Assessing Cervical Spine Response to Head-First Impact Using Vertebral Segments, Head-Neck, and Full-Body Computational Human Models

Abstract

Head-first impact (HFI), which can occur in automotive rollovers and sports collisions, is associated with a high risk of cervical spine injury. Cervical spine injuries from HFI such as fracture-dislocations frequently lead to severe spinal cord injuries and in some cases death, as reported in field data and epidemiology. Experimentally, isolated motion segments have been tested in compression and bending to mimic loads incurred in HFI, while cadaveric head and necks with torso surrogate masses (TSMs) and full body (FB) cadavers have been inverted and dropped to investigate HFI. However, isolated segment tests are limited in producing the complex kinematics of HFI, and TSM response has not been quantified with respect to FB testing. Recently, computational human body models (HBMs) have been developed to simulate humans in injurious loading conditions, but have only seen limited application in HFI. In this study, computational models were applied to investigate HFI, using an isolated vertebral segment model, an isolated head and neck with a TSM, and a contemporary FB human model. First, an existing and validated cervical motion segment model was loaded in combined compression and flexion relevant to HFI, to investigate the loads and moments associated with fracture-dislocation failures. Next, TSM and FB head-first impacts were modelled using a contemporary HBM in three postures (flexed, neutral, extended) at three impact velocities. Finally, the FB model was compared with a unique set of experimental full body cadaver HFI tests. In isolated segment loading, combined compression and flexion produced hard tissue failure patterns reported in fracture-dislocations. Fracture-dislocation was achieved by simultaneously rotating and translating the superior vertebra anteriorly. Comparing the isolated head and neck TSM and FB models, the TSM condition demonstrated higher neck forces, internal energy, and a larger volume of hard tissue failure compared to the FB models under the same impact conditions. Despite similar head contact forces between TSM and FB, the compliant thorax of the FB model reduced the neck forces by half, which significantly reduced corresponding energy stored in the neck tissues. The neutral and extended neck postures predicted higher neck forces due to facet joints engaging, while neck flexion in the flexed posture reduced neck forces by misaligning the spine from the impact. Finally, it was found that the FB model had similar head impact forces and comparable T1 rotation to cadaveric HFI experiments, but a high sensitivity to initial posture was identified. This study identified forces and moments that can create a fracture-dislocation in a motion segment using prescribed boundary conditions. The TSM and FB simulations demonstrated compression loads and moments of a similar magnitude to the motion segment, but differed in timing, generating higher axial loads leading to the onset of fracture in the spine. The neck loads were higher using the TSM boundary condition compared to the FB condition. Both TSM and FB models identified the importance of neck posture on response, showing that an initially extended neck posture leads to higher neck forces compared to a flexed posture. This study identified the importance of full body boundary conditions for the simulation of HFI, the complex dependency of kinematic and kinetic response on neck posture, providing model results in agreement with the small number of full body experiments. Further experimentation was recommended to provide detailed measurements necessary for model assessment and validation. Future computational studies will integrate the motion segment and FB results to improve the understanding of fracture-dislocation.