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SUPERVISOR
Peiman Mosaddegh,Mahmoud Kadkhodaei
پیمان مصدق (استاد راهنما) محمود کدخدایی الیادرانی (استاد راهنما)
 
STUDENT
Mohammad Jannesari Ladani
محمد جان نثاری لادانی

FACULTY - DEPARTMENT

دانشکده مهندسی مکانیک
DEGREE
Master of Science (MSc)
YEAR
1393

TITLE

Finite Element Modeling of Biomechanical Behavior of the Eye Globe Tissue by Using the Intraocular Pressure
:The eye globe is one of the most complicated and important organs in the body. The cornea, as a traarent tissue in front of the eye globe, is the most important optical structure of the eye which reflects the light onto the retina. Small alternations from physiological states due to changes in the biomechanics of the cornea reduce its visual performance. Therefore, it is necessary to have more information about the biomechanics of eye and in particular the cornea. Additionally, more knowledge about the corneal biomechanics makes the relevant surgeries more effective. Aqueous humor, which is generated by ciliary bodies locating in the eye globe, causes the posterior surface of cornea to be under a constant mechanical stress. This mechanical stress is called intraocular pressure (IOP) and is measured by devices called tonometers. High values of IOP may cause eye diseases such as Glaucoma, so it is essential to assess eye conditions by measuring IOP. There are different types of tonometry devices, among which non-contact tonometries are considered in the present study. Non-contact tonometers, including ORA and Corvis ST, not only are used to estimate intraocular pressure (IOP) in clinical surveys but also are utilized to evaluate biomechanical properties of the cornea or anterior eye. In this thesis, a finite element model (FEM) is developed in order to study the eye globe behavior during IOP measurement process. Understanding the material properties of the eye and specially the cornea is the first essential step for the modeling purposes. To calculate the material properties of the cornea, images from cornea deformation due to the applied air jet were acquired from Corvis ST and were converted to corneal deformation profiles by image processing techniques. By excluding the eye globe rigid body movement (retraction) from the total eye displacement, pure deformation of the cornea is obtained and is used to estimate the corneal material properties. However, as for the cornea, a realistic material model is still a controversial issue; this study tried to make this clearer. To this aim, the corneal biomechanical response is investigated by using a four-element viscoelastic model called Burgers model, which is characterized by two sets of ex-vivo and in-vivo experimental data. In the first approach, by using creep test data from the literature, viscoelastic material properties are calculated by employing the least square method. In the second approach, corneal pure deformation was used to calculate the material properties of the viscoelastic model. By calculating the retardation time, contribution of the material viscosity during the test is estimated. Although the results of the both approaches are quantitatively different, they qualitatively show that viscosity effects do not have much contribution in the cornea response during dynamic tests. As it is obvious that the cornea behaves like a nonlinear elastic material, the Neo-Hookian hyperelastic material model is selected for the FEA. Then, by employing inverse finite element method, which is a combination of FEM and an optimization algorithm, cornea biomechanical properties are evaluated in order to reach cornea pure deformation. The influence of IOP on the estimated corneal material properties is also studied in order to investigate the sensitivity of the estimated material properties to the IOP. In a similar way and by employing a simple one-dimensional model, the biomechanical properties of the fat tissue located behind eye are determined by combining an ODE solver and an optimization algorithm in order to determine the eye globe movement. Combination of these two models forms a model for the whole eye globe with the ability to study the behavior of the cornea in tonometry tests. Since ORA tonometer is more popular than other tonometers in Iran, this tonometer was chosen to be studied by finite element modeling. The effects of maximum applied air pressure, IOP, and corneal geometry on the behavior of the cornea were studied by utilizing this model. Studies show that corneal hysteresis (CH) introduced by ORA is related to none of the investigated parameters. It was also shown that IOP is more sensible to the maximum displacement of corneal apex than to the applanation times. Accordingly, it is suggested to use displacement of the corneal apex as an indirect parameter for IOP measurement. Keywords: Intraocular pressure, Cornea, Viscoelasticity, Inverse finite element model, ORA, Corvis ST
:چشم به‌عنوان یکی از مهم‌ترین اعضای بدن انسان نیازمند شناخت دقیق‌تر است. این شناخت باعث شناسایی بهتر بیماری‌ها و بهبود انجام فرآیند درمان می‌شود. هدف از انجام پژوهش حاضر بررسی رفتار بیومکانیکی چشم و به‌خصوص قرنیه در آزمون تونومتری ORA است. در گام اول با استفاده از تصاویر خروجی تونومتر کرویس و پردازش آن‌ها، تغییر شکل خالص قرنیه و جابه‌جایی بافت چربی پشت کره چشم محاسبه شد. از تغییر شکل خالص قرنیه برای تعیین خواص بافت قرنیه و از جابه‌جایی بافت چربی برای مدل‌سازی شرایط مرزی کل چشم استفاده می‌شود. از آن‌جایی که رفتار کلی چشم در آزمون‌های تونومتری به‌صورت مشخص تعیین نشده است، در ابتدا با استفاده از مدل ویسکوالاستیک برگر و مقایسه اختلاف‌فاز ایجاد شده بین بارگذاری و تغییر شکل، نوع رفتار قرنیه و بافت چربی مشخص گردید. در این بررسی مشخص گردید که ذات ویسکوز قرنیه به علت نرخ بالای بارگذاری تحریک نشده و قرنیه رفتاری الاستیک غیرخطی از خود نشان می‌دهد. همچنین نشان داده شد که بافت چربی پشت کره چشم، در حین انجام آزمون‌های تونومتری دینامیکی رفتاری ویسکوالاستیک از خود نشان می‌دهد. سپس با استفاده از روش المان محدود معکوس و با در نظر گرفتن تغییر شکل خالص قرنیه، خواص بافت قرنیه به‌گونه‌ای تعیین شد که قرنیه تغییر شکلی مشابه با تغییر شکل آزمون تونومتری کرویس داشته باشد. با انجام فرآیندی مشابه، خواص بافت چربی پشت کره چشم تعیین گردید. درنهایت با استفاده از نتایج به دست آمده مدل المان محدودی از آزمون تونومتری ORA ساخته شد و اثر هندسه، فشار درون‌چشمی و فشار هوای اعمالی بررسی گردید. با توجه به نتایج حاصله مشخص گردید که بیشینه جابه‌جایی نوک قرنیه معیار مناسب‌تری نسبت به فشار در زمان‌های صاف‌شدگی جهت سنجش فشار درون‌چشمی محسوب می‌شود.‌ همچنین به نظر می‌رسد که کمیت هیسترزیس قرنیه که هیچ وابستگی به کمیت‌های مورد بررسی ندارد، بیشتر از آن‌که خاصیتی ذاتی از قرنیه باشد، دینامیک سازه را نمایان می‌سازد. لغات کلیدی: فشار درون‌چشمی، قرنیه، ویسکوالاستیسیته، روش المان محدود معکوس، ORA، کرویس

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