The Dynamic Responses of the Cervical Spine: Buckling, End Conditions, and Tolerance in Compressive Impacts

This study explores the dynamics of head and cervical spine impact with the specific goals of determining the effects of head inertia and impact surface on injury risk. Head impact experiments were performed using unembalmed head and neck specimens from 22 cadavers. These included impacts onto compliant and a rigid surfaces with the surface oriented to produce both flexion and extension attitudes. Tests were conducted using a drop track system to produce impact velocities on the order of 3.2 m/s. Multiaxis transduction recorded the head impact forces, head accelerations, and the reactions at T1. The tests were also imaged at 1000 frames/sec. Injuries occurred 2 to 30 msec following head impact and prior to significant head motion. Head motions were not found to correlate with injury classification. Decoupling was observed between the head and T1, resulting in a lag in the force histories. Cervical spine loading due to head rebound constituted up to 54±16 percent of the total axial neck load for padded impacts and up to 38±30 percent for the rigid impacts. Dynamic buckling was also observed; including first order modes and transient higher order modes which shifted the structure from a primarily compressive mode of deformation to various bending modes. The average load at failure was 2243±572 N for males and 1061 ± 273 N for females. The difference between male and female tolerance was significant p = 0.0015). Impacts onto the padded surfaces produced significantly larger neck impulses (p = 0.00023) and a significantly greater frequency of cervical spine injuries than rigid impacts (p = 0.043). The impact angle was also correlated with injury risk (p < 0.00001). These experiments demonstrate that in the absence of head pocketing, the head mass can provide sufficient constraint to cause cervical spine injury. The buckling modes illustrate the kinematic complexity of cervical spine dynamics and may explain why injuries can occur at different vertebral levels and with widely varying mechanism in compressive head impacts. These experiments also suggest that highly deformable padded contact surfaces should be employed carefully in environments where there is the risk for cervical spine injury; however, the orientation of the head, neck, and torso relative to the impact surface is of greater significance.