A comparison of biomechanical mechanisms of whiplash injury from rear impacts.

Several hypotheses have been proposed to explain the mechanism of injury in whiplash including, pressure on nerve root ganglia, stretching of facet capsules, or damage to facet articular cartilage. These injury mechanisms have not been directly compared in the same study. A comparison could provide insight into the most likely mechanism of whiplash injury. Twenty eight volunteers underwent rear impacts with head and chest acceleration data collected. The same apparatus was used to test 11 cervico-thoracic human cadaveric spines with an instrumented headform attached. Head acceleration, individual vertebral kinematics from high speed video, local nerve root pressure, and facet joint contact pressures were collected during impacts. Each specimen was tested first at an impact acceleration similar to that of volunteers, who reported minimal or no symptoms after the test, then at double the acceleration. Head X (forward) and Z (upward) accelerations of cadaveric specimens were very similar in time sequence and magnitude to those of unprepared volunteers. Pressure around the lower cervical nerve roots ranged from 2.7kPa to 10kPa, and occurred generally after chest but before peak head acceleration. Facets at C4-5 and C5-6 had the highest probability (64% and 71% respectively) of pinching. Neither pressure rise nor pinching changed significantly with increased acceleration. Vertebral intersegmental extension rotations (4 ( o ) -9.5 ( o ) ) and posterior translations (3.7-8.9 mm) peaked near maximum head excursion into the head restraint, at the time of peak head acceleration. Vertebral shear translations showed the largest (and only significant) increases with increased impact acceleration. This data implies that facet shearing was most sensitive to the increased acceleration in this experiment and may be a primary mechanism of cervical spine injury in rear impacts.