Structure and Biomechanics of the Human Optic Nerve Head

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Primary open angle glaucoma (POAG) represents a serious and growing health problem accounting for ~12% of global blindness. Studies have identified age, ancestral group (racial background), and intra- ocular pressure (IOP) as significant risk factors for the development and progression of POAG. Our long- range goal is to understand the relationship between these established risk factors and the responsive loss of retinal ganglia cells that characterizes this disease. Current wisdom proposes that the biomechanical properties of the optic nerve head (ONH) and lamina cribrosa (LC) play a critical role in defining the pathology of POAG. In preliminary studies, we have identified that regions of the LC susceptible to glaucomatous change (inferior, superior) contain less collagen and show more pressure-induced deformation (more compliant) than other regions of the LC (nasal and temporal). Building on these findings, the objective of this proposal is to define the 3 dimensional biomechanical properties of the ONH by microscopically reconstructing the structural components and biomechanical properties across the LC at a micron scale and relate these properties to POAG risk factors. Based on our preliminary studies we propose the following testable hypotheses:1) That there are regional differences in both the structure and pressure-induced deformation of the LC such that regions susceptible to early glaucomatous change have decreased collagen and increased deformation; 2) That the regional differences in LC structure and pressure-induced deformation increase with age and vary with ancestral group, such that older individuals and those with ancestries more susceptible to POAG will have greater structural changes and show greater pressure induced deformation; 3) That the regional differences in LC structure and pressure-induced deformation are directly related to differences in the 3 dimensional, microscopic elastic modulus such that regions of the LC most susceptible to glaucomatous damage (and related to age and ancestry) will have significant differences in the elastic modulus compared to other regions. To test these hypotheses, we have developed innovative, state of the art technologies to globally assess the three dimensional (3D) structure and biomechanical properties of the human ONH with high resolution. These technologies take advantage of known non-linear optical affects that occur when high intensity photons generated by ultrafast lasers interact with tissue. Using these novel technologies we propose to study ex vivo human eyes from normal individuals and different ancestries at varying ages by the following Specific Aims: 1) Dynamically map in 4 dimensions (time and space) IOP induced changes in collagen fibril and elastic fiber structure in ex vivo human ONH using an artificial pressure chamber and an ultrafast laser; 2) Three dimensionally reconstruct the ONH at high resolution (0.9 mm lateral and 2 mm axial) to volumetrically measure the regional changes in structure and relate these to the measured pressure induce deformations in the same eye; 3) Measure the regional (superior vs. inferior etc.) biomechanical properties of ex vivo human ONH to relate structure to the biomechanical properties and susceptibility to POAG. We expect that these investigations will provide new, and critically important, information concerning the biomechanical properties of the human ONH and provide a clearer understanding of the risk factors for POAG.