Abstract
Purpose: :
To determine if local directional stiffness in the LC - induced by local beam orientation - influences IOP-related deformations, stress and strain within the ONH, and if so, how local directional stiffness interacts with other factors governing LC biomechanics.
Methods: :
An eye-specific 3D finite element model of the LC and sclera of a single normal monkey eye was constructed using geometric information from a digital 3D ONH reconstruction and scleral topography and thickness measurements. Models were run while altering the degree of LC anisotropy from no directional stiffness (isotropic) to highly direction-dependant stiffness (anisotropic). For the anisotropic cases, the LC was modeled with enhanced LC stiffness in the radial direction to simulate peripheral beams inserting radially into the scleral canal wall (Roberts MD, IOVS 2008). Additionally, we varied the LC elastic constant, scleral stiffness, and five geometric factors (scleral canal radius and eccentricity, LC position and thickness, and scleral thickness) simultaneously. 288 unique versions of the model were assessed for an IOP elevation from 10 to 15 mmHg to understand ONH biomechanics at physiologic IOP. A two-level full factorial analysis was used to rank the most important individual and interacting determinants of the LC mechanical response.
Results: :
The top three individual determinants of LC stress and tensile strain were the LC elastic constant, scleral stiffness, and LC anisotropy, respectively. Increasing LC anisotropy decreased scleral canal expansion, LC posterior deformation, and LC tensile and compressive strains significantly (P<0.0001). Interactions between LC anisotropy and scleral canal radius, LC elastic constant and LC position were also important determinants of LC strain and deformation (P<0.0001; Fig 1).
Conclusions: :
Our results suggest that LC beam orientation contributes significantly to ONH biomechanics, and that local LC beam orientation is optimized to reduce IOP-related deformation and strain. Local LC beam orientation should therefore be considered when modeling ONH connective tissue biomechanics.
Keywords: lamina cribrosa • optic nerve • intraocular pressure