A New Understanding of the Mechanism of Injury to the Pelvis and Lower Limbs in Blast

Abstract

Dismounted complex blast injury (DCBI) has been one of the most severe forms of trauma sustained in recent conflicts. This injury has been partially attributed to limb flail; however, the full causative mechanism has not yet been fully determined. Soil ejecta has been hypothesized as a significant contributor to the injury but remains untested. In this study, a small-animal model of gas-gun mediated high velocity sand blast was used to investigate this mechanism. The results demonstrated a correlation between increasing sand blast velocity and injury patterns of worsening severity across the trauma range. This study is the first to replicate high velocity sand blast and the first model to reproduce the pattern of injury seen in DCBI. These findings are consistent with clinical and battlefield data. They represent a significant change in the understanding of blast injury, producing a new mechanistic theory of traumatic amputation. This mechanism of traumatic amputation is shown to be high velocity sand blast causing the initial tissue disruption, with the following blast wind and resultant limb flail completing the amputation. These findings implicate high velocity sand blast, in addition to limb flail, as a critical mechanism of injury in the dismounted blast casualty.

Keywords: biomechanics; blast injury; fracture; military; mouse; sand; soil; traumatic amputation.

FIGURE 1

Experimental sand sizes used, scaled to human values, shown alongside ideally distributed particle sizes. a = human median value. b = human 85th centile. a = lower limit of experimental sand range. b = upper limit of experimental sand range. % pass (combined) describes the percentage of total volume of sand passing a specific sieve size; sieve size (mm) relates to the diameter of each hole within the sieve.

FIGURE 2

Gas gun with under-body sand blast mounting platform, fenestrated steel fences, and mouse. Mouse represented with model.

FIGURE 3

(A) Aerial view of schematic illustrating initial sand stream passing through offset fenestrated steel fences causing dispersion of the sand prior to impact with the specimen. (B) Oblique view of schematic illustrating initial sand stream passing through offset fenestrated steel fences causing dispersion of the sand prior to impact with the specimen. (C) Photograph showing the initial sand stream converted into multiple streams. (D) Photograph showing multiple streams dispersing into a widely distributed spread of high velocity sand.

FIGURE 4

Left: uninjured mouse. Right: mouse injured with sand blast at 252 m/s sustaining pelvic fractures with (A) sacroiliac joint disruption and (B) pubic rami fractures, (C) abdominal injury with free air in the abdomen, perineal injury, and (D) an open tibial fracture with surrounding extensive soft tissue loss. The increased density on the injured mouse represents sand debris.

FIGURE 5

(A–F) Injury risk curves for perineum injury (A), lower limb degloving (B), abdominal injury (C), traumatic amputation (D), and pelvic fracture (E) as a function of average sand velocity; 95% CI is represented with dashed lines. (F) Shows the respective v₂₅, v₅₀, and v₇₅ for each category of injury; 95% CI is represented with variability whiskers.

FIGURE 6

(A–D) The mechanism of injury of dismounted blast trauma. (A) Casualty stands on an IED which detonates, causing the initial blast wave to compress the surrounding soil. (B) Sand is ejected at high velocity toward the casualty, causing soft tissue degloving and skeletal disruption. (C) The casualty is impacted by the blast wind, resulting in lower limb flail with separation of the pubic symphysis. (D) The blast wind completes the amputation at the level of the initial disruption, whilst continued leg flail results in opening of the sacroiliac joint and vascular injury.