3D Finite Element Modeling of Cervical Musculature and its Effect on Neck Injury Prevention

Sammanfattning: Injuries to the head and neck are potentially the most severe injuries in humans, since they may damage the nervous system. In accidents, the cervical musculature stabilizes the neck in order to prevent injury to the spinal column and is also a potential site for acute muscle strain, resulting in neck pain. The musculature is consequently an important factor in the understanding of neck injuries. There is however a lack of data on muscle response and little is known about the dynamics of the individual muscles. In this thesis the numerical method of Finite Elements (FE) is used to examine the importance of musculature in accidental injuries. In order to study the influence of a continuum musculature, a 3D solid element muscle model with continuum mechanical material properties was developed. It was hypothesized that a 3D musculature model would improve the biofidelity of a numerical neck model by accounting for the passive compressive stiffness, mass inertia, and contact interfaces between muscles. A solid element representation would also enable the study of muscle tissue strain injuries.A solid element muscle model representing a 50th percentile male was created, based on the geometry from MRI, and incorporated into an existing FE model of the spine. The passive material response was modeled with nonlinear-elastic and viscoelastic properties derived from experimental tensile tests. The active forces were modeled with discrete Hill elements. In the first version of the model the passive solid element muscles were used together with separate active spring elements. In the second version the active elements were integrated in the solid mesh with coincident nodes. This combined element, called the Super-positioned Muscle Finite Element (SMFE), was evaluated for a single muscle model before it was incorporated in the more complex neck muscle model. The main limitation of the SMFE was that the serial connected Hill-type elements are unstable due to their individual force-length relationship. The instabilities in the SMFE were minimized by the addition of passive compressive stiffness from the solid element and by the decreased gradient of the force-length relation curve. The solid element musculature stabilized the vertebral column and reduced the predicted ligament strains during simulated impacts. The solid element compressive stiffness added to the passive stiffness of the cervical model. This decreased the need for additional active forces to reproduce the kinematic response of volunteers during impact. The active response of the SMFE improved model biofidelity and reduced buckling of muscles in compression. The solid element model predicted forces, strains, and energies for individual muscles and showed that the muscle response is dependent on impact direction and severity. For each impact direction, the model identified a few muscles as main load carriers that corresponded to muscles generating high EMG signals in volunteers. The single largest contributing factor to neck injury prediction was the muscle active forces. Muscle activation reduced the risk of injury in ligaments in high-energy impacts. The most urgent improvements of the solid element muscle model concerns: the stability of the SMFE; the boundary conditions from surrounding tissues; and more detailed representations of the myotendinous junctions. The model should also be more extensively validated for the kinematical response and for the muscle load predictions.It was concluded that a solid muscle model with continuum mechanical material properties improves the kinematical response and injury prediction of a FE neck model compared to a spring muscle model. The solid muscle model can predict muscle loads and provide insight to how muscle dynamics affect spinal stability as well as muscle acute strain injuries.