Investigation of Neck Posture and Muscle Activity on Cervical Spine Impact Kinematics Using a Finite Element Human Body Model

Abstract

Whiplash-associated disorders (WAD) define a broad range of symptoms affecting the neck such as pain and stiffness, reported in up to half of motor vehicle collisions. WAD are typically associated with, but not limited to, low-severity rear impacts. The high incidence of WAD and high socioeconomic cost have led to significant, but still inconclusive, efforts to better understand the associated causal injury mechanisms. Neck posture and muscle behaviour are known factors that contribute to neck injuries during low-severity vehicle impacts. Quantifying the effects of such parameters at the tissue level is challenging in experimental studies but may be informed by computational human body models (HBMs). However, three limitations in neck models have been identified: (1) neck muscle controllers were often tuned to a narrow set of specific load cases, (2) neck models were unable to predict the S-shape (upper cervical spine flexion) magnitude observed in experiments during rear impacts, and (3) defining tissue-level injury thresholds remain elusive for the neck. To address these challenges, three studies were defined for this thesis using a contemporary head and neck finite element model from an average-stature male HBM (Global Human Body Models Consortium (GHBMC)) with the aim of enhancing and evaluating the tissue-level response associated with WAD injury risk following rear impact. In the first study, a new closed-loop controller with a single set of parameters for neck muscle activation based on known reflex mechanisms was implemented in the GHBMC model. The updated model was assessed over a range of impact conditions. The closed-loop controller had an average cross-correlation to the experimental data of 0.699 for 14 load cases, including frontal, rear and lateral impacts, within 2% to 9% of previous calibrated open-loop approaches. In the second study, a novel methodology was developed to integrate pre-tension in the neck muscles based on experimental cadaveric and volunteer data and assessed in rear impact scenarios. Only the model with pre-tension achieved flexion of the upper cervical spine at the same magnitude as reported in impact tests with volunteers. Pre-tension increased the muscle tissue strain relative to cases with no pre-tension, and, in some cases, led to potentially injurious-level strains, reinforcing that the initial muscle strain is essential for evaluating WAD injury risk. In the third study, the methods from the first and second studies (closed-loop muscle activation controller and muscle pre-tension) were combined to assess possible WAD injury mechanisms based on tissue-level analysis of stresses and strains in 4g to 10g rear impacts. The existing injury metrics and tissue-level muscle strains identified that hyperextension was the main injurious phase in low-severity rear impacts. In addition, muscle pre-tension and activation changed the distribution of muscle strains, better representing the injury regions reported in the literature. New model developments and knowledge obtained from the three studies completed in this research can be generalizable to other HBMs and can be applied to evaluate the efficacy of vehicle safety systems, ultimately reducing injury risk and diminishing societal costs related to low-severity neck injury in the future. Further, the enhanced neck model developed in this work has identified possible areas of experimental interest for future neck injury research.