Finite element model of ocular injury in abusive head trauma

PURPOSE To develop a finite element analysis of the eye and orbit that can be subjected to virtual shaking forces. METHODS LS-DYNA computer software was used to design a finite element model of the human infant eve, including orbit, fat, sclera, retina, vitreous, and muscles. The orbit was modeled as a rigid solid; the sclera and retina as elastic shells; the vitreous as viscoelastic solid or Newtoman fluid; and fat as elastic or viscoelastic solid. Muscles were modeled as spring-damper systems. Orbit-fat, fat-sclera, sclera-retina, and vitreous nodes-retina interfaces were defined with the use of the tied surface-surface function in LS-DYNA. The model was subjected to angular acceleration pulses obtained from shaking tests of a biofidelic doll (Aprica 2.5 kg dummy). Parametric studies were conducted to evaluate the effect of varying the material properties of vitreous/fat on maximum stress and stress distribution. RESULTS With the vitreous modeled as a Newtonian fluid, the repeated acceleration-deceleration oscillatory motion characteristic of abusive head trauma (AHT) causes cumulative increases in the forces experienced at the vitreoretinal interface. Under these vitreous conditions, retinal stress maximums occur at the posterior pole and peripheral retina, where AHT retinal hemorrhage is most often found. CONCLUSIONS Our model offers an improvement on dummy and animal models in allowing analysis of the effect of shaking on ocular tissues. It can be used under certain material conditions to demonstrate progressive "stacking" of intraocular stresses in locations corresponding to typical AHT injury patterns, allowing a better understanding of the mechanisms of retinal hemorrhage patterns. (J AAPOS 2009-13:364-369)