Finite Element Modeling of Anterior Laminar Disinsertion in Early Experimental Glaucoma

Abstract

Abstract: : Purpose: The pattern and pathophysiologic mechanisms of early glaucomatous connective tissue damage remain uncertain. Continuum finite element modeling of the early glaucomatous monkey optic nerve head has reproduced early glaucomatous laminar hypercompliance by altering laminar material properties only. This study tests the hypothesis that modeling anterior laminar disinsertion (a partial separation of the lamina cribrosa from the sclera at the anterior border of its insertion), without imposing changes in material properties, can also produce hypercompliant deformations. Methods: Geometrically simplified finite element models of the sclera, lamina cribrosa, and dura of the monkey optic nerve head were constructed using Abaqus/CAE (HKS, Inc., RI)). The laminar–scleral insertion was modeled using TIE constraints to join opposing surfaces. A hypothetical separation was introduced by converting sectors of the TIE constraint to free surfaces loaded with elevated IOP. The sectors represented separation over only the anterior third of the insertion (not through–thickness). An initial separation over a 12 degree arc was introduced superiorly, then propagated over a 24 degree arc to model progression, and then over 360 degrees to represent a complete separation of the anterior border. Isotropic material properties of all tissues were taken from the literature. Results: The 360 degree defect resulted in 0–10 microns of separation. Stresses on the affected surfaces were reduced and stresses at the edges of the initial and propagated defects were increased. Most notably, posterior deformation of the intact lamina subject to 20 mmHg elevated IOP was approximately 29 microns relative to normal, while that of the model with a 360 degree anterior separation resulted in approximately 38 microns of posterior deformation relative to normal, or an approximate increase of 30%. Conclusion:Anterior laminar disinsertion may contribute significantly to experimentally observed laminar hypercompliance and should be considered a potential mechanism, along with material property changes, when investigating glaucomatous connective tissue damage of the lamina.

Keywords: lamina cribrosa • sclera • intraocular pressure