Biomechanics of human thoracolumbar spinal column trauma from vertical impact loading.

Recent studies suggest that dorsal spine injuries occur in motor vehicle crashes to restrained occupants. Compression/compression-flexion injuries occur in frontal crashes due to seat pan and vertical loading. While injuries, mechanisms and tolerances for neck injuries have been determined, thoraco-lumbar spine data are very limited. The objective of the study was to determine the biomechanical characteristics associated with such spinal injuries due to vertical loading. Upper thoracic (T2-T6), lower thoracic (T7-T11) and lumbar (T12-L5) columns from post mortem human surrogates were procured, fixed at the ends and dropped from three heights: the first two impacts designed as non-failure tests and the final was the failure test. Intermittent evaluations consisted of palpations and x-rays. Injuries were assessed using posttest x-rays and computed tomography scans. The age, stature, total body mass and body mass index of three PMHS were: 50 years, 164 cm, 66.9 kg, and 24.7 kg/m(2). The mean peak forces from 24 tests for the upper and lower thoracic and lumbar spines for varying drop heights ranged from 1.6 to 4.3, 1.3 to 5.1, and 1.3 to 6.7 kN, respectively. All peak forces increased with increasing drop heights. Injuries to the three spines included unstable vertebral body and posterior element (bipedicular and lamina) compression fractures and posterior complex disruptions. Logistic regression analysis indicated that peak forces of 3.4 and 3.7 kN are associated with 50% probability of fracture. These results indicate the initial tolerance limits of dorsal spines under vertical loading.