A Quasi-Static Analytical Sagittal Plane Model of the Cervical Spine in Extension and Compression 912917

Axial loading of the head-neck complex in a head first collision is a major cause of traumatic cervical spine and spinal cord injuries. It has been suggested (McElhaney, 1989) that cervical spine fracture is not observed when the head and neck are forced into extension. To evaluate this posture as an injury risk reducing strategy and to estimate the loading imposed on the structures of each cervical segment a quasi-static analytical sagittal plane model of the cervical spine in extension and compression was developed in conjunction with an instrumented physical test model. The modelled structures included the anterior longitudinal ligament (ALL) the Longus Colli (Lco) and Longus Capitis (Lca) muscles, the anterior musculature of the neck (Ma), the intervertebral discs (IVD) and the spinous processes (Sp). Input parameters included the compressive and shear forces and the moment of force obtained from the altanto-occipital junction of a Hybrid III anthropometric test dummy (ATD) subjected to face first collisions against a fixed barrier with the neck in extension. MRI scans of human subjects were used to determine muscle cross sectional areas, appropriate geometric scaling between the ATD neck and the human cervical spine, and appropriate moment arm lengths for the modelled structures. Moments were taken about the facet joint at each cervical level considered the mechanical fulcrum in full extension. Stress-strain relationships obtained from the literature were used to predict the passive muscle forces that occurred under impact and well before the possibility of modulating myoelectric activity. Model output included the load at each cervical spine segment level and the loads on each of the structures noted above. Based on a bareheaded ATD travelling at 1.8 m.s⁻¹ the model output suggests that the ALL and neck musculature support only small percentage of the compressive force (2.5-5%) while the spinous processes that are in contact are loaded to a significant proportion of their failure tolerance.