Vitreous strain during saccadic movements in an infant eye computational model

摘要

Background: A literature review of vitreous materials is formulated where material determination studies are considered in a finite element infant eye model. The purpose of this vitreous material comparison is to consider a vitreous material that would most represent a typical infant vitreous. The infant vitreous is more gel-like and viscous than an adult and would behave differently in a dynamic simulation. This vitreous viscosity has a direct impact with shearing forces in Shaken Baby Syndrome (SBS). The adhesion of the infant vitreous to the retina is considerably stronger than adult which also contributes to shearing forces and retinal hemorrhaging. A model of the infant eye is built based on geometry and materials from literature studies for use in a dynamic simulation modeling a saccade. The saccade is chosen as it represents a typical physiological response and the motion is well documented. The saccade is extremely fast and the quick motion makes it an optimal candidate in a computationally intensive model. The model utilizes a LaGrangian mesh when considering four vitreous material candidates — in keeping with a gel-like substance. In addition, the constitutive material models of the vitreous are also varied for the viscoelastic behavior. A comparison of the shear strain located at the equatorial wall is done between the candidate vitreous materials, as eye radius is proportional to the maximum shear. The vitreous materials implemented from the considered material determination studies vary greatly in compliancy. Some of the vitreous materials are not appropriate to use in a LaGrangian mesh due to high shear strains as a result of compliant shear moduli and lead to uncontrolled mesh shape. Modeling the vitreous requires selection of a constitutive model that is curve fitted to the given data and is representative of the material. In this closed-form infant eye model, materials requiring removal of the vitreous or assuming viscoelastic behavior demon--strated much higher levels of strain than minimally invasive and in vivo methods with assumptions of elastic behavior. Low strain levels, at the posterior eye, indicate high shearing forces that are especially damaging.