Blast Injury in the Spine: Dynamic Response Index Is Not an Appropriate Model for Predicting Injury

Abstract

BackgroundImprovised explosive devices are a common feature of recent asymmetric conflicts and there is a persistent landmine threat to military and humanitarian personnel. Assessment of injury risk to the spine in vehicles subjected to explosions was conducted using a standardized model, the Dynamic Response Index (DRI). However, the DRI was intended for evaluating aircraft ejection seats and has not been validated in blast conditions.Questions/purposesWe asked whether the injury patterns seen in blast are similar to those in aircraft ejection and therefore whether a single injury prediction model can be used for both situations.MethodsUK military victims of mounted blast (seated in a vehicle) were identified from the Joint Theatre Trauma Registry. Each had their initial CT scans reviewed to identify spinal fractures. A literature search identified a comparison population of ejected aircrew with spinal fractures. Seventy-eight blast victims were identified with 294 fractures. One hundred eighty-nine patients who had sustained aircraft ejection were identified with 258 fractures. The Kruskal-Wallis test was used to compare the population injury distributions and Fisher’s exact test was used to assess differences at each spinal level.ResultsThe distribution of injuries between blast and ejection was not similar. In the cervical spine, the relative risk of injury was 11.5 times higher in blast; in the lumbar spine the relative risk was 2.9 times higher in blast. In the thoracic spine, the relative risk was identical in blast and ejection. At most individual vertebral levels including the upper thoracic spine, there was a higher risk of injury in the blast population, but the opposite was true between T7 and T12, where the risk was higher in aircraft ejection.ConclusionsThe patterns of injury in blast and aircraft are different, suggesting that the two are mechanistically dissimilar. At most vertebral levels there is a higher relative risk of fracture in the blast population, but at the apex of the thoracic spine and in the lower thoracic spine, there is a higher risk in ejection victims. The differences in relative risk at different levels, and the resulting overall different injury patterns, suggest that a single model cannot be used to predict the risk of injury in ejection and blast.Clinical RelevanceA new model needs to be developed to aid in the design of mine-protected vehicles for future conflicts.

Biomechanical tolerance of whole lumbar spines in straightened posture subjected to axial acceleration

“…However, lower spine injuries account for only 1% of spinal injuries in civilians . Other studies supported this by demonstrating 53% of 266 spinal fractures attributed to underbody blast affected T12‐L5 levels . Caudal injury migration may be attributable to severe loading conditions in military personnel during aircraft ejections, helicopter crashes, or underbody blasts .…”

An improved spinal injury parameter model for underbody impulsive loading scenarios

“…It has also been observed that the injuries are predominantly distributed in the thoracolumbar spine. 1,14 It was also pointed out in Stemper et al 6 the importance of the inclusion of spine geometry and consideration of the eccentric loading on the spine to assess the occurrence of burst and wedge fractures of the vertebral segments. Thus, an injury parameter relating the compressions of all springs in between T1 and L5 were taken into account, and their sum was proposed as the injury parameter.…”

“…These are mainly caused due to high rate axial loading resulting in compression of vertebral height and increase in angulations of each vertebrae. Spurrier et al reported that wedge compression and burst fractures were most common in the blast loading scenarios and ejection injuries. It has also been observed that the injuries are predominantly distributed in the thoracolumbar spine .…”

“…Spurrier et al reported that wedge compression and burst fractures were most common in the blast loading scenarios and ejection injuries. It has also been observed that the injuries are predominantly distributed in the thoracolumbar spine . It was also pointed out in Stemper et al the importance of the inclusion of spine geometry and consideration of the eccentric loading on the spine to assess the occurrence of burst and wedge fractures of the vertebral segments.…”

“…Also, it was reported in Griffith that “DRI have no apparent practical utility in rocket assisted seat ejection system,” as the SDOF DRI model does not take the behaviour of the whole spine into account. Spurrier et al pointed out that DRI model is not an appropriate model of predicting injuries in underbody blasts due to the differences in injury pattern and pointed out the need for the ability of the risk prediction model to incorporate the posture of the spine into account. Thus, there is a need to develop alternate injury parameters that can perform better than the DRI model.…”