Biomechanics of human occupants in simulated rear crashes: documentation of neck injuries and comparison of injury criteria.

The objective of this study was to subject small female and large male cadavers to simulated rear impact, document soft-tissue injuries to the neck, determine the kinematics, forces and moments at the occipital condyles, and evaluate neck injury risks using peak force, peak tension and normalized tension-extension criteria. Five unembalmed intact human cadavers (four small females and one large male) were prepared using accelerometers and targets at the head, T1, iliac crest, and sacrum. The specimens were placed on a custom-designed seat without head restraint and subjected to rear impact using sled equipment. High-speed cameras were used for kinematic coverage. After the test, x-rays were obtained, computed tomography scans were taken, and anatomical sections were obtained using a cryomicrotome. Two female specimens were tested at 4.3 m/s (mean) and the other two were tested at 6.8 m/s (mean), and one large male specimen was subjected to 6.6 m/s velocity. One female specimen tested at 4.1 m/s did not sustain injury. All others produced injuries to soft tissue and joint-related structures that included tearing of the anterior longitudinal ligament, rupture of the ligamentum flavum, hematoma at the upper facet joint, anterior disc disruption at the lower spine, and facet joint capsule tear. Compressive forces (100 to 254 N) developed within 60 ms after impact. Tensile forces were higher (369 to 904) and developed later (149 to 211 ms). While peak shear forces (268 to 397 at 4.3 m/s and 257 to 525 N at 6.8 m/s) did not depend on velocity, peak tensile forces (369 to 391 N at 4.3 m/s and 672 to 904 N at 6.8 m/s) seemed to correlate with velocity. Peak extension moments ranged from 22.0 to 33.5 Nm at low velocity and 32.7 to 46.6 Nm at high velocity. All these biomechanical data attained their peaks in the extension phase (with very few exceptions), which ranged from 179 to 216 ms. The neck injury criterion, NIC, exceeded the suggested limit of 15 m(2)/s(2) in all specimens. Axial force and bending moment data were used to evaluate various neck injury criteria (N(ij), N(TE), peak tension and peak extension). The risk for AIS >/= 3 injury for the combined tension-extension criteria was 30 percent in one female specimen tested at 6.8 m/s. For the other specimens the risk of AIS >/= 3 injury was less than five percent using all criteria.