Part I:
Inflicted head injury by shaking trauma (IHI-ST) is often simulated to better understand the injury mechanisms and to analyze whether violent shaking can cause head injury in infants. Computational models are usually subjected to linear and rotational inputs to simulate shaking scenarios. In existing studies, the head’s rotation center is kept fixed over time during shaking. However, the infant’s head is unlikely to rotate around a fixed pivoting point in real life due to the flexibility of the infant’s neck and the external imposed shaking motion by the perpetrator. It is currently unknown how the location of the rotation center changes over time and how this manifests itself in the expression of the injury mechanisms associated with IHI-ST.
In this study, the variation of the rotation center of an infant’s head during shaking and its potential effect on injury mechanisms were analyzed. First, dynamics of the infant’s head were obtained in shaking experiments with an infant surrogate. Next, the variation of the rotation center was calculated and relations between characteristics of the participants and shaking variables were analyzed.
Key findings: during shaking the location of the head’s rotation center varied in both anterior-posterior and vertical direction with respect to the head, causing the head’s radius of curvature to vary six orders of magnitude. Therefore, head-dynamics and injury mechanisms underlying IHI-ST are possibly simulated incorrectly when using a fixed rotation center. It remains unclear how this affects the validity of IHI-ST injury risk assessments and the injury thresholds on which these assessments are based. Future research should therefore evaluate the performance of head-dynamic simulations regarding IHI-ST.

Part II:
Computational model simulations are extensively used to analyze inflicted head injury by shaking trauma in infants (IHI-ST). Infant head models are usually excited by dynamic inputs, which are applied to a specific point with respect to the head. In existing studies the load application point is assumed to be fixed over time; thereby neglecting spatiotemporal variation of the rotation center during shaking. Therefore, this assumption may be inappropriate, because the location of the heads’ rotation center is in fact not constant over time during shaking. It is unknown to what extent head dynamics are correctly simulated when using a fixed rotation center, hence simulation results regarding injury thresholds and shaking trauma assessment could be invalid.
In this study, loading-methods used in IHI-ST simulations were evaluated for their temporal accuracy in replicating external head-dynamics. First, a mathematical model incorporating spatiotemporal variation of the head’s rotation center was proposed. Secondly, head dynamics were calculated using the proposed mathematical model and existing model-loading methods. Finally, the calculated head dynamics were compared to a reference dataset.
Key findings: in all of the 29 cases from the reference dataset, implementation of a time-varying load application point resulted in an improved temporal replication of shaking dynamics compared to existing model-loading methods. Accelerations of the head in x- and z-direction had a two and four times smaller absolute error over a typical shake cycle than any previously existing finite element model (FEM) for IHI-ST. It remains unclear how implementation of a time-varying load application point affects the dynamics of fluids and tissues inside the skull. Future research should therefore focus on re-evaluating the results of IHI-ST assessment studies and injury threshold studies employing FEM head-models.