Responses of Human Surrogates to Simulated Rear Impact: Velocity and Level Dependent Facet Joint Kinematics

The objective of the present study was to determine the kinematics of the human head-neck complex with specific reference to posterior facet joints as a function of rear impact acceleration. Six intact human head-neck complexes were prepared by fixing the first thoracic vertebra in polymethylmethacrylate. The specimens were oriented such that the Frankfurt plane was horizontal and the cervico-thoracic disc was at an angle of 25 degrees to simulate the normal driving position. Retroreflective targets were inserted to the cervical vertebrae. The specimens were subjected to simulated rear impact accelerations using a minisled apparatus. A series of tests were conducted with velocities of 2.1, 4.6, 6.6, 9.3, and 12.4 km/h. In this study, to achieve the objective, results are presented on the facet joint motions at the C4–5, C5–6, and C6–7 levels as a function of change in velocity. Data were extracted from high-resolution high-speed video photography such that the compressive and sliding motions of the facet joints at the three levels were quantified and compared using statistical techniques. Results indicated that the cervical facet joints demonstrate compression of the most dorsal regions accompanied by distractions of the most ventral regions. Data demonstrated monotonically increasing variations of peak compression and peak sliding kinematics with increasing changes in velocity. However, C5–6 facet joints responded with higher magnitudes of compression at higher changes in velocity than their adjacent counterparts, but lower magnitudes at lower changes in velocity. These data may be of value in the determination of local changes in spinal component kinematics secondary to varying changes in rear impact acceleration.