Analysis of head impacts causing neck compression injury.

OBJECTIVE: Human cadavers have been subjected to inverted drop, linear, and pendulum impacts to the top of the head, causing neck compression injury. The data are not comparable on the basis of impact velocity because of differing impact masses and test conditions. This study analyzed the published biomechanical data and used peak head velocity to merge the datasets. Correlations were determined between biomechanical responses and serious injury (AIS 3+).
METHODS: Three studies were found involving 33 inverted drop tests and three others involving 42 linear or pendulum impacts to the top of a cadaver's head. Various biomechanical responses were measured in the tests. The datasets could not be meaningfully merged on the basis of impact velocity. The coefficient of restitution (e) was determined and the peak head velocity calculated for tests with missing data. This allowed the datasets to be merged and statistically analyzed for relationships between head velocity, impact force, and serious injury. Power functions were fit to the biomechanical data, t-tests conducted for significant differences in injury, and logit risk functions determined.
RESULTS: The coefficient of restitution was e = 0.24 +/- 0.16 (n = 19) for the drop tests and e = 0.21 +/- 0.12 (n = 20) for the impact tests. Peak head velocity was 22% higher than the impact velocity for the drop tests but -20% lower in the impact tests. Head velocity averaged 6.32 +/- 1.29 m/s (n = 51) causing serious injury and 3.75 +/- 2.16 m/s (n = 24) without injury (t = 5.39, p = 0.00001, df = 31). Impact force was 7,382 +/- 3,632 N with injury and 3,760 +/- 3,528 N without (t = 3.95, p = 0.0003, df = 42). A power function fit the impact force versus head velocity data (F = 374V(h)(1.565), R(2) = 0.758).
CONCLUSION: Peak head velocity was determined for inverted drop and impact tests as a means of merging and analyzing cadaver data on serious injury for impacts to the top of the head. There are relationships between head velocity, impact force, and serious injury. A 15% risk of serious injury is at 2.3 m/s (5.1 mph) head velocity and 50% risk at 4.2 m/s (9.4 mph); however, more data are needed in the 2-4 m/s head velocity range to clarify injury risks. In addition, many factors influence the risk of the neck injury, including the age and physical condition of the person; orientation of the head, neck, and torso; and location of impact and interface.