Sensitivity of material model parameters on finite element models of infant head impacts.

Finite element (FE) models of human infant heads can be used in forensic investigations to infer whether a given pattern of head injuries could have resulted from a hypothetical scenario. This requires accurate models of the behaviour of the head tissues. Material models for human infant head tissues have been developed using experimental data from both infant and adult tissues. Experimental data for infants are scarce due to ethical considerations. To guide future experimental work, a sensitivity analysis of the material model parameters was conducted on a FE model of an infant occipital head impact. A simplified head geometry, consisting of the scalp, skull, suture and brain, was impacted onto a rigid anvil at a speed equivalent to a drop height of 0.3 m. The scalp, suture and brain were represented using hyperelastic material models, while an isotropic elastic model was used for the skull. Three hundred simulations were performed, with the material model parameters varied in each. Spearman's rank correlation was used to determine the influence of each parameter on selected outputs which predict injury level. The elastic modulus and Poisson's ratio for the skull were the most important parameters, followed by the hyperelastic constants for the brain, scalp and suture. It is recommended that future research prioritises increasing experimental datasets of skull elastic modulus, especially at higher loading rates, followed by obtaining data for the skull Poisson's ratio. The results from this sensitivity analysis can ensure that future experimental work makes the best use of scarce tissues.

Preliminary observations of the sequence of damage in excised human juvenile cranial bone at speeds equivalent to falls from 1.6 m.

There is much debate within the forensic community around the indications that suggest a head injury sustained by a child resulted from abusive head trauma, rather than from accidental causes, especially when a fall from low height is the explanation given by a caregiver. To better understand this problem, finite element models of the paediatric head have been and continue to be developed. These models require material models that fit the behaviour of paediatric head tissues under dynamic loading conditions. Currently, the highest loading rate for which skull data exists is 2.81 ms-1. This study improves on this by providing preliminary experimental data for a loading rate of 5.65 ± 0.14 ms-1, equivalent to a fall of 1.6 m. Eleven specimens of paediatric cranial bone (frontal, occipital, parietal and temporal) from seven donors (age range 3 weeks to 18 years) were tested in three-point bending with an impactor of radius 2 mm. It was found that prompt brittle fracture with virtually no bending occurs in all specimens but those aged 3 weeks old, where bending preceded brittle fracture. The maximum impact force increased with age (or thickness) and was higher in occipital bone. Energy absorbed to failure followed a similar trend, with values 0.11 and 0.35 mJ/mm3 for age 3 weeks, agreeing with previously published static tests, increasing with age up to 9 mJ/mm3 for 18-year-old occipital bone. The preliminary data provided here can help analysts improve paediatric head finite element models that can be used to provide better predictions of the nature of head injuries from both a biomechanical and forensic point of view.

Finite element models and material data for analysis of infant head impacts.

Finite element (FE) models of the infant human head may be used to discriminate injury patterns resulting from accidents (e.g. falls) and from abusive head trauma (AHT). Existing FE models of infant head impacts are reviewed. Reliability of the material models is the major limitation currently. Infant head tissue properties differ from adults (notably in suture stiffness and strain-to-failure), change with age, and experimental data is scarce. The available data on scalp, cranial bone, dura, and brain are reviewed. Data is most scarce for living brain. All infant head model to date, except one, have used linear elastic models for all tissues except the brain (viscoelastic or Ogden hyperelastic), and do not capture the full complexity of tissue response, but the predicted whole-head response may be of acceptable accuracy. Recent work by Li, Sandler and Kleiven has used hyperelastic models for scalp and dura, and an orthotropic model for bone. There is a need to simulate falls from greater than one metre, and blunt force impacts.