Wednesday, September 26, 2018 1:00 PM – 2:00 PM Spinal Trauma: 50. Military lumbar injuries from vertical loading are more unstable than civilian automobile trauma: field and biomechanical studies

BACKGROUND CONTEXT Many clinical and biomechanical studies have delineated the injuries and injury mechanisms to the thoracolumbar spine in the civilian population. The stability of the spine is assessed based on the three-column, AO, and other classifications. Thoracolumbar injuries are also prevalent in military populations due to recent events such as underbody blast loading from improvised explosive devices, IEDs. These injuries have not been reproduced in a realistic laboratory setting. The stability associated with military injuries and their comparison with the civilian automobile trauma have not been addressed. PURPOSE Reproduce military injuries, determine their mechanisms and compare with the civilian trauma. STUDY DESIGN/SETTING Field data using motor vehicle databases and biomechanical study using a human cadaver model simulating military loads. PATIENT SAMPLE Fourteen human cadaver specimens and civilian field data. OUTCOME MEASURES Injuries to human cadaver specimens, classification based on clinical scales, and identification of injuries to the civilian trauma population. METHODS Real-world data consisted of an analysis of motor vehicle crash injuries to restrained occupants using US national databases for the years 1993-2012. Thoracolumbar injuries and their patterns were obtained from medical records, X-ray images, and CT scans. For the biomechanical study, 14 human cadaver thoracolumbar spines (T12-S1) were procured, pretest CT scans were taken, and QCT-based BMD were obtained. They were fixed at the ends using PMMA, and they were positioned according to the military seating posture, that is the T12 midbody was oriented at 5° above the horizontal, and the line joining the center of T12-L1 and L5-S1 discs was aligned at a posterior angle of 12° from the vertical line. X-ray studies were taken in the aligned posture before loading, while the specimen was still in the impacting device. The specimens were accelerated from S1 along the caudal to cephalad direction. The impact trajectory was applied from a custom vertical accelerator, simulating underbody blast conditions, with velocities up to 9 m/s and time to peak within 0.002 to 0.01 seconds. Forces and bending moments were recorded at the distal and proximal ends using six-axis load cells. Post-test X-ray images and CT were obtained, and dissection was done. Injuries were identified and classified using standard classification systems. RESULTS In the field data study, the thoracolumbar junction was found to be most susceptible to fractures, more commonly at the L1 level (30%). The vertebral body compression was most common (37%) followed by burst (33%) fractures. In the biomechanical study, L1 was most frequently involved in the injuries, and vertebral body compression and burst fractures occurred in all specimens. However, 58% had burst fractures with posterior column involvements, and 43% had fractures at more than one level and all were contiguous. These burst fracture injuries were more severe and typically of the three-column type, ie, unstable. CONCLUSIONS Similarities of burst fractures were noted in military and civilian populations; however, there was an increased incidence of L1 injuries in the military and a higher occurrence of injuries to the posterior columns in the military. The proposed mechanism is that the personal protective equipment required by the military may alter the normal seating posture, exposing the individual to altered axial load patterns allowing for greater involvement of the posterior column resulting in more unstable injuries with potential consequences. Because of the greater instability, MRI should be considered to assess the status of the adjacent vertebral bodies and ligamentous structures. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.