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Title: Microstructure based numerical model of ocular globe
Author: Zhou, D.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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The human eye has been intensively explored regarding its structure and optical property as a precise optic system to receive external information. Many treatments were invented and improved to help those who suffered from visual disability or ocular diseases. Contemporary progress of experimental technique and computer methods enable scientists to understand further and even predict the biomechanical behaviour of the ocular globe. Nevertheless, it is still a challenge to accurately describe the material characteristics of ocular tissue and the realistic mechanical response. The research was conducted to develop a microstructure-based full ocular model by considering the regional variation of fibril distribution. The tissue microstructure is mainly the architecture of embedded collagen fibrils in this thesis. The microstructure findings by using X-ray indicated the potential to incorporate realistic fibril measurements that can dominate the material behaviour of ocular tissue in the numerical model. A method was developed to reconstruct X-ray maps and ensured the regularity of the fibril data covering whole eye globes. A comprehensive quantitative study was accomplished on the preference of fibril alignment based on seven reconstructed X-ray maps. Results showed that 62±2% of fibrils were aligned within 45? sectors surrounding the vertical and horizontal directions. In contrast, more than one-third of the total fibril content was concentrated along the circumferential direction at the limbus (37±2%) and around the optic nerve head (35±2%). The insertion locations of the four recti muscles exhibited a preference in the meridional direction near the equator (39±2%). Defining fibril distribution in the ocular numerical model required the fitting of X-ray fibril data. Zernike polynomials at order 10 produced the most accurate fitting results of fibril density by considering the bias and overfitting. Fine-mesh finite element (FE) models with eye-specific geometry were built and supported by a user-defined material model, which considered the regional variation of fibril density and orientation. The models were then used, in an iterative inverse modelling study to derive the material parameters that represent the experimental behaviour of ocular tissues with ages between 50 and 90 years. The technique called Particle Swarm Optimisation (PSO) was used to interact with finite element solver to perform the inverse analysis. The PSO algorithm was validated through ocular biomechanics related tests. The results showed PSO was competent to provide reliable and stable optimisation process. Sensitivity analysis showed that reducing the number of directions that represented the anisotropy of collagen fibril orientation at each X-ray scattering measurement point from 180 to 16 would have limited and insignificant effect on the FE solution (0.08%). Inverse analysis resulted in age-related stiffening material parameters that provided a close match with experimental intraocular pressure-deformation behaviour with an RMS of error between 3.6% and 4.3%. Fibril reorientation is of great importance to make the tissue adaptive to the surrounding mechanical environment. This thesis analysed the fibril distribution across the cornea strip at the strain level from 0% to 8%. A relationship between uniaxial strain and changes of fibril density at different orientation was formulated into linear equations that were adopted in the constitutive model to control the anisotropy of the material. Based on the strain regime, the relationship was proportionally applied to each orientation. The reorientation algorithm was validated using the experimental measurements. Three FE ocular models with different amount of tissue removed were simulated and their results of fibril distribution after cutting were compared. The reorientation study confirmed fibrils tend to align in the direction of cutting (5.16% increase in fibril density) to enhance the stiffness and withstand more considerable stress in refractive surgery. The algorithm was potential to be adopted in a complicated surgical model to consider the alteration in material behaviour and provide reliable simulation results. To conclude, a constitutive material model based on distributions of collagen fibril density and orientation has been developed to enable the accurate representation of the biomechanical behaviour of ocular tissues. The model offers a high level of control of stiffness and anisotropy across the ocular globe, and therefore has the potential for use in planning surgical and medical procedures. The success of applying them can benefit the improvement of tonometry and the planning of refractive surgery.
Supervisor: Elsheikh, Ahmed ; Abass, Ahmed Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral