Abusive head trauma (AHT) is a prominent cause of death and disability in children in the United States. Retinal hemorrhage (RH) is often used to diagnose AHT, but injury mechanisms and thresholds are unknown. One goal of our research is to develop a finite element (FE) model of the human infant eye to evaluate changes in retinal stress and strain during infant head trauma. However, there are no published data characterizing age-dependent material properties of ocular tissues.

To characterize age and strain-rate dependent properties, we tested sclera and retina from preterm, infant, and adult sheep according to two uniaxial tensile test protocols. In general, scleral strength decreased with age, whereas no age effect was found for the retina. Sclera and retina had a stiffer elastic response when tested at higher strain-rates. Anterior sclera was stiffer than posterior sclera.

In preparation to collect human tissue, viable storage techniques and postmortem time frames for material testing were determined. Pediatric scleral specimens were evaluated up to 24 hours postmortem. Retinal and scleral fresh, frozen-then-thawed, and fixed specimens were also evaluated. Adult sclera maintains its integrity for 24 hours, but immature sclera softened after 10 hours postmortem. Freezing then thawing had minimal effect on the material properties of retina and sclera suggesting this may be a suitable shipping method for the pediatric ocular tissues.

The mechanical data were used to determine appropriate constitutive models for the sclera and retina. The material models were implemented into a FE model of the eye and validated against experimental ocular inflation tests. Finally, a whole model was generated to represent an infant eye subjected to shaking. Vitreoretinal interaction parameters were varied to analyze the changes in retinal stress and strain. Interaction parameters minimally affected retinal stress and strain. Overall, the equatorial retina experienced the greatest stress and strain. Stress and strain increased with the addition of shaking cycles. The anterior retina experienced greater strain than the posterior region after the first cycle and for the remaining rotation sequence. With additional refinement, these models will be valuable to investigate potential injury mechanisms of RH and potentially differentiate abuse-related RH.