Neck injury response to direct head impact.

Previous in vivo studies have observed flexion of the upper or upper/middle cervical spine and extension at inferior spinal levels due to direct head impacts. These studies hypothesized that hyperflexion may contribute to injury of the upper or middle cervical spine during real-life head impact. Our objectives were to determine the cervical spine injury response to direct head impact, document injuries, and compare our results with previously reported in vivo data. Our model consisted of a human cadaver neck (n=6) mounted to the torso of a rear impact dummy and carrying a surrogate head. Rearward force was applied to the model's forehead using a cable and pulley system and free-falling mass of 3.6kg followed by 16.7kg. High-speed digital cameras tracked head, vertebral, and pelvic motions. Average peak spinal rotations observed during impact were statistically compared (P<0.05) to physiological ranges obtained from intact flexibility tests. Peak head impact force was 249 and 504N for the 3.6 and 16.7kg free-falling masses, respectively. Occipital condyle loads reached 205.3N posterior shear, 331.4N compression, and 7.4Nm extension moment. We observed significant increases in intervertebral extension peaks above physiologic at C6/7 (26.3° vs. 5.7°) and C7/T1 (29.7° vs. 4.6°) and macroscopic ligamentous and osseous injuries at C6 through T1 due to the 504N impacts. Our results indicate that a rearward head shear force causes complex neck loads of posterior shear, compression, and extension moment sufficient to injure the lower cervical spine. Real-life neck injuries due to motor vehicle crashes, sports impacts, or falls are likely due to combined loads transferred to the neck by direct head impact and torso inertial loads.