinear mechanical properties of the blood vessel based on the results of experimental data and numerical simulation.Methods Pressure loading was applied on the blood vessel to obtain the experimental data of in vitro holistic blood vessel by using the self-designed device. The finite element model of vessel inflation was established by supposing that material characteristics of the blood vessel were in corresponding with the hyperelastic Ogden model. Mechanical properties of the blood vessel were then computed by the reverse method based on these experimental data and simulated results .Results The first-order and second-order Ogden material parameters of the rabbit abdominal aorta were identified, in which α=10.86±1.98 for the first-order Ogden material model. The mechanical properties of the rabbit abdominal aorta could be characterized as the hyperelastic material. Conclusions The inverse method based on the experimental measurement and numerical simulation can be used to identify the nonlinear mechanical properties of the blood vessel.
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Objective To obtain pressure-volume relatioship for the rabbit eye in vivo. Method Physiological salt solution was injected with the rate of 20 μL/min through the limbus to the anterior chamber of the rabbit eye for 100 min and the intraocular pressure (IOP) was recorded. Results The relationship between IOP and injection time could be fitted to a segmented function with a characteristic point called IOPg. The ocular rigidity coefficients before and after this IOPg were (4.02±0.86) mmH2O/μL and (2.43±0.94) mmH2O/μL, respectively (1 mmH2O=9.8 Pa), showing significant difference. Conclusions IOPg existed in all curves of IOP and injection time and the ocular rigidity coefficients were dependent on the injection rate and position of IOPg. Parameters of the fit function between IOP and injection time have definite physiological significance.
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Objective To investigate the interface pressure on above knee residual limb at mid stance during walking，so as to provide basis for the establishment of measurement and evaluation system for the above-knee socket. Methods Based on CT images of the femur, soft tissue and above knee socket of the patient with residual limb, the three-dimensional model was first built, and then to assemble them according to changes of the hip joint in a gait cycle. The finite element model was then established to simulate the loading conditions at mid stance during a gait cycle, and the effect of pre-stress was investigated by nonlinear large deformation analysis. Results If considering the effects such as joint changes and pre-stress, the maximum value by calculation for normal interface pressure distributed on the terminal of stump was 257.66 kPa, which could better match the maximum stress actually measured by the Mflex Sensor Distributing System as 258.9 kPa. Conclusions The three-dimensional finite element model with considering the factors such as hip joint change, pre-stress and friction could effectively simulate the stress condition of patient with residual limb during walking.
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Objective To measure the iris elastic modulus. Method Using the self designed device, the information of in vitro holistic iris deformation of rabbits were obtained with the load pressure increasing gradually from 0 Pa to 600 Pa, and the heights of such iris were computed. The corresponding model based on experimental data was established and analyzed by ANSYS finite element software, and the iris elastic modulus was computed by a reverse method. Results The elastic modulus is 6.1 kPa at 100 Pa load pressure. ConclusionsThe iris material can be considered as linear elastic model when the pressure difference between the anterior chamber and posterior chamber is low.