Abstract
Abstract: :
Purpose: Mechanical damage to retinal ganglion cells may play a role in the development of glaucomatous optic neuropathy, and inter–individual anatomic variability may predispose certain optic nerve heads (ONHs) to damage. Our goal was to study the biomechanical environment within the human ONH, focussing on individual–specific models. Methods: Human eyes with no known history of optic nerve disease were obtained post mortem and imaged using scanning laser tomography at different IOPs, as previously described. The paired eyes were then fixed at 5 or 50 mmHg, embedded in JB4 plastic, serially sectioned (∼100 µm), and stained with Picrisirius red and Solochrome cyanin. Serial light micrographs were aligned using a non–rigid transformation algorithm based on fiducial landmarks and used to build 3D models of the ONH (Amira 3.0, TGS Inc. France). This involved tissue segmentation, 3D geometry construction, insertion of the ONH region into an ocular shell and meshing for finite element calculations. Models consisted of five components: sclera, lamina cribrosa, pre–laminar and post–laminar neural tissue and pia mater. All tissues were assumed to be homogeneous, incompressible and linearly elastic with Young’s moduli of 3 MPa, 0.3 MPa, 0.03 MPa, 0.03 MPa and 3 MPa respectively. IOP–induced tissue deformation and mechanical stresses were computed using the commercial code ANSYS (ANSYS Inc, USA). Results: Six individual–specific ONH models have been constructed. The relatively stiff sclera and pia mater bear the highest stresses, with stress concentrations occurring at the opening of the scleral canal and at the point of attachment of the pia mater to the sclera and lamina cribrosa. The relatively compliant neural and laminar tissues suffer most of the strains, with strain levels in the lamina cribrosa reaching 13% and being maximal in a rim around the centre. Agreement between computed IOP–induced vitreo–retinal interface deformations and those measured with the HRT were good. Conclusions: Using anatomically realistic models of the ONH, we show that biologically significant levels of strain are experienced by human ONH tissues under pathologic levels of IOP. Ongoing work is evaluating inter–individual differences in ONH biomechanics. The model geometry construction process presents a new opportunity for the visualization and study of optic nerve head biomechanics in humans.
Keywords: computational modeling • intraocular pressure • optic disc