A computational model of the human head and cervical spine for dynamic impact simulation

Injury to the human neck is a frequent consequence of automobile accidents and has been a significant public health problem for many years. The term `whiplash' has been used to describe these injuries in which the sudden differential movement between the head and torso leads to abnormal motions within the neck causing damage to its soft tissue components. Although many different theories have been proposed, no definitive answer on the cause of `whiplash' injury has yet been established and the exact mechanisms of injury remain unclear. Biomechanical research is ongoing in the field of impact analysis with many different experimental and computational methods being used to try and determine the mechanisms of injury. Experimental research and mathematically based computer modelling are continually used to study the behaviour of the head and neck, particularly its response to trauma during automobile impacts. The rationale behind the research described in this thesis is that a computational model of the human head and neck, capable of simulating the dynamic response to automobile impacts, could help explain neck injury mechanisms. The objective of the research has been to develop a model that_,, can accurately predict the resulting head-neck motion in response to acceleration impacts of various directions and severities. This thesis presents the development and validation of a three-dimensional computational model of the human head and cervical spine. The novelty of the work is in the detailed representation of the various components of the neck. The model comprises nine rigid bodies with detailed geometry representing the head, seven vertebrae of the neck and the first thoracic vertebra. The rigid bodies are interconnected by spring and damper constraints representing the soft-tissues of the neck. 19 muscle groups are included in the model with the ability to curve around the cervical vertebrae during neck bending. Muscle mechanics are handled by an external application providing both passive and active muscle behaviour. The major findings of the research are: From the analysis of frontal and lateral impacts it is shown that the inclusion of active muscle behaviour is essential in predicting the head-neck response to impact. With passive properties the response of the head-neck model is analogous to the response of cadaveric specimens where the influence of active musculature is absent. Analysis of the local loads in the soft-tissue components of the model during the frontal impact with active musculature revealed a clear peak in force in the majority of ligaments and in the intervertebral discs very early in the impact before any forward rotation of the head had occurred. For the case of rear-end impact simulations it has been shown for the first time that the inclusion of active musculature has little effect on the rotation of the head and neck but significantly alters the internal loading of the soft-tissue components of the neck.