Biomechanics of Optic Nerve Tethering in Adduction

A novel hypothesis of optic nerve tethering, which represents a possible cause of primary open-angle glaucoma with normal intraocular pressure, has been motivated by magnetic resonance imaging studies performed in multiple eccentric gaze positions. To evaluate the hypothesis, this thesis reviews the basic anatomical structures of the eye, explores the biomechanical characteristics of ocular tissues, and observes and simulates the phenomenon of optic nerve tethering during adducting eye rotation. While there are many different secondary forms of glaucoma, this thesis mainly considers a pressure-independent contribution to primary open-angle glaucoma. Chapter 1 explains the current state of the glaucoma population, which is steadily increasing every year, and points out what may be a systematic narrowness in thinking in ophthalmology, which emphasizes intraocular pressure as the only known treatment of primary open-angle glaucoma. In Chapter 2, the function, principle, and structure of the ocular globe and optic nerve are briefly reviewed, and the mechanical characteristics of the bilayered optic nerve sheath are explored in depth. In Chapter 3, the mechanical properties of human ocular tissues, which are essential for understanding the biomechanical behavior of the ocular globe, are characterized through uniaxial micro-tensile testing. As an extended study, tensile experiments with and without preconditioning are performed on human ocular tissues to test the necessity for this preconditioning. In addition to the tensile testing, an opto-mechanical correlation between birefringence and tangent modulus of the ocular tissue is discussed as a possible non-invasive method of characterizing tissue. Chapter 4 introduces the hypothesis of optic nerve tethering and summarizes observations from related previous imaging studies. The finite element analysis used in this paper predicts deformation of the optic nerve head and surrounding tissues due to traction by the optic nerve during adduction. After demonstrating the simulation of optic nerve tethering, the thesis explains the pathological connection between optic nerve tethering and normal tension glaucoma. Lastly, chapter 5 concludes and suggests possible treatments for optic nerve tethering in adduction that might be applied if and when the hypothesis is confirmed in the future